* Data Set OverviewFull resolution basic image data records (BIDR) consist of SAR(synthetic aperture radar) normalized backscatter data (sigma0) at 13.78GHz (Ku band) acquired in one pass of the Cassini spacecraft near Titan.The primary image data and the additional 'backplanes' of geometry andbeam mask are provided at 256 pixels per degree on Titan's surface(approximately 175 m per pixel).The USGS will produce 128-pixel/degree versions of all the floatingpoint BIDR images (backplanes and primary image). USGS will alsoproduce 8-bit versions of primary (incidence angle corrected sigma0)image BIDRs at 256, 128, 32, 8, and 2 pixel per degree resolutions.The primary image data and the backplanes are provided on a rectangulargrid which is an oblique cylindrical coordinate system whose equatoris the nadir track of the spacecraft on the current pass and the zeroof longitude is the point of closest approach. Two of the backplanesare longitude and latitude in standard Titan coordinates.A particular index pair specifying a location in the data grid providesdata for the same point for each full resolution file and similarly forthe averaged 128 pixel per degree files (primary image and backplanes).For more information about the format and content of the BIDR files,see the Cassini Radar Basic Image Data Record Software InterfaceSpecification, JPL D-27889. A copy of the document is located onthis volume as file BIDRSIS.PDF in the DOCUMENT directory.* ParametersThe original image data and the 128 pixel per degree data are floating-point values of sigma0 in linear units (not dB). Lower resolution imagesare 1-byte scaled values in dB.The data are (1) Primary dataset (e.g., incidence angle corrected sigma0) in 32-bit floating-point format (linear scale values, not in dB). (2) Sigma0 without incidence angle correction in 32-bit floating point format (linear scale values, not in dB). (3) Incidence angle, floating point values in degrees. (4) Latitude, floating point values in degrees. (5) Longitude (positive-west), floating point values in degrees. (6) Beam mask, 8 bit values. (7) Number of looks, 32 bit integer values.* ProcessingThe 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 files will be converted into 8-bit formats at various resolutions bymembers of the Astrogeology Team of the U. S. Geological Survey (USGS) inFlagstaff, Arizona. The processing used to produce the 8-bit formats isthe same as that done for the Cassini Radar Digital Map products asdescribed in the Cassini Radar Digital Map Software InterfaceSpecification.* DataSee Parameters.* Ancillary DataNo ancillary data beyond the collection of BIDR files is needed tointerpret the images.* Coordinate SystemThe oblique cylindrical coordinate system is fully described in thedata set map projection catalog DSMAP.CAT, which is located in theCATALOG directory on this volume.* SoftwareNo software is provided within this volume. Cassini BIDR files can be readusing PDS image handling software or by using the Integrated Software forImagers and Spectrometers (ISIS) developed by the Astrogeology Team of theU.S. Geological Survey. ISIS may be of use for scientists wishing toread, display, manipulate, and analyze BIDR data. ISIS including C andFORTRAN source code is in the public domain and versions configured to rununder Solaris or Red Hat Linux may be obtained by qualified researchersfrom (http://astrogeology.wr.usgs.gov/Projects/ISIS). Documentation forthe system is available from the same website.* Media/FormatThe 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 OverviewThe BIDR data set contains synthetic aperture radar (SAR) images andcoregistered backplanes of geometry and beam mask. (See DATA_SET_DESCbelow.) The data has been calibrated and empirical range and dopplercorrections have been applied in order to minimize image artifacts dueto errors in pointing and emphemeris knowledge. The images are elongatedstrips. The long (along track) dimension corresponds to the motion of thespacecraft during the observation. The short (cross track) dimensioncorresponds to the combined width of the 5 radar beams.* ReviewBIDR data will be validated internally by the Cassini Radar Team priorto each release of data to the PDS. The overall data set organizationwill also be peer reviewed once by the PDS prior to the release of thefirst volume.* Data Coverage and QualityCurrently 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. The BIDRdata set contains SAR image data only. Other Cassini RADAR data suchas altimetry, scatterometry, and radiometry are available in the SBDR,LBDR, and ABDR data sets.The SAR image pixel values are normalized radar cross-section, a unitlessquantity related to the slope, roughness, and composition of the observedsurface. We currently estimate the accuracy of this quantity to be + or -3 dB absolute and + or - 2 dB relative for the majority of image pixels.(See image artifacts discussion in the Limitations section.) The fullresolution BIDRs are oversampled to insure no loss of information content.The intrinsic spatial resolution of the data varies from 300 m for theportion of the swath obtained at closest approach to 1 - 2 km at theextreme ends of the swath. Location error of the pixels is dominatedby ephemeris and pointing knowledge errors and is expected to be lessthan 5 km throughout the mission.* LimitationsEphemeris 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.