The phase control system is described. Potential sources of phase error are identified and the performance leading to selection of the allowable phase error for each source is summarized. The pilot transmitter, the effects of ionospheric, the master slave returnable timing system (MSRTS), the SPS receiver, and the high power amplifier for dc to microwave conversion are considered separately. Design parameters of the pilot transmitter and spacetenna transponder are presented

Chie, C. M.

Kantak, A. V.

Lindsey, W. C.

English

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NASA U-/0-3'13
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{NASA — Cat-167393) SOLAR LOWER SATELLITE
ANTENNA PHASE CONTROL SYSTEM HARDWARE
SIMULATION, RASE 14: VOLUME 1	 EXECUTIV E
SUMMARY Final Report (.Li.nCom Corp,,
Pasadena, Calif.) 22 p HC A02/MF A01
N81-33612
Unclas
03/44 27696
P.O. Box 2793D, Pasadena, Calif. 91105
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OCT 1,c)81
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SOLAR POWER SATELLITE ANTENNA PHASE CONTROL SYSTEM
HARDWARE SIMULATION PHASE IV
VOLUME I. EXECUTIVE SUMMARY
PREPARED BY
NASA LYNDON B. JOHNSON SPACE CENTER
HOUSTON, TX	 77058
TECHNICAL MONITOR: JACK SEYL
CONTRACT NO. NAS9-16097
PREPARED BY
W. C. LINDSEY
A. V. KANTAK
C. M. CHIE
LINCOM CORPORATION
P.O. BOX 2793D
PASADENA, CA 91105
MARCH 1981	
1
TR-0381-1280
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1, INI ODUCTION
The Solar Power Satellite offers a potentially economic means of
producing sonar based electric power which is virtually limitless in
nature.
A critical requirement for the proposed Solar Power Satellite (SPS)
concept is the ability of the satellite to beam and focus microwave
energy to a predetermined spot located on the earth's surface from a
geosynchronous orbit of 36,000 kin. in altitude and with a transmission
efficiency in excess of 90%. To form a coherent microwave beam to
achieve the desired transmission efficiency a precise phase control of
the EM wave during its passage through the entire circuit is
necessary. The major elements required in the operation of an SPS
r
system which employs retrodirectivity as a means of automai;icaliy
forming and pointing the beam to the appropriate spot on the earth is
shown in Figure 1.1. A block diagram representation of the system is
shown in Figure 1.2.
As shown in the figure, the EM wave circuit includes: (1) the
pilot transmitting antenna, (2) the power transmitting antenna on the
satellite (spacetenna), and (3) the power receiving antenna on the earth
(rectenna). The pilot transmitting antenna, situated,at the center of
the rectenna emits a pilot tone which is used by the spacetenna to
direct the high power beam so that it comes to focus at the rectenna.
As mentioned earlier, the phase control of the EM wave is of
paramount importance for the proper operation of the SPS system. Each
block in the block diagram of Figure 1.2 is a potential ohase
disturbance source. From previous work on the SPS system it is agreed
that a total phase error of lo o at the spacetenna radiation ports would
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allow one to obtain an acceptable transmission efficiency in excess of
90%. The breakdown of the phase error in terms of all the potential
sources of phase error in the SPS system so that the RMS phase error at
the spacetenna radiation port is not greater than loo is shown in Figure
7.2. Subsequent sections describe each source of phase error and give
the performance leading to selection of the allowable phase errors for
the sources.
The executive summary of accomplishments and pertinent results is
organized into five sections, each one is summarized as follows.
2. PILOT TRANSMITTER
The SPS MPTX system is deemed to have a retrodirective self
cohering antenna system working with a pilot beam from the rectenna
center. The pilot waveform will be a NRZ pseudonoise code amplitude
modulated onto a subcarrier of 40 MHz. This subcarrier amplitude
modulates the uplink carrier at 2.45 GHz. The pilot transmitter system
functional block diagram is depicted in Figure 2.1.
The purpose of spread spectrum code modulation is several fold.
First it provides link security, second it provides a multiple access	 I nr
capability for the operation of a network of SPSs and third, the
antijamming protection is provided for both intentional radio frequency
interference (RFI) and unintentional RFI such as those arisin g from
neighboring SPS on an adjacent orbit.
Fig. 2.2 shows the spectrum of the uplink CV) pilot tone. The
subcarrier modulation shifts the spectrum of the NRZ PN code by 40 MHz
on either side of the uplink carrier frequency of 2.45 GHz suppressing
the carrier component. This shifting of the code spectrum is used to
effectively reduce the noise at the output of the front end of thenMPTX
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system. On the MPTX system the suppressed carrier is regnerated, its
phase is used to form the self cohering retrodirected downlink power
beam.
As can be seen from the Fig. 2.1 the clock phase noise as well as
the noise added by the system component will have a corresponding !chase
jitter introduced in the uplink pilot beam. This phase jitter
constitutes one part of the total phase jitter suffered by the entire
