From August 1982 through November 1984, X-band downlink (8.4-GHz) system noise temperature measurements were made on the DSN 64-m antennas during tracking periods. Statistics of these noise temperature values are needed by the DSN and by spacecraft mission planners to assess antenna, receiving, and telemetry system needs, present performance, and future performance. These measurements were made using the DSN Mark III precision power monitor noise-adding radiometers located at each station. It is found that for DSS 43 and DSS 63, at the 90% cumulative distribution level, equivalent zenith noise temperature values fall between those presented in the earlier (1977) and present (1983) versions of DSN/Flight Project design documents. Noise temperatures measured for DSS 14 (Goldstone) are higher than those given in existing design documents and this disagreement will be investigated as a diagnostic of possible PPM or receiving system performance problems

Andres, E. M.

Slobin, S. D.

English

NASA Technical Reports Server

TDA Progress Report 42-87 July-September I986 
PPM/NAR 8.4-GHz Noise Temperature Statistics for DSN 
64-Meter Antennas, 1982- 1984 
S. D. Slobin and E. M. Andres 
Radio Frequency and Microwave Subsystems Section 
From August 1982 through November 1984, X-band downlink (8.4-GHz) system noise 
iemperature meiisurements were made on the DSh'54-m antennas during tracking periods. 
Statistics of these noise temperature values are needed by the DSN and by spacecraft 
mission planners to assess antenna, receiving, and telemetry system needs, present per- 
formance, and future performance. These measurements were made using the DSN 
Mark III precision power monitor noise-adding radiometers located at each station. I t  is 
found that for DSS 43 and DSS 63, a t  the 90% cumulative distribution level, equivalent 
zenith noise temperature values fall between those presented in the earlier (1977) and 
present (1 983) versions of  DSNIFlight Project design documents. Noise temperatures 
measured for DSS 14 (Goldstone) are higher than those given in existing design docu- 
ments and this disagreement will be investigated as a diagnostic of possible PPM or 
receiving system performance problems. 
1. Introduction 
The precision power monitors (PPMs) installed on the three 
DSN 64-m antennas use a noise-adding radiometer (NAR) to  
monitor the system noise temperature, and a signal level esti- 
mator (SLE) to monitor the spacecraft signal level. Descrip- 
tions of these instruments will be published in the TDA 
Progress Report in the future. A description and analysis of 
NAR operation are given in Refs. 1 and 2. Over the two-year 
period from August 1982 through November 1984, over 
240,000 system noise temperature measurements were made 
using the NARs during spacecraft tracking periods. While noise 
temperature variations are primarily due to weather effects, 
these variations may also be indicative of problems in the 
microwave/receiver systems. Thus, the PPM can serve as a 
useful real-time diagnostic indicator of station performance. 
During Mark IV implementation, the three 64-m DSN 
stations were not operational for the full period of August 
1982 through November 1984. The valid data periods for the 
stations are as follows: 
DSS 14: September 1982-June 1983 
DSS 43: August 1982-September 1984 
DSS 63: August 1982-November 1984 
Because of the shorter data collection period, DSS 14 statistics 
may not be directly comparable to those of DSS 4 3  and 
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. _. 
DSS 63. In particular, the months of July and August (which 
are missing) are characterized by high absolute humidity and 
thunderstorm activity. 
Typically, an NAR determines system operating noise 
temperature (Top) by pulsing a noise diode on and off and 
injecting this square-wave noise into the main signal channel 
of the receiving system. In the DSN application described 
above, a typical noise diode modulation rate is 10 Hz and the 
injected noise is typically -1 K. Two fundamental equations 
govern NAR operation. They are, for Top: 
where 
To, = system operating noise temperature, K (defined at 
TN = noise diode injected noise temperature, K (defined 
