PDS_VERSION_ID     = PDS3

RECORD_TYPE        = STREAM
OBJECT             = TEXT
  NOTE             = "Known errors and/or anomalies in the volumes"
  PUBLICATION_DATE = 2018-07-12
END_OBJECT         = TEXT
END

ERRORS AND/OR ANOMALIES IN THE CURRENT VOLUME

Volume CORADR_0285, Version 02
------------------------------

1.  The Cassini Radar Transition file (EXTRAS/CRT_285_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 defined for this application
as just the average brightness temperature in the measured beam out to 2
degrees from the beam axis, and does not allow for possible contributions from
the far sidelobes (sidelobes outside of 2 degreees).  The archived value will
include an additional contribution if the  far sidelobes happen to fall on 
other than cold sky.   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.  A more detailed explanation will be found in Janssen et al.,
2009. 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.The submitted volume CORADR_0143 has an erroneous INDEX and CUMINDEX entry
for BIFQE10N073_D143_I049S01_V02. 

12.Every volume from CORADR_0144 to CORADR_0229 has missing CUMINDEX entries 
for Iapetus_049_3.  

13. There are some invalid resistive load counts at the very beginning which
result in incorrect antenna temperatures.

14. Volumes CORADR_0265 and CORADR_0266 have missing CUMINDEX entries for 
COARDR_0035_V03.

15. Even though radiometry measurements were made using all 5 beams during
regular SAR segments, only beam 3 brightness temperature results are
archived due to calibration challenges of the other four beams.

16. Because the LBDR product was longer than 2 GB, the product has been 
split into two files.  The filenames and product ID's have an extra 
three characters (_P1 or _P2) to denote part 1 and part 2. This does not
conform to the SIS. 

17. Even though the ABDR product's size did not exceed 2 GB, the ABDR file
was split into 2 to provide results from 2 different altimetry modes. ABDR
file 1 contains altimetry results collected in auto-gain mode and ABDR
file 2 contains altimetry results collected with a fixed attenuator setting.

18. Usually calibrated backscatter for beams 1,2,4 and 5 are computed by 
JPL and beam 3 is computed by Stanford University. For this observation, 
beam 3 calibrated backscatter during times -138 mins to -71 mins and 
+90 mins to +155 mins from closest approach is computed by JPL.

19. The rot_velocity_target_y field is incorrectly assigned in all Cassini
RADAR SDBR, ABDR, and LBDR products, but the angular velocity vector of the
spacecraft can be computed independently from the time series of the 3 
spacecraft body frame axes, sc_X_target, sc_Y_target, and sc_Z_target.

20. The Beammask and Number of Looks BIDR backplanes have a value for 
SAMPLE_TYPE="UNSIGNED INTEGER" instead of "UNSIGNED_INTEGER". This field 
needs to be fixed by the user before being ingested by ISIS software.

21. The AAREADME file that is included as part of each PDS volume
has a section addressing the version number of all the data products.
The AAREADME file for flybys TA-T30 had erroneously reported
that the volumes with PRODUCT_VERSION_ID 02 for flybys T36-T77
represent products processed with the non-synchronous pole, instead
of reporting as being processed by the synchronous pole. This
comment is corrected in the AAREADME files in later flybys.


ERRORS AND/OR ANOMALIES IN PREVIOUS VOLUMES

1. The last field of a few records in the ABDR SUMMARY file 
corresponding to Flyby T8 and T91 have unusually large numbers. These 
erroneous values will not be fixed.Instead, the user can exclude these 
data points while using results from the ABDR SUMMARY file.