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Report  No.  FAA-RD-  73-49 

INVESTIGATION  OF  SITE  COVERAGE  AND  ASSOCIATED 

PROBLEMS  AT  THE  O'HARE  AIRPORT,  CHICAGO,  ILLINOIS, 

ENROUTE  RADAR  BEACON  TEST  SITE 


U56  in 


George  F.  Spingler 


APRIL  1973         V^ 


INTERIM  REPORT 


Document  is  available  to  the  public  through 
the  National  Technical  Information  Service, 
Springfield,    Virginia     22151 


Prepared  for 

IRAN  DEPARTMENT  OF  TRANSPORTATION 

FEDERAL  AVIATION  ADMINISTRATION 

TL 
7X5.  iT7  Systems  Research  &  Development  Service 

Washington  D.  C,  20591 


iransporwioh 

CENTER 
LIBRARY 


NOTICE 

This  document  is  disseminated  under  the  sponsorship 
of  the  Department  of  Transportation  in  the  interest  of 
information  exchange.     The  United  States  Government 
assumes  no  liability  for  its  contents  or  use  thereof. 


\ 


1 .     Report  No. 

FAA-RD-73-49 


2.     Government   Accession  No. 


3.     Re 


ge 


3  5556  031  147283 


4.     Title  and  Subtitle 

•^INVESTIGATION  OF  SITE  COVERAGE  AND  ASSOCIATED 

PROBLEMS  AT  THE  O'HARE  AIRPORT,  CHICAGO,  ILLINOIS, 
ENROUTE  RADAR  BEACON  TEST  SITE^) 


5.  Report  Date 


April  1973 


6.     Performing  Organization  Code 


7.     Author's) 

George  F.  Spingler 


8.     Performing  Organization   Report  No. 

FAA-NA-73-28 


9.     Performing  Organization  Name  and   Address 

^-'Federal  Aviation  Administration 

i-National  Aviation  Facilities  Experimental  Center 
Atlantic  City,  New  Jersey   08405 


10.     Work   Unit  No.   (TRAIS) 


1  1  .      Contract  or  Grant  Nc 


Project   No.    031-241-020 


12.     Sponsoring  Agency  Nome  and  Address 

Department  of  Transportation 
Federal  Aviation  Administration 
Systems  Research  and  Development  Service 
Washington  D.C.   20591 


13.      Type  of   Report  and  Period   Covered 

Interim 
September  1972-September  1972 


14.     Sponsoring   Agency  Code 


15.     Supplementary  Notes 


16.     Abstract 

A  temporary  beacon  test  site  was  installed  adjacent  to  the  Chicago,  Illinois, 
O'Hare  Airport  and  operational  tests  were  conducted  to  determine  its  suita- 
bility for  possible  use  as  a  future  enroute  radar  beacon  site.   Photographic 
data  were  collected  using  "targets-of -opportunity"  flying  within  the  coverage 
area  of  the  test  site.   The  data  were  analyzed  at  NAFEC  to  determine  the 
extent  of  the  radar  beacon  coverage  and  further  scrutinized  to  uncover  any 
anomolies  which  might  derogate  the  operation  of  an  enroute  radar  beacon  site 
installed  at  the  test  location.   As  a  result  of  the  initial  data  analysis, 
flight  tests  were  conducted  in  the  vicinity  of  the  O'Hare  Enroute  Radar  Beacon 
Test  Site  using  a  NAFEC  jet  aircraft.   The  NAFEC  flight  tests  confirmed  the 
originally-suspected  problem  areas  and  provided  additional  justification  for 
linking  the  anomolies  to  the  vertical  radiation  pattern  of  the  standard  radar 
beacon  directional  antenna.   The  total  test  effort  showed  that:   (1)  the 
procedure  of  using  a  temporary  beacon  test  facility  to  determine  coverage  and 
problem  area  of  future  radar  beacon  sites  was  sound,  and  (2)  that  this  pro- 
cedure should  be  utilized  whenever  there  is  some  question  about  the  adequacy 
of  the  coverage  that  a  future  site  might  provide. 


17.  Key  Words 

Portable  beacon  test  site 

ATCRBS 

Site  coverage 

Temporary  beacon  test  facility 


18.  Distribution  Stotement 


Document  is  available  to  the  public 
through  the  National  Technical  Information 
Service,  Springfield,  Virginia  22151 


19.     Security  Classif.   (of  this   report) 

Unclassified 


20.     Security  Classif.  (of  this  page) 

Unclassified 


21.  No.  of  Pages 

61 


22.     Price 

$3.00     PC 

$    .95  MF 


Form  DOT  F  1700.7    (8-72) 


Reproduction  of  completed  page  authorized 


PREFACE 


I  wish  to  express  my  appreciation  to  my  associates  from  the  Great  Lakes 
Region  who  participated  in  the  testing  of  the  O'Hare  Airport  Enroute  Radar 
Beacon  Test  Site. 

I  also  wish  to  thank  Air  Traffic  Controllers  Robert  J.  Lucas,  (NAFEC) , 
Duane  L.  Johnson  (NAFEC),  and  Joseph  Chaloka  (Great  Lakes  Region);  pilots 
Kenneth  B.  Johnson,  Jesse  S.  Terry,  and  Fredrick  G.  Auer ;  photographers 
John  J.  Bradley  and  James  P.  McGrail;  engineer  Barry  J.  Saltzman;  and 
technicians  Stanley  L.  Scull  and  Joseph  H.  Reed  for  their  much  needed 
assistance. 


in 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

CARLI:  Consortium  of  Academic  and  Research  Libraries  in  Illinois 


http://www.archive.org/details/investigationofsOOspin 


TABLE  OF  CONTENTS 


INTRODUCTION 


Page 


Purpose  1 

Background  1 

Description  of  Site  Equipment  1 

DISCUSSION  4 

General  4 

Description  of  Flight  Tests  7 

Targets-of-Opportunity  7 

Flight  Inspection  Aircraft  Flight  Tests  7 

High-Altitude  Flight  Tests  7 

Low-Altitude  Flight  Tests  9 

NAFEC  Aircraft  Flight  Tests  9 

Results  of  the  Flight  Tests  14 

Results  of  the  Targets-of-Opportunity  Tests  14 

Site  Coverage  14 

Reflected  Beacon  Replies  20 

Vertical  Lobing  23 

Results  of  FIDO  Aircraft  Flight  Tests  25 

High-Altitude  Flight  Tests  25 

Low-Altitude  Flight  Tests  25 

Results  of  NAFEC  Aircraft  Flight  Tests  28 

CAUSES  OF  SITE  PROBLEMS  AND  POSSIBLE  SOLUTIONS  40 

Reflected  Beacon  Reply  Problems  40 

Vertical  Lobing  Problems  44 

Future  Expansion  of  the  Beacon  Test  Site  Area  45 

CONCLUSIONS  47 

APPENDIX 


v 


LIST  OF  ILLUSTRATIONS 


Figure  Page 

1  Aerial  Photograph  Showing  Relationship  of  the  2 

Beacon  Test  Site  to  the  O'Hare  Airport 

2  Layout  of  Beacon  Test  Site  Showing  Temporary  3 

Tower  and  Equipment 

3  Equipment  Layout  as  Installed  Inside  the  5 

Equipment  Van 

4  Flight  Path  of  FIDO  High-Altitude  Flight  Test  8 

5  Flight  Path  of  FIDO  Low-Altitude  Flight  Test  10 

Conducted  21  September  in  the  Morning 

6  Flight  Path  of  FIDO  Low-Altitude  Flight  Test  11 

Conducted  21  September  in  the  Afternoon 

7  Flight  Path  of  FIDO  Low-Altitude  Flight  Test  12 

Conducted  22  September  in  the  Morning 

8  ,   NAFEC  Aircraft  Convair  880M  Used  for  Flight  Tests        13 

9  Flight  Path  of  NAFEC  Aircraft  Coverage  Flight  Tests      15 

10  Two  Examples  of  Reflected  Beacon  Replies  Recorded        21 

South  of  the  Beacon  Test  Site  During  Targets-of- 
Opportunity  Tests 

11  Two  Examples  of  Reflected  Beacon  Replies  Recorded        22 

North  of  the  Beacon  Test  Site  During  Targets-of- 
Opportunity 

12  Loss  of  Beacon  Replies  Due  to  Vertical  Lobing  on         24 

the  13°  Radial  of  the  Beacon  Test  Site 

13  Loss  of  Beacon  Replies  Due  to  Vertical  Lobing  on         26 

the  84°  Radial  of  the  Beacon  Test  Site 

14  Loss  of  Beacon  Replies  Due  to  .Vertical  Lobing  on         27 

the  172°  Radial  of  the  Beacon  Test  Site 

15  Airborne  System  Equipment  Configuration  Installed        29 

in  NAFEC  Aircraft  N  112 


VI 


LIST  OF  ILLUSTRATIONS  (continued) 

Figure  Page 

16  Synchronization  System  Block  Diagram  30 

17  Modified  Standard  Gain  Horn  Antenna  That  was  Used        31 

at  the  Beacon  Test  Site  to  Transmit  the  "Synchronizing" 
Interrogation 

18  Data  From  the  Beacon  Test  Site  That  was  Recorded,  in      33 

the  NAFEC  Aircraft,  Using  the  Synchronizing  System 

19  Reflected  Beacon  Replies  Recorded  South  of  the  Beacon    35 

Test  Site  During  the  NAFEC  Aircraft  Flight  Tests 

20  Reflected  Beacon  Replies  Recorded  North  of  the  Beacon    36 

Test  Site  During  the  NAFEC  Aircraft  Flight  Tests 

21  Loss  of  Beacon  Replies  Due  to  Vertical  Lobing  on  13°     38 

Radial  of  the  Beacon  Test  Site  During  NAFEC  Aircraft 
Flight  Tests 

22  Loss  of  Beacon  Replies  Due  to  Vertical  Lobing  on  84°     39 

Radial  of  the  Beacon  Test  Site  During  NAFEC  Aircraft 
Flight  Tests 

23  Reduction  of  Beacon  Replies  Due  to  Vertical  Lobing  on    41 

170°  Radial  of  the  Beacon  Test  Site  During  NAFEC 
Aircraft  Flight  Tests 

24  Panoramic  View  of  Terrain  Surrounding  the  Beacon  Test    42 

Site  From  a  Height  of  75  Feet 

25  Future  Development  Planned  for  Beacon  Test  Site  Area     46 


vix 


INTRODUCTION 


PURPOSE. 

The  purpose  of  this  effort  was  to  conduct  tests  in  the  vicinity  of  the  Chicago 
O'Hare  Airport  to  determine  the  suitability  of  a  test  site  location  for  possible 
use  as  a  future  enroute  radar  beacon  site.   The  coverage  (within  150  miles)  of 
the  O'Hare  Airport  Radar  Beacon  Test  Site  was  investigated  as  well  as  any 
problems  which  might  derogate  the  operation  of  an  enroute  radar  beacon  site 
installed  at  the  test  location  in  the  future. 

BACKGROUND. 

In  August  1971,   the  Central  Region  of  the  Federal  Aviation  Administration  (FAA) 
requested  that  the  National  Aviation  Facilities  Experimental  Center  (NAFEC) 
participate  in  an  investigation  of  reflected  radar  beacon  replies  at  the 
McCook  Enroute  Radar  Site.   Flight  tests  were  conducted  within  the  coverage 
area  of  the  McCook  Site  using  a  NAFEC  aircraft.   The  result  of  the  flight 
tests  indicated  that  the  occurrence  of  reflected  replies  from  nearby  buildings 
was  so  extensive  that  the  reflected  reply  problem  could  not  be  resolved  unless 
the  site  was  relocated. 

