Deep space tracking stations on different continents were arrayed during the encounter of the International Cometary Explorer (ICE) spacecraft with the comet Giacobini-Zinner during September 9 through 12, 1985. This is the first time that telemetry signals received on different continents have been combined to enhance signal to noise ratio. The arraying was done in non-real time using the method of symbol stream combining. The improvement in signal to noise ratio was typically 2 dB over the stronger of the two stations in each array

Hurd, W. J.

Pollara, F.

Russell, M. D.

Siev, B.

Winter, P. U.

English

NASA Technical Reports Server

N 8 6 - 2 2 8 0
TDA Progress Report 42-84 October-December 1985
Intercontinental Antenna Arraying by Symbol Stream
Combining at ICE Giacobini-Zinner Encounter
W. J. Hurd, F. Pollara, M. D. Russell, and B. Siev
Communications Systems Research Section
P. U. Winter
DSN Data Systems Section
Deep space tracking stations on different continents were arrayed during the encounter
of the International Cometary Explorer (ICE) spacecraft with the comet Giacobini-Zinner
during September 9 through 12,1985. This is the first time that telemetry signals received
on different continents have been combined to enhance signal-to-noise ratio. The arraying
was done in non-real time using the method of symbol stream combining. The improve-
ment in signal-to-noise ratio was typically 2 dB over the stronger of the two stations in
each array.
I. Introduction
Symbol stream combining is a promising method of antenna
arraying to enhance telemetry performance. Its mam advan-
tage over baseband combining is that much less data need to
be transmitted from the stations to the combiner location.
For example, for the Giacobini-Zinner data rate of 2048 sym-
bols/s, the data rate for symbol stream combining is 16 kilo-
bits per second (kb/s), whereas the data rate for baseband
combining is approximately 90 Mb/s, or 6 MHz analog band-
width. The savings are 3 to 4 orders of magnitude either in
ground communications link capacity, for a real-time system,
or in data storage capacity, for a non-real-time system (Ref. 1).
The theoretical performance of symbol stream and base-
band combining is the same, except for minor differences in
synchronization losses and in data transmission losses in a
real-time baseband combiner with an analog ground link. On
balance, there is a slight advantage for symbol stream com-
bining of 0.2 dB ±0.2 dB (Ref. 1).
The International Cometary Explorer (ICE) spacecraft
encounter with the comet Giacobini-Zinner presented an ideal
opportunity to demonstrate symbol stream combining, and,
at the same time, to provide enhanced reliability to the mis-
sion. As ICE passed through the comet tail, on September 11,
1985, the spacecraft was visible from the DSN complexes at
Goldstone, California, and Madrid, Spam, as well as from the
Arecibo Radio/Radar Observatory in Puerto Rico. Neither
Goldstone nor Madrid has sufficient antenna aperture to relia-
bly receive the signal at 1024 bits per second (b/s) (2048 sym-
bols/s), in the event of signal loss due to the comet tail. Thus,
Arecibo acted as a real-time backup capability, and symbol
stream combining between Goldstone and Madrid provided a
non-real-time backup capability with sufficient antenna aper-
220
https://ntrs.nasa.gov/search.jsp?R=19860013336 2020-03-20T15:03:57+00:00Z
ture to provide reliable signal reception in the event of the
above-mentioned degradations.
Symbol stream combining was also performed between
Usuda, Japan, and Goldstone, between Canberra, Australia,
and Usuda, and between Canberra and Goldstone. An addi-
tional benefit to the combining system is that it recorded the
telemetry symbols prior to decoding, thus providing redun-
dancy in case of failure of the real-time decoders.
II. System Description
Figure 1 shows the overall block diagram of the non-real-
time symbol stream combining system. Data are recorded in
real time to magnetic tape at the Deep Space Network (DSN)
stations and Usuda. The tapes are then brought to JPL where
the symbols from station pairs are combined on a VAX 750
computer.
The combining software produces a tape of combined data
in the same format as the original tapes. Statistics for both
input tapes and the output tape are also produced by the
