Cassini Mission- Radiometry Scans of Saturn Rings

- Akhilesh Mishra

Outline

• About the mission • Composition of Saturn Rings• Cassini RADAR radiometer• Microwave Passive Radiometry of

the Saturn rings

Cassini – Huygens

• Launched in October 15, 1997, three missions till now.

• Named for two 17th-century scientists, Giovanni Domenico Cassini of Italy and Christiaan Huygens of Holland

• 12 instruments – CIRS, RADAR, ISS • RADAR- SAR, Altimeter and Radiometer at

13.78 GHz • Huygens probe- 6 intruments ,

landed on Titan on Jan 14, 2005

Saturn Rings Ring features

C ring17500 km

B ring25 580 km

A ring14 580 km

NASA/JPL/Space Science Institute

Cass

ini

divi

sion

65 700 km

F rin

g

NASA/JPL/University of Colorado

•  Composed of 99.9 percent pure water ice with a smattering of impurities

• Thickness between 10m to 1km• Particle size – 1cm to 10m• Rings have a sparse atmosphere consisting

O2, H2

Cassini Radiometer• RADAR instrument operates in burst mode- active and passive

segments• Dickie Switching Technique- Microwave switch to select sky signal

and internal reference blackbody signal. • Durations of burst cycles variable• Radiometer segment extended to 1s with 25ms integration over

reference load. • Key Challenges- Large beamwidth compared to resolution of

imaging instruments, gain and baseline drift, sidelobe contribution , large number of obsevations

Calibration

• The calibration factor to convert output voltage into corresponding Brightness Temp in K.

• Scale is linear as it lies in the Rayleigh Jeans Approximation regime for the Planck function.

• Low temp ref- Cold sky (2.7K) and High temp ref- internal load- Baseline drift compensated - Gain variations and due to instrument warm up and solar

ilumination compensated• Measurements from Titan used as reference, comparing

measurements with Huygens data (physical temperature of surface 93.7 K)

-Side lobes error signal (30% of main beam) were removed • Slow linear drift of gain due to ageing of receiver components.

Radiative Transfer calculations-• Based on Juno Microwave Radiative Transfer (JAMRT) program. • More complex for Saturn as compared to its rings(NH3 absorbtion )• Saturn Rings are extended targets.• Optical thickness of rings much greater and variable than Saturn’s

atmosphere.

Fig1- Antenna Radiation Pattern ..[1]

Fig-2 Initial Steps in the calibration process illustrated for the Dec 2009 map. Each 1s measurement from this observing campaign is plotted as a single point in all panel. Top to bottom the panels show respectively- (a) the time- ordered data with an initial scale calibration but without a baseline correction (b) Model convolution s over the reference model to obtain estimates of the sidelobe contributions (c) A derived zero offset correction (d) The final calibrated and baseline adjusted antenna temperatures. .. [3]

Fig3- Cassini RADAR high(A) and low(B) resolution scans of Saturn’s rings. ..[4]

Fig4- Calibrated antenna temperatures (A) and initially processed brightness temperatures for a low resolution rings observation(B) ..[4]

Fig 5- Processed brightness temperatures for high resolution scans acquired at 20° (left ) and 4° (right) ring opening angles. ..[4]

References 1. West, Anderson, Boehmer, Borgarelli, Callahan, Elachi, Yonggyu Gim,

Hamilton, Hensley, Janssen, Johnson, Kelleher, Lorenz, Ostro, Roth, Shaffer, Stiles, Wall, Wye, and Zebker. "Cassini RADAR Sequence Planning and Instrument Performance." IEEE Transactions on Geoscience and Remote Sensing 47.6 (2009): 1777-795. Web.

2. “Cassini Solstice Mission”, http://saturn.jpl.nasa.gov/3. M.A. Janssen, A.P. Ingersoll, M.D. Allison , S.Gulkis, A.L. Laraia, K.H. Baines

, S.G. Edgington , Y.Z. Anderson . “Saturn’s thermal emissionat 2.2 cm wavelength as imaged by the Cassini RADAR radiometer.” Icarus – Elsevier

4. Z.Zhang, A.G> Hayes, M.A. Janssen, J.N . Cuzzi , P.D. Nicholson “Observing Saturn’s Rings in the Microwave with Cassini”. Vol.8 EPSC2013- 126, 2013

5. Alberto Flanders, Linda Spiker, Ryuji Morishima, Stuart Pilorz, Xedric Leyrat, Nicolas Altobelli, Shawn Brooks, Scott G. Edgington “Brightness of Saturn’s rings with decreasing solar elevation.” Planetry and Space Science - Elsevier