The  Central  Region  had  been  looking  for  a  new  site  for  the  McCook  Enroute 
Radar  Site  prior  to  the  NAFEC  investigation  and  this  effort  was  continued 
when  the  Central  Region,  in  early  1972,  turned  over  the  responsibility  for 
the  McCook  Enroute  Radar  Site  to  the  Great  Lakes  Region. 

The  beacon  test  site  property,  owned  by  the  city  of  Chicago  (see  Figure  1) , 
was  located  adjacent  to  the  runway  complex  of  the  O'Hare  Airport.   The  property 
is  presently  being  used  as  an  agricultural  nursery. 

Operational  tests  were  conducted  at  the  O'Hare  Airport  Radar  Beacon  Test 

Site  (beacon  test  site)  using  targets-of-opportunity ,  flight  inspection  aircraft, 

and  a  NAFEC  aircraft.   The  result  of  these  tests  are  documented  herein. 

DESCRIPTION  OF  SITE  EQUIPMENT. 

In  order  to  simulate  a  radar  beacon  directional  antenna  installed  on  a  con- 
ventional 50-foot  enroute  radar  tower,  a  70-foot  tower  was  fabricated  at 
the  test  site  using  typical  construction  scaffolding  (see  Figure  2) .  Since 
the  tests  were  conducted  at  radar  beacon  frequencies  only,  an  enroute  radar 
antenna  was  not  required.  The  Air  Traffic  Control  Beacon  Interrogator  (ATCBI) 
Directional  Antenna,  Type  FA-8043,  was  mounted  directly  on  top  of  the  scaf- 
folding tower  on  an  Antenna  Pedestal,  Type  AB-294/FPS-8 . 

An  omnidirectional  antenna  was  also  installed  at  the  test  site  so  that 
Improved  3-Pulse  Side  Lobe  Suppression  (SLS)  could  be  implemented.   The 
Ommnidirectional  Antenna,  Type  FA-8044,  was  installed  on  the  tower  so  that 
the  bottom  of  the  omnidirectional  antenna  was  at  the  same  vertical  height  as 


FIGURE     1. 


AERIAL  PHOTOGRAPH  SHOWING  RELATIONSHIP  OF  THE 
BEACON  TEST  SITE  OF  THE  O'HARE  AIRPORT 


DIRECTIONAL 
ANTENNA 
TYPE  FA-8043 


OMNI- DIRECTIONAL 
ANTENNA  TYPE  FA- 8044 


FIGURE    2. 


LAYOUT  OF  BEACON  TEST  SITE  SHOWING  TEMPORARY 
TOWER  AND  EQUIPMENT 


the  top  of  the  directional  antenna.   The  omnidirectional  antenna  was  also 
installed  so  that  the  centers  of  the  omnidirectional  and  directional  antennas 
were  separated  in  the  horizontal  plane  by  approximately  15  feet. 

An  equipment  van  (26'  x  8')  was  used  at  the  test  site  to  house  the  radar  beacon 
interrogator,  defruiter,  decoder  and  indicator  equipment.   Power  for  all  of 
the  test  site  equipment  was  supplied  by  a  25KVA  3-phase  110/208  volts  Portable 
Generator  (see  Figure  2).   The  mobile  van,  with  its  equipment,  the  antenna  ped- 
estal and  the  antennas  were  all  part  of  a  Mobile  Enroute  Radar  Facility  (MERF) 
loaned  to  the  Great  Lakes  Region  by  the  Aeronautical  Center,  Oklahoma  City, 
Oklahoma,  to  implement  the  beacon  test  site. 

The  radar  beacon  equipment  installed  in  the  mobile  equipment  van  was  used 
to  provide  a  video  display  for  observation  by  an  air  traffic  controller  and 
simultaneously  photographed  for  future  reduction  and  analysis  of  data  (see 
Figure  3).   The  main  and  standby  interrogators  were  Air  Traffic  Control  Beacon 
Interrogators,  Model  ATCBI-4.   The  Model  ATCBI-4  interrogators  provided  the 
normal,  Mode  3/A  only,  beacon  interrogator  functions  as  well  as  improved 
3-pulse  SLS  operation.   Removal  of  non-synchronous  radar  beacon  replies  result- 
ing from  other  interrogators  was  accomplished  by  utilizing  an  Interference 
Blanker,  Type  MX-8757/UPX,  on  both  the  main  and  standby  channels. 

Decoding  of  the  interrogator  output  was  performed  within  the  mobile  equipment 
van  by  the  main  or  standby  Common  (Decoder)  Rack,  Type  FA-6193.   Selection  of 
the  video  that  was  displayed  on  the  Console  Cabinet,  Type  CA-4080A  (ARSR-1  dis- 
play), was  controlled  by  a  Master  Control  Box  (10-Channel) ,  Type  FA-6191A. 


DISCUSSION 


GENERAL . 

Prior  to  conducting  coverage  tests  at  the  beacon  test  site,  the  interrogation 
mode  of  the  ATCBI-4  interrogators  was  set  for  Mode  3/A  only  and  the  pulse 
repetition  frequency  (PRF)  of  the  interrogators  was  set  for  350  interrogations 
per  second.   The  pulse  parameters  of  both  interrogators  were  also  adjusted 
so  they  met  the  U.  S.  National  Aviation  Standard  for  the  Mark  X  (SIF)  Air 
Traffic  Control  Radar  Beacon  System  (ATCRBS)  characteristics. 

The  average  pulse  power  for  two  pulses  was  measured  on  the  directional  antenna 
transmission  line  at  the  transmitter  and  at  the  directional  antenna  to  deter- 
mine the  attenuation  afforded  the  radar  beacon  interrogations  by  the  trans- 
mission line.   The  average  power  at  the  transmitter  measured  8.4  milliwatts  (mw) 
through  20dB  of  additional  attenuation  (1,500  watts  peak  power),  while  the 
average  power  at  the  directional  antenna  measured  1.4  mw  through  the  same  20dB 
of  additional  attenuation  (250  watts  peak  power) .   This  difference  in  power 
indicated  that  the  RG-8U  coaxial  cable,  RG-218U  coaxial  cable,  and  pedestal 
rotary  joint  attenuated  the  radar  beacon  directional  antenna  signal  by  7.78dB. 


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Receiver  tangential  sensitivity  measurements  were  made  on  both  channels  of 
the  ATCBI-4  interrogator  and  the  Sensitivity  Time  Control  (STC)  curve  was 
set  to  36dB  attenuation  on  each  receiver. 

The  measurements  were  as  follows: 

Channel  1. 

Receiver  tangential  sensitivity:  -88.8dBm 

STC  curve  readings : 

Reduced  Receiver 


Delay 

(ys) 

Attenuation(dB) 

Sensitivity  (-dBm) 

15 

36.0* 

52.8 

27 

30.6 

58.2 

52 

25.0 

63.8 

102 

19.0 

69.8 

200 

12.8 

76.0 

398 

6.7 

82.1 

794 

0 

88.8 

Channel  2. 

Receiver  tangential  sensitivity:  -87.6dBm 
STC  curve  readings: 


Reduced  Receiver 


Delay  (ys) 

Attenuation  (dB) 

Sensitivity  (-dBm) 

15 

36.2* 

51.4 

27 

30.0 

57.6 

52 

24.0 

63.6 

102 

18.0 

69.6 

200 

11.5 

76.1 

398 

5.5 

82.1 

794 

0 

87.6 

*The  dB  attenuation  at  15  us  delay  designates  the  value  of  the  STC  curve. 

Measurements  were  made  of  the  radar  beacon  system  overall  sensitivity  by 
determining  the  minimum  signal  at  the  receiver  input  that  would  be  displayed 
on  the  ARSR-1  Plan  Position  Indicator  (PPI)  display.   In  this  manner,  the 
effects  of  the  receiver,  interference  blanker  (defruiter) ,  decoder  and  display 
circuitry  were  all  taken  into  account.   The  minimum  receiver  input  signal 
that  was  displayed  on  the  ARSR-1  dPI  Display  measured  -83dBm  on  both  the 
main  and  standby  channels. 


DESCRIPTION  OF  FLIGHT  TESTS. 

Three  types  of  flight  tests  were  performed  during  September  and  October  1972, 
to  determine  the  radar  beacon  coverage  of  the  beacon  test  site.   They  were: 

1.  Targets-of-opportunity , 

2.  Flight  Inspection  District  Office  (FIDO)  high-  and  low-altitude 
aircraft  flight  tests,  and 

3.  NAFEC  aircraft  flight  tests. 

All  of  the  flight  tests  were  observed  by  an  air  traffic  controller  from  either 
the  Great  Lakes  Region  or  NAFEC.   The  35  mm  camera  (Figure  3)  that  was  mounted 
on  the  ARSR-1  PPI  display  contained  a  semi-transparent,  periscope-type  mirror 
which  allowed  simultaneous  viewing  of  the  PPI  display  by  the  air  traffic  con- 
troller and  recording  of  data  by  the  camera.   During  all  of  the  flight  tests, 
the  air  traffic  controllers  recorded  any  unusual  occurrences;  e.g.,  loss 
of  replies,  reflected  replies  or  change  of  altitude. 

TARGETS-OF-OPPORTUNITY . 

The  major  portion  of  the  targets-of-opportunity  were  recorded  on  Friday, 
15  September,  and  Friday,  22  September,  between  the  hours  of  1500  and  2000 
hours.   This  is  the  time  when  the  traffic  reaches  a  maximum  in  the  Chicago 
O'Hare  Airport  area.   The  high  level  of  air  traffic  during  these  times  provided 
a  complete  but  random  coverage  of  all  portions  of  the  airways  within  the  beacon 
test  site  coverage  area. 

FIDO  AIRCRAFT  FLIGHT  TESTS. 

HIGH-ALTITUDE  FLIGHT  TESTS.   High-altitude  flight  tests  of  the  beacon  test 
site  were  conducted  using  a  FIDO  aircraft  on  20  September  1972,  between  1400 
and  1800  hours.   An  Aero  Commander,  Type  AC21,  was  flown  to  the  area  from 
Oklahoma  City,  Oklahoma,  for  this  purpose.   The  route  of  the  flight  tests 
are  shown  in  Figure  4  along  with  any  pertinent  flight  test  information.   The 
test  was  broken  into  two  segments  to  allow  the  aircraft  to  replenish  the 
fuel  that  was  consumed  on  the  flight  from  Oklahoma  City.   The  segmenting 
of  the  flight  test  allowed  the  aircraft  to  enter  the  test  area  at  an  altitude 
of  29,000  feet  (FL  290),  but  also  caused  an  interruption  in  the  high-altitude 
testing  in  the  area  of  Green  Bay,  Wisconsin. 

From  Figure  4,  it  can  be  seen  that  the  initial  portion  of  the  flight 
test  commenced  over  the  Capital  VORTAC  and  ended  in  Green  Bay,  where  the 
pilot  landed  the  aircraft  to  refuel.   The  major  portion  of  this  leg  of  the 
flight  test  was  flown  at  26,000  feet  (FL  260)  until  the  pilot  descended  the 
aircraft  to  land  at  Green  Bay. 

After  refueling,  the  pilot  climbed  the  aircraft  to  27,000  feet  (FL  270) 
where  it  remained  until  the  end  of  the  flight  test.   The  pilot  diverted  the 
aircraft  over  the  Wheatland  Intersection  from  its  intended  course,  the  175° 
radial  of  the  Northbrook  VORTAC,  to  the  Woodland  Intersection.   The  diversion 


NAUTICAL  MILES 


FIGURE    4.         FLIGHT  PATH  OF  FIDO  HIGH- ALTITUDE  FLIGHT  TEST 

8 


allowed  for  a  more  complete  high-altitude  flight  test  coverage  while  still 
allowing  a  sufficient  reserve  of  fuel  to  land  the  aircraft  at  the  W.K. 
Kellogg  Airport  in  Battle  Creek,  Michigan,  after  the  flight  test  was  com- 
pleted.  Prior  to  ending  the  tests,  the  aircraft  was  flown  over  the  beacon 
test  site  so  that  the  overhead  coverage  could  be  measured  at  27,000  feet 
(FL  270). 

