Cassini Radar

Exploring Titan

Contents

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

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)

The Cassini Radar System

High-GainAntenna (HGA)

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 kmMastrogiuseppe (2014)

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)

Types of RADAR modes

From Thesis of Lauren Wye (2011)

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

The monostatic radar equationIf 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 = P

r − P

n

Ps received echo signal power

Pr total received power

Pn mean noise power

From Wye (2011)

With the effective antenna aperture Ar = λ2Gt / 4π

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)

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)

Pulses

● N ~ 50 for SAR● Interpulse separation

– Not possible at high incidence angle

– Impacts noise modeling → uncertainty

From Wye (2011)

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

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

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

Ligeia Mare

Kraken Mare

Northern Lakes

PungaMare

T91

Kraken Mare

Small lakes

Kraken Mare

Mývatn Lacus

Oneida Lacus

Waikare Lacus

T91 Fly-By ISIS-GRASS

BIDR scales (F&D)

Kraken Mare main Peninsula

Ligeia Mare

Mechanical erosion(Not chemical)

Black et al (2012)

Ligeia Mare IslandMare 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)

Sub Equatorial Dunes

13% of Titan

Matured features 1.3km width 2.7km crest spacing

Hydrocarbon chains>1m/s saltationEquinox weather

Savage et al, (2014)

T95 Fly-ByISIS-GRASS

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)

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.

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

Thank you

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