The control system achieves beam forming by adjusting the phases of the individual transmitters onboard the solar power satellite. To specify the correct amount of adjustments, the phases of the power beams from each individual transmitter arriving at the rectanna center are measured, the appropriate corrections determined, and then relayed to the satellite. The functional operation of the ground based phase control concept is summarized. The key issues examined were: measurement waveform design and selection; phase measurement pilot reference design and selection; uplink phase corrections command link format and design; and system synchronization techniques

Chie, C. M.

English

NASA Technical Reports Server

C O H E R E N T  MULTIPLE T O N E  TECHNIQUE F O R  
G R O U N D  BASED SPS PHASE CONTROL* 
C. M .  Chie 
Li nCom Corporation 
1.0 Introduction 
The ground based phase control concept has been under study a t  
LinCom as an a l t e rna t ive  aproach to  the reference SPS phase control 
system (See Refs. 1 ,2 ,3) .  The de ta i l s  of the ground based phase control 
system study are documented i n  Ref. 4. In t h i s  short  paper we surnrnarize 
the coherent multiple tone technique used for  the ground based phase 
meastirement waveform design and phase control system. 
2.0 Ground-Based Phase Control Concept 
The ground based phase control system achieves beam forming by 
adjusting the phases of the individual transmitters on board SPS. The 
phase adjustments are controlled by ground commands. To specify the 
correct  amount of adjustments, the phases of the power beams from each 
individual transmitter arriving a t  the rectenna center must be measured, 
the appropriate corrections determined ( t o  ensure tha t  a1 l power beams 
arr ive  a t  the same phase) and relayed to  the SPS. The proposed scheme 
to be considered i s  sequential i n  nature, i .e., the phase measurement i s  
performed one a t  a time for  each individual transmitter a t  approximately 
one-second intervals  (measurement time a1 1 ocated i s  10 psec) ,  The phase 
corrections are updated once every secocd. A 10-bit  phase quantization 
for the corrections giving 0.35' resolution i s  envisioned. The up1 ink 
command data ra te  i s  on the order of 10 Mbps. The functional operation 
of the ground-based phase control concept i s  summarized i n  Fig. 1. As 
evident from the f igure ,  the key issues tha t  need t o  be addressed are: 
*This work was performed a t  LinCom Corporation for  the NASA Johnson 
Space Center, Houston, TX, under Contract No. NAS9-15782. 
https://ntrs.nasa.gov/search.jsp?R=19810008027 2020-03-21T15:13:20+00:00Z
RECTENNA CENTER 
PHASE ClEASUREIs!ENT 
PHASE ERROR 
UPDATE ALGORITHlci 
30;'IPlUN I  CAT I  OPJ 
a- 
Figu re  1. Ground Based Phase Control  Systern Concept with 
I4ajor Funct ional  Blocks. 
CONTROL 
CENTER ( C C )  
- 
INTERFACE BUS 
SPACETENP!A 
PHASE REFERENCE 
DOIdNL I N K  
[$=.--Pi 
t h PHASE O F  i- 
(1) measurement waveform design and selection,  
(2 )  phase measurement p i lo t  reference design and selection,  
(3)  uplink phase corrections command 1 ink format and design, and 
( 4 )  system synchronization techniques. 
3.0 Two-Tone Phase Measurement Scheme with Coherent Subcarrier 
In the basic two-tone measurement scheme, two side tones a t  fo 5 Af 
are  transmitted from the s a t e l l i t e  t o  the ground receiver. A phantom 
ca r r i e r  can be reconstructed from the sidetones by passing the signal 
through a squaring c i rcu i t .  The output will then have a CW component 
w i t h  frequency 2f0 and a phase component equal t o  + @ J ~ ) ,  where 
and $2 are the channel induced phase s h i f t s  a t  fo + Af and fo - A f ,  
respectively. This phase s h i f t  i s  very close t o  double the one that  
would have occurred i f  the down1 ink signal were a single sinusoid a t  
frequency fo. I f  we divide the 2f0 component by two, we obtain the 
