The passage of the International Cometary Explorer (ICE) through the tail of comet Giacobini-Zinner took place on September 11, 1985, at approximately 11:04 GMT. The signal-to-noise ratio of the data received from the ICE spacecraft during the comet encounter was improved by arraying the 64-m antenna channels A and B (RCP and LCP) with the two 34-m antennas. Specially designed combiners were built to combine the signals received by the three antennas at the different DSN sites to ensure that the spacecraft's weak signal was received. Although the ICE spacecraft was built with a 5-W transmitter and with a small antenna designed to provide data from no farther than 1 million miles, these combiners provided enough signal margin during the encounter to receive the ICE transmitted data from within the tail of comet Giacobini-Zinner, 44 million miles from earth

Foster, C.

Marina, M.

English

NASA Technical Reports Server

l TDA Progress Report 42-91 July-September 1987 
Analysis of the ICE Combiner for Multiple Antenna Arraying 
C. Foster and M. Marina 
Radio Frequency and Microwave Subsystems Section 
1 1. Introduction 
The passage o f  the International Cometary Explorer (ICE) through the tail of comet 
Giacobini-Zinner took place on September 11, 1985, at  approximately 11:04 GMT. 
The signal-to-noise ratio of  the data received from the ICE spacecraft during the comet 
encounter was improved by arraying the 64-m antenna channels A and B (RCP and LCP) 
with the two 34-m antennas. Specially designed combiners were built to combine the sig- 
nals received by the three antennas at the different DSN sites to ensure that the space- 
craft’s weak signal was received. Although the ICE spacecraft was built with a 5-W trans- 
mitter and with a small antenna designed to provide data from no farther than I million 
miles, these combiners provided enough signal margin during the encounter to receive the 
ICE transmitted data from within the tail of the comet Giacobini-Zinner, 44 million 
miles from Earth. 
ICE telemetry link analysis showed that reception from the 
ICE at the Ciacobini-Zinner encounter at the desired 1024-bps 
data rate would be made feasible by combining the two down- 
links (channels A and B) received by the DSN 64-m antenna 
[ l ]  . The addition of two 34-m antennas to this array would 
provide an additional margin of 0.8 dB to the received signal 
and would reduce effects caused by weather, etc. (see Table 1). 
Five combiners were built for the DSN to support the capa- 
bility to array together the telemetry output of the 34-m an- 
tennas at DSS-12 and DSS-15 and the 64-m antenna RCP and 
LCP channels. The combiners were multi-input-port (two, 
three, or four port), single- or triple-output-port devices to 
allow operational flexibility for ICE operations and testing 
, 
i 
I (Fig. l). 
II. The Combiners 
The function of the combiners is to accept baseband sig- 
nals from the telemetry phase detector’s output; to weight the 
signals appropriately in accordance with their respective SNRs 
to provide the maximum signal-to-noise ratio at the output; 
to sum the signals; and to provide the combined output sig- 
nal to prime and backup telemetry data streams. The block 
diagram in Fig. 2 shows how this was accomplished. Baseband 
signals were passed through variable attenuators which were 
used to set the weighting factors to provide the maximum 
signal-to-noise ratio at the output of the summing amplifier. 
Current amplifiers were used to provide the combined output 
signal for prime and backup telemetry data streams. 
Table 1 lists the relevant parameters for the four-input-port 
combiner for the ICE-GZ encounter. 
269 
https://ntrs.nasa.gov/search.jsp?R=19880003322 2020-03-20T09:15:26+00:00Z
111. ICE Encounter Support 
Two combiners were operated at each DSN complex dur- 
ing the encounter, the prime unit combining all available 
signals and a backup combining only the two strong signals I 
from the 64-m antenna (Fig. 2). 
Baseband receiver telemetry phase detector outputs were 
patched directly into an input port, and the output of the 
combiners was patched directly to a telemetry string consist- 