system input reference plane) 
at system input reference plane) 
Y = (v,  + CIV:)/(V, + CIV;). ratio 
V ,  = detector output voltage, noise diode on, V 
V ,  = detector output voltage, noise diode off, V 
CY = detector nonlinearity constant, V-l (= 0 in an 
ideal detector) 
and for noise temperature resolution: 
ATOP = 2Top( l+  T O p I T N Y f i  (2 
where 
7 = measurement time, s 
B = predetection bandwidth, Hz 
For DSN operational use, the value of this injected signal 
level (T,) is kept low to avoid contamination of the very low 
system noise temperature (-20-25 K). On the other hand, for 
high resolution (small ATop),  a large value of TN is required. 
In operation, the PPM performs the following steps to  
determine Top: 
(1) The PPM switches the waveguide system to an ambient 
load (-300 K) and calibrates a large (-50 K) noise 
diode according to 
(3) 
and 
TOP. amb = T, t TE (4) 
where 
TE = TM + TF (equivalent noise temperature of 
the receiver, K) 
TM = maser noise temperature, K 
TF = followup receiver noise temperature, K 
T, = physical temperature of ambient 
termination. K. 
(2) The PPM switches the waveguide out the horn and uses 
the calibrated 50 K noise diode to  determine Top on 
the cold sky according to Eq. (1). 
(3) Knowing the sky Top, the PPM chooses a smaller 
operational noise diode to give a Top resolution 
(ATop) of 0.1 dB (2.33%) of the value determined in 
Step (2). 
Thus, for example, if cold sky Top  is determined to  be 40 K, 
the required AT,, is 0.93 K, and the TN chosen for opera- 
tional use will have a value somewhat greater than 0.35 K for 
an integration time of 10 seconds and a bandwidth of lo7 Hz. 
In this case. a 0.5 K noise diode would be chosen. This amount 
of injected noise will not seriously degrade receiving system 
performance. 
Clearly, an inaccurate calibration of the large noise diode 
when looking at the ambient load will result in errors in 
determining Top. A possible source of calibration error 
is due to receiver nonlinearities (saturation) at large Top  
e 3 0 0  K). In this case, the measured Y factor will be smaller 
than it should be, and the value of TN determined from 
Eq. (3) will thus be too low. This will ultimately result in a 
lower than actual determination of Top according to Eq. (1). 
II. Data Reduction 
Tapes of PPM data were generated monthly for each 
64-m station. Only the X-band data were analyzed, as S-band 
weather effects are minimal and contribute insignificantly to  
increased system noise temperature. The data contained 
numerous PPM diagnostics and antenna tracking parameters, 
including date, time, spacecraft identification, maser assembly 
number, receiver assembly number, system noise temperature, 
standard deviation of Top,  and antenna pointing (local hour- 
angle and declination). For each data point, antenna azimuth 
and elevation values were calculated based on the station's 
186 
latitude and the local hour-angle and declination of the space- 
craft. Each data point (typically one every 40 seconds) was 
examined for validity according to  the following criteria: 
(1) Top between 10 K and 300 K. 
(2) Nonzero local hour-angle and declination. 
(3) A changing (nonstuck) hour-angle. 
(4) Elevation greater than 3 deg. 
(5) Spacecraft declination in the range of t 6 0  deg to  
(6) Standard deviation of Top  between 0 and 2 K. 
-60 deg. 
Short of invoking overly sophisticated logic or hand-picking 
the data points, it was felt that the six criteria above would 
sort out the vast majority of bad data points. Of the 282,547 
total points recorded, 37,715 (or 13.35%) were discarded as 
failing one or more of the above criteria. Table 1 shows the 
percentage of points taken on each spacecraft and the percen- 
tage of points taken in each 10-deg elevation-angle interval. 
Initial inspection of the histogram of noise temperature 
values showed a small cluster of points between 10 and 14 K 
for DSS 14, and an “abnormal looking” distribution between 
17 and 20 K for DSS 63. In each of these cases, the data 
points were discarded. They amounted to 1.5% and 3% of the 
totals, respectively. No adjustment of this sort was made for 
the DSS 43 data points. 