$l++2 average phase Unfortunately, the divide by two c i r cu i t  resul ts  
i n  a 0" - 180" ambiguity. 
4.0 Four-Tone Phase- Measurement Scheme 
The four-tone measurement scheme given in Fig. 2 i s  a simple 
modification of the two-tone scheme. Basically, we f i r s t  use 
frequencies a t  fo +_ 2Af for  phase .error measurement with introduces a 
ambiguity. Then we use frequencies a t  fo  t Af for  ambiguity 
resolution. The scheme works as follows. The transmitted signal a t  the 
i n p u t  t o  the transmitting antenna i s  (neglecting mu1 tip1 icat ive  
constants)  
PHASE ERROR/F.!!B IGUITY 
RESOLUTION DATA 
F igure2 .  Four-Tone Phase Measurement Scheme. 
where e i  includes the commandable phase s h i f t ,  i s  the ambiguity 
fo  introduced by the divide by N c i r cu i t ,  f = -, and a = 0,1,2 depending N 
on whether the PM i s  in the power mode ( I ) ,  ambiguity resolution mode 
( 2 ) ,  or phase e r ro r  measurement mode ( 3 ) .  A t  the receiver on the 
ground, 
e a 28 ka 
s 2 ( t )  = cos[w ( I +  R)t + (1+ + -- + v+(e) I  0 N 
where q+(.E) and *(a) are the phase s h i f t s  introduced by the channel. 
The reference signal s 3 ( t )  i s  given by 
21ram 
s 3 ( t )  = cos[wo(l+ i)t + ( I +  ;)eR + -N 1 
e II 21rkrn + C O S [ U ~ ( ~ -  W)t + (1- N ) O R  - 7 N 
where eR i s  the phase of the ground reference, and i s  the ambiguity 
introduced by the ground divide by N c i r cu i t .  If the operations are  
synchronized, we can then measure u p  to  modulo ZIT a t  the output of the 
measurement c i r c u i t ,  the phases 
a 27~a Y + ( L )  + ( I +  N)(ei -eR)  + -N ( k - m )  = +,(a) + znM+(e) (1 
e 21ra y ( a )  + ( I  - - )  - 0  ) - -- ( k )  = +-(a)  + 21rM ( t )  
- N 1 R  N (2)  
Actually, i n  (1) and (2 ) ,  ++ (a )  and +-( a7 are the measured phases and 
M+( a)  and M ( a )  are integers so tha t  the absolute values of ++(a)  and 
+ - ( a )  can be res t r i c ted  t o  7 ~ .  Note tha t  we are in teres ted i n  
determining [q+(ll)+c~_(11)]/2 modulo 27~. For a=2, we know from (1) and 
(2 )  tha t  
Now i f  we can resolve whether [~+(2)+M-(2)1 i s  even or odd, we can 
determine [ q + ( 2 ) + ~ ( 2 ) 1 / 2  + ( e i - e R )  modulo 21r. T h i s  information i s  
Figure 3 .  SPS Ground Bascd Phase Control Functional Block Diaqram Showing 
Systcs~ Tiniirig I l ierarchy.  
134  
p rov i ded  by comparing 
2 471 ( 1  - ( 1  = - B ( ~ i - ~ R )  - $k-n) + (+(I) - (-(I) +[M+(l)-M-(l)]Zn 
(4) 
If A f  i s  designed p rope r l y  ( a f  < 50 MHz) t h e  l e f t  hand s ide  o f  ( 5 )  and 
(5 )  a re  n e a r l y  equal. See Ref. 4 f o r  a  d iscuss ion  on ionospher ic  
e f f e c t s .  Equat ing (4)  and (5 )  we have 
( + ( 2 ) 3 - ( 2 )  
+ [M+(2)-M-(Z)l 71 1  - ( 1 )  (mod ) ( 6 )  
Since we can measure $ + ( a ) ,  we can determine from (6 )  whether [M+(2)- 
- 
M-(211 i s  odd o r  even. Th is  then determines whether [ ~+ (2 )+M- (2 )1  i s  
odd o r  even, s i nce  [M+(2)-M-(2)1 + [M+(2)+M-(2)1 = M+(2) must be even. 
Wi th  t h i s  i n f o rma t i on ,  we can so lve  f o r  [ q+ (2 )+~~ - (2 )1 /2  + (ei-BR) modulo 
271 i n  (3 ) .  
5.0 Base1 i ne System f o r  Ground-Based Phase Cont ro l  
The implementat ion o f  t h e  ground-based phase c o n t r o l  concept i s  
determined by t h e  phase c o n t r o l  waveform designs employed. Based on our 
waveform se l ec t i ons ,  f u n c t i o n a l  subsystems t o  implement t h e  ground-based 
phase c o n t r o l  concept are i d e n t i f i e d  and f u n c t i o n a l  l y  represented. The 
r e s u l t a n t  ground-based phase c o n t r o l  f u n c t i o n a l  b lock  diagram i s  
dep ic ted  i n  F i g .  3  and inc ludes:  
%Sate1 1 i t e  Signal  Processing 
aTime-Frequency Cont ro l  
oProcess i  ng Cont ro l  Center 
eSignal D i s t r i b u t i o n  Network 
cProcess i  ng Power Module 
eDownl i n k  P i  1 o t  Transmi t t e r  
1 3 5  