LOW-ALTITUDE  FLIGHT  TEST.   Low-altitude  flight  tests  of  the  beacon  test  site 
were  conducted  using  a  FIDO  aircraft  on  21  and  22  September  1972.   A  Douglas 
DC-3  aircraft  was  flown  to  the  area  from  Battle  Creek,  Michigan,  for  this 
purpose.   The  low-altitude  flight  tests  were  so  extensive  that  2  days  were 
required  to  complete  the  testing.   A  morning  and  afternoon  flight  test  was 
conducted  on  21  September,  and  a  morning  flight  test  was  required  on 
22  September.   The  routes  of  the  flight  tests  are  shown  in  Figures  5,  6,  and  7 
along  with  the  flight  test  altitude  of  each  path.   The  figures  are  presented 
in  a  chronological  order  starting  with  the  morning  of  21  September. 

All  of  the  low-altitude  flight  tests  were  made  at  altitudes  between  2,500 
and  5,000  feet.   One  segment  of  the  flight  test,  during  the  morning  of 
21  September,  extended  to  a  maximum  range  of  62  nmi  from  the  beacon  test  site. 
This  occurred  over  the  Vermillion  Intersection  where  a  change  in  altitude 
from  4,000  feet  to  2,500  feet  was  also  requested.   The  descent  maneuver  com- 
bined with  the  extreme  range  was  made  to  determine  the  minimum  coverage  in 
this  area. 

Before  the  FIDO  aircraft  completed  the  flight  test  on  the  morning  of 
21  September,  the  aircraft  was  landed  at  the  O'Hare  Airport  for  fuel.   After 
refueling,  the  flight  test  was  continued  until  lunch  time.   A  beacon  test 
site  overhead-flight  was  included  as  part  of  the  afternoon  low-altitude  flight 
tests  on  21  September. 

During  the  flight  tests  on  the  morning  of  22  September,  the  improved  3-pulse 
SLS  was  turned  off.   This  occurred  when  the  aircraft  was  in  the  vicinity  of 
the  Crib  Intersection,  and  was  done  to  determine  whether  the  improved  3-pulse 
SLS  had  caused  any  loss  of  replies  or  changes  in  the  system  operation.   The 
FIDO  flight  test  aircraft  was  also  diverted  during  the  morning  flight  test 
in  the  vicinity  of  the  Chicago  Heights  VORTAC.   The  pilot  was  concerned  about 
the  operation  of  some  of  the  aircraft's  equipment  and  placed  the  aircraft 
in  a  holding  pattern  while  he  requested  maintenance  consultation. 

NAFEC  AIRCRAFT  FLIGHT  TESTS. 

The  flight  tests  that  were  conducted  at  the  radar  beacon  test  site,  utilizing 
a  NAFEC  aircraft,  were  made  during  the  week  of  2  October  1972.   A  Convair 
880  M  Jet  Aircraft  (FAA  N-112)  (see  Figure  8)  was  used  to  verify  problem 
areas  uncovered  during  the  analysis  of  the  targets-of-opportunity  data  and 
the  FIDO  flight  test  data.   The  analysis  of  the  targets-of-opportunity  data 
and  the  FIDO  flight  test  data  took  place  during  the  week  of  25  September. 


FIGURE    5. 


NAUTICAL  MILES 

FLIGHT  PATH  OF  FIDO  LOW- ALTITUDE  FLIGHT  TEST 
CONDUCTED  21  SEPTEMBER  IN  THE  MORNING 

10 


NAUTICAL  MILES 

FIGURE     6.         FLIGHT  PATH  OF  FIDO  LOW- ALTITUDE  FLIGHT  TEST 
CONDUCTED  21  SEPTEMBER  IN  THE  AFTERNOON 

11 


NAUTICAL  MILES 

FIGURE    7.         FLIGHT  PATH  OF  FIDO  LOW- ALTITUDE  FLIGHT  TEST 
CONDUCTED  22  SEPTEMBER  IN  THE  MORNING 

12 


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Two  areas  where  reflected  beacon  replies  occurred  were  Investigated  using 
the  NAFEC  aircraft  and  three  areas  where  vertical  lobing  occurred  were  also 
flight  tested.   Prior  to  the  completion  of  the  flight  testing  on  2  October, 
the  aircraft  was  flown  over  the  beacon  test  site  at  41,000  feet  to  measure 
the  overhead  coverage  of  the  site  at  this  altitude.   An  orbital  flight  test 
also  conducted  on  3  October  (see  Figure  9) .   The  radius  of  this  flight  test 
was  150  nmi  with  the  NAFEC  aircraft  flying  at  an  altitude  of  27,000  feet 
(FL  270)  . 

RESULTS  OF  THE  FLIGHT  TESTS. 

Even  though  the  air  traffic  controllers  did  an  excellent  task  of  observing 
the  beacon  test  site  PPI  display,  they  could  not  study  all  portions  of  the 
PPI  display  with  equal  measure.   This  was  particularly  true  when  more  than 
one  aircraft  was  displayed  at  a  time;  e.g.,  targets-of-opportunity  data. 
When  the  filmed  data  was  reviewed  at  NAFEC,  during  the  week  of  25  September, 
it  was  rerun  many  times.   Each  time  the  film  was  run,  a  different  portion 
of  the  filmed  PPI  display  was  studied.   This  resulted  in  the  detection  of 
more  problem  areas  than  was  originally  suspected. 

RESULTS  OF  THE  TARGETS-OF-OPPORTUNITY  TESTS. 

SITE  COVERAGE.   On  14  September  1972,  the  air  traffic  controllers  at  the  beacon 
test  site  recorded  target  information  while  observing  targets-of-opportunity. 
The  aircraft  type,  position,  and  altitude  of  displayed  replies  were  coordi- 
nated with  the  Chicago  Air  Route  Traffic  Control  Center  located  at  Aurora, 
Illinois.   Observations  were  made  simultaneously  at  the  Chicago  Center  and 
the  beacon  test  site.   The  Chicago  Center  observed  the  McCook  ARSR-2  radar 
beacon  display,  while  the  beacon  test  site  observed  their  radar  beacon  dis- 
play.  The  results  of  this  radar  beacon  coverage  coordination  was  as  follows: 

RANGE  SELECTED  -  100  NMI 

Hours  -  1000  to  1130 

Aircraft 

Type  Position  Altitude 

B707  5  Mi  West  of  the  TADPOLE  Intersection  11,000 

BE99  TADPOLE  Intersection                ,  8,000 

BE80  15   Mi  North   East   of    the  MIU  VOR  9,000 

AC21  10  Mi  North  East  of  the  SBN  VOR  FL-230 

Cessna  310  Grand  Beach  Intersection  7,000 

G159  5  Mi  North  East  of  TADPOLE  Intersection.  14,000 


14 


FIGURE    9.         FLIGHT  PATH  OF  NAFEC  AIRCRAFT  COVERAGE 
FLIGHT  TESTS 


15 


Aircraft 
Type 

BE55 


PA28 

BE90 

Cessna  411 
Twin  Otter 
Cessna  182 
Unknown 
DC  9 

Cessna  182 
PA28 
Cessna  172 

B727 
CV88 
BE55 
BE80 

Unknown 

Twin  Otter 
BE90 
PA28 
C119 


Position  Altitude 

12  Mi  East  of  the  Sunfish  Intersection  8,000 

(NOTE:  Ident.  Feature  Loud  and  Clear) 

10  Mi  North  East  of  the  Taylor  9,000 
Intersection 

Grand  Beach  Intersection  5,000 

10  Mi  North  East  of  the  RBS  VOR  11,000 

10  Mi  East  of  the  RBS  VOR  8,000 

5  Mi  North  West  of  the  CGT  VOR  3,000 

5  Mi  North  East  of  Musky  Intersection  7,000 

2  Mi  North  East  of  ORD  2,500 

5  Mi  West  of  the  VPZ  Airport  2,500 
10  Mi  East  of  the  0X1  VOR  6,000 
12  Mi  South  East  of  the  0X1  VOR  4,000 

Hours  -  1245  to  1415 

6  Mi  North  West  of  the  JOT  VOR  7,000 
12  Mi  North  West  of  the  RFD  VOR  17,000 
4  Mi  North  of  the  Lowell  Intersection  5,000 
10  Mi  North  East  of  the  BDF  VOR  7,000 

(NOTE:  Ident.  Feature  Loud  and  Clear) 

7  Mi  South  East  of  the  Kentland  9,000 
Intersection 

15  Mi  South  West  of  the  JOT  VOR  6,000 

15  Mi  North  East  of  the  BDF  VOR  7,000 

10  Mi  North  of  the  BDF  VOR  6,000 

3  Mi  South  of  the  Lakewood  4,000 
Intersection 


16 


Aircraft 
Type 

UHI  Helicopter 


Fairchild 

C119 

BE90 

G159 

Cessna  172 

NA265 

Unknown 

Unknown 

BE55 

BE99 

AC68 

P2 
BE55 


Position 


Altitude 


3,000 


45  Mi  West  of  ORD 
(Ident.  Loud  and  Clear) 

10  Mi  South  East  of  the  PLL  VOR  9,000 
Malta  Intersection  4,000 

15  Mi  North  East  of  the  JVL  VOR  3,500 
10  Mi  North  East  of  the  0X1  VOR  13,000 
10  Mi  West  of  the  0X1  VOR  4,000 
8  Mi  West  of  the  GSH  VOR  11,000 
15  Mi  West  of  the  SBN  VOR  6,500 
7  Mi  North  East  of  Benton  Harbor   7,500 

4,000 


2  Mi  East  of  the  Lowell 
Intersection 

5  Mi  East  of  the  Zoro 
Intersection 

3  Mi  South  of  the  Manteno 
Intersection 

Musky  Intersection 

5  Mi  West  of  the  EON  VOR 


5,000 

Climbing  to 
17,000 

5,000 

4,000 


Radar  beacon  coverage  recordings  were  also  made  by  the  Air  Traffic  Controllers, 
at  the  beacon  test  site,  on  15  September  1972.   The  individual  radar  beacon 
replies  were  grouped  by  transponder  code  and  not  by  exact  altitude.  Observations 
were  made  of  the  azimuth,  direction  of  flight,  and  the  maximum  range  that 
the  reply  could  be  tracked.   The  results  of  these  coverage  tests  were  as  follows 


17 


RANGE  SELECTED  -  200  NMI 
HOURS:  1000  to  1145 
Code  1100  (Surface  to  flight  level  230) 
Azimuth  (Degrees)        Maximum  Range  (nmi) 


360 
170 
280 
330 
040 
085 
220 
190 
110 
095 


150 
150 
160 
125 
110 
145 
155 
95 
120 
130 


Code  1500  (Surface  to  flight  level  230) 


085 
100 
090 
110 
170 
190 
220 
260 
360 


70 

85 

110 

100 
90 
80 
70 
65 
85 


Code  1700  (Surface  to  flight  level  240) 


350 
080 
070 
100 
110 
170 
240 
270 
280 


60 
70 
80 
90 

100 
85 

100 
90 

110 


Direction 

North 
South 
West 

North  West 
North  East 
East 

South  West 
South 
East 
South  East 


East 

East 

East 

East 

South 

South 

South  West 

West 

North 


North 

East 

East 

East 

East 

South 

South  West 

West 

West 


18 


Codes  2100  and  2300  (Above  flight  level  240) 

Azimuth  (Degrees)        Maximum  Range  (nmi)  Direction 

160  160  South 

130  150  South  East 

300  160  North  West 

170  170  South 

080  160  East 

275  165  West 

220  160  South  West 

290  165  North  West 

130  160  South  East 

Code  1200  (Surface  to  9,500  Feet) 

210  80  South  West 

190  50  South 

160  90  South 

320  110  North  West 

070  90  North  East 

130  120  South  East 

Code  1400  (Between  10,500  and  17,500  Feet) 

100  145  East 

260  110  West 

280  100  West 

350  150  North 


19 


REFLECTED  BEACON  REPLIES. 

