An analysis of the atmospheric refraction corrections at the DSS-13 34-m diameter beam waveguide (BWG) antenna for the period Jul. - Dec. 1990 is presented. The current Deep Space Network (DSN) atmospheric refraction model and its sensitivity with respect to sensor accuracy are reviewed. Refraction corrections based on actual atmospheric parameters are compared with the DSS-13 station default corrections for the six-month period. Average blind-pointing improvement during the worst month would have amounted to 5 mdeg at 10 deg elevation using actual surface weather values. This would have resulted in an average gain improvement of 1.1 dB

Alvarez, L. S.

Perez-Borroto, I. M.

English

NASA Technical Reports Server

TDAProgressReport42-111 November15,1992
Atmospheric Refraction Correction for Ka-Band Blind
Pointing on the DSS-13 Beam Waveguide Antenna
I. M. Perez-Borroto and L. S. Alvarez
Ground Antennas and Facilities Engineering Section
An analysis of the atmospheric refraction corrections at the DSS-13 34-m diame-
ter beam waveguide antenna for the period covering July through December 1990 is
presented. The current DSN atmospheric refraction model and its sensitivity with
respect to sensor accuracy are reviewed. Refraction corrections based on actual at-
mospheric parameters are compared with the DSS-13 station default corrections for
the six-month period. Average blind-pointing improvement during the worst month
would have amounted to 5 mdeg at 10 deg elevation using actual surface weather
values. This would have resulted in an average gain improvement of 1.1 dB.
I. Introduction
The bending of radio waves as they pass obliquely
through the atmosphere can produce large pointing er-
rors. Currently in the DSN, a refraction model (referred
to here as the Lanyi model 1 is used to compute a correc-
tion which reduces antenna beam-pointing errors in ele-
vation. The inputs to the refraction model are predicted
elevation angle, surface atmospheric pressure, surface tem-
perature, and surface relative humidity. The use of actual
weather parameter inputs from antenna station instrumen-
tation, instead of from default values, will significantly
improve blind-pointing accuracy. This article investigates
the magnitude of the pointing improvement by analyzing
1 G. Lanyi, "Atmospheric Refraction Corrections to Antenna Point-
ing at 1 Millidegree Accuracy," JPL Interoffice Memorandum
335.3-89-026, Jet Propulsion Laboratory, Pasadena, California,
March 24, 1989.
real atmospheric conditions at the DSS-13 beam waveguide
(BWG) antenna.
During the phase 1 testing period [1] at DSS 13, gain
and pointing calibrations were conducted from June 1990
through January 1991. Throughout this period the Lanyi
refraction model was used to compute and apply refraction
corrections based on station default weather parameters
coded in the Antenna Controller Subassembly (ACS). Sta-
tion weather instrumentation readings, not interfaced with
the antenna-pointing system at the time, were also logged
at ten-minute intervals. These atmospheric readings al-
low analysis and comparison of actual weather-based re-
fraction corrections with the fixed-parameter corrections
during the experiment period.
It was anticipated that actual beam-pointing error mea-
surements made on stellar radio sources during the phase 1
62
https://ntrs.nasa.gov/search.jsp?R=19930009714 2020-03-17T07:21:34+00:00Z
testing would enable assessment of the reduction of actual
elevation pointing errors. Unfortunately, only a limited
number of measurements were made at the lower elevation
angles, and they appeared to be of only marginal quality.
Due to the nonrepeatable nature and poor quality of these
pointing measurements, they were deemed unreliable for
the purpose of analysis. In this article, estimated improve-
ment in the refraction pointing error correction at DSS
13 is based strictly on the deviation of the logged meteo-
rological parameters (pressure, temperature, and relative
humidity) from the default DSS-13 weather parameters.
The data analysis to be presented will span the real-
time weather input measurement period between July 1
and December 31, 1990. The data acquisition and reduc-
tion techniques are described. The Lanyi refraction model,
along with its sensitivity to input weather parameters, is
reviewed. It is shown that the magnitude of the refrac-
tion pointing error correction based on actual atmospheric
parameters deviates significantly from the default, on the
