PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT NOTE = "Known errors and/or anomalies in the volumes" PUBLICATION_DATE = 2009-11-30 END_OBJECT = TEXT END ERRORS AND/OR ANOMALIES IN THE CURRENT VOLUME Volume CORADR_0143, Version 02 ------------------------------ 1. The Cassini Radar Transition file (EXTRAS/CRT_143_V02.TAB) contains no information about ScanStart and ScanEnd transitions. 2. The uncompressed LBDR and BIDR products have attached labels. Normally, a file that has been compressed with ZIP would have been generated without an attached label. 3. In the volume index table (INDEX/INDEX.TAB), double quotes enclose all the date/time values. Normally, PDS date/time values are not quoted, but quoting makes parsing easier for some applications. 4. The HTML documents in the DOCUMENT directory contain HTML character codes that, while all-ASCII, are not easily interpretable by someone who is reading the HTML documents as text documents. For example, "α" represents the lower-case Greek character "alpha" and is rendered as such in a web browser. Equivalent and more legible character codes (e.g., "&#alpha;") are available as of the HTML 4.0 specification but cannot be used here, as PDS requires HTML documents to comply to the HTML 3.2 specification. 5. Antenna temperature, brightness temperature, and receiver temperature are defined in Janssen, M. A., "An Introduction to the Passive Microwave Remote Sensing of Atmospheres," Chapter 1 in Atmospheric Remote Sensing by Microwave Radiometry, (M. Janssen, ed.), pp. 1-35, Wiley & Sons, New York (1993). The archived value in the SBDR and LBDR files gives uncalibrated antenna temperature in units of Kelvin. The best current algorithm for correcting the archived antenna temperatures is Ta_corrected = Ta_archive * ( 0.920 - 0.0041*( t - 1.90 ) ) where t = time in years and fractional years since 2004.0 (0 UTC on 1 Jan 2004) This algorithm is based on the radioastronomical flux scale at 2-cm wavelength by direct comparison of distant (unresolved) Titan measurements by the Cassini radiometer with VLA measurements of Titan reported by Butler and Gurwell, 2004. This algorithm will improve with time as more distant Titan measurements are obtained and as more radioastronomical comparison sources are included. Butler, B. J., Gurwell, M. A. 2004. Radio Wavelength Observations of Titan with the VLA. Bull. Am. Astron. Soc. 36, 6.04. This algorithm applies also to all preceeding volumes. 6. Ideally the calibrated antenna temperature is referenced to cold sky at 2.7 K, although no guarantee is made that this zero-level accounting has been correctly made. Also, the antenna temperature is not necessarily the brightness temperature seen in the main beam; e.g., there will be an offset if the sidelobes happen to fall on other than cold sky. The antenna pattern has low but significant gain at large angles from the boresight which can introduce thermal power from other targets besides the intended target in the main lobe. In particular, there is an offset to be expected and accounted for when an extended source like Titan or Saturn is observed from a close distance. System gain is the quantity that multiplies the raw sky counts to convert to the uncalibrated Kelvin scale. Receiver_temp is the receiver noise temperature Tr (comparison made at internal reference switch). The receiver temperature plus the antenna temperature is equal to the total signal (raw counts times system gain). 7. ant_temp_std is a measure of the rms uncertainty of Ta, and is only an estimation. It is obtained as the standard deviation of Ta for three points, Ta(I-1), Ta(I), and Ta(I+1), and is a useful measure that identifies questionable data. For example, it gets large when the beam is sweeping across a brightness discontinuity. 8. All BIDRs except for the byte-valued backscatter images (BIB*.IMG) were produced by JPL. The byte-valued backscatter images were produced by USGS. The USGS BIDRs have unit strings in their labels which are all CAPS. The other BIDRs have lower case units. According to the PDS dictionary, both are acceptable. 9. Checksums were not computed for floating point valued BIDRs. The CHECKSUM keyword was assigned a zero value. 10. In the primary BIDR images a technique was used to remove systematic errors due to thermal noise and BAQ compression during downlink. The technique appears to have a slight negative bias for very low SNR, i.e. when the signal power is less than one tenth the thermal noise power. Without noise subtraction, backscatter values are artificially high and always positive. With noise subtraction, negative values occur both due to the aforementioned bias and due to residual random error. Byte-valued BIDRs are produced by transforming all data to Decibels by taking the logarithm and then multiplying by 10. Data is clipped below a minimum value (typically -20 dB) determined by the OFFSET keyword in the attached label. All negative backscatter values or value belows 0.01 (-20 dB)are assigned to the minimum pixel value. Anyone interested in viewing data below this threshold needs to use the REAL-VALUED backplanes. 11. Geolocation of the SAR imagery is performed on a sphere of radius 718 km. The actual shape of Iapetus is slighlty non-spherical with deviations up to 35 km from the nominal sphere. Geolocation for each pixel is performed by intersecting the equi-doppler cone, a sphere of radius equal to the range centered at the spacecraft location, and the spherical surface of Iapetus. Location errors of approximately 40 km can occur due to the non-spherical shape of Iapetus. Spacecraft ephemeris information in the SBDR's and LBDR's along with the correct shape model of Iapetus could be used to correct the geolocation. 12. Altimetry data was acquired but no ABDR products were generated and no height data is included in the SBDR and LBDR products. Raw altimeter echo data is available in the LBDR. 13. The incidence angle corrected (BIF*.IMG) BIDR file is not included because no reliable relationship between incidence angle and radar bacscatter has been determined for Iapetus. 14. Note that the resolution of these BIDR files is 16 pixels/degree unlike Titan flybys which are typically 256 pixels/degree and 128 pixels/degree. ERRORS AND/OR ANOMALIES IN PREVIOUS VOLUMES Not Applicable