RADAR Operating in Radiometer Mode

This map shows the pattern of Cassini spacecrafts well-controlled up-and-down nodding movements. These made it possible for the microwave radiometer to gather global measurements of Saturn's atmosphere, reflected in the example data

Cassini radar instrument: radiometer mode

In the radiometry mode, the RADAR operated as a passive instrument, simply recording the energy emanating from the surface of the planet. This mode operated at 1 kilobit per second at 13.78 GHz passive Ku-bandwidth with 7 to 310 km resolution and 135 MHz bandwidth. The antenna was set to the central (and smallest) radar beam, beam 3. The radiometric segment was expanded to observe the target for the entire 1-s period except for a 25 ms integration on the reference load in each cycle. These choices provided the beamwidth and sensitivity given below.

RADAR (an acronym that stands for "radio detection and ranging") can operate in three ways: imaging, altimetry and radiometry. For this data a 1-s duration burst cycle in which the active segment was eliminated and the antenna was set to the central (and smallest) radar beam, beam 3 was used. The radiometric segment was expanded to observe the target for the entire 1-s period except for a 25 ms integration on the reference load in each cycle, These choices provided the beamwidth and sensitivity given below.

Nominal radiometer characteristics.
Frequency			13.78 GHz
Wavelength			2.18 cm
Polarization			1 linear
Radiometer bandpass			135 MHz
Measurement noise			0.026 K/pHz
Beam full width at half-power (beam 3)			0.36" Circular

Cassini Radiometer Data

Derived data

This data set is comprised of well-calibrated, high-resolution global maps of Saturn’s thermal emission at 2.2-cm wavelength obtained by the Cassini RADAR radiometer through the Prime and Equinox Cassini missions, a period covering approximately six years. The absolute brightness temperature calibration of 2% achieved is more than twice better than for all previous microwave observations reported for Saturn, and the spatial resolution and sensitivity achieved each represent nearly an order of magnitude improvement. The brightness temperature of Saturn in the microwave region depends on the distribution of ammonia, which our radiative transfer modeling shows is the only significant source of absorption in Saturn’s atmosphere at 2.2-cm wavelength. At this wavelength the thermal emission comes from just below and within the ammonia cloud-forming region and yields information about atmospheric circulations and ammonia cloud-forming processes.

These maps are presented as residuals compared to a fully saturated model atmosphere in hydrostatic equilibrium. Also included are time-ordered data detailing the modeling results, referencing the midpoint of each radiometric integration period.

Observational Approach

The RADAR radiometer was used to observe Saturn during five equatorial periapsis passes occurring between 2005 and 2011 for the purpose of mapping its 2.2-cm thermal emission. Observational parameters are given below.



Derived Data

Raw Data

The raw data are archived by the PDS Imaging node at JPL and USGS-Flagstaff.

References

Elachi, C., et al, 2004. RADAR: The Cassini Titan radar mapper. Space Science Reviews 115, 71-110. DOI:10.1007/s11214-004-1438-9.

Janssen, M.A., et al., 2009, Titan’s surface at 2.2-cm wavelength imaged by the Cassini RADAR radiometer: calibration and first results. Icarus, 200, 222-239, DOI: 10.1016/j.icarus.2008.10.017

Janssen, M.A., et al., 2013, Saturn’s Thermal Emission at 2.2-cm Wavelength as Imaged by the Cassini RADAR Radiometer, Icarus, Volume 226, Issue 1, p. 522-535, DOI:10.1016/j.icarus.2013.06.008.

Moeckel, C, Janssen, M, De Pater, I, (2019) A re-analysis of the Jovian radio emission as seen by Cassini-RADAR and evidence for time variability, Icarus, 321, 994-1012, DOI: 10.1016/j.icarus.2018.12.013.

Citation

Janssen (2021) Saturn's Thermal Emission at 2.2-cm from Cassini RADAR Radiometry, NASA Planetary Data System, DOI XXXXXXXXX.