average, as the elevation tracking angle decreases. The Ka-
band gain loss corresponding to deviations in the correc-
tions emphasizes the need for accurate, real-time weather
inputs to the DSN refraction correction algorithm.
AEL = f(P, T, RH, EL) (1)
where
AEL = change in elevation pointing
P - surface pressure
T = surface temperature
RH = surface relative humidity
EL = predicted elevation angle
The DSN default surface weather parameters, which
were used at DSS 13 are
P = 901.09 mbar
T = 22.07 deg C
RH = 31.57 percent
Figure 1 shows the magnitude of the refraction correc-
tions at DSS 13 as a function of elevation angle, using the
default weather parameters.
II. Refraction Correction in the DSN
A. Background
Until 1990, the DSN used the Berman-Rockwell refrac-
tion model defined in [2,3]. In 1990, the Lanyi model was
implemented in the DSN, as well as at the DSS-13 34-m
BWG antenna. A detailed comparison of the models can
be obtained through the references. The main motiva-
tion for implementing the Lanyi model was its estimated
improvement in accuracy, especially at the low-elevation
angles. The estimated Lanyi model accuracy is stated to
be better than 0.5 mdeg at elevation angles greater than
6 deg, assuming that accurate and current measurements
of the atmosphere are provided to the antenna controller.
In terms of the current DSN Media Calibration Subsys-
tem (DMD) weather instrumentation specifications, this
could produce an error of 1.5 to 4.0 mdeg at 6 deg ele-
vation for average and extreme DSN water vapor content,
respectively. Such pointing errors may seem small but are
significant when contemplating error budgets for Ka-band
beamwidths (17 mdeg for 34-m antennas and 9 mdeg for
70-m antennas).
B. The Lanyi Refraction Model
The Lanyi refraction model has the basic functional
form of
Ill. DSS-13 Surface Atmosphere Measure-
ments
A. Weather Instrumentation and Measurement
Analysis
Daily records of surface atmospheric conditions at
DSS 13 were collected from July through December 1990.
These time-tagged measurements provide the basis for
computation of actual refraction corrections, which were
used for comparison with the default corrections. It is thus
necessary to try to assess the quality of the measurements.
Two sensors for each weather parameter were opera-
tional during the period of interest, and their observations
were recorded every 10 rain by the station weather data
acquisition computer. The accuracy specifications of the
instruments are stated to be
Pressure: 800 to 1100 mbar, -4-0.3 mbar
Temperature: -50 to +80 deg C, -4-0.1 deg C
Relative humidity: 0 to 80 percent, zl:2 percent, and
80 to 100 percent, -/-3 percent
The differences between the two sensors for each measure-
ment (25,329 data points) were computed. The average
63
difference between the two sensors for all days was com-
puted to be
Pressure: 0.0493 mbar (a = 0.0791 mbar)
Temperature: 0.0710 deg C (a = 0.1428 deg C)
Relative humidity: 3.9 percent (_ = 3.7 percent)
Both the pressure and temperature sensor pairs agreed ex-
tremely well during this period, while at least one relative
humidity probe appeared to be biased. The differences
also display large variations. To illustrate the variability
in the humidity sensors, Fig. 2 shows the average daily
difference between the humidity observations for days 267
through 321. The readings from sensor 2 are always larger
than those registered by sensor 1. The true relative hu-
midity value may lie between the sensor readings, or one
or both of the sensors may be biased high or low. These
discrepancies in humidity can map into significant differ-
ences in refraction correction, especially at low elevation
angles. To minimize the impact on the refraction correc-
tion analysis, the atmospheric measurements were filtered
in the following manner: Points were eliminated when the
difference between the two measurements from each of the
sensors was greater than twice the stated DSS-13 accuracy
specification. Approximately twenty percent of the data
points were removed in this manner.
B. Sensor Error Propagation
As noted, uncertainties in all three surface weather
measurements will propagate into errors in the computed
refraction corrections. To quantify the correction degra-
dation, the sum of the squares of the partial derivatives of
the refraction model with respect to each input parameter
were computed.
A first-order approximation to the Lanyi model, which
is adequate for sensitivity analysis, is given by
