Cassini RADAR Earth Swingby Page

Last modified 10/5/99

under construction!

Groundtrack, Incidence Angle Profile, and preliminary processed radiometer and backscatter data . Time ticks are estimated beam center footprints marked in seconds after closest approach. NB these are predicts based on ephemeris prior to final course corrections. Thanks to my immediate contacts Oti Liepack, Richard West and Mahta Moghaddam (JPL) for reducing the data : but recognize that the whole engineering team worked very hard for this!

Submitted AGU abstract


Ralph Lorenz1, Charles Elachi2, The Cassini Radar Science Team2

1 LPL, University of Arizona 2 Jet Propulsion Laboratory, Pasadena, CA 91109

On 18 August 1999, the Cassini RADAR instrument observed the Pacific Ocean and South America during the spacecraft flyby of the Earth. The observations comprise passive radiometry and a modified scatterometer mode, both at Ku band (13.8 GHz). The instrument operated as planned and several features are apparent in the data, notably the coastal transition between land and sea, perhaps prophetic of observations to be made of Saturn's satellite Titan which may have seas of liquid hydrocarbons. As the beam footprint crossed onto land, the microwave brightness temperature increased abruptly from the low-emissivity ocean surface to the much higher value over land. Variations in brightness temperature were seen corresponding to the colder, elevated terrain of the Andes compared with the plains to the East. The scatterometer also showed an increase in backscatter on crossing onto land. A double-peaked backscatter later in the traverse - also seen (inverted) in the radiometer data - may be related to the Brazilian highlands. Further analysis of the data, and correlation with other datasets (high resolution topography, NSCAT backscatter etc.) is underway.

Comparison of Cassini 13.8 GHz radiometry with SSMI data at 19 GHz

Sea:Land transition is obvious (ESB data is shifted by 17 seconds to make the curves line up : subsequent pointing reconstruction should fix this. Absolute calibration is preliminary - data for the ocean appears to lie between the 19GHz horizontal and vertical polarizations.

Interesting that the apparent change in slope at 680s is apparent in the SSMI data, and so is probably a real ocean circulation effect.

Over land the Cassini data is consistently cooler than for SSMI - possibly a wavelength dependence, or calibration offset. Note the drip at around 830 seconds - the Altiplano. The broad peak in the SSMI data may indicate a pointing reconstruction deficiency - the beam was in fact looking at a higher (cooler) region than the predictions (and the corresponding SSMI observations) indicated.

Double dip at 1100 seconds is well-resolved and correlates with the Brazilian highlands. Note that this feature, and the western edge of the Altiplano are more subdued in the Cassini data because the Cassini footprint (1/200 radians at 10000km = 50km/cos(incidence)) is rather larger than the 0.33 degree = 35km resolution of SSMI.
Ralph - note to myself - try this with a smoothed version of the SSMI data

Thanks to Ralph Ferraro of NOAA for providing the comparison dataset (Ascending orbit (730pm LST) 0.33 degree brightness temperature : average of the period 15-21 August) see NOAA/SSMI page for details

Comparison of Cassini 13.8 GHz Backscatter with Seasat data

Seasat Altimeter/Scatterometer (SASS) 13.5 GHz geocoded product from Brigham Young University was sampled with the predicted instrument pointing to generate this comparison. This is a 1/8 degree product (14km), at 40 degree incidence.

(I will try to obtain the corresponding NSCAT product)

The SASS product is set to zero over the sea, so has an artificially sharp edge. I cannot yet explain the apparent timing discrepancy. (And this is after the 17s shift - see above). The peak at 820s is striking - I'll try and find out what it is.

It is encouraging that the backscatter is in general within a couple of dB of what it should be. As we move along the groundtrack, the SASS data shows a fall in backscatter - presumably due to smoother terrain (at 40 deg) - consistent with the increase in CASSINI backscatter at lower incidence.

The double peak (see also the radiometry data) at 1100s is interesting, and apparently does not appear in the SASS data (although NB Cassini was only 10 degrees from vertical at this point). The spike at 1150km is interesting (I am tempted to ascribe it to a city! - will need the final pointing to determine this) Also, most city development will have taken place since Seasat (1978)

Some light may be shed on this by examining other datasets - the landcover type derived from Normalized Difference Vegetation Index (NDVI) indicates a change in vegetation roughly at this double peak

NDVI Landcover type (see also map below)
6 is wooded grassland
8 is bare ground
9 is shrubs and bare ground
11 is broadlead deciduous forest and woodland

The corresponding topography profile from the ETOPO5 5-degree (9km) digital Elevation Model is shown below

Other DataSets of this region

Seasat Backscatter (45 deg incidence) Map

NSCAT Backscatter (45 deg incidence)

NB this is an image, not a geocoded product - need to get that

SSMI 19 GHz Radiometry Map

EOPO5 Digital Elevation Model (Available from NOAA This is square root of altitude

whereas this version has a wraparound greyscale - indicates surface slopes/roughness

The 1 degree gridded landcover map (below) is from Devries and Townshend

check out more info on the Cassini Home Page and the Cassini Radar Page