2.3.5 ISPM LAN File Trailer Definition

Please Refer to the Appendix for JPL SIS Reference, July 27, 1991. The following section shows a memo to Harry Woo at the Jet Propulsion Lab, from TPA.

Subject: Algorithm for extraction of ISPM data pool items from the LAN telemetry.

By negotiation of Lanzerotti with the SWT, the LAN contribution to the data pool will consist of 6 particle fluxes. These are to be derived from time averages of the decompressed count rates which appear in the LAN data stream. The flux is obtained by multiplying the time-averaged count rate by a conversion factor which we provide here. We reserve the right to adjust the conversion factors based on flight experience, so the data pool generating software should be constructed to allow for easy modification of these numbers.

Algorithm to Obtain Data Pool Items from the LAN Telemetry Stream
(Revised 1/16/84, T.P. Armstrong, add missing data handling)

The count rates required appear in a block of 478 bytes of Rate and CA information which is repeated 5 times during the LAN data cycle (4 formats). The block of 478 bytes contains repetitions of the data pool rates at the following offsets from its beginning (on a 0-477 byte basis):

The 5 repetitions of the Rate and CA block within the 4 format LAN data cycle are located as follows:

Note: Byte counts are on a 0-639 basis for the 640 bytes per format of LAN information in the telemetry stream. Each occurrence of a log compressed count rate is one byte. The decompressed rate is obtained by

Rate = (16+E*8+F*4+G*2+H)*2**(8*A+4*B+2*C+D-1)

where the compressed byte was ABCDEFGH. If the compressed byte has a value between 00 and 0F (HEX), no decompression is necessary--the value stands.

Data validity: At the beginning of a run when the LAN experiment may have been powered down, it is necessary to wait for 12 formats after the power on flags show "1" for valid data. The power on flags are in:

Byte 2, bit 0
Byte 4, bits 4,5,6    All formats.

Also to be checked are the valid data group flags in the status trailer (bytes 636-637 of format 3).

These flags must show "0" for valid data:

(bit counts within bytes given on 0-7 basis with 0 as the most significant bit)
(byte counts within formats given on a 0-639 basis)
(format cycle runs 0,1,2,3)

Missing data

Power on flags. When these are not present in the data stream, it should be assumed that they have not changed from the last valid occurrence.

Status trailer flag

When one of these is not present in the data stream, it should be assumed that it is invalid. This would result in the elimination of the corresponding data repetition group from the pool algorithm average.

Data group repetition

When any part of a repetition is missing from the data stream, the entire repetition should be flushed (all channels).

Next: Section 2.3.6 ISPM LAN File Trailer Definition