AEL = Xo - Ztotl (Rsin_(EL))
tan(EL)
(2)
where
Xo = Xary + Xwet = total surface refractivity
Ztot = Za,.v + Zwet = total zenith path delay
R = Earth radius
EL = the uncorrected elevation angle
The dry and wet surface refractivities and dry and wet
zenith path delays can be determined from surface mea-
surements of pressure, temperature, and relative humidity.
The DSS-13 weather instrumentation specifications were
then input as uncertainties and yielded an error of 0.71
mdeg in the computed refraction correction at 10 deg ele-
vation. Thus, the sensor error propagation would not be a
major problem in this current study if all the sensors were
within their accuracy limits.
Figure 3 shows the DSS-13 rss refraction correction
error at 10 deg elevation due to relative humidity un-
certainty, using the default weather parameters and the
given pressure and temperature sensor specifications. It
is seen that when the sensor error increases above 3 per-
cent, refraction error correction on the millidegree level is
unachievable.
IV. Refraction Correction Analysis
A. Computed Refraction Corrections
Refraction corrections were computed at 1-hr intervals
for the atmospheric measurement set spanning the six
months (3446 points). In order to examine the variability
of the computed corrections over this period, the extreme
ranges of the sensor readings are considered. By setting
two of the three input variables (pressure, temperature,
and relative humidity) to the DSS-13 default values and
entering the extreme points listed below into the refraction
model, the correction ranges listed in Table 1, in millide-
grees, are computed.
As seen in that table, the change in refraction correction
due to relative humidity is about 17 times greater than
that due to pressure and about 3 times greater than that
due to temperature.
B. Effect on Gain
The absolute differences between the default refraction
corrections and those corresponding to actual weather pa-
rameters were computed. The resultant values are as-
sumed to be improvements in the beam-pointing accuracy
for blind pointing if the real-time surface weather obser-
vations were used in the refraction correction for the six-
month period. For all the hour-interval atmospheric mea-
surements, the absolute difference is computed at elevation
increments of 5 deg. Figure 4 illustrates the differences for
the month of October 1990, which had the highest aver-
age refraction difference from the default refraction values.
64
Note that for this particular month, very few actual refrac-
tion corrections equaled the default corrections (absolute
difference = 0). Thus, rarely would good blind pointing be
achieved, and the average pointing errors at low elevation
angles would be rather large (4.8 millidegrees). Expected
beam-pointing improvement (using real weather inputs)
increases significantly as the elevation angle is decreased.
To summarize the whole six-month period, statistics
were computed for the entire data set at 5-deg increments.
Figure 5 shows the means and standard deviations of the
absolute differences. At 10 deg elevation, the refraction
pointing error should, on the average, be reduced by 4
mdeg, with a 2.4-mdeg 1-a variation. The expected DSS-
13 Ka-band gain degradation corresponding to the average
differences is shown in Fig. 6. X-band gain loss would be
less than 0.1 dB. The large magnitude of the average gain
loss at the lower elevations stresses the need for accurate,
real-time weather inputs for refraction correction during
Ka-band tracking operations.
Figure 7 shows the means of the absolute correction
differences for the months of August and October, when
the smallest and largest average differences were observed,
respectively. Figure 8 illustrates the corresponding Ka-
band gain loss. The plots indicate that on the average,
the DSS-13 default weather parameters matched the ac-
tual atmospheric conditions best in August and worst in
October.
VoConclusions
An analysis of the atmospheric refraction correction at
the DSS-13 BWG antenna for the period covering July
through December 1990 has been presented. The Lanyi
refraction model and its sensitivity with respect to sen-
sor error were reviewed. It was shown that the present
specifications on the DSS-13 weather instrumentation are
sufficient to provide submillidegree refraction correction,
however, performance will sharply degrade when the rela-
tive humidity sensors fail to meet their specified accuracy.