During  the  analysis  of  the  targets-of -opportunity  data  at  NAFEC,  it  was  noted 
that  aircraft  replies  suddenly  appeared  on  the  display  and  then  just  as  suddenly 
disappeared.   Some  of  these  replies  were  approximately  60  to  80  nmi  south 
of  the  beacon  test  site.   From  past  experience  it  was  assumed  that  these  were 
reflected  beacon  replies.   The  criteria  for  determining  whether  a  certain 
beacon  reply  is,  in  reality,  a  reflected  beacon  reply  are  as  follows: 

-  The  range  of  the  reflected  reply  is  always  greater  than  the  range 
of  the  normal  beacon  reply.   Sometimes  the  added  range  is  so 
insignificant  that  the  difference  is  undetectable  on  the  PPI  dis- 
play.  (The  flight  path  of  the  aircraft,  during  the  rotation  time 
of  the  antenna,  must  not  be  ignored  in  calculating  this  range),  and 

-  The  reply  code  of  the  normal  beacon  reply  and  the  reply  code  of 
the  reflected  beacon  reply  must  be  identical;  e.g.,  (a)  both 
should  display  a  single-slash  or  both  should  display  a  double-slash, 
(b)  if  the  normal  reply  shows  the  "identification"  feature,  the 
reflected  reply  should  also  show  the  "identification"  feature. 

When  this  criteria  was  applied  to  the  intermittent  aircraft  replies  that 
occurred  at  the  ranges  of  60  to  80  nmi  south  of  the  beacon  test  site,  the 
existence  of  reflected  beacon  replies  in  this  area  was  confirmed.   These 
reflected  beacon  replies  extended  from  203°  to  172°,  as  the  aircraft  flew 
from  354°  to  026°.   The  reflected  beacon  replies  did  not  occur  every  rotation 
of  the  antenna,  as  the  normal  aircraft  beacon  reply  covered  the  above  azimuth 
changes,  but  a  large  number  of  reflected  replies  did  occur  during  such  flights. 

Figure  10  shows  two  examples  of  reflected  replies  that  were  recorded  on 
15  September  in  this  area.   These  photographs  were  selected  because  both  the 
normal  and  the  reflected  beacon  replies  show  the  "identification"  feature 
which  confirmed  the  reflected  beacon  reply.   The  normal  beacon  reply  was 
normally  double-slashed;  then  showed  the  "identification"  feature  for  two 
antenna  rotations;  and  finally  returned  to  a  double-slashed  return.   The 
reflected  reply  followed  the  same  sequence. 

When  the  film  data  collected  on  15  September  was  analyzed  at  NAFEC,  a  persist- 
ent "flashing"  of  replies  was  noted  on  the  display  90  to  150  nmi  north  of 
the  beacon  test  site.   These  intermittent  reflected  beacon  replies  were  recorded 
between  the  azimuths  of  10°  to  15°.   The  reflected  beacon  replies  in  this  area 
were  traced  to  aircraft  flying,  at  approximately  the  same  range,  in  the 
vicinity  of  the  85°  radial.   The  identification  was  made  by  comparing  double- 
slashed  codes  and  replies  utilizing  the  "identification"  feature.   Figure  11 
shows  two  examples  of  reflected  replies  that  appeared  in  this  area. 

The  reflected  beacon  replies  that  occurred  between  the  ranges  of  90  and  150  nmi 
and  azimuths  of  10°  to  15°  did  not  persist  for  more  than  four  or  five  antenna 
rotations.   Most  of  the  reflected  replies  in  this  area  lasted  for  only  one 
or  two  antenna  rotations.   These  reflected  beacon  replies  were  far  less  persistent 
than  the  reflected  replies  that  occurred  south  of  the  beacon  test  site. 

20 


FIGURE     10. 


TWO  EXAMPLES  OF  REFLECTED  BEACON  REPLIES 
RECORDED  SOUTH  OF  THE  BEACON  TEST  SITE 
DURING  TARGETS-OF-OPPORTUNITY  TESTS 

21 


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22 


VERTICAL  LOBING.   During  the  analysis  of  the  targets-of-opportunity  data 
at  NAFEC,  the  beacon  replies  were  seen  to  diminish  in  width  as  the  aircraft 
flew  through  certain  coverage  areas.   Sometimes  this  decrease  in  the  width 
of  the  beacon  reply  was  so  severe  that  the  beacon  reply  was  not  seen  for  a 
number  of  antenna  rotations.   If  the  aircraft  was  changing  course,  the  loss 
of  beacon  replies  could  have  been  attributed  to  the  shielding  of  the  aircraft 
antenna  due  to  the  turn  maneuver.   But  in  certain  areas,  the  beacon  reply 
was  lost  consistently  when  aircraft  flew  through  this  area  even  though  the 
aircraft  was  not  turning. 

From  past  experience  obtained  from  analyzing  field  site  data,  it  was  assumed 
that  the  narrowing  and  complete  loss  of  beacon  replies  could,  in  this  case, 
be  attributed  to  propagation  vertical  lobing.   Vertical  lobing  is  a  phenomenon 
that  is  produced  by  the  reception  of  a  radar  beacon  interrogation  or  reply 
over  multiple  propagation  paths.   If  two  propagation  paths  are  available  from 
the  ground  antenna  to  the  aircraft  antenna  a  cancellation  of  signal  intensity 
will  occur  when  the  phasing  of  the  two  signals  differ  by  approximately  180° . 
The  intense  signals  that  are  reflected  from  large  flat  areas  of  terrain,  due 
to  the  terrain  being  illuminated  by  the  broad  vertical  pattern  of  the  direc- 
tional antenna,  can  cause  very  serious  vertical  lobing  problems. 

During  the  analysis  of  the  targets-of-opportunity  data  at  NAFEC,  it  was  noted 
that  aircraft  replies  were  lost  north  of  the  beacon  test  site  as  aircraft 
flew  along  Jet  Airway  38-106.   The  loss  of  replies  occurred  at  10°,  13°  and 
20°  to  a  large  number  of  aircraft  that  flew  through  the  area.   The  range  where 
the  beacon  replies  were  lost  on  Jet  Airway  38-106  was  approximately  145  nmi 
from  the  beacon  test  site.   Figure  12  shows  an  example  of  the  vertical  lobing 
that  occurred  in  this  area.   A  series  of  photographs  are  shown  so  that  the 
loss  of  the  beacon  reply  could  be  associated  with  the  position  of  the  aircraft 
prior  to  and  after  the  time  the  beacon  reply  was  lost.   The  aircraft  reply 
that  exhibited  vertical  lobing  was  circled  on  the  figure  for  easier 
identification. 

The  reason  that  all  of  the  replies  from  aircraft  that  flew  through  the  vertical 
lobing  area  were  not  lost  is  that  the  reduction  of  signal  intensity,  due  to 
vertical  lobing,  is  dependent  upon  the  vertical  angle  of  the  aircraft  relative 
to  the  beacon  test  site.   The  altitude  of  the  aircraft  that  were  seen  on  the 
PPI  display  during  the  analysis  of  the  targets-of-opportunity  data  was  not 
known.   Therefore,  the  vertical  angle  of  the  aircraft  relative  to  the  beacon 
test  site  was  also  unknown.   Two  aircraft  that  appear  at  almost  identical 
ranges  and  azimuths  could  still  be  separated  by  altitude.   This  would  mean 
that  the  two  aircraft  were  at  different  vertical  angles  relative  to  the  beacon 
test  site. 

Another  factor  that  should  not  be  overlooked  in  analyzing  flight  test  data, 
is  that  all  aircraft  transponders  do  not  have  equal  receiver  sensitivity  or 
output  power.   The  receiver  sensitivity  and  output  power  are  significant 
factors  in  the  determination  of  why  vertical  lobing  can  reduce  the  beacon 
reply  of  one  aircraft  and  not  another  even  though  the  two  aircraft  are  flying 
at  similar  ranges  and  azimuths. 


23 


p 

^ 
% 

FIGURE     12. 


LOSS  OF  BEACON  REPLIES  DUE  TO  VERTICAL  LOBING 
ON  THE  13°  RADIAL  OF  THE  BEACON  TEST  SITE 


24 


During  the  analysis  of  the  targets-of-opportunity  data,  vertical  lobing  was 
also  noted  in  the  vicinity  of  the  71°  to  89°  radial,  at  ranges  approximat- 
ing 100  to  138  nmi.   Aircraft  flying  on  Jet  Airways  94-547  and  584  covered 
these  radials  during  their  outbound  and  inbound  flights.   The  three  azimuths 
where  the  vertical  lobing  was  most  intense  were  71°,  84°,  and  89°.   Vertical 
lobing  was  noted  at  two  ranges  on  the  84°  and  89°  radials.   Only  one  area 
of  vertical  lobing  was  noted  on  the  71°  radial,  this  occurred  at  130  nmi. 
Figure  13  shows  an  example  of  vertical  lobing  that  took  place  on  the  84° 
radial  at  100  nmi. 

Some  loss  of  radar  beacon  replies  was  observed  on  the  targets-of-opportunity 
data  when  aircraft  flew  in  the  vicinity  of  the  164°  radial  at  ranges  between 
140  to  157  nmi.  and  on  the  172°  radial  at  ranges  approximating  142  nmi.  The 
loss  of  beacon  replies  did  not  occur  too  often  in  this  area  and  it  was  ques- 
tionable whether  the  loss  of  replies  in  this  area  was  really  very  serious. 
Figure  14  shows  an  example  of  the  loss  of  beacon  replies  that  occurred  in 
the  vicinity  of  the  172°  radial  at  a  range  of  142  nmi. 

RESULTS  OF  FIDO  AIRCRAFT  FLIGHT  TESTS. 

HIGH-ALTITUDE  FLIGHT  TESTS.   The  results  of  the  high-altitude  flight  tests, 
using  the  FIDO  Aero  Commander,  Type  AC21,  reconfirmed  the  occurrence  of  ver- 
tical lobing  in  the  beacon  test  site  coverage,  but  no  positive  areas  of 
reflected  replies  were  recorded.   The  loss  of  the  FIDO  aircraft  beacon  return 
was  discounted  during  turn  maneuvers,  but  there  were  five  instances  when  the 
beacon  reply  was  lost  while  the  aircraft  was  not  turning. 

Loss  of  the  FIDO  aircraft  beacon  reply  occurred  at  azimuths  of  310°,  331°, 
10°,  13°  and  19°.   All  of  these  losses  of  replies  occurred  at  ranges  between 
110  and  143  nmi. 

The  loss  of  the  FIDO  aircraft  beacon  reply  at  10°  and  13°  reconfirmed  losses 

of  radar  beacon  replies  that  had  been  previously  recorded  at  these  same  azimuths 

during  the  targets-of-opportunity  tests. 

At  the  very  end  of  the  high-altitude  flight  tests,  using  a  FIDO  aircraft, 
the  aircraft  was  flown  directly  over  the  beacon  test  site  at  an  altitude  of 
27,000  feet.   During  this  maneuver,  the  flight  test  aircraft  reply  was  lost 
at  a  slant  range  of  5  nmi  on  one  side  of  the  site  and  reappeared  at  a  slant 
range  of  4  nmi  on  the  other  side  of  the  site. 

