The approach taken to test a completed DSCS communications satellite on a system level is described. Areas to be described are measuring RF isolation of separate communications subsystems and a test method which insures that one RF subsystem does not interfere with another. In addition, the method of complying with MIL-STD-1541 in the area of demonstrating safety of electroexplosive devices in an RF field is discussed

Massaro, M.

Sargent, J.

English

NASA Technical Reports Server

N81-141 41
R. F. TESTING OF THE THIRD GENERATION DEFENSE COmmUNICATION SATELLITE
John Sargent *
Dr. Michael Massaro **
Abstract
This paper will describe the approach ta_n to test, on a system level, /
a completed DSCS Communicat_¢ns Satellite. Areas to be described are meas-
uring RF isolation of separate communicati_is subsystems and a test method
which insures that one RF subsystem does not interfere with another. In
addition, the method of complying with MIL-STD-1541 in the area of demonstra:-
ing safety of electroezploslve devices in an RF field will be discussed.
INTRODUCTION
In January of 1977 a contract for the third generation Defense Satellite
,. Communication System was awarded to the General Electric Space Division.
This newer generation satellite is unique compared to earlier generations in
that it has a six channel transponder designed for both frequency and time
domain access operation and real-time commandable uplink and downlink multi-
beam antennas. On the satellite the six SHF channels can be received via one
of two earth coverage horns, or a 61 element, switchable multibeam antenna
(MBA) and transmitted via one of two earth coverage horns, a gimballed high-
gain reflector antenna (GDA), or one of two 19 element switchable multibeam
antennas. In addition, there are two omnidirectional S-Band antennas for TT&C
operation, and a Single Channel Transponder operating at UHF and SHF. Thus,
there are a total of twelve antennas, two SHF earth coverage Transmit horns,
two earth coverage Receive horns, one 61MBA Receive antenna, a CDA Transmit
antenna, a UHF Transmit and Receive Antenna, and finally, two S-Land omnl
antennas.
The RF testing of the DSCS III satellite represents a unique challenge
from a technical standpoint, as well as from an implementation standpoint re-
garding needed facilities and test equipment.
This paper presents the test philosophy, test approach and test acceptance
criteria and expected test results of the DSCS III satellite RF tests. In par-
ticular, the following tests will be discussed:
• Antenna Isolation
• RF Self Compatibility Performance
• Radiated Electro Magnetic Susceptibility
• Intermodulatlon Product Generation
All of the above satellite tests will be performed in a large anechoic room
which wa_ designed to absorb most of the RF energy radiated from the spacecraft
in order to closely duplicate free space. The above tests are being conducted
to verify basic program design performance parameters or to determine compliance
* System Test Engineer ** EMC Engineer
General Electric Space Division
Valley Forge, Pc.
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1981005626-033
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with the design and test requirements of MIL-STD-1541, which has been applied
to the program. The antenna isolation test and the RF self compatibility
performance rest are performed to satisfy both requirements; whereas, the
Radiated EMISM test satisfies only the latter, and the IMP test the _ormer.
Figure i - DSCS Satellite
DSCS III Satellite Description
The Defense Communications Satellite, due for launch early _n 1981, is
a third generation communication satellite which, during its i0 year design
llfe, will act as a repeater in space for our defense forces. The satellite,
which is presented _n Figure i, is a three-axis stabilized synchronous orbit
satellite whose body size measures 6 1/2 feet wide (38 feet including solar
array), 6.75 feet long and 6.5 feet high, weighing 2200 founds. Ten communi-
cation antennas are mounted on its earth viewing face. Antenne - operating at
X-band consist of two receive earth coverage horns, two transmit earth cov-
erage horns, two 19 beam transmitter antennas, one 61 beam receive antenna and
one gimball dish. At UHF there are separate receive and transmit antennas.
At S-band there are two antennas, one on earth viewing and one on space facing
side used for telemetry and command functions. The north face o_ he satel-
lite contains the communications and the south panel, which contains the power
controller and regulation unit, attitude control, command decoder, telemetry
unit, data encrypters, decrypters and batteries. All subsystems are fully re-
dundant and may be cross strapped. Frequency spectrum covered by the communi-
cations payload is 7900 to 8400 _z for X-band receive, 7250 to 7750 M_z for
X-band transmit. This transmit band is divided into 6 bands or channels whose
downlink transmit power is provided by two 40 watt and four i0 watt TWT's.
There are two standard SGLS channels number 12 and 16 for S-band TT&C control.
OR O' VALPAG,
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1981005626-034