ing of a Subcarrier Demodulator Assembly (SDA), a Symbol 
Synchronizer Assembly (SSA), and a Telemetry Processor 
Assembly (TPA). 
The combiner required external monitoring to validate its 
performance. (Separate telemetry strings provided assessments 
of proper performance.) The SSA measured signal-to-noise 
ratios, and the sequential decoder provided a symbol error 
rate statistic. Thus, when array testing was under way, all 
available telemetry strings were used to measure configuration 
setup conditions and long-term performance. 
I 
Baseband signals were combined using optimum combining 
coefficients {a,} so that the output SNR was the sum of the 
input SNRs. Appendix A describes the expression used for 
combining coefficients {a,  1. 
For the ICE-GZ encounter, the combining coefficients were 
normalized so that the total output power from the combiners 
was approximately the same as the baseband power levels 
(”6 dBm), to be within the acceptable range for the SDAs: 
{a i }  = ai k; 
The arrival time difference for signals at the SPC-10 (DSS- 
14/15/12) combiners has two main components. One compo- 
nent is the transport delay from the receiver input on the 
antenna to the input port of the combiner. The transport 
delay for DSS-14 and DSS-15 is approximately 1.3 ps and can 
be ignored. The transport delay for DSS-14 and DSS-12 is 
approximately 5 5  ps, since the stations are 16.5 km apart. 
Because the ICE combiner does not compensate for this delay, 
the result is 0.2 dB loss in SNR when a signal arrives at each 
antenna at the same time. 
The second component is the geometric time-of-arrival 
difference for the baseline between the DSS-14 and DSS-12 
antennas. For the ICE-GZ encounter, DSS-12 will contribute 
about 0.3 dB to the net symbol signal-to-noise ratio (SSNR) 
from spacecraft rise to meridian crossing, but its contributions 
will deteriorate to nearly 0.1 dB by spacecraft set. This expec- 
tation was supported by observation during the test pass on 
July 26, 1985, at a bit rate of 1024 bps. 
For the time near comet encounter, the arrival time delay 
for DSS-12 to DSS-14 followed approximately the following: 
Spacecraft Position T (DSS-12/DSS-14) 
Rise 6 PS 
Meridian 12 ps 
Set -35 ys 
Appendix B derives the expected SNR degradation due to 
arrival time dephasing during the Giacobini-Zinner comet 
encounter. 
IV. Conclusions 
Figure 3 shows the observed performance at Goldstone 
using the three- and four-input-port combiners for this time 
interval. The actual improvement in signal-to-noise ratio real- 
ized by the ICE resistive combiners can be estimated from the 
plots of the symbol error rate (SER) and the SSNR performed 
on DOY 185 (1985) using the three-input-port combiner to 
combine DSS-14 (channels A and B) with DSS-12. These plots 
are shown in Fig. 4. 
At this time, the ICE spacecraft was operating at a bit rate 
of 512 bps. The sum of the DSS-14 channels A and B, with 
SSNRs of 1.3 dB and -0.2 dB, yields a theoretical combined 
SSNR of 3.68 dB, while the observed SSNR was 3.6 dB. Thus, 
the actual improvement due to antenna arraying using the ICE 
resistive combiner was within 0.1 dB of the calculated predic- 
tion. The combiner is currently being used to support Pioneer 
and ICE spacecraft. 
Reference 
[ l ]  N. A. Fanelli, L. Efron, and R. J .  Muellerschoen, “ICE Second Halley Radial: TDA 
Mission Support and DSN Operations,” TDA Progress Report 42-87, vol. July- 
September 1986, Jet Propulsion Laboratory, Pasadena, California, pp. 285-290, 
November 15, 1986. 
270 
Table 1. Symbol definitions and values 
Symbol Description Value 
SSNR at DSS-14 (RCP) 
SSNR at DSS-14 (LCP) 
SSNR at DSS-12 
SSNR at DSS-15 
Symbol rate 
Baseband bandwidth 
DSS-14 (RCP) 
DSS-14 (LCP) 
DSS-12 
DSS-15 
Combined SSNR (DSS-14 only) 
Combined SSNR (DSS-14,12, 15) 
0 dB 
-1 dB 
-1 dB 
-8 dB 
2048 sps 
730 kHz 
0.52 
0.48 
0.16 
0.14 
2.55 dB 
3.34 dB 
271 
CDSCC I GDSCC I MDSCC 
2-PORT 
COMB1 NE R 
4-PORT 
COMBINER 
3-PORT 2-PORT 
COMBINER COMBINER +:a2' STRING 1 
BIU 
+ I 
STRING 
NOCC ECS 
Fig. 1. ICE encounter support configuration 
r-------- 1 
I 
DSS-14 
I 
I 
ATTENUATOR 