system. A list of parameter selection for the pilot signal is included
in a later article.
The phase jitter added by the pilot transmitter to the pilot is
required to be less than 1 0. As we will see later, phase jitter in the
m crowive radiating system is one major cause of loss of power transfer
efficiency of the Solar Power Satellite.
3. IONOSPNERICS
The downlink power beam of the proposed SPS at geosynchronous
altitude is to be formed by the retrodirective phase conjugation
technique. The uplink pilot beam providing tho phase information to the
conjugator circuit passes through the heated ionosphere introducing the
phase jitter to the phase of the pilot wave.
The heating produced by the downlink pilot beam would be of the
underdense type, i.e., it is a case of heating of plasma with a
microwave frequency greater than the critical frequency of the plasma,
so that the heating wave penetrates the ionosphere. This underdense
heating of the ionosphere creates irregularities in the ionsophere
termed striations. The striations size depends on the angle between
downlink power beam and the magnetic field of the plasma. Because of
V	 the horizontal dimension of the SPS beam in the ionosphere is about S
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FIGURE 3.1. GEOMETRY OF SCINTILLATION PROBLEM.
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Km, the striations of width 5 Km or less would be of any concern 14., o the
SPS t;:"A ink pilot beam. If the ionospheric irregularities have scales
equal to or less than the Fresnel zone associated with the pilot beam
then the diffractive scattering of the pilot beam will occur while if
the scales are larger thdn the Fresnel zone the refrartive scattering
will dominate diffractive scattering.
Fig. 3.1 illustrates that in the presence of fluctuations of
ionospheric electron concentration an impinging plain wave suffers phase
fluctuations as it emerges from the striation layer. For small electron
density fluctuations the emerging uplink pilot undergoes only phase
fluctuations without any fluctuations in the intensity. As the wave
front propagates towards the spacetenna, the phase mixing occurs and as
a direct consequence, the spatial intensity fluctuations may also
develop.
These processes affecting the phase of the uplink pilot are largely
unknown and hence a need for experimentation exists. However from
whatever data at hand it seems reasonable to assume a maximum phase
jitter of 30 on the uplink pilot received at spacetenna conjugation
ports.
4. MASTER SLAVE RETURNABLE TIMING SYSTEM (MSRTS)
As mentionc.1 earlier, solar power satellite employs retro-
directivity as the means of automatically forming and pointing the high
power microwave downlink beam to the rectenna site. The need for a
constant phase reference necessary for the retrodirective operation of
the spacetenna is comprehended by the reference phase distribution
system shown in Fig. 4.1.
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The reference phase distribution system 4 s fin^,-,, Iemonted in the form
of a phase distribution tree structure.	 This tree is deemod to have
four levels having 16, 16, 16, and R5 branches per each node at levels
1, 2, 3 and 4 respectively. The iiiajor purpose of the tree structure is
to electronically compensate for the phase shift due to the transition
path lengths from the center of the spacetenna to each phase control
center (PCC) in each subarray. From which the constant phase is fed to
each receiver for the purpose of conjugation. The electronic phase
compensation is accomplished using the master slave returnable timing
system (MSRTS) technique.
Figure 4.2 shows tho electronic components of the MSRTS system in
block diagram form. The master signal at the center of the spacetenna
is fed to the frequency double ►, at point A in the figure. The master
phase is regenerated at point B which is frequency doubled and then is
used to supply the regenerated master phase to the phase control centers
in the next level in the phase distribution tree.
The MSRTS phase distribution system has several components such as
tile pilot signal receiver at the center of spacetenna, power splitters
in the phase distribution tree, oscillators 
in 
the MSRTS, etc., each
component adding to the phase jitter of the radiated power wave. The
total	 rms phase jitter of the system is required to be less than 7.50.
This is the phase jitter observed at the input of Cie conjugator at each
power amplifier tube.	 The phase jitter below 7.5 0 will	 be required to
achieve power transfer efficiency of in excess of 90%.
V, SPS RECEIVER
The basic idea behind the SPS receiver is to receive the uplink
pilot, regenerate the suppressed carrier and use the phase of the
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Figure 5.1 shows the basic loops required in the receiver in block
diagram form. These loops are (1) the code tracking loop (PN despreader
loop), (2) the carrier tracking loop (pilot phase tracking loop) and (3)
the PA phase control loop. Analytical models for the above loops as
well as the front end filter performance are formulated.
The signal and noise spectra at the input to the transponder are
shown in Fig. 5.2. The signal plus noise is filtered by the front end
filter. The filter is of notch type and the main purpose of it being to