In addition, there is a single channel transponder whose UHF downlink
communicates to all AFSATCOM equipped force elements. Access to AFSATCOM
transponder is via UHF; X-band direct, X or S-band TT&C. DSCS III has two
beacons operating with identical simultaneous modulation. Telemetry is im-
pressed on the beacon modulation using an interplex modulation of two tel-
emetry streams modulated on two subcarriers. One telemetry stream is time
shared between a PN code and SCT telemetry data. Main telemetry is 61K bits
in length with a i K BPS rate. The Transmit and Receive frequencies are in-
dicated in Figure 2.
Designation of Tests /
Early in the program ground rules were agreed to which determine which
tests were to be automated and which were to be manual. These ground rules
were:
a) Are the measurements of a repetitive nature, such as transponder
power output across the band?
b) Will the automation reduce overall test time?
c) Will the test be used repetitively on ea'_h satellite, or is it just
for qualification of one satellite?
d) Will the additional complexity of the test which includes computer
operators and software programmers be recoverable in shortened test time?
Thus, a five-hour manual test shortened to one hour through automation may
not be cost effective if it takes 4 times the number of people to run the test.
SATELLITE SYSTEM RF TESTS
Antenna Isolat fon
I_l l 7280 l0 _ $ IIM 75
$ ?o,67 75 4 8285 t0
S 0"_70 60
H1MJ _ 7365 60
t 7560 10 _ 1 8005 60
2 nOlO 30
_ 7510 60 6 7925 50
6 7725 50
IEACQI( 7605 _ | m(_S 60
IOSO 60
tC_l $ ?aJ57 75 $ |17_ ?$
$ 76¢3 60 4 8,285 60|y.._c_¢ ?600
US_ I 360 g0
nt_. 1 7580 6O 1' l_? 20
1 7560 60 8 I 1007 1813
T 225? 2277
Figure 2
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1981005626-035
-q
Since the spectrum of frequencies of the satellite is so broad band,
there exist many possibilities for one subsystem to interfere wlth another
by means of spurious responses, intermodulatlon products, or adjacent channel
noise. To prevent thls from happening, the harmonic content of each trans-
mitter is minimized by careful design. By measuring the satelllte's receiver
thresholds - that is - the minimum signal necessary to operate the s)stem and
a tt=nsmitter's fundamental and harmonic output at a specific frequency, the
magnitude of isolation which must exist between the two on-board RF subsystems
to prevent intrasystem interference can be determined.
/
On the DSCS satellite there exist high field strengths from all trans-
mit antennas, and therefore large isolation values were required of the an-
tenna systems. It was decided to measure isolation across the antenna's
entire operating band to insure that any peaks in the antenna's coupling
from one antenna to another would not be overlooked.
There are many ways to make isolation measurements - phase lock re-
ceivers, network analyzers, and tracking generators. Each system was eval-
uated for ease of use and minimization of set up time. It was decided that
because of the wide band of frequencies to be covered, the sweep generator
and spectrum analyzer approach would be employed. There are over 50 combin-
ations of pairs of antenna measurements to be made. Since the sweep generator
power source had to be coupled to many antenna ports, and since there would
be frequent connection changes, the use of waveguide as the test signal path
was impractical. Therefore, all antenna ports using wavoguide are transl-
tioned to coax for connection to the test equipment. To prevent the sensi-
tive spectrum analyzer from measuring the leakage from coax used for the
tests and misLaking it for antenna leakage, solid shield short length hellax
cable will be used for interconnectlon. Reflected energy from the walls of
the facility are expected to be low and in the order of -70 dBc, as shown in
Table I. Being so low in amplitude, they will not interfere with the isola-
tion measurement.
[ 1BAND PATH ANT. SIDE LOBE ABSORBER TOTAL ATT.
UHF 30 32 I0 70
S 42 6 25 73
X 45 50 30 125
TABLE I - RF ISOLATION FROM FACILITY
Although the test Is repetitive In nature and thereby qualifies for
automation, it wlll be performed manually because the test will only be done
on the Qual vehicle. Antenna isolation is affected by antenna position, type
of antenna geometric layout, operating frequencies and load matching networks.
27
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1981005626-036
These relationships should not change from satellite to satellite. Prelim-
inary design data indicates that isolation is i0 dB better than acceptance
levels, and therefore there is little chance of risk in performing this test
on only one satellite.
RF SELF COI_ATIBILITY PERFO_LNCE
Up to the point in time where this test is performed, the satellite has
been tested in a hard wire configuration where antennas were not connected /
to operating subsystems. During this test the satellite will radiate through
its antennas and system performance tests via air link for the first time.
The purpose of the test is a) to insure there is no mutual RF interference
between two subsystems - that is - satellite receivers listening to their own