An attempt was made to create a zenith noise temperature 
model from the elevation-angle (nonzenith) points in the raw 
data. A l/sin(elevation) model was used. The method was 
as follows: 
(1) The lowest Top  value in each station’s valid data was 
assumed to be the best weather and highest elevation 
angle point of the test period. 
(2) From the value above, a zenith ground noise con- 
tribution (3 K) and zenith clear sky atmosphere 
contribution were subtracted. The remainder is the 
constant antenna noise contribution due tu liie horn, 
waveguide, maser, and cosmic background contribu- 
tion (considered to  be constant over a wide range of 
conditions). 
(3) An equivalent zenith system noise temperature 
(TQr”,go) is calculated according to the equation 
where 
Top, e = Top at elevation angle 0 (raw data) 
ATground = decrease in ground contribution 
when moving to  zenith 
- - Tground,e - 3.0 
Tconst = constant antenna contribution deter- 
mined in Step (2) 
T ~ ~ ~ ~ ~ , ~  = 3.0 t 5.0 [(go - e)/90] 
This technique attempts to  create a uniform condition (zenith- 
looking) by which the three 64-m antennas can be compared. 
111. Results 
Figure 1 shows the probability density function (histogram) 
of the X-band system noise temperature for the three 64-m 
stations. The integral of the PDF, over all noise temperatures, 
is by definition 1.0. In the data set presented here, all data 
points below 20 K have been removed as described above. 
Note that DSS 43 shows a narrower range of system noise 
temperatures than do the other two stations. This does not 
necessarily indicate that “better weather” existed at DSS 43. 
Because of the preponderance of Voyager-related points (and 
Jupiter, Saturn, and Uranus have had significant southerly 
declinations for the past several years), DSS 43 would have 
tracked generally at higher elevation angles than the other 
two sites. This high elevation bias would result in lower 
atmospheric noise temperature values, even during clear sky 
conditions. 
Figure 2 shows the cumulative distribution (CD) of system 
noise temperature (the integral of the histograms in Fig. 1) at 
the three 64-m sites. These CDs are for all valid data and repre- 
sent a mix of elevation angles, spacecraft, and tracking periods. 
If one were to blindly infer weather statistics from these 
curves, it would appear that DSS 63 has the best weather, 
and DSS 14 the worst. At the 90% “confidence level” 
(CD = 0.90), the system noise temperatures are 44.5 K 
(DSS 14), 41.4 K (DSS 43), and 38.5 K (DSS 63). It must be 
reiterated that the three stations do not have similar distri- 
butions of elevation angle (see Table l ) ,  and that data were 
not collected for a long enough time period to  generate truly 
long-term statistics. 
Figure 3 shows the equivalent zenith CDs for the distri- 
butions in Fig. 2. The- method of generating these points is 
described in Section 11. The equivalent zenith points for Fig. 3 
were generated one by one; the distributions in Fig. 2 were not 
187 
modified as a whole to create the zenith distributions. Also 
shown in Fig. 3 are system noise temperature CDs given in 
both the present (1983) and earlier (1977, “old 810-5”) 
versions of DSN/Flight Project interface design documents’. 
The 810-5 curves are presented for comparison only; it is pre- 
mature to attempt a replacement of the 810-5 models with the 
relatively short-term PPM measurements. The statistics of the 
older interface design documents are still being used in the 
Voyager telemetry prediction and performance (TPAP) 
computer program. It has been noted that the TPAP program, 
using the “old 810-5” model, generally predicts worse telem- 
etry performance than is actually observed. Indeed, at the 90% 
confidence level (a commonly used performance reference 
point), the Top values at both DSS 43 and DSS 63  were pre- 
dicted (old 810-5) to be higher than those measured by the 
PPM and higher than those predicted in the present version of 
the DSN/Flight Project design documents. At DSS 14, how- 
ever, the measured PPM values appear to be substantially 
higher than both design-document models. If t h s  is indeed an 
erroneous result. one possible explanation is that the short 
