The Cassini Radar system on the Cassini spacecraft provided synthetic aperture radar imaging of Titan's surface. It operated in different modes like imaging, altimetry, and radiometry. Imaging produced BIDR products which showed various geological features on Titan like lakes, dunes, and craters. Analysis of these features provided insights into Titan's methane-ethane based hydrologic cycle and surface processes like erosion. The Cassini mission significantly expanded understanding of Saturn's moon Titan through radar observations over more than a decade.

1. Cassini Radar
Exploring Titan

2. Contents
● Introduction
● Cassini Radar system
● BIDR product overview
● Surface of Titan
● Conclusion

3. Introduction
● Cassini spacecraft launched 1997
● Saturn orbiter 2004-07-01
●
Hugyens on Titan 2005-01-14
● Multiple missions
– Saturn, heliosphere, testing relativity
– Icy moons, Saturn's rings, etc.
– Enceladus jets and subsurface ocean
● 10+ years of orbiting Saturn
– 2017 Solstice (dunes moving at equinox season)

4. The Cassini Radar System
High-Gain
Antenna (HGA)

5. The Cassini Radar System
● Wavelength is 13.78 GHz (Ku-band)
● Synthetic Aperture Radar Imager
– 0.35 to 1.7 km
● Altimeter
– 24 to 27 km horizontal
– 90 to 150 m vertical
● Radiometer
– Passive
– 7 to 310 km
Mastrogiuseppe (2014)

6. Modes of operation
● Imaging:
– Timing pulses at many incidence angles
● Altimeter:
– Single focus pulse timing
● Backscatter:
– Returned intensity gives surface properties
● Radiometer:
– Emitted noise from Titan atmosphere (calib)

7. Types of RADAR modes
From Thesis of Lauren Wye (2011)

8. Synthetic Aperture Radar
● Doppler-Shifted (DS) signal frequency
– Relative motion of HGA & Titan surface
● “Focusing” uses two DS states of object
– Higher frequency if moving closer
– Lower frequency if moving apart
– Illumination of target point isolated
● Synthetic Aperture
– “Virtual length” of antenna
– Relative velocity displacement

9. The monostatic radar equation
If Tx and Rx are collocated (same fixed antenna), then:
Pt
total transmitted power (48.084 W),
Gt
on-axis antenna gain (50.7 dB),
Ar
effective aperture area of the receiving antenna (4.43 m2
)
R distance (range) between the radar and the target
Ps
= Pr
− Pn
Ps
received echo signal power
Pr
total received power
Pn
mean noise power
From Wye (2011)
With the effective antenna
aperture Ar = λ2Gt / 4π

10. Transmitted power, interception
● PtGt is the Transmitted energy
– Isotropic spherical wave
– Spherical spreading loss (1 / 4πR2)
– Attenuation sphere has radius R
● Interception on target surface by σ
– Energy absorption
– Energy isotropically re-radiated
– Spherical spreading loss (1 / 4πR2)
● Reception by the Antenna
From Wye (2011)

11. Radar Cross-Section (σ)
● Inherent property of the target, units are m2
● Reflectivity
– dielectric properties
● Directivity
– physical structure (size, shape)
– at scales relative to the illuminating wavelength
● Other parameters
– illuminating wavelength
– viewing geometry
– polarization configuration
Proper characterization of the RCS’s response
to incidence and azimuth angle variation
helps to eliminate the viewing geometry dependence
From Wye (2011)

12. Pulses
● N ~ 50 for SAR
● Interpulse separation
– Not possible at high incidence angle
– Impacts noise modeling → uncertainty
From Wye (2011)

13. Noise
● Mean noise power level
– receiver electronics thermal noise (mostly, Prec)
– received target radiation thermal noise (less, Pa)
● System noise power
– Psys = Prec + Pa
● The ideal receiver system
– large front-end gain
– receiver thermal noise power unaffected by any back-end gain changes
– In this scenario, Prec constant for a particular receiver bandwidth
– need to calculate the noise power once for each receiver filter
● Not the case in Cassini RADAR

14. BIDR products in PDS
● SAR image from a single Titan pass
● Raw processing a “formidable undertaking”
● PDS raster format
– IMG data file in binary format
– LBL metadata file in text format
– Imports in ISIS directly

15. Integrated Software
for Imagers & Spectrometers
● isis.astrogeology.usgs.gov
“Manipulate imagery collected by current and past NASA and
International planetary missions sent throughout our Solar System”
● Works in Linux & Mac
● Command Line Interface
pds2isis from=BIDR*.IMG to=out.cub
● Display, mosaick GUI

16. Ligeia Mare
Kraken Mare
Northern Lakes
Punga
Mare

17. T91
Kraken Mare
Small lakes
Kraken Mare
Mývatn Lacus
Oneida Lacus
Waikare Lacus
T91 Fly-By ISIS-GRASS

18. BIDR scales (F&D)
Kraken Mare main Peninsula

19. Ligeia Mare
Mechanical erosion
(Not chemical)
Black et al (2012)

20. Ligeia Mare Island
Mare characteristics (not only from BIDR)
● Strong specular reflection, no waves
- 1mm rms (Zebker et al., 2014)
● Extremely transparent (Mastrogiuseppe, 2014)
- Suspended particles < 0.1%
- 160 m maximum depth
Dissolution/Precipitation play? (personal thought)

21. Sub Equatorial Dunes
13% of Titan
Matured features
1.3km width
2.7km crest spacing
Hydrocarbon chains
>1m/s saltation
Equinox weather
Savage et al, (2014)
T95 Fly-By
ISIS-GRASS

22. Craters
● Wood et al. (2010)
– 5 confirmed craters, 44 potential (+ E Xanadu)
● Neish et al (2012)
– 5 confirmed > 20km diameter studied
– Eolian infill ? (see Forsberg-Taylor, 2004)
● Giliam & Jurdy (2014)
– Connection crater<>subsurface water
Menrva
(20.1°N 87.2°W)

23. Various Geological Features
● Lopes et al. . (2012)
– Hot cross bun from faults (38.5N, 203W)
● Stofan et al. (2008)
– South pole complex surface morphology (T39)
● Wood (2011)
– Caldera/Maar volcanism at the poles (?)
● More features recorded here:
https://en.wikipedia.org/wiki/List_of_geological_features_on_Titan
Maar: phreatovolcanism, explosive, often making lakes on Earth
Arcūs, Faculae, Fluctūs, Flumina, Insulae, Labyrinthi,
Large ring features, Maculae, Montes, Planitia, etc.

24. Conclusions
● SAR imaging on Cassini probe
● BIDR products explore surface of Titan
● Methane & Ethane circulation
● Eolian, evaporative, rain processes
● Seas, lakes, dunes, geomorphology
● Strange, fascinating World at 92 Kelvin

25. Thank you
Credit for half of the images:
NASA
clicops.org
& collaborators