pass the NRZ PN spread pilot signal sidebands without distortion at the
same time rejecting the self and mutual coupling noises affecting the
performance of the transponder. Figure 4.2 illustrates the amplitude
response of the front end filters.
The filtered signal plus noise is then fed to the code tracking
loop. The code tracking loop is one chip time tau-dither loop which
sequentially correlates the retarded and advanced versions of locally
generated code with the incoming code. The error after passing through
the arm filter drives the loop VCO which controls the local code
generation. When the loop locks, the error between the incoming code
and the local code is negligible (there is no message on the code). The
loop bandwidth and the arm filter bandwidth are of importance in
determining the code tracking error. Following table on page 17 gives
the values selected for these parameters.
Output of the front end filter is also fed to the carrier tracking
loop. The main function of this loop is to reproduce the suppressed
,
carrier and track its phase. The phase fluctuations at the output of
this loop is directly related to the various noise processes through the
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closed loop transfer function of the loop. The important parameters
determining the performance of the loofa are the chip rate of the code,
the front end filter bandwidth and the phase lock loop bandwidth. A
computer program termed SOLARSIM is developed to enable performance
tradeoffs for the entire solar power satellite system and the
recommended values for tire pertinent design parameters of ti!e receiver
are given in a table later on.
6. HIGH P014E;R AMPLIFIER
The do to inicrowave conversion unit is the high Gower amplifier.
Fig. 4.1 shows the position of Lire high power amplifier in tiro phase
control hierarchy. The output of phase conjugation circuits serve as
inputs to the high Dower amplifier. The klystron amplifier in the power
transponder generates a phase noise profile that can add significantly
to the downlink carrier phase error if it is unattended for. Phase
noise added by the power amplifier can be reduced to acceptable levels
if one puts a stabilization loop around the amplifier as shown in Fig.
4.1
The loop around the klystron amplifier tracks the noise components
around the carrier frequency. Only these noise coinponents which have
Fourier frequencies greater than the loop bandwidth will be transmitted
to the power beam. When the phase noise rejection is our only
consideration, the obvious solution is to widen the loop bandwidth as
much as possible but this also cr ens up the bandwidth for other noises
in the transponder. Recommended value for the bandwidth is given in gibe
table later on.
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7. SUMMARY OF RESULTS
The important findings on the pilot transmitter and spacetenna
transponder design parameters can be summarized as follows:
Pilot Transmitter
EIRP = 93.3 dBW
PN Chip Rate - 10 Mcps
Chip Time = 0.1 ijseR:
Code Repeats After 10,000 bits
Front End
6 Pole Butterworth Bandpass Filters
Center Frequencies = 2410 MHz and 2490 M11z
3 dB Bandwidth ;3 20 MHz
3 dB Notch Bandwidth m 40 Mitz
Code Tracking Loop
6 Pole Butterworth Bandpass Arm Filter
3 dB Bandwidth - 3 kHz
Code Loop Bandwidth = 10 1]z
Phase Jitter of Code Tracking Loop ,< 1% Chip Time
Carrier Tracking Loop
3 dB Bandwidth of the High Pass Filter = 20 MHz
Carrier Loop Bandwidth = 10 Hz
Phase Jitter of Carrier Tracking Loop _< 0.40
Power Amplifier Stabilization Loop
lR Loop Bandwidth > 10 kHz
In arriving at these values, we have extensively used the
capabilities of SOLARSIM to perform the necessary tradeoffs. These
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results are generated using a model of RFI with coupling coefficients of
-20 d8 for the self as well as mutual coupling of the radiators.
Every i*em in tis4 SPS system described in above pages introduces a
phase jitter to the phase of the Downlink power beam. This phase ,jitter
is one primary cause of degradation of power transfer efficiency. There
are other efficiency degrading effects such as antenna tilts, tilt
jitters, radiating and conjugation point jitter to name a few. Fig. 7.1
shows the contribution of system components to the phase jitter suffered
by the downlink power beam and the resulting power transfer efficiency
from it. In the calculation of power transfer efficiency the other
efficiency degrading effects than the phase noise are held to zero.
Figure 7.2 shows the power transfer efficiency as .a function of the
total rms phase error. As can be seen from this figure, at 10.5 0 of
phase jitter the efficiency is about 90.5%.
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MECHANICAL PERFECT SPACETENNA
94
10 d8 CURRENT TAPER
93	 SPS HEIGHT FROM RECTENNA CENTER
36,000 kip
92
91
90
89
88
87
86
85
84
83
82
81
80 1 	TOTAL RMS PHASE EkROR (deg)i
4	 6	 8	 10	 "12	 14	 16	 18	 20
Fig, 7.2,	 SPS Power Transfer Efficiency vs the Total rms Phase Error,
r-.16-ill(f(0111
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POWER TRANSFER
EFFICIENCY
-19-


Solar Power Satellite antenna phase control system hardware simulation, phase 4: Volume 1: Executive summary

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