data gathering period (10 months) has somehow biased the 
data with worse than average weather conditions. Also, the 
method of creating the equivalent zenith model may be at 
fault. but then the zenith statistics at all stations would be 
’Deep Space Network/Flight Project Interface Design Handbook, 
TDA document 810-5, Rev. D, TCI-40, Rev. B, pp. 1-31 (internal 
document), Jet Propulsion Laboratory, Pasadena, Calif., 1977 and 
1983. 
wrong in the same way, not just those at Goldstone. Other 
possible sources of DSS 14 errors are receiver system linearity, 
unstable noise diode outputs, defective PPM operation, or sys- 
tematic, incorrect operational calibration of the PPM. Further 
data collection and analysis using the Mark IV PPMs will help 
to resolve this question. 
IV. Conclusions 
Several years of system noise temperature data were col- 
lected using the noise-adding radiometers in the PPMs at the 
three DSN 64-m stations. At the 90% confidence level, it is 
seen that the equivalent zenith Top measurements made by 
the PPM fall between the earlier (1977) DSN/Flight Project 
Interface Design Document model (used for Voyager) and the 
more “optimistic” model developed in 1983. The zenith 
results presented for Goldstone (DSS 14) show substantially 
higher Top values than either the 1977 or 1983 models. The 
cause of this is presently unknown, but may be related to the 
short period of data recording. Qualitative observations of 
weather at the three 64-m sites indicate that Goldstone has the 
most benign weather conditions and should experience the 
lowest noise temperature increases. The resulting statistics, 
then, are puzzling, and may indicate problems with the PPM 
itself, the receiving system, or with certain operational pro- 
cedures. These problems will be fully investigated in order to 
guarantee reliable and accurate data output from these instru- 
ments in the future. 
References 
1. Batelaan, P. D., Goldstein, R. M.,  and Stelzried, C. T., “A Noise Adding Radiometer 
for Use in the DSN,” Space Programs Summary 37-65, Vol. 11, Jet Propulsion Labora- 
tory, Pasadena, Calif., pp. 66-69. 
2 .  Stelzried, C., “Noise Adding Radiometer Performance Analysis,” TDA Progress Report 
42-59, Jet Propulsion Laboratory, Pasadena, Calif., October 15, 1980, pp. 98-106. 
188 
Table 1. Fractional occurrence of spacecraft tracks and antenna elevation angle 
Parameter DSS 14 DSS 43 DSS 63 All Stations 
Spacecraft 
12 (Pioneer 12) 
23 (Pioneer 10) 
24 (Pioneer 11) 
26 (Viking Lander 1) 
31 (Voyager 1) 
32 (Voyager 2) 
90 (Helios 1) 
Elevation Angle, deg 
0 -10 
10 - 20 
20 - 30 
30 - 4 0  
40 - 50 
50 - 6 0  
60 - 70 
70 - 80 
80 - 9 0  
Point Count 
Total 
Good 
Bad 
0.0881 
0.0036 
0.0026 
0.0001 
0.4040 
0.5015 
0.0001 
0.0336 
0.1467 
0.2098 
0.3249 
0.1011 
0.1124 
0.0705 
0.0009 
0.0000 
49568 
47516 (95.86%) 
2052 (4.14%) 
0.1307 
0.0034 
0.0001 
0.0000 
0.3163 
0.5492 
0.0003 
0.0158 
0.1084 
0.1361 
0.1700 
0.2221 
0.1163 
0.1445 
0.0867 
0.0000 
98433 
76820 (78.04%) 
21613 (21.96%) 
0.0390 
0.0031 
0.0000 
0.0000 
0.4952 
0.4627 
0.0000 
0.0392 
0.1940 
0.3023 
0.1866 
0.1012 
0.1667 
0.0096 
0.0005 
0.0000 
134546 
120496 (89.56%) 
14050 (10.44%) 
0.0773 
0.0033 
0.0005 
0.00004 
0.4213 
0.4973 
0.0001 
0.0308 
0.1580 
0.2322 
0.2082 
0.1391 
0.1403 
0.0638 
0.0276 
0.0000 
282547 
244832 (86.65%) 
37715 (13.35%) 
189 
0.20 
0.20 I I I I I I I I I I I 
I I I I I I 1 I I 
(a) DSS 14 
(c) DSS 63 
0.12 
SYSTEM NOISE TEMPERATURE, K 
Fig. 1. X-band system noise temperature, probability density 
function (histogram) 
190 
!.e 
0.9 
0.8 
0.7 
0.6 
0.5 
0.4 
0.3 
0.2 
z 
0.1 
3 
K + 
0 
w 
+ 
3 
3 
0 
m 
Y O  
1.0 
4 
0.9 
0.8 
0.7 
0.6 
0.5 
0.4 
0.3 
0.2 
0.1 
0 
SYSTEM NOISE TEMPERATURE, K 
Fig. 2. X-band system noise temperature, cumulative distribution, all elevation angks 
191 
1 .o 
0.9 
0.8 
0.7 
0.6 
0.5 
0.4 
0.3 
0.2 
0.1 
C 
1 
( c )  DSS 63 
1 1 1 1 1 1 1 
i i 
SYSTEM NOISE TEMPERATURE, K 
Fig. 3. Equivalent zenith X-band system noise temperature, cumulative distribution, with 8105 models for comparison 
192 


PPM/NAR 8.4-GHz noise temperature statistics for DSN 64-meter antennas, 1982-1984

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