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
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.
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
(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
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
