N456ORB.TXT - Format of Nimbus 4, 5, and 6 Orbit files.
This document has been adapted from N456_ORBIT_TAPES.DOC which was
produced when the original magnetic tapes were copied to optical disk.
OVERALL STRUCTURE
-----------------
* The original data has now been merged into one file for each experiment.
* Data was originally stored in 12 bit 'words' and now occupies the 12
least significant bits of a 16 bit word. - Data range is 0 - 4095.
* The byte ordering within 16 bit integers follows the dec convention of
least significant byte first on the "tape".
DATA RECORD
-----------
WORD FUNCTION
0 3654 (7106 Octal) Sync Code repeated twice per block/record
1 3654 " " " " " " " "
2 Block Length (Total number of words in block, all inclusive)
3 Block #
4 Identifier See note (1)
5,6 Orbit # See note (2)
7 Northbound equatorial longitude (deg East*8) See note (3)
8 Southbound equatorial longitude (deg East*8) " " "
9 Nominal day of data See note (4)
10 Nominal year of data " " "
11 Number of channels (NCHANS)
12-35 Channel numbers See note (5)
36-76 Northbound data for 1st channel - 80S, 76S, ..., 80N
77-117 Southbound data for 1st channel - 80N, 76N, ..., 80S
118-199 Next channel
* The above format is repeated for each of the NCHANS channels.
36 + NCHANS*82 2321 End of block marker (EOB)
37 + NCHANS*82 Checksum
Zero in the data array means no (or bad) data.
NOTES
-----
Each file contains one block for each orbit of the satellite. This block
contains data at 4 degree latitude crossings, starting at 80 degrees South and
going around in time order to 80 degrees South. Every orbit is present in the
"tape" although a proportion will contain no data because of the 'blind' orbits.
- The use of the word 'orbit' here strictly means exactly one orbit from 80
degrees South to 80 degrees South. It should not be confused with the use of
'orbit' by the Nimbus project where it can mean the data returned during a
play-back, which can begin and end at arbitrary latitudes.
(1) IDENTIFIERS
-----------
The fifth word of every block containing orbit data is 470 decimal.
(2) ORBIT NUMBER
------------
This can take a value up to 32,768 ( = 15 bits).
WORD: 6 BITS: 0-2 most significant bit (MSB)
7 0-11 least significant bit (LSB)
The orbit number used here is that appearing in the Nimbus equatorial
crossing lists for the northbound (day) side. Note that frame-by-frame data are
often labelled with an orbit number which refers to the orbit when the tape-
recorder is replayed (sometime several orbits after acquisition) but this
definition has no bearing on what is given here.
(4) NOMINAL DATE
------------
The nominal day and year refer to the nominal date for which the data were
processed. This may be up to one different from the actual date since up to
about two orbits either side of the orbits for a particular day were processed
with that day's data. Where a block contains no data, the nominal date is set
to 0 0.
(5) CHANNEL NUMBERS
---------------
Spare slots are set to zero.
(3) EQUATORIAL CROSSING LONGITUDES
------------------------------
Range of values: 0 - 8*360 degrees.
Given that successive orbits are about 26 degrees further west, the
southbound equatorial crossing longitude, which occurs half an orbit after the
northbound equatorial crossing longitude, will be about (180 - 13) = 167 degrees
east of it.
If required, the longitude decrement per orbit should be determined by
inspecting individual pairs of orbits, or from listings published by the
Nimbus project. This decrement changes with time for a given satellite,
particularly Nimbus 6 where gas ejected from the LRIR (Limb Radiance Inversion
Radiometer) during the first eight months affected the orbital dynamics.
Scaling Factors
---------------
The radiance unit is (mW/m^2/ster/cm^-1).
All radiance channels are multiplied by 16, except for the Nimbus 5 SCR
channel C4D which is multiplied by 20.
For the Nimbus 6 PMR, the eigenfunction coefficients are scaled as follows:
1st - * (16/4.8) (i.e. multiply by (4.8/16) to bring to values, in
radiance units, compatible with the first eigenfunction
vector, or multiply by (4.8/16)/sqrt(59) to bring to the
mean radiance over the scan)
2nd - 6th - * (16/2.4) + 2048 (i.e. subtract 2048 and multiply by (2.4/16)
to bring to values of coefficients
compatible with the eigenfunction vectors)