Data from the ICE probe to comet Giacobini-Zinner are analyzed to determine the effects of spacecraft rotation upon the signal to noise ratio (SNR) for the two channels of data. In addition, long-term variations from sources other than rotations are considered. Results include a pronounced SNR variation over a period of three seconds (one rotation) and a lesser effect over a two minute period (possibly due to the receiving antenna conscan)

Nadeau, T.

English

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N 8 6 - 2 2 8 0 6
TDA Progress Report 42-84 October-December 1985
Periodic Variations in the Signal-to-Noise Ratios of
Signals Received From the ICE Spacecraft
T. Nadeau
Communications Systems Research Section
Data from the ICE probe to comet Giacobim-Zinner are analyzed to determine the
effects of spacecraft rotation upon the signal-to-noise ratio (SNR) for the two channels
of data. In addition, long-term variations from sources other than rotation are considered.
Results include a pronounced SNR variation over a period of three seconds (one rotation)
and a lesser effect over a two-minute period (possibly due to the receiving antenna
conscan).
I. Introduction
The use of a former Earth satellite to probe the tail of the
comet Giacobim-Zinner has recently been accomplished. The
probe, known as International Cometary Explorer (ICE),
transmits data from two 4-lobed antennas mounted on a boom
emanating from the top of the spacecraft Figure 1 shows the
positions of the probe relative to the Earth and the probe's
two antennas. The rotation of the probe causes a rotating
doughnut-shaped antenna pattern, with a small skew angle,
to be observed from the Earth station(s)
The spacecraft transmits identical data symbols on two
S-band carriers, channel A at 2270 MHz and channel B at
2217 MHz. This article examines the received symbol signal-
to-noise ratios (SNRs) for channels A and B in order to deter-
mine the periodic components of the SNR time variations.
II. Description of Data
The experiment was based on data received from ICE at
DSS-14 from 1125 Z to 1203 Z on 1985 DOY 148. The
detected symbol streams quantized to 8 bits at the output of
the symbol synchronizer assemblies (SSAs) were recorded
simultaneously for channels A and B on the Telemetry
Processing Assembly (TPA) computers using software devel-
oped for symbol stream combining (Ref. 1). Initial processing
was to estimate the symbol SNRs as a function of time for the
two channels using the moment method (Refs. 2, 3). This
processing also produced sample absolute moments and sample
second moments for the two channels. These data were
produced by summing over 57 data symbols, received from the
probe at a rate of 1024 symbols per second (symb/s). This
pre-sum interval is short enough that the actual SNR is almost
constant within it, yet long enough to make subsequent
processing tractable.
III. Short-Term Variation
The rate of rotation of the spacecraft is known to be about
19.65 rpm, or 0.3275 Hz, and each channel's antenna consists
of four approximately equally spaced lobes. Therefore, peaks
and troughs in the received SNR occur at about 1.31 Hz or
four times the rotational rate.
214
https://ntrs.nasa.gov/search.jsp?R=19860013335 2020-03-20T15:03:45+00:00Z
The first step in the data analysis was to accurately deter-
mine the frequency of the dominant periodic component.
This was done by calculating the energy at closely spaced
frequencies near 1.31 Hz, for a data record 83 seconds in
length. The final estimate of the true dominant frequency,
averaged over the first (absolute) and second moment streams
of the A and B channels, was 1.3108 Hz. Thus, 0 3277 Hz or
one-fourth of the dominant frequency was the actual rotational
frequency. Note that this result assumes that the symbol rate
was exactly 1024 symb/s.
The next step was to determine the Fourier coefficients of
each data stream at harmonics of the fundamental, 0.3277 Hz.
Reconstruction of the time functions of the four statistics
from these harmonics resulted in smoothed estimates of the
periodic time variations. Then smoothed estimates of the
symbol SNR were determined at each time step of the recon-
structed A and B channels using the unbiased moment method
(Refs. 2, 3).
The relative magnitudes of the first 20 Fourier coefficients
of the A- and B-channel second moment data determined
which of the coefficients were statistically significant Using
the average SNR estimates and the mean square levels of the
input data, the statistics were calculated for the Fourier
coefficients at frequencies with no non-random energy. In
the second moment data, the means and standard deviations
of the amplitude of Fourier coefficients with noise only
were 121 and 048 for channel A, and 0.86 and 0.34 for
channel B. The decision to use the fourth and eighth har-
monics of the 0.3277 Hz fundamental was based on the data
in Table 1. Harmonics four and eight are greater than the mean
plus three standard deviations, and are clearly significant.
Some others were of borderline significance, but were not
used
The resultant A- and B-channel SNRs, reconstructed using
the fourth and eighth harmonics, are illustrated in Fig. 2.
The A-channel SNR was on the average 1.5-dB stronger than
