4.7 Spin Reference Pulse

 

4.7.6 Reconstitution on the Ground of the SRP

 

The time when the Sun Reference Pulse is issued to the users onboard the spacecraft (experiments and AOCS) is transmitted on the telemetry as a datation event. The main difference between a "classical" event datation and the SRP is that a "classical" datation gives the time of an event which took place before the telemetry channel is sampled. In the case of the SRP this event could as well be a past or future event.

 

This is further complicated by the non-monotonicity of the datation counter, which recycles every 32 sec. (16 bits, LSB=2048 sec).

 

The reconstitution on ground of the absolute time when the SRP was or will be issued should take into account these factors. For this purpose two additional parameters are telemetered. These are the spin period associated with the SRP and the spin counter which is incremented every time that the SRP is issued (it recycles every 256 SRP). This counter is not needed for the reconstitution, but could be used for assessing the consistency of the data.

 

The flow chart in Figure 4.54 can be used for reconstitution under the following conditions:

1. Telemetry only scientific mode
2. Spin period less than 16 sec.

 

The following definitions are used in the flowchart:

 

SRD	Datation of the SRP
OBTC	Onboard time, LSB 2 sec.; the 4 LSB are 0 except in Eng. format
PER	Spin period, LSB=1/2048 sec
CYCLE	Period of recycling of the datation counter, 32 sec.

 

Figure 4.54 Flow chart for the reconstitution of the SRP

 

 

 

Next: Chapter 4.7.7 Error on the SRP Determination