LOW-ALTITUDE  FLIGHT  TESTS.   During  the  low-altitude  flight  tests,  using  a  FIDO 
aircraft,  the  aircraft  remained  within  62  nmi  of  the  beacon  test  site.   Because 
the  range  of  the  flight  test  aircraft  did  not  extend  beyond  the  effective 
range  of  the  improved  3-pulse  SLS  system,  there  were  no  reflected  beacon  replies 
recorded.   Some  reflected  replies  could  have  occurred  if  there  was  a  reflecting 
surface  less  than  1,000  feet  from  the  beacon  test  site  antenna,  but  there 
was  no  indication  that  any  efficient  reflecting  surface  was  within  this  range. 
The  major  contribution  of  the  low-altitude  FIDO  flight  tests  was  to  confirm 
the  beacon  test  site  coverage. 


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Some  "selected  code"  breakthrough  that  resembled  reflected  beacon  replies, 
was  seen  during  the  low-altitude  FIDO  flight  tests,  but  further  investi- 
gation showed  that  these  returns  were  due  to  code  garbling  that  occurred  between 
two  unrelated  aircraft  replies.   The  flight  test  aircraft  was  also  lost  a 
number  of  times  during  turn  maneuvers  when  the  aircraft  antenna  was  shielded 
by  the  aircraft  wings  or  fuselage. 

The  low-altitude  flight  tests  included  a  station  overflight  at  2,500  feet. 
During  this  flight  maneuver,  the  aircraft  beacon  reply  was  lost  at  a  slant 
range  of  3/4  of  a  nmi  on  one  side  of  the  test  site  and  reappeared  at  a 
slant  range  of  3/4  of  a  nmi  on  the  other  side  of  the  test  site. 

RESULTS  OF  NAFEC  AIRCRAFT  FLIGHT  TESTS . 

Two  different  airborne  equipment  configurations  were  used  during  the  NAFEC 
aircraft  flight  tests.   The  first  equipment  configuration  used  the  aircraft's 
calibrated  transponder,  a  Bendix  Corp.  Type  TRU-1  Serial  No.  55,  which  was 
interrogated  by,  and  responded  to,  the  beacon  test  site  normal  Mode  3/A 
interrogations.   The  antenna  for  this  transponder  was  installed  on  the 
bottom  of  the  aircraft  beneath  the  cockpit. 

The  second  equipment  configuration  that  was  used  during  the  NAFEC  aircraft 
flight  tests,  consisted  of  an  airborne  and  ground-based  synchronization  system. 
This  system  was  used  ONLY  during  vertical  lobing  flight  tests.   A  photograph 
of  the  airborne  portion  of  this  system  equipment  configuration  is  shown  in 
Figure  15.   A  Radio  Corporation  of  America  (RCA)  Transponder,  Type  2.3NAlb/ 
RT-1  Serial  No.  1014,  was  used  for  the  airborne  portion  of  the  system. 
The  location  of  the  antenna  (Type  237Z-1) ,  that  was  used  with  this  transponder, 
is  shown  in  Figure  8. 

A  block  diagram  of  the  entire  equipment  configuration,  that  was  used  during 
the  vertical  lobing  flight  tests  is  shown  in  Figure  16.   This  airborne  and 
ground-based  system  was  used  as  a  synchronizing  system  to  ensure  that  only 
the  interrogations  generated  by  the  beacon  test  site  were  recorded  in  the 
aircraft,  on  the  digital  recorder. 

The  ground-base  equipment  for  this  system  consisted  of  a  transmitter,  logic 
circuitry  and  an  antenna.   The  transmitter  was  a  Power  Pulsed  Signal  Source, 
Model  PG  5KA,  fabricated  by  Applied  Microwave  Laboratory  Inc.,  which  was  capable 
of  producing  5  KW  pulses  at  1030  MHz.   The  transmitter  was  triggered  by  three 
pulses  supplied  by  the  logic  circuitry.   The  first  two  pulses  were  separated 
by  25  Us  to  form  a  Mode  D  radar  beacon  interrogation.   The  third  pulse  (parity 
pulse)  was  transmitted  10  ys  later,  to  prevent  capture  of  the  airborne  equip- 
ment circuitry  by  false  Mode  D  interrogation  pulse  spacings  that  might  be 
occasionally  produced  by  two  nonsynchronous  interrogators.   The  antenna  that 
was  used  for  the  ground-based  portion  of  the  system  was  a  Scientific  Atlanta 
Standard  Gain  Horn,  Model  11-0.9  (see  Figure  17).   The  antenna  was  modified 
at  NAFEC,  by  increasing  the  flare  length.   This  provided  increased  directivity 
and  gain. 

The  ground-based  portion  of  the  system  used  the  "Beacon  Sync"  of  the  ATCBI-4 

interrogator  and  the  "North  Mark"  of  the  antenna  system  together  with 

the  logic  circuitry,  to  generate  60  3-pulse  "synchronizing"  interrogations 

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for  each  antenna  rotation.   The  logic  circuitry  provided  the  means  for  not 
only  controlling  the  precise  azimuth  where  the  60  "synchronizing"  interroga- 
tions began,  but  also  supplied  a  strobe  for  the  PPI  display.   The  timing  of 
the  60  3-pulse  "synchronizing"  interrogations  were  made  to  coincide  with  the 
interrogation  of  the  flight  test  aircraft  by  the  main  beam  of  the  directional 
antenna.   This  was  accomplished  by  shifting  the  position  of  the  strobe  on 
the  PPI  display.   The  strobe  was  placed  on  the  counterclockwise  edge  of  the 
flight  test  aircraft  reply.   This  positioning  of  the  strobe  caused  the  next 
60  3-pulse  "synchronizing"  interrogations  to  be  transmitted  to  the  flight 
test  aircraft,  approximately  12  us,  prior  to  the  transmission  of  the 
normal  Mode  3/A  interrogation  from  the  directional  antenna. 

The  airborne  portion  of  the  synchronizing  system,  (Figure  16),  consisted 
of  an  antenna,  transponder,  logic  circuitry,  analog-to-digital  converter  and 
a  digital  recorder.   The  reception  of  the  Mode  D  interrogation  by  the  trans- 
ponder produced  a  trigger  for  the  logic  circuitry  which  was  "ANDED"  with  the 
parity  pulse.   The  pulse  that  was  produced  by  the  "AND"  circuit  was  delayed 
to  coincide  with  the  Pi  pulse  of  the  normal  Mode  3/A  beacon  site  interrogation, 
so  that  the  amplitude  of  the  Pi  interrogation  pulse  could  be  sampled.   The 
analog-to-digital  converter  changed  the  amplitude  of  the  Pi  interrogation 
pulse  into  a  digital  code  which  was  recorded  on  the  digital  recorder  along 
with  the  time  of  the  recording. 

In  order  to  record  the  amplitude  of  60  consecutive  Pi  interrogation  pulses 
in  the  main  beam  of  the  beacon  test  site  directional  antenna,  the  digitized 
amplitude  of  each  Pi  interrogation  pulse  was  stored  in  a  7  x  60  bit  shift 
register.   After  a  time  interval  equivalent  to  five  interrogation  periods  had 
elapsed  without  receiving  a  Mode  D  interrogation,  the  digitized  amplitude  of 
the  Pi  interrogation  pulses  that  were  stored  in  the  shift  register  were 
transferred  to  the  digital  recorder  and  printed.   Figure  18  shows  a 
typical  recording  produced  in  this  manner.   The  time  of  recording  is  shown 
on  the  left  in  hours,  minutes,  seconds  and  tenths  of  seconds  while  the  data 
is  recorded  on  the  right.   Each  of  the  two  digits  of  data  can  assume  a  value 
between  zero  and  *.   An  increase  or  decrease  of  one  in  the  numerical  value  of 
the  number  in  the  far  righthand  column  indicated  a  change  in  the  pulse 
amplitude  of  16  millivolts.   A  recorded  one  was  equivalent  to  16  millivolts, 
while  a  recorded  one*  was  equivalent  to  496  millivolts.   The  series  being  0, 
1,2, 3,4,5,6, 7,8,9,  +  ,-,V, A,  ft,*.   Figure  18  shows  data  that  was  recorded  during 
the  vertical  lobing  flight  testing.   The  recorded  data  show  how  the  ampli- 
tude of  the  directional  antenna  main  beam  varied  with  azimuth. 

A  system  calibration  was  performed,  at  NAFEC,  prior  to  departure  for  Chicago, 
to  allow  converting  the  transponder  input,  in  -dBm,  to  the  appropriate 
digits  that  were  recorded  on  the  digital  recorder.   The  calibration  tests 
showed  that  a  -64  dBm  level  at  the  input  to  the  transponder  was  recorded  as 
a  three  on  the  digital  recorder,  while  an  input  level  of  -15  dBm  was  recorded 
as  a  69.  The  synchronization  system  was  effective  to  ranges  beyond  150  nmi 
but  the  amplitude  of  the  normal  Mode  3/A  interrogation  P^  pulse  could  not 
be  recorded  at  levels  below  -65dBm.   This  limited  the  effective  range  of  the 
entire  system  to  approximately  120  nmi.   This  limitation  was  caused  by  a 
Honeywell  3C  Schmitt  Trigger  Circuit  which  converted  the  Mode  3/A  video  pulse. 

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DATA  FROM  THE  BEACON  TEST  SITE  THAT  WAS 
RECORDED,    IN  THE  NAFEC  AIRCRAFT,    USING 
THE  SYNCHRONIZING  SYSTEM 


33 


A  more  sensitive  Schmitt  Trigger  is  needed  to  extend  the  range  of  the  system 
to  at  least  150  nmi .   The  system  was  still  effective  in  verifying  vertical 
lobing  that  took  place  in  Chicago  at  ranges  less  than  120  nmi. 

The  flight  tests  that  were  conducted,  using  NAFEC  aircraft  N-112  on  2  October, 
were  flown  to  reconfirm  reflected  beacon  replies  that  were  obtained  during 
the  targets-of -opportunity  tests.   The  NAFEC  aircraft  flight  tests  were  made 
at  ranges  approximating  64-80  nmi  in  an  attempt  to  reproduce  reflected 
replies  south  of  the  beacon  test  site.   The  flight  tests  were  made  at  25,000, 
27,000,  29,000  and  31,000  feet.   All  of  the  flight  tests  produced  some  refelcted 
beacon  replies  at  azimuths  from  205°  to  150°  as  the  aircraft  was  flown  between 
site  azimuths  of  345°  to  24°.   Figure  19  shows  two  examples  of  dual  reflections 
that  occurred  during  these  flight  tests. 

The  NAFEC  aircraft  was  flown  directly  over  the  beacon  test  site  at  41,000  feet 
on  2  October  1972.   During  this  maneuver,  the  aircraft  replies  were  lost  at 
344°  at  a  slant  range  of  9.5  nmi  and  were  received,  after  the  station  cross- 
ing, at  169°  at  a  slant  range  of  9  nmi. 

Flight  tests  were  conducted  on  3  October  in  the  vicinity  of  the  beacon  test 
site  82°  to  90°  radials.   The  purpose  of  these  flight  tests  was  to  produce 
reflected  beacon  replies  north  of  the  beacon  test  site  which  would  recon- 
firm the  reflected  beacon  replies  recorded  during  the  targets-of-opportunity 
tests.   During  the  NAFEC  aircraft  flight  tests  at  altitudes  between  24,000  and 
31,000  feet,  reflected  beacon  replies  were  recorded  between  6°  and  15°.   The 
reflected  replies  that  occurred  at  an  azimuth  of  15°  reconfirmed  the  reflected 
replies  recorded  during  the  targets-of-opportunity  tests  at  the  same  radial. 
The  reflected  replies  that  occurred  at  an  azimuth  of  15°  were  produced  as 
the  aircraft  was  flown  in  the  vicinity  of  the  83°  radial.   Figure  20  shows 
reflected  beacon  replies  that  occurred  at  15°  and  355°  when  the  NAFEC  air- 
craft was  flown  at  83°  and  99°,  respectively.   The  reflected  replies  that 
occurred  at  355°  were  produced  during  the  orbital  flight  test  that  was  con- 
ducted at  the  end  of  the  flight  testing  on  3  October. 