.Up1 ink Command Receiver 
@Ground Based Signal Processing 
e P  i 1 o t  Beacon Receiver 
@Ca l ib r a t i on  Receiver 
sPhase Measurement Unit 
@Synchronization Unit  
*Phase Update A1 gori  thm 
-Data Processing Unit  
cUplink Command Transmi t te r  
The ground-based system envisioned employs sa te1  1 i t e  based 
frequencyl t iming re fe rence  with an IF frequency of 490 MHz. A 4-tone 
measurement scheme using f requenc ies  a t  2,450 2 9.57 MHz and 2,450 9 
19.14 KHz i s  s e l ec t ed .  Each power nodule devotes 10 usec  per  second f o r  
phase c o r r e c t i o n  measurement, represen t ing  a minimal 1 oss  i n  t o t a l  power 
t ransmi t ted .  Two f requenc ies  a r e  chosen f o r  t h e  downlink and one 
frequency f o r  uplink; t he  downlink p i l o t  s igna l  c en t e r  frequency i s  s e t  
a t  4.9 GHz. 
O u r  prel  imi nary i nves t i ga t i on  i n d i c a t e s  t h a t  t h e  e f f e c t s  of power 
beam i n t e r f e r e n c e  and thermal noise on t he  phase measurement e r r o r  can 
be control  l e d  t o  a to1 erabl  e 1 eve1 . The ground based system can a1 so  
funct ion i f  t h e  ionosphere i s  nonturbulent  i n  na ture  and t he  sate11 i t e ' s  
t i l t  r a t e  i s  l i m i t e d  t o  0.5 m%/sec. 
6.0 Limiting Fac tors  of t he  Feasbi l  i t y  of Ground-Based Phase Control 
Svstem 
The f e a s i b i l i t y  of t he  ground-based phase cont ro l  concept becomes 
unclear i f  t h e  condi t ions  on t h e  ionosphere and t he  s a t e l l i t e  motion a r e  
not met. The ground-based phase cont ro l  system can only c o r r e c t  f o r  
random phase f l u c t u a t i o n s  which have a c o r r e l a t i o n  time t h a t  i s  l a r g e  
compared w i t h  1.25 sec. The no ise  components which a re  f a s t e r  than 1.25 
sec i s  uncompensated f o r  and r e s u l t  i n  a  degradat ion on t r ansm iss i on  
e f f i c i e n c y .  Un fo r t una te l y ,  measured ionosphere da ta  which i s  su i  tab1 e 
f o r  t h e  SPS system i s  n o t  r e a d i l y  ava i l ab l e .  (Most data a re  concerned 
w i t h  s p a t i a l  c o r r e l a t i o n s  r a t h e r  than temporal c o r r e l a t i o n s .  A1 so, most 
data a re  measured from 1 ow o r b i t  sate1 1 i t e s  r a t h e r  than geos ta t ionary  
sate11 i tes. ) The o t h e r  1  i m i  t i n g  f a c t o r  i s  t h e  s t a t i s t i c a l  behav io r  o f  
t he  random p o i n t i n g  e r r o r  e x h i b i t e d  by t h e  spacetenna. Again, t h e  f a s t  
component o f  t h i s  e r r o r  i s  n o t  co r rec ted  f o r  and i t  c o n t r i b u t e s  t o  
e f f i c i e n c y  degradat ion. A t  t h i s  p o i n t ,  we f e e l  t h a t  t he  development and 
s p e c i f i c a t i o n  o f  models f o r  i onospher i c  phase d is tu rbance  and s a t e l l i t e  
mot ion i s  essen t i a l .  It i s  hoped t h a t  our  f i n d i n g s  can serve as a 
g u i d e l i n e  f o r  any p a r a l l e l  e f f o r t s  i n  s t udy i ng  these two fac to rs .  
REFERENCES 
1. Lindsey, W. C . ,  and Kantak, A. V., "Automatic Phase Control i n  
Sol a r  Power Sate1 1 i t e  Systems," Prepared f o r  NASAIJSC, 
TR-7809-0977, September 1977, LinCom Corporation, Pasadena, CA. 
2. Lindsey, W. C.,  "A So la r  Power S a t e l l i t e  Transmission System 
Incorporat ing Automatic Beam Forming, S teer ing  and Phase Control ,I1 
Prepared f o r  NASAIJSC, June,  1978, LinCom Corporation, Pasadena, 
CA. 
3 .  Lindsey, W. C.,  and Kantak, A. V., "SPS Phase Control System 
Performance v ia  Analyt ical  Simulation," Prepared f o r  NASAIJSC, 
.February, 1979, LinCom Corporation, Pasadena, CA. 
4. Chie, C. M . ,  "A Ground Based Phase Control System f o r  t h e  Solar  
Power Sate1 1 i t e , "  Prepared f o r  NASAIJSC, January, 1979. 


Coherent multiple tone technique for ground based SPS control

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