combining software. These statistics include input and output
symbol signal-to-noise ratios (SNRs), signal levels and dc
offsets and data stream alignment points. The combined out-
put symbol stream is then decoded in a VAX 750 computer,
and an output tape of decoded data is produced in the same
standard format as for data decoded at DSN stations.
A. Station Configurations
All data received for the non-real-time combining process,
except for Australia, were produced using the 64-m antennas
in a stand-alone mode and a Telemetry Processing Assembly
(TPA) Modcomp Computer at each station. The ICE space-
craft transmits data from two antennas mounted on top of the
spacecraft. Identical data symbols are transmitted on two
S-band carriers: channel A at 2270 MHz and channel B at
2217 MHz. Except for day of year (DOY) 255, the spacecraft
data rate was 1024 b/s. On DOY 25 5 the data rate was 512 b/s.
From both Spain and Goldstone, data from channels A
and B were received on one 64-m antenna, and combined
using resistor networks. Data received from Usuda were chan-
nel A only. Data received from Australia were channel A only,
with one 64-m antenna and one 34-m antenna arrayed. Fig-
ure 2 shows the configuration of the Madrid and Goldstone
stations and the resistor combining network.
B. Recording
Soft symbols quantized to 8 bits were recorded at Usuda
using a MARK III telemetry string and at Goldstone, Spain,
and Australia using MARK IV telemetry strings. The MARK HI
and MARK IV TPA software were modified to accommodate
the soft symbol recording requirements. These modifications
included changes to the TPA/SSA coupler commands so that
the coupler would pass the full 16-bit quantized symbol from
the Symbol Synchronizer Assemblies (SSAs) to the TPAs. The
modified software strips off the high order 8 bits, which are
merely the extended sign of the symbol value, packs the low
order 8 bits into 16-bit words, and stores these words into
buffers in the standard Original Data Record (ODR) format
shown in Figure TLM-3-13-2 of the 820-13 document. These
buffers are written to the magnetic tape. One tape record
comprises five of these buffers, for a total of 2850 symbols
per record. Tape handling, which included the ping-ponging
between the two TPA tape drives and the operator prompts
to the Local Monitor and Control (LMC), is performed in the
standard TPA fashion. The soft symbol recording software is
invoked from the LMC with the same commands that invoke
the standard TPA software.
C. Combining
The combiner program was written in VAX/VMS FORTRAN
and can take inputs from tape, disk or a combination of tape
and disk. The program performs three functions: alignment,
SNR and weight estimation, and combining.
The data are stored on tape in ODR format. The two
incoming streams of records must be shifted relative to one
another because the different geographical locations of the
stations result in different arrival times of the same symbols
at different stations. Data which are recorded when two
stations were receiving spacecraft data simultaneously are
combined by the program.
The alignment is done in two steps. First, a rough alignment
is performed using the time tags in the headers of the ODR
record. The time tags indicate the ground received time of the
last bit in a block of data and are accurate to 1 ms. Once the
tapes from each station have been aligned using the time
tags, the difference between two adjacent time tags on the
same tape is used to compute the symbol alignment positions
within a data block. The data are then separated from the
header portion of the record and placed in a separate array
for the second step, fine alignment. The symbols from the two
tapes are cross correlated for the 2850 lags closest to zero
delay, as predicted from the above alignment. These values are
used to compute a correlation threshold. The point at which
the greatest correlation value occurs is the symbol alignment
point, provided that this peak value exceeds the threshold.
SNRs and weights are computed for each data stream, for
every record. The algorithm to compute SNR and symbol
stream weights uses sums of absolutes and sums of squares and
221
is described in Ref. 2, The 2850 symbols used to estimate
SNRs and combined weights are sufficient to result in a loss