on-board transmitters, and b) the RF field around the satellite does not leak
into a performing subsystem and effect its operation. The first is detected
by spectrum analysis of the transmitted signal. The second is measured by
offsets in telemetry from those values recorded in a baseline measurement.
An optimistic test plan would exercise the spacecraft in every possible
configuration in all modes and insure proper operation, but considering the
fact that the 6-channel comm transponder alone has 155,000 possible operating
modes, this is impractical. Therefore, we must scope our tests to only those
configurations which can realistically be affected. A study was performed
and ground rules made to simplify the tasks necessary for an uncompromising
compatibility test. A summary of this study was as follows:
a) Attitude control system is affected by RF because the sensitive
sun and earth sensors measuring satellite attitude are withi: the radiation
field of the antennas.
b) The Single Channel Transponder's receiver may incorrectly AGC due
to RF leakage entering the X or UHF receivers.
c) The X-band co_aunicatlons channels may be affected by RF leakage
into their mixer stages and cause spurious outputs within their channels.
d) Power subsystem may incorrectly regular= due to RF leakage onto
power controller sense lines.
To test for these effects and others, the satellite i_ configured by air
link to a minimum po_er configuration and each subsystem is powered up by
itself, and baseline measurements recorded. As an example, the attitude
control system is initialized to a stable null condition and all sensor out-
puts recorded. The single channel transponder is configured by UHF and is
placed in a bypass mode where it operates as a true transponder. In this
configuration the uplink power is varied and a bit error rate curve is gener-
ated. Comm transponder outputs in each channel are monitored by spectrum
analyzer under no uplink and noise power measurements made. Command thresh-
old measurements are made on the command system and minimum uplink power
points are established for S-band and X-band commanding. Beacon modulation
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1981005626-037
spectrum measurements are made and center frequency is accurately established.
These tests are the_. repeated on _ach subsyste_ _ile all.other systems
are fully on. If there is any interference, it will show up on that subsystem
by a delta in performance as measured either directly on a spectrum analyzer
or on telemetry by changes from baseline, values previously recorded.
Radiated EMISM Spray Test - Figure 3
The RF Radiated Spray Test must demonstrate that the various subsystems
of the vehicle could operate within their sp(cified performance limits if
the environmental field-strength_ were increased 6 dB above their nominal
levels. That is, electromagnetic radiated interference safety margins,
(EMISM) must be established for the various subsystems of the spacecraft.
An EMISH must be demonstrated for the following coupling modes:
(I) Transmit antenna-to-component case radiated field coupling.
(2) Transmit antenna-to-wire bundle and cable radiated field coupling.
(3) Transmit antenna-to-electroexplosive device case and wire harness
radiated field coupling.
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Figure 3
29
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1981005626-038
This test is accomplished by first measuring the ambient fields around
the vehicle at the transmitter fundamental frequencies and large harmonics.
It was decided to use a Radiation Hazard probe (RADHAZ) type instrument to
measure the ambient fields. RADHAZ probes are ty_tcally broadband, omnidi-
rectional and polarization independent, and are thus ideal for probing unknown
field distributions, iiowever, RADHAZ probes have low sensitivity with mini-
mum discernible fields on the order of about 2.0 V/m. Consequently, this
limit was used to establish the lower limit of the intensity of the spray
field at a level of 5 V/m.
After establishing the ambient field levels, the S/C is then sprayed a J
leve! that is 6 dB greater in intensity at each of the measured frequencies.
During the spray test at each frequency, selected critical performance par-
ameters are monitored to determine if acceptance limits have been exceeded.
For example, for the £HF transponders, spurious sideband levels are measured.
For the TT&C system X-Band and S-Band downlink telemetry is monitored for up-
sets and changes. For the Single Channel Transponder system, a Bit error
rate test is performed.
The test is performed for the Electro-explosive device system (EED) by
first detemining the largest environmental field strength that the EED
system, EED device, case and control cable will experience. The environ-
ment _n this case refers to the fields that exist at the launch pad, the
fields produced by pre-launch tests, or the fields produced by the launch
vehicle. The test is performed to ensure that the RF environment does not
cause a false firing of the EED. A 20 dB safety margin must be established,