I 
DSS-14 
SUMMING 
AMPLIFIER 
AMPLIFIER P-4 ATTENUATOR DSS- 12 I 
DSS-15 
Fig. 2. Four-input-port combiner block diagram 
272 
10 - 1  I 1 I I 
9 -; - 
8 -  ;*: 
7 - :. 
6 -  :. - 
5. - 
- 
. e. - 
e r: 
d 
5 -  *. . 
s. .. 
3 -  . A& 
.. 
:\ ....*!* % - 
.* . . % 4 -  .. '....;,- 
8 I I I I 
TPA 2 
7 - A T 1 2 0 0 2 :  0 ADJUSTED PREDICTS - 
6 - AND 8 (-0.2 dB) * +3.7 dB - 512 bps SUM OF A (+1.3 dB) 
DSS-14 lA/8)/15/12 DSS-14 (NB) 
ARRAY 
0800 0900 1000 1100 1200 0900 1000 1100 1200 
TIME, hours UTC TIME, hours UTC 
c 
C 
1 1 1 I :I 1 3-PORT C O M B l N E k .  TPA 2 
10 
8.5% - -0.2 dB 
.*-:*: 
8 8  
~ 5 %  9 +1.3 dB 
r; L OBSERVED (A + B) = +3.6 d 8  -I 
:; 
0 " .$V!: 4 
I .-. - - 1  I I 1 
1100 1200 1300 1400 1500 
TIME. hours UTC 
Fig. 4. Performance of Goldstone at 512 bps on 1985 DOY 185 
Fig. 3. Observed ICE link performance at encounter (comet tail 
crossing = 1104 2 on DOY 254): (a) four-port combiner; 
(b) three-port combiner 
273 
Appendix A 
Arraying Methodology 
If we have n Gaussian voltage sources with means uK and 
standard deviation UK, we define the signal-to-noise power 
ratioR, as 
is the combined array SNR and 
P A = u;: t u;: 
is the combined output channel total power. 
The optimum combining ratios {a,} t o  maximize RA with 
respect t o  the {aK } are found by  taking a R A / a a K  = 4. where u, is the mean integrated symbol voltage, (I, is the rms 
integrated symbol noise voltage, and the channel total power is 
Thus: 
These Gaussian sources model the integrated symbol volt- 
age distribution from each of n telemetry chains with indepen- 
dent noise 
And using (A2) and (A3), we get 
signal voltage density. Now, if we combine the signals with n 
combining coefficients { aK } , we get, for signal voltages that 
add coherently: 
From ( A l )  and (A2), we get 
(A10) 1 
I 
n 
' A  = ' K  ' K  
K = 1  
From (AS) and (A6), we get 
for noise powers that add incoherently: 
UA =$!E- l t 2 R A  
n 
('45) 
Thus: 
K = l  
Then 
4 
2.; 
RA = -  
274 
In the case where PK = P A ,  we get 
Using these values for (aK }, we have 
RA = 4 
2.; 
- K = l  - 
n 
K = l  
So for the above choice of {aK }, the output SNR is the sum 
of the input SNRs. Note that the combined SNR RA is the 
same if all ( 0 1 ~ ' s )  are multiplied by some constant value. 
In particular, we can normalize so the largest aK is equal to 
unity EK = aK/aR ;aR = max {aK }. 
, 
275 
Appendix B 
Degradation Due to Dephasing 
During the Giacobini-Zinner comet encounter, the ICE 
spacecraft was operating at a bit rate (rB) of 1024 bps with 
a rate 1/2 code and biphase modulation format. 
The symbol period T,  = l/r, where rs is the symbol rate 
where rs = 2rb. The Manchester transition period T, is 
where rB = l / T B .  
Now let p x s  be the symbol transition probability and let p x m  
be the Manchester transition probability. 
Thus, 
so the average number of transitions per second N, is 
Pxm - 
TX 
N, = - - 4p,, rB (B3) 
for PN symbols (p,, 2. 0.5) N, 3rg. 
With no dephasing, the combined output signal uA(0) 
becomes: 
and the in-phase power 
Now let any of the K t h  channels be dephased by 'kK 
seconds (see Fig. SI). Then the combined output  signal 
voltage uA(QK) will be: 
= X u K  ( 1  -2Nx W K I )  w h e r e K =  1 ,  
K 
while the output noise power CIA' is unaffected. 
Thus: 
where 
I- 1 
. . .  
Using In ( 1  - x )  % -x when x << 1 .  then the amplitude loss in 
nats of the combined output  signal becomes 
~ 
i 
I 
and the power loss I 
I 
( B 1 1 )  ' 
I 
I 
276 
In the case where 2Nx 'kK aK uK / uA << 1 is not true, then 
we have: 
and from Eq. B9 we have 
U A ( \ k K )  = U A ( 0 )  ( 1 - 2 N x  'k K : j 2  - = U j ( 0 )  x 
u p K )  = U J O )  1 - 2Nx 'kK -- (B13) 
Thus, the SNR degradation is given by X .  
t 
Example: An array configuration with no dephasing and a bit 
rate (rB) of 1024 bps has a theoretical combined output SSNR 
of RA = 3.35 dB. Dephasing one channel (whose contribution 
is approximately 0.45 dB to the net SSNR) by 50 micro- 
seconds will degrade the SSNR by 0.2 dB. 
277 


Analysis of the ICE combiner for multiple antenna arraying

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