Refraction corrections based on actual atmospheric pa-
rameters from the six-month period were computed and
compared with the DSS-13 station default corrections.
The average worst-month differences between the correc-
tions was 5 mdeg at 10 deg elevation (Fig. 7). The cor-
responding average Ka-band gain loss expected using the
DSS-13 default weather parameters during this period was
thus 1.1 dB at that elevation (Fig. 8). The X-band gain
loss would be approximately 0.1 dB. This significant gain
loss mandates the need for accurate, real-time weather-
parameter-based refraction correction for future Ka-band
tracking operations.
Acknowledgment
Steve Slobin is acknowledged for providing many helpful technical suggestions
and encouragement.
References
[1] S. D. Slobin, T. Y. Otoshi, L. S. Alvarez, M. J. Britcliffe, S. R. Stewart, and
M. M. Franco, "Efficiency Calibration of the DSS 13 34-Meter Beam Waveguide
Antenna at 8.45 and 32 GHz," TDA Progress Report 42-106, vol. April-June
1991, Jet Propulsion Laboratory, Pasadena, California, pp. 283-297, August 15,
1991.
[2] A. L. Berman and S. T. Rockwell, New Optical and Radio Frequency Angular
Tropospheric Refraction Models for Deep Space Applications, Technical Report
32-1601, Jet Propulsion Laboratory, Pasadena, California, November 1, 1975.
[3] A. L. Berman, "Modification of the DSN Radio Frequency Angular Tropospheric
Refraction Model," DSN Progress Report 42-38, vo]. January-February 1977, Jet
Propulsion Laboratory, Pasadena, California, pp. 184-186, April 15, 1977.
65
Table 1. Effect of measured weather extremes on calculated refraction correction.
Parameter Refraction correction, mdeg
lO-deg elevation 20-deg elevation 30-deg elevation
Pressure, 883 to 907 mbar
Temperature, -9.7 to 37.7 deg C
Relative humidity, 4 to 99 percent
83.6-85.6 41.7--42.7 26.5-27.1
84.3-95.1 42.1-47.4 26.7--30.1
75.2 - 109.7 37.6- 54.6 23.9- 34.6
oILl
rr
n-
O
O
Z
O
I--
O
<
PC
LLLU
¢r
10el
140_
120
_ooW
8O
6O
4oi
20
i
0__
I JO _- J20 30 40 50 60 70 80 90
ELEVATION ANGLE, deg
Fig. 1. Lanyi angular refraction correction model for DSS-13 default atmospheric parameters.
12
10
E
8
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I
W
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5
w 4
rr
0
260
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270 280 290 300 310 320
DAY OF YEAR, 1990
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Fig. 2. Dally average difference between relative humidity probe readings.
330
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40f3,5
"_ 3,0
z
O
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(o
_u 2.5rr
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O
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J
I i L i
2 4 6 8 10
RELATIVE HUMIDITY SENSOR ERROR, percent
Fig. 3. Error In computed correctlon at 10 dog elevation, due to relatlve humidity Instrument
error, using default parameters.
14
12
E
LU
o
z
uJ
cE
Lu
iz
u,
Q
10 20 30 40 50 60 70 80 96
ELEVATION ANGLE, dog
Fig. 4. Absolute difference between actual weather-based refraction corrections and default
corrections for October 1990.
68
5,0
4,5
4.0
3,5
O
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<
3.0
LU
a
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rr 2,5
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STANDARD DEVIATIONS _'_-- --_..
1990
I L L10 20 3b _0 _ 60
ELEVATION ANGLE, cleg
70 80 90
Fig. 5. Mean and standard deviation of the absolute difference between default and actual
refraction corrections during July-December 1990.
-0.2
-06 !
-0.7 i
0
m -0,3
q_
o5
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o
z -0.4
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,o 2b _o _o _o 6b _o 8'0 9o
ELEVATION ANGLE, deg
Fig. 6. Gain loss tor refraction pointing error, July-December 1990.
69
E
U.I
(o
z
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LU
LL
LL
Z
<
01
0
_ BER 1990
AUGUST 1990__
10 2'0 3'o 40 50 60 70 80 90
ELEVATION ANGLE, deg
Fig. 7. Mean of the absolute difference between default and absolute refraction
corrections, August and October 1990.
-0,2
-0.4
r-n
(,9
O _).6
Z
<
-08
-1.0
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AUGUST 1990 _-
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CT ER 1990
ELEVATION ANGLE, deg
Fig. 8. Gain loss for refraction pointing error, August and October 1990.
90
70


Atmospheric refraction correction for Ka-band blind pointing on the DSS-13 beam waveguide antenna

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