When using Deep Space Station (DSS)-14 in a monostatic configuration, radar observations of Titan show that Titan is a diffuse reflector with a relative radar cross section of 0.14 +/- 0.03. No hot spots were observed

Goldstein, R. M.

Jurgens, R. F.

English

NASA Technical Reports Server

FDAProgressReport 42-109 May 15, 1992
z-/-V /
N92-2938§
DSN Observations of Titan
f
R. M. Goldstein
Telecommunications Science and Engineering Division
R, F, Jurgens
Communications Systems Research Section
When using DSS 14 in a monostatic configuration, radar observations of Titan
show that Titan is a diffuse reflector with a relative radar cross section of 0.14
-4-0.03. No hot spots were observed.
I. Background
Last summer, there was a favorable opportunity to ob-
serve Titan with the Goldstone radar. Detection of radar
echoes and the study of their characteristics are expected
to be very helpful in the design of the Cassini radar instru-
ment and in planning for its data-acquisition scenarios.
Previous observations by Muhleman, Grossman, But-
: ler, and Slade [1], using the Goldstone transmitter and the
Very Large Array receivers, revealed reftectivities of 0.38,
0.78, and 0.25 percent on successive nights. Later mea-
surements have shown that the hot spot may be variable
in time and/or location.
A campaign was undertaken to use the upgraded Gold-
stone radar, in the monostatic mode, to observe such areas
on Titan.
II. The Experiment
Sixteen nights at DSS 14 were initially scheduled for
this experiment. This would have enabled the examina-
tion of each 22-deg time zone of longitude around Titan.
An observation night consisted of one 2.5-hour period for
transmission followed by an equal period for reception.
The signal to noise ratio for a hot spot was expected to be
greater than 10 for a single night's observation.
Each observation started with an antenna-pointing test
using Saturn as a radiometric source. After verifica-
tion that Saturn increased the system temperature by
1.6 kelvins, the antenna was swung to nearby Titan and
transmission was started.
Frequency hopping was used, so that every 30 seconds
the transmitter would jump 10 kHz in frequency. After
three hops, the transmitter would return to the original
frequency. This technique permits accurate background
removal, which is required for detection of weak signals.
During the receive time, the signals were filtered to
40-kHz bandwidth, digitally sampled, Fourier transformed
to 39-Hz resolution, detected, and summed. The summed
power spectra for each hop period were stored on floppy
disks. The above procedure was performed on-line with
the newly installed orbital debris equipment.
A daytime test run, using the planet Mercury as a re-
flector, proved that all systems were working. Even so, all
16 nights of observing time were wasted. The problem,
which will be described subsequently, was in tlle pointing
of the antenna.
377
https://ntrs.nasa.gov/search.jsp?R=19920020145 2020-03-24T07:15:17+00:00Z
BecauseoftheimportanceofTitandata,sixadditional
nightsofobservationswerescheduled.Fiveofthesenights
producedusefuldata.Tile (weighted)averageradarchar-
acteristicswereasfollows:
Radiatedpower= 468kW
Antenna diameter = 70 m
Antenna efficiency = 0.68
System temperature = 18.0 kelvins
Range to Titan = 9.22 AU
_Vavelength = 3.5 cm
Figure 1 presents the results: one spectrogram for each
night of successful observation. The spectrograms have
been smoothed to a final resolution of 235 Itz. For clarity,
the vertical scales are different for each night. A dotted
reference curve shows the echo expected from the average
relative cross section (0.35) reported by Muhleman, et al.
[1].
The bottom curve of Fig. 1 is the weighted average of
the other five observations and represents all the results
of the experiment. It constitutes a weak (5.00") detec-
tion of Titan, and gives a radar relative cross section of
0.14 4-0.03.
Titan appears to be a diffuse scatterer, with most of
the disk contributing to the echo. There was no visible
specular component, no central peak in the spectrogram,
such as exists for tile terrestrial planets. Neither was there
any evidence of the sought-after hot spots.
Iil. Problems
The support given to this experiment by tile Goldstone
Radio Science group and the DSN personnel was excel-
lent. There were, however, problems encountered whose
solution would improve certain hardware and/or software
deficiencies.
(1) For 15 of the scheduled nights, the antenna was off-
point because the command "PLAN" was absent
from the control stream. This command works like
"Simon Says"; although the antenna moved away
from Saturn when the coordinates were changed, it
did not then move to Titan since Simon didn't say to
do so. There was no feedback to tile experimenters,
so there was no way they could know a problem ex-:
isted.
(2) On one of the remaining nights, the antenna was off-
point because the time was entered with four digits
instead of six. Once again, since there was no feed-
back about what the machine was actually doing, no
one could know that the problem existed.
(3) On two occasions, the antenna went to brake, caus-
ing minor outages.
The Goldstone Solar System Radar would benefit from
some provision for the experimenters to monitor the actual
pointing of the antenna.
Reference
[1] D. O. Muhleman, A, W. Grossman, B. J. Butler, and M. A. Slade, "Radar
Reflectivity of Titan," Science, vol. 248, pp. 975-980, May 25, 1990.
378
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Ell
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Fig. 1. Spectrograms for each night of successful observation.
379


DSN observations of Titan

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