due to weighting errors of less than 0.02 dB. SNR variations
caused by rotation of the spacecraft transmitting antenna are
described in Ref. 3.
In the combining step the data symbols and weights are
used to produce a single output data stream. To combine the
two streams, two aligned symbols are each multiplied by their
weights and added together to produce the combined output
symbol. This process is repeated for each symbol to produce a
combined output stream. The combined data are placed back
into the data portions of the first tape's record. This record is
then written out to tape, ready for decoding. The SNR of the
combined symbols is estimated using the same algorithm as
for the input symbols, and the combining efficiency is calcu-
lated. This efficiency (in dB) is the sum of the input SNRs,
less the SNR of the combined symbols.
D. Decoding
Decoding is performed by a software decoder written in
"C" language on a VAX 750 computer. This sequential decoder
is based on the Fano algorithm and uses soft quantized input
symbols, as described in Ref. 4.
The decoder also performs the task of frame synchroniza-
tion, by correlating the channel symbols with a known "tail
pattern." For the ICE code, the total frame size is 1024 bits
(2048 channel symbols), where 984 bits are information and
40 are tail. The particular tail pattern used is (at the encoder
input):
{0,0,0,1,0,0,1,0,1,1,1,1,1,1,0,0,1,0,0,0,0,0,0,1,1,
0,0,1,1,1,1,1,1,0,1,1,1,1,1,0}
Since the code rate is 1/2 and the constraint length K is 24,
there are 80 channel symbols due to the tail, of which
80-2(A"-1) = 34
form a fixed pattern, independent of the data. Synchroniza-
tion is based on these 34 fixed symbols:
{-1,1,1,1,1,1,1 ,-1,1 ,-1,1,1,1,1 ,-1 ,-1,1,1 ,-1 ,-1,1,
1 ,-1,1 ,-1 ,-1 ,-1 -1 ,-1 -1 -1 -1 -1 -1,}
and is continuously checked and reacquired if necessary.
The decoder has two kinds of output: one is the actual
decoded data and the other is statistics on the probability of
frame deletion, average number of computations per frame,
and information on which frames were deleted.
E. Output Tape
The output decoded data are written to a tape in DSN-
Goddard Space Flight Center (GSFC) format.
III. Results
All data received simultaneously from station pairs were
combined successfully and decoded. Figure 3 shows the DOY
and times of data received. Encounter day was September 11,
1985, DOY 254. For days 252, 253, 254, and 255, data were
received from Goldstone and Usuda. Data were received from
Spain and Australia for DOY 254. Overlap times were approx-
imately 1.5 hours for Goldstone and Usuda, 5 hours for Gold-
stone and Spain, 4 hours for Goldstone and Australia, and
4 hours for Usuda and Australia.
Figures 4 through 7 were produced from data generated
by the combiner program. They show the SNR of the two
input symbol streams, the SNR of the combined symbols and
the efficiency of the combining process, as functions of time.
Figures 4(a), (b), (c), and (d) show the Usuda-Goldstone
results for the four days. All data were at a bit rate of 1024 b/s
except for DOY 255. At the 1024-b/s data rate, the symbol
SNR for Usuda is typically 2 dB, increasing slightly with time
as the antenna elevation angle increases. Performance of the
telemetry string used at Goldstone for symbol recording was
somewhat weaker than that used for real-time data. The SNR
from the Goldstone 64-m antenna decreases from approxi-
mately 0.2 dB to -0.7 dB as the elevation angle decreases.
The combining efficiency is approximately -0.3 dB. A short
signal outage is apparent at Usuda on DOY 253. The results
at 512 b/s, DOY 255, are similar but with 3 dB higher SNRs.
Figure 5 shows the Goldstone-Spam results for DOY 254.
The symbol SNR increases at Goldstone and decreases at
Spain as the elevation angles change. The combined SNR is
typically 3.7 dB, and the efficiency is -0.1 dB. There were
several short intervals of lower SNR at Spain and one such
interval at Goldstone.
Figure 6 shows the Goldstone-Australia results for DOY 254.
Results are similar to the above, with more change in SNR
due to antenna elevation. The combined SNR is typically
2.7 dB, and the efficiency is -0.3 dB.
Figure 7 shows the Usuda-Australia results for DOY 254.
Because Usuda and Canberra are close to each other longi-
tudinally, the symbol SNRs increase and decrease together.