the B-channel SNR. The variations ranged over approximately
1.4 dB peak-to-peak for the A-channel and 2.0 dB for the
B-channel. This effect was obviously due to the combined
probe rotation effect and the 4-lobed antenna pattern. The
short-term variations of the A and B channels were nearly
in phase.
IV. Long-Term Variation
A 24-minute span of data (472 spacecraft rotations) was
used to determine if other, long-term, variations in SNR
existed. The observation of the symbol error rate (SER) by
J. W. Layland had shown a variation with a principal period of
about two minutes. The mean SNR of each rotation was found
for channels A and B; Fourier analysis of this 472-point data
stream yielded a strong variation with a period of 132 s
(frequency 0.00758 Hz). This variation is shown in Fig. 3.
Table 2 lists the Fourier coefficients for the first 20 har-
monics of the 0.00758 long-term fundamental frequency. The
first harmonic is in general one order of magnitude larger than
any other harmonic; statistical analysis of these coefficients
was not necessary since the first harmonic so clearly dominated
all other harmonics listed
The A-channel mean SNR varied over a range of about
0.3 dB peak-to-peak while the B-channel mean SNR varied
over a range of about 0.6 dB. The reconstruction using the
first harmonic of 0.00758 Hz was sufficiently accurate to
represent the long-term variation in SNR. The variations of
the two channels were in phase The 132-s period is consistent
with the conscan period of the ground-based antenna system
of approximately 128 seconds.
V. Conclusions
There are significant short-term and long-term variations
in the SNR of signals received from the ICE spacecraft The
dominant short-term effects are at 1.31 Hz and 2.62 Hz, the
fourth and eighth harmonics of the spacecraft rotation rate.
The dominant long-term effect is at 0.00758 Hz, which is
approximately the receiving antenna conscan rate. The short-
term variations of period 3.05 seconds are much greater than
the long-term effect. Short-term variations are 1.4 dB peak-
to-peak for the A channel and 2.0 dB for the B channel,
whereas long-term variations are 0.3 dB for the A channel
and 0.6 dB for the B channel
215
References
1. 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, CA, Aug. 15,1984, pp. 115-121.
2. Vo, Q., "Signal-to-Noise Ratio and Combiner Weight Estimation for Symbol Stream
Combining," TDA Progress Report 42-76, Jet Propulsion Laboratory, Pasadena, CA,
February 15, 1984, pp 86-98
3. Layland, J W., "On S/N Estimation," Space Program Summary 37-48, Volume III,
Jet Propulsion Laboratory, Pasadena, CA, Dec 31, 1967, pp. 209-212
216
Table 1. Fourier coefficients of short-term variation of ICE SNR, i, = 0.3277 Hz
(1125 2, DOY 148)
Channel Harmonic = n
A, Sum of Absolutes 0
1
2
3
4
5
6
7
8
B, Sum of Absolutes 0
1
2
3
4
5
6
7
8
A, Sum of Squares 0
1
2
3
4
5
6
7
8
B, Sum of Squares 0
1
2
3
4
5
6
7
8
^»
20.423
-0020
0.009
0080
-0.037
0.060
0.075
0.035
-0.153
14.959
0.023
0.083
0023
-0268
0.066
0.044
0.004
-0.080
545 348
-0929
1.001
4201
-1.580
2.908
2596
2.236
-6899
308 269
0.389
2872
0474
-9.818
1.990
1.182
0.420
-2.975
*n
0000
0.041
-0064
-0028
-0.357
-0.034
-0007
-0.004
0.067
0.000
0.065
-0.071
-0.058
-0.489
0.026
0.006
0.015
0.028
0.000
1.089
-2.813
-0.924
-15.649
-2.095
-0603
-1.104
2229
0.000
1.886
-2692
-2151
-16.780
1068
0197
0.380
1.141
Magnitude
20.423
0045
0.065
0.085
0.359
0069
0075
0036
0.167
14,959
0.068
0.109
0.063
0558
0.071
0.044
0015
0084
545.348
1.431
2.986
4302
15729
3.584
2.665
2.494
7.250
308 269
1925
3.936
2.203
19441
2.259
1.198
0566
3 186
217
Table 2. Fourier coefficients of long-term variation of ICE SNR, f, = 0.007576 Hz
(1125 Z to 1149 Z, DOY 148)
Channel Harmonic = n
A 0
1
2
3
4
5
6
1
8
9
10
11
12
13
14
15
16
17
18
19
20
B 0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
An
13800
-0.0094
-00019
-0.0001
0.0044
-0.0033
-0 0006
00019
0.0007
-0.0025
00002
0.0020
0.0009
-0 0007
0.0006
-0.0037
-0.0027
QQQ22
-0.0021
-0 0005
-0.0025
0.9500
-0.0182
-0.0004
0.0005
-00013
0.0021
00000
00048
-0.0010
00015
-0.0027
-0 0029
0.0033
00032
0.0012
00013
-0 0009
-0.0001
-0 0004
-0.0022
0.0002
Bn
0.0000
-0.0183
0.0004
0.0006
-0 0034
0.0020
-0.0007
00043
-0.0030
-00018
0.0006
0.0000
00010
0.0037
-o'.ooie
-0.0011
0.0027
-0,0021
0.0015
0.0008
00029
00000
-0.0301
0.0020
-0.0020
00023
00010
-0 0009
-0.0019
-0 0007
0.0002
-0 0020
-0.0020
00006
-00012
00010
0.0006
0.0022
0.0009
-0.0014
-0.0002
-0 0028
Magnitude
1.3800
0.0205
0.0020
0.0006
0.0056
0.0038
0.0009
0.0048
0.0031
0.0031
00006
0.0020
00013
0.0038
0.0017
0.0038
0.0038
0.0030
0.0026
0.0010
00038
0.9500
0.0352
00021
0.0021
00026
00023
0.0009
0.0052
00012
0.0015
00034
0.0035
0.0033
00034
0.0015
00015
0.0024
00010
00014
0.0022
0.0028
218
BCHANNEL ANTENNA
A CHANNEL ANTENNA
GROUND
STATION
RECEIVER
-EARTH
HORIZON
Fig. 1. ICE spacecraft orientation
-2
CHANNEL A
CHANNELS
TIME, s
Fig. 2. SNRs for channels A and B vs time (1125 Z, DOY 148)
-2
CHANNEL A
CHANNELS
50 100
TIME, s
Fig. 3. SNRs for channels A and B vs time (1125 2 to 1149 Z,
DOY 148)
219


Periodic variations in the signal-to-noise ratios of signals received from the ICE spacecraft

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