Vertical  lobing  was  also  recorded  during  the  NAFEC  aircraft  flight  tests  that 
took  place  on  3  October.   The  most  severe  case  of  vertical  lobing  occurred 
as  the  aircraft  was  flown  on  the  77°  and  84°  radials. 

The  orbital  flight  test  that  was  flown  at  27,000  feet  on  3  October  was  con- 
ducted to  investigate  the  beacon  test  site  coverage  and  determine  if  obstruc- 
tions on  the  site  horizon  would  interfere  with  the  beacon  test  site  coverage. 
The  only  extended  loss  of  the  NAFEC  aircraft  beacon  reply  that  did  occur  during 
the  orbital  flight  was  not  due  to  obstructions  but  due  to  a   change  in  the 
transponder  code  that  was  requested  by  the  Indianapolis,  Indiana,  Center. 
The  code  change  was  not  coordinated  with  the  beacon  test  site.   This  caused 
a  loss  of  the  NAFEC  aircraft  beacon  reply  for  10  antenna  rotations  between 
the  site  azimuths  of  180°  and  175°.   During  the  orbital  flight  test,  reflected 
beacon  replies  were  recorded  on  the  355°,  1°,  5°  and  10°  radials  as  the  NAFEC 
aircraft  was  flown  through  site  azimuths  of  99°,  87°,  87°  and  267°,  respectively, 


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Vertical  lobing  caused  a  loss  of  the  beacon  reply  in  the  northern  sector 
at  358°  and  15°  during  the  orbital  flight  test.   A  more  detailed  listing 
of  the  reflected  beacon  replies  that  occurred  during  the  flight  testing  on 
2  and  3  October  can  be  found  in  the  Appendix. 

Flight  tests,  using  the  NAFEC  aircraft,  were  conducted  on  4  and  5  October, 
to  investigate  areas  that  had  produced  a  loss  or  narrowing  of  beacon  replies 
during  the  targets-of-opportunity  tests.   The  first  area  that  was  investi- 
gated, in  an  attempt  to  record  vertical  lobing,  was  north  of  the  beacon  test 
site.   The  flight  tests  were  made  in  the  vicinity  of  the  10°  to  18°  radials 
at  an  altitude  of  25,000  feet.   The  major  portion  of  the  beacon  replies  that 
were  lost  during  these  tests  were  between  98  and  153  nmi  on  the  10°  and  16° 
radials.   Some  beacon  replies  were  also  lost  between  98  and  133  nmi  on  the 
13°  radial  of  the  beacon  test  site.   Figure  21  shows  a  loss  of  beacon  replies 
that  occurred  on  the  13°  radial  at  125  nmi  due  to  vertical  lobing.   A  series 
of  three  photographs  are  shown  to  illustrate  where  the  aircraft  reply 
appeared  prior  to  and  after  the  loss  of  replies  occurred. 

During  the  morning  of  4  October,  the  timing  signal  that  was  used  for  the  digital 
recorder  clock  was  supplied  directly  from  the  aircraft's  60-Hertz  power  source. 
This  introduced  an  error  due  to  the  60-Hertz  power  source  frequency  shift 
which  occurred  over  a  period  of  time.   A  Time-Mark  Generator,  Tektronix  Type 
2901,  used  to  provide  a  very  accurate  timing  signal  for  the  digital  recorder 
clock,  was  not  functioning  during  the  morning  flight  on  4  October.   The  time- 
mark  generator  problem  was  resolved  at  noon  on  4  October,  and  the  generator 
functioned  normally  for  the  remainder  of  the  flight  tests. 

Flight  tests  were  conducted,  in  the  afternoon  of  4  October,  on  the  beacon 
test  site  radials  between  60°  and  75°.   A  large  portion  of  the  losses  of  beacon 
replies  that  occurred  during  the  targets-of-opportunity  tests  were  lost  on 
the  71°,  84°  and  89°  radials.   Since  extensive  flight  testing  was  accomplished 
on  the  84°  radial  during  the  flight  tests  that  were  performed  on  3  October, 
flight  testing  on  the  84°  radial  was  not  repeated.   On  3  October,  losses  and 
narrowing  of  targets  were  detected  on  the  84°  radial  at  120  nmi  (Figure  22) . 
Some  loss  of  beacon  replies  was  also  recorded  on  3  October  on  the  84°  radial 
at  110,  155  and  170  nmi.   Losses  and  narrowing  of  beacon  replies  were  recorded 
on  3  October  at  ranges  of  115,  123  and  133  nmi  on  the  77°  radial  and  at  135  nmi 
on  the  89°  radial  (see  Appendix) . 

The  flight  tests  that  were  conducted,  during  the  afternoon  of  4  October,  showed 
definite  loss  and  narrowing  of  beacon  replies  on  the  71°  radial  at  ranges 
of  65,  106,  115,  126  and  134  nmi.   Some  narrowing  of  beacon  replies  was 
recorded  on  the  60°  radial  at  71  nmi. 

During  the  flight  testing  that  was  conducted  on  4  October,  some  reflected 

beacon  replies  were  recorded.  The  reflected  replies  occurred  between  201° 

and  177°  when  the  aircraft  was  flown  between  357°  and  21°  at  ranges  between 
51  and  76  nmi  (see  Appendix) . 


37 


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39 


The  flight  testing  that  was  conducted  on  5  October,  using  the  NAFEC  aircraft, 
commenced  by  investigating  the  165°  to  175°  beacon  test  site  radials  at  27,000 
to  29,000  feet.   The  flight  tests  were  made  in  an  attempt  to  produce  losses 
of  beacon  replies  in  this  area,  but  no  complete  loss  of  beacon  replies  was 
noted  except  during  turn  maneuvers.   Some  narrowing  of  the  beacon  reply  was 
recorded  on  the  162°  radial  at  72  nmi  and  on  the  170°  radial  at  79  nmi . 
Figure  23  shows  the  narrowing  of  the  beacon  reply  that  occurred  on  the  170° 
radial  at  79  nmi. 

At  the  completion  of  the  flight  testing  in  the  vicinity  of  the  165°  to  175° 
radials,  the  weather  at  the  0'Hare  Airport  was  below  the  minimum  safe  landing 
requirements  so  the  aircraft  could  not  land.   Flight  testing  was  continued 
on  the  83°  radial  of  the  beacon  test  site  awaiting  a  weather  change.   After 
the  flight  test  was  completed,  the  aircraft  was  flown  to  Green  Bay,  Wisconsin, 
via  the  Peacock  Intersection.   By  this  time  the  weather  had  cleared  and  the 
aircraft  was  flown  back  to  the  O'Hare  Airport  where  it  was  landed.   During 
the  maneuvering  after  the  aircraft  left  the  83°  radial,  the  pilot  did  not 
try  to  maintain  the  aircraft  in  straight  and  level  flight  as  he  had  done 
prior  to  this  time.   This  resulted  in  more  serious  losses  of  beacon  replies 
during  the  later  portion  of  the  flight  testing  on  5  October  1972. 

CAUSES  OF  SITE  PROBLEMS  AND  POSSIBLE  SOLUTIONS. 

The  problems  that  were  observed  during  the  flight  testing  and  analysis  of 
data  recorded  at  the  beacon  test  site  can  be  divided  into  two  groups.   There 
were  beacon  replies  due  to  reflections  and  loss  or  narrowing  of  the  beacon 
replies.   Both  of  these  problem  areas  can  be  related  to  the  broad  vertical 
pattern  of  the  directional  antenna,  particularly  the  loss  or  narrowing  of 
the  beacon  replies.   If  the  energy  radiated  from  the  directional  antenna 
could  have  been  confined  to  vertical  angles  above  the  horizon,  the  beacon 
test  site  would  have  been  virtually  free  of  problems.  Considerable  FAA  research 
and  development  effort  is  presently  being  expended  in  the  development  of  radar 
beacon  antennas  that  radiate  a  controlled  vertical  pattern.   This  is  in  the 
hope  of  eliminating  or  greatly  reducing  future  beacon  problems  due  to  reflected 
replies  and  the  loss  of  beacon  replies  due  to  vertical  lobing. 

REFLECTED  BEACON  REPLY  PROBLEMS.    The  reflected  beacon  replies,  that  were 
recorded  at  the  beacon  test  site,  were  limited  mainly  to  two  areas.   These 
areas  were  both  north  and  south  of  the  beacon  test  site.   The  reflected  replies 
that  occurred  in  the  south  were  confined  to  site  azimuths  between  150°  and 
205°  (see  Appendix  ).   From  the  panoramic  photographs  of  Figure  24  that  were 
taken  at  the  beacon  test  site  at  a  height  of  75  feet,  it  can  be  seen  that 
the  reflecting  surface  is  either  the  fence  along  Irving  Park  Road  or  the 
buildings  and  boxcars  in  the  freight  yard  across  Irving  Park  Road. 

The  replies  that  were  caused  by  the  reflecting  surface  in  the  south  were  limited 
to  ranges  between  53  and  80  nmi.   These  reflected  replies  should  have  been 
eliminated  by  the  improved  3-Pulse  SLS  system  radiation  but  the  data  shows 
that  the  system  capability  was  limited  to  approximately  53  nmi.   Normally, 
the  improved  3-Pulse  SLS  system  is  effective  to  ranges  approximating  80  nmi. 

40 


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42 


The  reason  for  the  restricted  range  at  the  beacon  test  site  was  thought 
to  be  caused  by  the  type  of  transmission  line  used  on  the  omnidirectional 
antenna.   Prior  to  the  erection  of  the  beacon  test  site,  it  was  agreed  that 
RG-197  (low-loss)  cable  would  be  used  on  both  the  directional  and 
omnidirectional  antennas.   A  normal  enroute  radar  site  uses  7/8-inch  low- 
loss  cable,  and  in  some  instances  1-5/8  inch  low-loss  cable,  to  feed  the 
directional  and  omnidirectional  antennas.   RG-218U  coaxial  cable  was  used 
at  the  beacon  test  site.   The  RG-218U  coaxial  cable  attenuates  radar  beacon 
frequencies  the  same  as  RG-17U  coaxial  cable,  which  is  4.4  dB  per  100  feet. 
The  loss  afforded  the  radar  beacon  transmission  by  the  7/8  inch  low-loss  cable 
is  nominally  1 . 5  dB  per  100  feet  while  the  loss  afforded  by  the  1-5/8  inch 
low-loss  cable  is  nominally  .78  dB  per  100  feet. 

If  the  1-5/8  inch  low-loss  cable  were  used  to  feed  the  omnidirectional  antenna 
at  the  beacon  test  site,  the  range  of  the  improved  3-pulse  SLS  system  would 
have  been  extended  by  a  factor  of  1.52.   This  would  have  increased  the  minimum 
range  of  the  improved  3-pulse  SLS  system  from  53  nmi  to  80  nmi  and  would 
have  probably  eliminated  the  reflected  beacon  replies  that  occurred  south  of 
the  beacon  test  site.   If  some  reflected  replies  remained  after  the  trans- 
mission line  cable  was  changed,  then  the  power  input  to  the  omnidirectional 
antenna  could  be  increased  while  the  directional  antenna  input  remained  the 
same.   Since  both  the  directional  and  omnidirectional  antenna  radiation 
power  would  probably  be  increased  by  the  changing  of  transmission  lines 
to  both  antennas,  the  power  input  to  the  omnidirectional  antenna  will  prob- 
ably have  to  be  increased  by  3  dB  to  ensure  that  the  reflected  beacon  replies 
south  of  the  beacon  test  site  were  eliminated. 

