* Data Set Overview The SARTopo file contains surface height estimates in profiles betweentwo SAR beams. Each row in the file contains height estimates and ancillarydata that correspond to a set of pixels in the BIDR image. For more information about the format and content of the SARTopo files,see the Cassini Radar Burst Ordered Data Product (BIDR) SoftwareInterface Specification, JPL D-27899. A copy of the document is located onthis volume as file BIDRSIS.PDF in the DOCUMENT directory.  * Parameters A complete listing of the parameters can be found in the SIS.  * Processing The encoded raw active mode data in the Burst Ordered Data Product(LBDR) files is decoded and processed into SAR images by the CassiniRADAR Instrument Team. Each of the five beams is treated separatelyby typical SAR processing (azimuth and range compression). The imagestrip is then produced by selecting the best beam for each pixel. Theselection is performed by choosing the beam with the most looks. Inthe event of a tie the highest SNR beam is chosen. The pixel values in the primary BIDR images will be normalizedbackscatter cross-section values corrected for incidence angle effects.The incidence angle correction algorithm is TBD and will be determinedjointly by the Cassini RADAR Instrument and Science Teams after enoughdata is acquired to estimate the variation of sigma0 due to incidenceangle over the surface of Titan. Until the final correction algorithm isdetermined, the primary BIDR image will be computed with a preliminaryincidence angle correction. The exact form of this correction can befound in the PDS label for the primary BIDR image file. These BIDR files are used to generate SARTopo files (CSV format) thatcontain height estimates of Titan in profiles between two SAR beamswithin a single SAR image. Each row in the file contains height estimatesand ancillary data that correspond to a set of pixels in the BIDR image.A seperate SARTopo file is produced for each SAR beam overlap profile.The SARTopo implementation yields 1-5 profiles in each SAR pass that are10 km by 1000s of km long.   * Data See Parameters.  * Ancillary Data There are no ancillary data needed to use the SARTopo files.  * Coordinate System The oblique cylindrical coordinate system is fully described in thedata set map projection catalog DSMAP.CAT, which is located in theCATALOG directory on this volume.  * Software No software is provided within this volume.  * Media/Format The data are provided on media as determined by PDS. The BIDR data filesare ZIPPED as described in the PDS standard. Detached labels are providedfor the ZIPPED files; the ZIPPED files also include their attached labels.Detailed formats are defined within the attached labels of the files andin the Cassini Radar Basic Image Data Record Software InterfaceSpecification.

* Confidence Level Overview The SARTopo data product has been produced using BIDR data sets that havebeen calibrated. Empirical range and doppler corrections have been applied toBIDR product being used in order to minimize image artifacts due to errors inpointing and emphemeris knowledge.  * Review SARTopo data will be validated internally by the Cassini Radar Team prior toeach release of data to the PDS. The overall data set organization willalso be peer reviewed once by the PDS prior to the release of the firstvolume. * Data Coverage and Quality Currently 20 SAR observations comprising roughly 20% of the surface ofTitan are planned for the nominal Cassini mission. BIDR data will beobtained for each such observation. SAR observations of Titan occurat irregular intervals throughout the mission. Each observation isnominally 40 minutes long. Shorter-duration cases occur whenever RADARshares a Titan observation with another Cassini instrument. SARtopo datawill be available for observations that have BIDR data. The SARtopo file contains estimated surface heights (and relatedmeasurements) of Titan in profiles between two SAR beams within a singleSAR image. SARTopo files will only be produced for Titan flybys that have SAR imaging. * Limitations Ephemeris error is expected to improve throughout the mission. Sincethere is no plan to recompute the ephemeris of previous observations asnew measurements are obtained, however, earlier observations may havepoorer location accuracy. The dominant calibration error term is error in our knowledge of thegain of the attenuators in the receiver. Engineering tests are currentlyplanned to improve our knowledge of the attenuator gains. SAR images are prone to artifacts such as: 1) poor calibrations at the edge of the beams2) extreme noisiness in regions with a small number of overlapping measurements (looks)3) scalloping and antenna calibration misalignment due to errors in ephemeris and pointing knowledge4) distorted imagery near the ends of the swath due to non-optimal pointing while turning to make altimeter measurements. Artifacts of type 2 and 4 are unavoidable. Types 1 and 3 can be minimizedby empirically correcting range and doppler biases in the data. We havesought to minimize the occurrence of these artifacts in the BIDR images,but there are still noticeable seams between beams and poor calibrationnear the edges of the swath. The artifact minimization algorithm does notattempt to use the estimated range and doppler biases to correct ephemerisor pointing. For this reason location errors of a few kilometers arepossible. The TA Titan Observation on October 25, 2004, was the first SARobservation. It has some special features. The first (earliest) thirdof the swath has noticeable scalloping in the along track dimension andis noisier than the rest of the image. This portion of the swath hasnon-optimal viewing geometry and a small number of looks due to pointingconstraints for other Cassini instruments. A substantial portion of thelast (latest) third of the swath is in low resolution SAR mode. Thismode coarsens the cross track resolution of the image by a factor of 2.It was used on TA in order to accommodate conservative SNR expectations.In the same region of the image, an experiment was performed in which thedata compression technique typically used by SAR was turned off. Due todata volume and data rate constraints this resulted in fewer looks andnoisier data.