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,
"&#945;" 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