Some  experimentation  could  also  be  performed  on  the  fence  along  Irving  Park 
Road.   An  additional  fence  could  be  installed  on  the  beacon  test  site  side  of 
the  original  fence,  and  tilted  as  described  in  NAFEC  Report  No.  NA-69-36 
entitled  "Experimentation  and  Analysis  of  Siting  Criteria,"   dated  September 
1969,  pages  85  through  89.   Tilting  the  fence  at  approximately  one-half  the 
Brewster  Angle  would  reflect  the  energy  that  strikes  the  fence  into  the 
ground  where  it  would  be  absorbed. 

The  reflected  beacon  replies,  that  were  recorded  to  the  north  of  the  beacon 
test  site  were  confined  to  site  azimuths  between  355°  and  15°  (see  Appendix). 
The  range  at  which  these  reflected  beacon  replies  occurred  varied  between 
105  and  178  nmi  which  is  far  beyond  the  range  of  the  improved  3-pulse  SLS 
system.   The  reflecting  surface  that  was  thought  to  be  responsible  for  pro- 
ducing these  reflected  replies  was  the  airline  hangars  approximately  2  to  3 
nmi  to  the  north  of  the  beacon  test  site. 

Most  of  the  reflected  replies  that  occurred  north  of  the  beacon  test  site 
were  caused  by  aircraft  flying  at  site  radials  between  82°  and  99°.   One 
isolated  case  was  recorded  when  an  aircraft  flying  at  267°  caused  a  reflected 
reply  at  10°.   Since  most  of  these  reflected  replies  are  caused  by  aircraft 
flying  in  the  vicinity  of  the  84°  radial  (see  Appendix) ,  a  system  could  be 


43 


installed  at  the  beacon  test  to  increase  the  range  of  the  improved  3-pulse 
SLS  system  in  just  this  one  sector.   A  NAFEC  Data  Report  entitled  "Investiga- 
tion of  Reflected  Reply  Problem  at  Trevose,  Pennsylvania,  Enroute  Radar 
Beacon  Site,"  dated  January  1972,  and  associated  with  Project  No.  031-241-01X, 
deals  with  this  same  problem  that  occurred  at  the  Trevose,  Pennsylvania, 
ARSR-2  Site.   A  horn  antenna  (similar  to  that  shown  in  Figure  17)  was  used  to 
provide  increased  gain  for  the  omnidirectional  antenna  radiations  in  a  certain 
sector  where  aircraft  were  flying  when  reflected  replies  were  produced. 

VERTICAL  LQBING  PROBLEMS.   The  vertical  lobing  problems  that  were  encountered 
during  the  investigation  of  the  beacon  test  site  can  be  grouped  into  three 
areas.   Vertical  lobing  caused  a  loss  or  narrowing  of  replies  at  site  azimuths 
in  the  vicinity  of  the  13°,  84°  and  170°  radials.   The  vertical  lobing  that 
occurred,  in  the  vicinity  of  the  170°  radial,  was  rather  minor  and  no  further 
discussion  will  be  addressed  to  this  area. 

The  vertical  lobing  that  caused  a  loss  or  narrowing  of  beacon  replies  in  the 
northern  sector  and  in  the  vicinity  of  the  84°  radial  was  moderate  in  severity 
and  was  thought  to  be  caused  by  reflections  from  the  smooth  terrain  of  the 
O'Hare  Airport.   The  drainage  lake  on  the  O'Hare  Airport  was  thought  to  play 
a  major  role  in  the  loss  of  replies  that  occurred  in  the  vicinity  of  the  84° 
radial . 

Almost  all  of  the  loss  or  narrowing  of  beacon  replies  occurred  at  ranges 
beyond  100  nmi .   This  was  due  to:  (1)  a  decrease  in  the  intensity  of  the  bea- 
con signals  with  increased  range,  (2)  an  increase  in  the  reflection  coeffi- 
cient of  the  terrain  at  the  lower  angles  of  propagation,  and  (3)  an  increase 
in  the  "apparent"  smoothness  of  the  terrain  at  angles  closer  to  the  horizon. 
If  the  transmission  line  that  feeds  the  directional  antenna  were  made  of  a 
low-loss  1-5/8  inch  coaxial  cable,  fewer  beacon  replies  would  be  lost  or 
narrowed.  At  first  it  might  seem  that  increasing  the  radiated  power  also 
increases  the  terrain  reflection  and  nothing  is  gained.   But,  suppose  a  1-volt 
signal  was  originally  received  in  the  aircraft  direct  from  the  site  and  a 
. 4-volt  signal  was  reflected  from  the  terrain  and  then  received  in  the  aircraft 
The  resultant  signal  would  be  .6  volts  when  the  direct  and  reflected  signals 
were  180°  out-of -phase .   Now,  suppose  we  increase  the  power  output  of  the 
site,  in  fact,  quadruple  it.   The  signal  received  in  the  aircraft  direct 
from  the  site  increases  to  2  volts  (6  dB)  and  naturally  the  reflected  signal 
is  also  doubled  to  .8  volts.   But,  now  the  resultant  signal  would  be  1.2 
volts  when  the  direct  and  reflected  signals  were  180°  out-of -phase .   This 
means  that  increasing  the  power  output  of  the  directional  antenna  by  6  dB 
will  result  in  doubling  the  range  where  the  vertical  lobing  begins. 

By  changing  the  directional  antenna  cable  from  the  RG-218U  coaxial  cable 
which  was  used  during  the  test  to  a  1-5-/8  inch  low-loss  coaxial  cable,  the 
effective  power  output  of  the  directional  antenna  would  be  increased  by  more 
than  3  dB.   This  increase  in  effective  power  could  extend  the  range  of  the 
beacon  signals  by  a  factor  of  1.4.   If  nothing  else,  this  should  emphasize 
the  necessity  of  using  low-loss  coaxial  cable  at  enroute  radar  beacon  sites 
and  using  this  same  type  of  cable  for  testing  of  prospective  radar  beacon 
sites . 

44 


Besides  increasing  the  power  output  of  the  directional  antenna,  there  is 
really  very  little  that  can  be  done  to  reduce  the  vertical  lobing  problem  at 
any  future  O'Hare  Airport  enroute  radar  site.   The  terrain  that  is  responsible 
for  the  vertical  lobing  problem  is  so  extensive  that  only  a  directional 
antenna  with  a  controlled  vertical  pattern  would  provide  the  total  solution 
required. 

FUTURE  EXPANSION  OF  THE  BEACON  TEST  SITE  AREA. 

At  the  present  time,  a  storm  water  control  and  recreation  preserve  is  planned 
for  the  area  surrounding  the  beacon  test  site.   Figure  25  shows  a  detailed 
plan  of  the  lake,  athletic  field  and  picnic  area  that  is  expected  to  be  devel- 
oped.  When  the  lake  area  is  completely  flooded,  vertical  lobing  problems 
could  start  at  site  azimuths  in  the  vicinity  of  the  300°  radial  where  none 
exist  at  this  time. 

Figure  25  shows  a  U.S.  Government  Post  Office  Building  approximately  1,000  feet 
southeast  of  the  beacon  test  site.   A  two-floor  structure  has  been  proposed 
for  this  building.   There  should  be  no  problem  with  this  building  if  the 
structure  is  limited  to  just  two  floors.   If  a  reflection  problem  were  to 
develop  in  the  future,  due  to  the  Post  Office  building,  a  screen  of  shrubs 
or  short  trees  could  be  used  to  shield  the  building  from  the  radar  beacon 
site  radiation. 

The  area  designated  on  Figure  25  as  "Plane  Watch  Hill"  may  also  form  an 
obstruction  at  site  azimuths  in  the  vicinity  of  the  260°  radial.   The  develop- 
ment plan  shows  contour  lines  which  seem  to  indicate  that  a  rather  large  hill 
may  be  formed  in  the  future,  along  with  an  observation  area  on  the  top  of  this 
hill.   A  detailed  study  of  the  elevations  planned  for  "Plane  Watch  Hill" 
should  be  undertaken  prior  to  making  any  firm  decisions  on  the  acceptability 
of  the  area  as  the  site  for  a  future  O'Hare  Airport  enroute  radar  beacon 
site. 


45 


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IRVING    PARK    ROAD 


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O'HARE    STORM  WATER    CONTROL  - 
RECREATION  PRESERVE 
DEVELOPMENT    PLAN 


FIGURE    25. 


FUTURE  DEVELOPMENT  PLANNED  FOR  BEACON 
TEST  SITE  AREA 


46 


CONCLUSIONS 


Based  on  the  test  data  that  was  collected  within  the  coverage  area  of  the 
O'Hare  Airport  enroute  radar  beacon  test  site,  it  is  concluded  that: 

1.  The  flat  terrain  and  hangars  of  the  adjacent  airport  make  the  O'Hare 
Airport  enroute  radar  beacon  test  site  something  less  than  an  ideal  site  from 
the  standpoint  of  siting  criteria. 

2.  Any  future  construction  in  the  vicinity  of  the  radar  beacon  site  could 
limit  the  site  coverage  and  increase  the  occurrence  of  reflected  beacon  replies 

3.  Detailed  coverage  information  can  be  obtained,  for  the  siting  of  future 
radar  beacon  sites,  through  the  use  of  temporary  test  facility  similar  to 
that  used  at  the  O'Hare  Airport  enroute  radar  beacon  test  site. 

4.  The  existence  and  intensity  of  abnormalities,  such  as  vertical  lobing 
and  reflected  beacon  replies,  can  be  determined  through  the  use  of  flight 
tests  using  the  temporary  test  facility. 

5.  The  use  of  coaxial  transmission  cable,  other  than  that  normally 
installed  at  radar  beacon  sites,  required  that  an  intuitive  interpretation 
be  performed  on  the  test  data  in  lieu  of  a  direct  one-to-one  correlation. 


47 


APPENDIX 


The  following  reflected  beacon  replies  were  noted  during  the  analysis  of  the 
O'Hare  Airport  enroute  radar  beacon  test  site  data  using  NAFEC  Aircraft  N-112 

2  October  1972 

Aircraft  Azimuth   Reflection  Azimuth  Aircraft  Range   Reflection 
Frame      In  Degrees         In  Degrees       Nautical  Miles   Range  NMI 


60 

000 

198 

66 

66 

62 

003 

197 

66 

66 

68 

009 

192 

70 

70 

73 

013 

186 

72 

72 

79 

018 

180 

76 

76 

80 

019 

179 

76 

76 

147 

024 

176 

80 

80 

153 

022 

179 

78 

78 

158 

017 

199 

75 

75 

161 

015 

194 

72 

72 

166 

012 

192 

68 

68 

167 

010 

164 

6  7 

6  7 

169 

009 

192 

65 

65 

170 

008 

193 

64 

64 

171 

007 

193 

64 

64 

197 

345 

165 

68 

68 

216 

000 

200 

70 

70 

217 

003 

199 

70 

70 

218 

002 

197 

70 

70 

224 

009 

190 

57 

67 

227 

013 

190 

66 

66 

231-1 

017 

184 

65 

65 

231-2 

017 

199 

6  5 

66 

232 

019 

155 

65 

65 

235-1 

021 

181 

66 

66 

235-2 

021 

150 

66 

75 

284 

025 

175 

8U 

80 

288 

022 

177 

78 

79 

295 

018 

198 

72 

72 

302 

011 

189 

71 

71 

307 

007 

193 

68 

68 

312 

003 

198 

68 

68 

314 

001 

199 

68 

68 

315 

000 

200 

69 

69 

316 

359 

200 

69 

69 

317 

359 

201 

70 

70 

318 

358 

202 

70 

70 

320 

357 

205 

70 

70 

A-l 


Aircraft  flew  over  the  site  at  41,000  feet  altitude  to  determine  the  overhead 
coverage  of  the  site.   The  following  azimuth  and  ranges  were  observed: 

Lost  radar  beacon  reply  —  344°  azimuth  at  9.5  nmi  slant  range. 
Regained  radar  beacon  reply  —  169°  azimuth  at  9.0  nmi  slant  range. 

