4.6 Validity Checks of the LAN Data Stream

 

Document source: R. E. Gold (LAN-082-85)
Document date: January 21, 1985

 

Processing the LAN data received on the Experiment Data Record (EDR) tapes from JPL must include validity checks as one of the first operations. Quality checks of the data reception and decoding by JPL are included in the EDR headers. Quality checks of the LAN data itself are included in the status preamble and status trailer words of the LAN logical record.

 

The status preamble words generally refer to the state of the spacecraft and the LAN subsystems. Changes in these status words usually affect an entire 4-format logical record. The status trailer words generally report spin-by-spin data quality.

 

The LAN data stream must be validated first on a 4-format basis, followed by a spin-by-spin validation within a 4-format period. The following order is suggested for the data reduction algorithm.

 

I. 4-Format Validity Checks

 

1. Power-on Telltale - This bit must be on during format zero and must remain on for valid data. Following the first appearance of the power-on bit, the rate and pulse height data are guaranteed valid after two 4-format pulses have passed (MFSA data after 3 4-format pulses).

If the power-on bit disappears, indicating LAN has been turned off, the data will be valid through the last full spin pair prior to the last format which had a power-on bit.

 

2. ROM/RAM Mode - This bit shows whether the standard internal data processing program is operating. Data are not valid following a switch to RAM mode. After switching back to the ROM mode, validity is assured after the second 4-format pulse (third pulse for MFSA data).

 

3. Power-Down Telltale - Data are valid through the last full spin pair prior to the last format with valid power-up bit. Rate and PHA data are valid following power-up only after the appropriate subsystem power bits have been on for at least one full format (i.e., starting at the second appearance of the subsystem power-on bit). MFSA data will be valid after the second 4-format pulse at which both PHA power-on bit and the power bit for the appropriate analog subsystem have appeared. Rate and pulse height data should only be used in full spin pairs.

 

4. Reset Telltale - This bit indicates reinitialization of the LAN microprocessor. Data validity conditions after a reset are the same as those for the reappearance of the Power-On Telltale.

 

5. Node Change Bit - This bit is set when the spacecraft bit rate changes. Rate and PHA data will be valid after the second subsequent 4-format pulse (third pulse for MFSA).

 

II. Spin Validity Checks

 

1. Valid Data Bits, Spin Groups 1-10 - A value of one indicates that the corresponding spin group has invalid rate and PHA data. The rate and pulse height data should be accepted only by spin pairs. MFSA data for the 4-format period is invalid when any spin group is invalid. Remember that this MFSA data is read out during the next 4-format period.

 

2. Sectoring Mode Bits, Spin Groups 1-10 - These bits indicate whether the data were accumulated in sun-sectored or time-sectored mode. Time sectored data do not correspond to fixed positions in space and care should be taken when summing these data.

 

Data should be considered invalid in the spin group immediately prior to, and the spin group immediately following, a change in the sectoring mode. The effect on MFSA data is the same as with the valid data bits above.

 

3. First Spin Group Bit - A value of one indicates spin group one has invalid data. Spin pair 1 and 2 should be dropped in the rate and pulse height data. The MFSA data, which are read out during the next 4-format period, are not valid.

 

4. Last Spin Count - If spins are dropped during a 4-format data collection period, these bits show the 1ast whole spin during which data were accumulated. The table below shows the meaning of the 1ast spin count at each spacecraft bit rate.

 

S/C Bit Rate	Mode ID	Nominal Last  Spin Count	Value of Last Spin Bits	Range of Spins	Spins in  "Spin Pair"
1024	11	9	spins -1	1-10	2
512	10	19	spins -5	1-20	4
256	01	39	spins -25	1-40	8
128	00	79	spins -65	1-80	16

 

5. Command Status 2 - These bits show which subsystems are powered. When a subsystem is turned off by command, data are valid for the full 4-format period prior to the enactment of the command (MFSA data are read out during the following 4-format period). When the subsystems are turned off by an overload, the rules for the Power-Down Telltale apply.

 

If the experiment is operating normally and a subsystem is turned on by command, the associated rate or PHA data are valid from the 4-format pulse at which the command is enacted. MFSA data collection will be valid from this point, but it is read out after the following 4-format pulse.

 

The subsystems that must be powered for each channel type are shown below.

 

Channel Names	Required Subsystems
P'	M’F’
E’, FP’	M’F’
P	MF
E, FP	MF
DE	BC
W, Z	W
C1, D1, C2, D2, P	W, PHA
MFSA
Schedule 0, 6	PHA, MF
Schedule 1, 2, 3, 4, 5, 7	PHA, M’F’

 

 

Next: Chapter 4.7 Spin Reference Pulse