i.e., the EED must be sprayed with an effective field that is a numerical
factor of i0 higher than the maximum ambient environmental field. For DSCS
Iii the maximum overage environmental field strength was a level of 60 V/m
produced by the launch vehicle. Since the test field strength for this case
(600 V/m) was enormous, an effective field strength of 600 V/m can be pro-
duced bF removing the shield from the EED harnesses and spraying at a field
strength of 6 V/m. This reduction in the spray intensity is based on the
fact that the shields can produce 40 dB of attenuation to external fields.
The harness is sprayed at this level for 2 months and a non firing of the EED
is the pass criteria.
Internal Product (I_ Testing)
_en a non linear electrical device on the satellite is simultaneously
sprayed with RF at two different frequencies, doubling and mixing actions
take place which can produce and self generate a third frequency. If this
self generated signal lies within a satellite's receive pass band, and is
strong enough to be "seen" by the satellite's receive antenna, the satellite
will amplify this signal and rebroadcast it on the downlink. This extra sig-
nal is capable of saturating the transponder's power and rendering - in
extreme cases - the channel useless. Examples of elements which cause IMP
generation are loose waveguide points, dissimilar metals in waveguide switches,
and antenna elements poorly joined. They are usually parts which are exposed
to high power.
('. 30
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1981005626-039
Recognizing the importance of this test, it is planned to be performed
three t_mes in the sat=illte test cycle. First, after initial sateilSte as-
sembly in order to prove workmanship quality. Second, after acoustic noi=e
exposure to prove structural integrity, and a third time after pyro shock ex-
posure. Since this test is so discretely repetitive and must be performed
on every satellite, it is a prime candidate for automation.
To perform this test a "full up" satellite is required. The test must
be air linked in order to allow the I_ to ring around into a receive an-
tenna port.
The test philosophy is as follows. An intermod measurement pair is se-
lected, such as those shown in Figure 4.
Xmit Xmit Rec Rec.
Ant. Freq. Ant. Freq.
Dish 7335 Earth Horn 8060
Earth Horn 7750 Earth Horn 7950
I_ IMP/R D_/Xmit Imp
Rec. Ant. Freq. Freq. Ant
M Beam 8165 7440 Earth Horn i
Figure 4
Two uplink signal generators are progran_ed to 8060 and 7950 CHz and
the signals sent to the satellite are received on earth horn #I. The sig-
nals are translated 7_0 MHz down in frequency, amplified and transmitted from
the satellite dish and earth horn #I antennas. If there is a non linear Ib_
generator, it will mix the two transmit frequencies and the resultant third
order internal product will fall at 8165 MHz. This is a receive frequency
IMP which will be translated down 750 _{z, amplified and rebroadcast on 7440
CHz. It is to this frequency the spectrum analyzer is tuned. Baseline noise
power measurements out of the satellite are made at this flequency at a band-
width of i0 KHz to determine amplitude of pure noise with no uplink established.
Once this noise power is quantized, the two uplink signals are sent and power
again measured at the IMP frequency. If power increases 8 dB w_th uplink sig-
nals present, there is an indication of an intermod product signal.
Considerable care in the selection of test equipment has been exercised
to insure that the uplink signal path does not in itself generate IMP products.
Two separate uplink antennas will be used to keep the two uplink test sig-
nals separate until they ultimately mix within the satellite. Low power from
the uplink signal generators - in the order of -30 dBM - will be used for the
test signals to prevent IM products from being generated on the uplink and
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1981005626-040
close coupling to the receive antennas will 3chieve immunity to generating
IM products on the uplink.
By testing all possible pairs of transmit and recei,e ports across all
possible IMP frequencies, we will eliminate any "surprise" signals from being
self generated by the satellite after launch.
CONCLUSIONS
As in every s_itellite test program there exists a myriad of component
and subsystem tests which prcve proper subsystem design. However, it is only
at the system level test where proper satellite operation is proven. This
is accomplished by verifying test parameters under true orbit simulation.
In this DSCS program, w_th regard tc RF system test parameters, we
have attempted to assume nothing. We have replaced "test by analysis" with
analysis then test. By completely testing a satellite in a mode which dupli-
cates its operation in orbit, then linking this with testing for a proper
margin of proper system operation. _ high level of confidence for correct sys-
tem operation is achieved.
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1981005626-041


R. F. testing of the third generation defense communication satellite

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