3  October  1972 

Aircraft  Azimuth  Reflection  Azimuth  Aircraft  Range   Reflection 
Frame     In  Degrees    In  Degrees     Nautical  Miles   Range  nmi 


124 

085 

013 

178 

178 

188 

090 

006 

116 

116 

189 

090 

007 

115 

115 

190 

090 

007 

114 

114 

394 

083 

015 

105 

105 

Reflected  replies  obtained  during  the  150-nmi  orbital  flight  test: 

010 
010 
355 
356 
355 
005 
001 
005 
010 


898 

267 

899 

268 

1258 

099 

1259 

097 

1260 

099 

1278 

087 

1280 

087 

1281 

086 

1291 

082 

145 

145 

145 

145 

145 

145 

146 

149 

146 

149 

146 

148 

145 

147 

145 

147 

145 

147 

4  October  1972 


70 

357 

73 

000 

74 

001 

75 

001 

77 

001 

80 

005 

81 

005 

85 

005 

242 

012 

247 

012 

252 

013 

262-1 

015 

262-2 

015 

265 

019 

267 

020 

268 

021 

272 

021 

273 

021 

201  61  62 

200  62  63 

199  62  63 
198  63  64 
197  64  65 
195  66  67 
194  66  67 
192  69  70 
186  72  72 
186  65  ;  66 
188  61  61 
180  74  76 

200  74  77 
180  76  77 

177  59  59 

178  60  60 
180  62  63 

179  63  64 


A- 2 


Aircraft  Azimuth  Reflection  Azimuth  Aircraft  Range   Reflection 
Frame     In  Degrees    In  Degrees     Nautical  Miles   Range  nmi 


274 

020 

179 

64 

65 

474 

016 

185 

5b 

57 

475 

015 

188 

54 

55 

476 

015 

187 

53 

54 

477 

014 

188 

54 

55 

478 

012 

188 

54 

55 

479 

009 

190 

55 

56 

480 

008 

192 

5  5 

56 

481 

006 

193 

56 

57 

482 

006 

195 

56 

57 

483 

005 

195 

5  7 

58 

500 

012 

187 

64 

65 

The  following  loss  or  narrowing  of  beacon  replies  were  observed  during  the 
analysis  of  the  O'Hare  Airport  Enroute  Radar  Beacon  Test  Site  data  using 
NAFEC  Aircraft  N-112: 

3  October  1972 


Frame  Degrees       Nautical  Miles            Comments 

77  84  120  Normal  Target 

78  84  121  Weak  Target 

79  84  122  Very  Weak  Target 

80  84  123  Normal  Target 

81  84  124  Weak  Target 

82  84  125  Very  Weak  Target 

83  84  126  Normal  Target 

84  84  127  Weak  Target 

85  84  No  Target  Return 

86  84  129  Normal  Target 

87  84  130  Very  Weak  Target 

88  84  131  Normal  Target 

103  84  153  Normal  Target 

104  84  154  Weak  Target 

105  84  No  Target  Return 

106  84  156  Very  Weak  Target 

107  84  No  Target  Return 

108  84  158  Weak  Target 

109  84  159  Normal  Target 

114  84  165  Normal  Target 

115  84  166  Weak  Target 

116  84  No  Target  Return 

117  84  168  Very  Weak  Target 

118  84  169  Normal  Target 


A- 3 


Frame  Degrees       Nautical  Miles  Comments 

119  84  No  Target  Return 

120  84  171  Weak  Target 

121  84  173  Weak  Target 

122  84  No  Target  Return 

123  84  176  Weak  Target 

124  84  177  Normal  Target 

291  77  114  Normal  Target 

292  77  No  Target  Return 

293  77  115  Normal  Target 

299  77  122  Normal  Target 

300  77  No  Target  Return 

301  77  125  Normal  Target 

307  77  131  Normal  Target 

308  77  132  Weak  Target 

309  77  133  Weak  Target 

310  77  134  Normal  Target 

455  89  132  Normal  Target 

456  89  133  Very  Weak  Target 

457  89  134  Normal  Target 

458  89  135  Very  Weak  Target 

459  89  136  Normal  Target 

632  17  150  Normal  Target 

633  16  150  Weak  Target 

634  16  No  Target  Return 

635  15  149  Normal  Target 

669  001  148  Normal  Target 

670  000  148  Weak  Target 

671  359  148  Weak  Target 

672  358  148  Normal  Target 

675  357.5  148  Normal  Target 

676  357  148  Very  Weak  Target 

677  356.5  148  Weak  Target 

678  356  148  Very  Weak  Target 

679  355.5  148  Weak  Target 

680  355  148  Normal  Target 

713  341  148  Normal  Target 

714  340.5  148  Weak  Target 

715  340  148  Normal  Target 


A-4 


Frame  Degrees       Nautical  Miles  Comments 

1103  170.5  146  Normal  Target 

1104  170  146  Weak  Target 

1105  169.5  146  Normal  Target 

1106  169  146  Weak  Target 

1107  168.5  146  Normal  Target 

4  October  1972  (Morning) 

116  10  101  Normal  Target 

117  10  102  Weak  Target 

118  10  103  Weak  Target 

119  10  104  Normal  Target 

120  10  105  Normal  Target 

121  10  106  Normal  Target 

122  10  No  Target  Return 

123  10  No  Target  Return 

124  10  109  Normal  Target 

125  10  110  Normal  Target 

126  10  111  Weak  Target 

127  10  112  Weak  Target 

128  10  113  Normal  Target 

129  10  114  Weak  Target 

130  10  115  Normal  Target 

137  10  123  Normal  Target 

138  10  124  Weak  Target 

139  10  No  Target  Return 

140  10  126  Very  Weak  Target 

141  10  127  Very  Weak  Target 

142  10  No  Target  Return 

143  10  129  Very  Weak  Target 

144  10  130  Weak  Target 

145  10  131  Weak  Target 

146  10  132  Normal  Target 

153  10  140  Normal  Target 

154  10  No  Target  Return 

155  10  No  Target  Return 

156  10  143  Weak  Target 

175  13  133  Normal  Target 

176  13  132  Weak  Target 

177  13  131  Normal  Target 

184  13  126  Normal  Target 

185  13  No  Target  Return 

186  13  124  Normal  Target 


A- 5 


Frame  Degrees       Nautical  Miles            Comments 

211  13  101  Normal  Target 

212  13  100  Very  Weak  Target 

213  13  99  Very  Weak  Target 

214  13  98  Normal  Target 

306  16  98  Normal  Target 

307  16  No  Target  Return 

308  16  101  Weak  Target 

309  16  102  Normal  Target 

310  16  103  Very  Weak  Target 

311  16  104  Normal  Target 

317  16  110  Normal  Target 

318  16  No  Target  Return 

319  16  No  Target  Return 

320  16  113  Normal  Target 

322  16  115  Normal  Target 

323  16  116  Weak  Target 

324  16  117  Weak  Target 

325  16  118  Normal  Target 

330  16  123  Normal  Target 

331  16  124  Very  Weak  Target 

332  16  125  Weak  Target 

333  16  126  Weak  Target 

334  16  127  Very  Weak  Target 

335  16  128  Very  Weak  Target 

336  16  129  Weak  Target 

337  16  No  Target  Return 

338  16  131  Normal  Target 

347  16  142  Normal  Target 

348  16  143  Weak  Target 

349  16  No  Target  Return 

350  16  145  Weak  Target 

351  16  146  Normal  Target 

353  16  148  Normal  Target 

354  16  149  Weak  Target 

355  16  No  Target  Return 

356  16  No  Target  Return 

357  16  152 .  Weak  Target 

358  16  153  Normal  Target 

417  16  112  Normal  Target 

418  16  111  Very  Weak  Target 

419  16  110  Very  Weak  Target 

420  16  109  Normal  Target 


A- 6 


Azimu 

th 

In 

Range  In 

Deg 

rees 

Nautical  Miles 

16 

104 

lb 

103 

lb 

16 

101 

16 

100 

16 

16 

98 

Frame  Degrees       Nautical  Miles  Comments 

426  16  104  Normal  Target 

427  16  103  Weak  Target 


428  16  No  Target  Return 

429  16  101  Normal  Target 

430  16  100  Normal  Target 

431  16  No  Target  Return 

432  16  98  Normal  Target 

4  October  1972  (Afternoon) 

256  71  113  Normal  Target 

257  71  No  Target  Return 

258  71  115  Very  Weak  Target 

259  71  116  Weak  Target 

260  71  No  Target  Return 

261  71  119  Normal  Target 

267  71  125  Normal  Target 

268  71  126  Very  Weak  Target 

269  71  127  Normal  Target 

273  71  131  Normal  Target 

274  71  132  Weak  Target 

275  71  No  Target  Return 

276  71  No  Target  Return 

277  71  135  Weak  Target 

278  71  136  Normal  Target 

357  60  73  Normal  Target 

358  60  72  Weak  Target 

359  60  71  Weak  Target 

360  60  70  Normal  Target 

387  71  63  Normal  Target 

388  71  64  Weak  Target 

389  71  65  Very  Weak  Target 

390  71  66  Broken  Target 

391  71  67  Normal  Target 

426  71  105  Normal  Target 

427  71  No  Target  Return 

428  71  No  Target  Return 

429  71  No  Target  Return 

430  71  No  Target  Return 

431  71  No  Target  Return 

432  71  110  Normal  Target 


A- 7 


Frame 

434 
435 
436 
437 
438 
439 
440 

445 
446 
447 
448 

450 
451 
452 
453 
454 
455 
456 


Azimuth 

In 

Range  In 

Degrees 

Nautical  Miles 

71 

112 

71 

113 

71 

114.5 

71 

116 

71 

117.5 

71 

119 

71 

120 

71 

125 

71 

126 

71 

71 

128 

71 

130 

71 

131 

71 

71 

71 

135 

71 

136 

71 

137 

Comments 

Normal  Target 
Weak  Target 
Weak  Target 
Normal  Target 
Weak  Target 
Weak  Target 
Normal  Target 

Normal  Target 
Weak  Target 
No  Target  Return 
Normal  Target 

Normal  Target 
Weak  Target 
No  Target  Return 
No  Target  Return 
Weak  Target 
Weak  Target 
Normal  Target 


5  October  1972 


203 
204 
205 
206 


162 
162 
162 
162 


70 
71 
72 
73 


Normal  Target 
Weak  Target 
Weak  Target 
Normal  Target 


526 
527 
528 
529 
530 
531 


170 
170 
170 
170 
170 
170 


81 
80 

79 
78 
77 
76 


Normal  Target 
Weak  Target 
Very  Weak  Target 
Normal  Target 
Weak  Target 
Normal  Target 


1231 
1232 
1233 
1234 
1235 
1236 
1237 
1238 
1239 
1240 


25 
23 


143 
143 


20 


142 


Normal  Target 
Weak  Target 
No  Target  Return 
No  Target  Return 
No  Target  Return 
No  Target  Return 
No  Target  Return 
No  Target  Return 
No  Target  Return 
Normal  Target 


A-8 


Azimuth  In  Range  In 

Frame  Degrees       Nautical  Miles           Comments 

1247  17  142              Normal  Target 

1248  No  Target  Return 

1249  No  Target  Return 

1250  No  Target  Return 

1251  15  142              Normal  Target 

1266  10  142              Normal  Target 

1267  9  142              Weak  Target 

1268  8  142              Weak  Target 

1269  No  Target  Return 

1270  No  Target  Return 

1271  6  142              Normal  Target 


A- 9