The combined SNR is typically 3.7 dB, and the efficiency
is -0.3 dB.
222
A. Combiner Efficiency
The combining efficiencies varied from -0.1 to -0.3 dB.
There is some small loss in the combining process, due to
imperfect weighting of the data from the different stations
and due to quantization, but these losses are believed to be
less than 0.1 dB. There is some error both in defining and in
measuring the symbol SNRs, because the signals are time-
varying and the measurement assumes constant SNR. Another
possible cause of inefficiency is the time-varying nature of the
signal strengths, and the differences in this time-varying nature
for the different stations. These differences are due to recep-
tion of channel A only at some stations and channels A and B
at other stations, and due to ground antenna conscan. Exam-
ples of these variations are given by Nadeau (Ref. 3). Exact
characterization of the effects of these variations on combiner
efficiency is outside the scope of the present work.
B. Decoding Results
The software decoder and synchronization system was
used successfully to decode a number of combined data tapes.
Due to the high SNR, no frame deletions occurred in
decoding the combined symbols, nor did frame synchroniza-
tion errors occur. This confirmed that the data had been pro-
perly combined, formatted, and correctly interfaced with the
decoder. Except for Usuda, the uncombined symbol streams
did not have high enough SNRs for this error-free decoding.
This dramatically illustrates the usefulness Of symbol stream
combining.
IV. Summary
This work establishes the feasibility and the utility of
intercontinental antenna arraying by symbol stream combin-
ing. Data were taken at four tracking complexes in California,
Japan, Spain, and Australia. Data were successfully combined
on all four antenna pairs where there was mutual visibility
of the spacecraft. Most important, all data were successfully
decoded, demonstrating the reliability and applicability of
the system.
References
l.Hurd, W. J., Reder, L. J., and Russell, M. D., "Symbol-Stream Combiner: Description
and Demonstration Plans," TDA Progress Report 42-78, Jet Propulsion Laboratory,
Pasadena, Calif., pp. 115-121, Aug. 15,1984.
2. Vo, Q., "Signal-to-Noise Ratio and Combiner Weight Estimation for Symbol Stream
Combining," TDA Progress Report 42-76, Jet Propulsion Laboratory, Pasadena,
Calif., pp. 86-98, Feb. 15,1984.
3. Nadeau, T., "Periodic Variations in SNR of Signals Received from ICE Spacecraft,"
TDA Progress Report 42-84, this issue.
4. Pollara, F., "A Software Simulation Study of a Sequential Decoder Using the Fano
Algorithm," TDA Progress Report 42-81, Jet Propulsion Laboratory, Pasadena,
Calif., pp. 40-46, May 15,1985.
223
SSA TPAMODCOMP
STATION 1
SSA TPAMODCOMP
STATION2
 ODR FORMAT
Fig. 1. Non-real-time symbol stream combiner block diagram
USER TAPE
GSFC
FORMAT
B
MHz
A
2270
MHz
BASEBAND
DETECTOR
RESISTOR
COMBINER
NETWORK
BASEBAND
DETECTOR
RECORDING
SOFTWARE
SDA = SUBCARRIER DEMODULATOR ASSEMBLY
SSA = SYMBOL SYNCHRONIZER ASSEMBLY
TPA = TELEMETRY PROCESSING ASSEMBLY
Fig. 2. Symbol stream recording hardware block diagram
DOY 252
USUDA
GOLDSTONE
DOY 253
USUDA
GOLDSTONE
DOY 254'
USUDA
AUSTRALIA
GOLDSTONE
SPAIN
I I I I I I I I I I I I I I I I I I I I I I
I I I I I I I I I I I I I I I I I
0 2
•ENCOUNTER DAY
6 8 10 12 14 16
GROUND RECEIVED TIME, UTC
Fig. 3. Tracking coverage
18 20 22
224
(a) OOY252
O
2
HI
O
EC
O
£ USUDA
O GOLDSTONE
O COMBINED
D EFFICIENCY
i i i i _j t i
-2 I 1 I I I 1 I I 1 I I I I I
1900 2100
Fig. 4. Combining results for Usuda and Goldstone
225
(c) DOY254
0 -
£ USUDA
O GOLDSTONE
O COMBINED
D EFFICIENCY
-2
(d) DOY255
|
1900 1950 2000
TIME, h
2050 2100
Fig. 4 (contd)
226
DOY 254
A SPAIN
O GOLDSTONE
O COMBINED
D EFFICIENCY
oc
O
IE
z
-2
0800 0900 1000
TIME, h
1100 1200
Fig. 5. Combining results for Goldstone and Spain
O
z
oc
O
a:
DOY 254
A AUSTRALIA
O GOLDSTONE
O COMBINED
D EFFICIENCY
-2
1700 1800 1900 2000
TIME, h
Fig.6. Combining results for Goldstone and Australia
227
GO
•O
>"
CJ
z
LU
0
o
cc
5
l^a^aa^fe^aaAa^aM^^
-2
DOY 254
A USUDA
O AUSTRALIA
O COMBINED
O EFFICIENCY
I
1900 2000 2100
TIME, h
2200 2300
Fig. 7. Combining results for Usuda and Australia
228


Intercontinental antenna arraying by symbol stream combining at ICE Giacobini-Zinner encounter

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