4.7 Spin Reference Pulse

 

4.7.4 Onboard Generation of the SRP

 

The sun pulses are dated in the Data Handling Subsystem (accuracy is 1/2048 sec.) for transmission to ground and used also for computing the Spin Reference Pulse. At low Solar Aspect Angle, even a small mutation could invalidate the SRP if raw data were used for its determination. Therefore the design of the SRP generation was based on a digital filtering of the times obtained by the sun crossing.

 

The SRP program implemented in the D/H software (called SPIN) is based on the datation times corresponding to the four pulses:

 

MSL1, MST1, MSL2, MST2

 

Depending on the solar aspect angle, only two pulses may be present (single crossing) instead of the four (double crossing). When initially scheduled, the SPIN program will work as single crossing. Double crossing is only selected by Telecommand.

 

The SPIN program finds the mid-crossing point by averaging MSL1 and MSL2 in the case of single crossing, or in the case of double crossing the average of the four datation pulses and this average shifted by -90 degrees. After that, to minimize the effects of notation, the time of occurrence of the next SRP is computed by a filter algorithm which takes into account the previous value of the spin period.

 

The flow chart of this filter is given in Figure 4.52, where the symbols used have the following meaning:

 

T(MSL1,...,MST2)	datation times
S	raw SRP
A	sensor selection constant
C	correction constant
Q	expected SRP from previous loop
E(n)	difference between raw and expected SRP
SRP(n)	filter output, time at which the SRP should occur
K1,K2	filter constants (both 1/16)
P(n)	estimated spin period
X(n)	time of SRP delayed by one estimated spin period
Q	actual time between two SRP

 

Figure 4.52 SRP filter flow chart

 

 

The SPIN program is designed also to perform the following functions:

 

• Automatic recognition of the sensor selected. The SRP is incremented by 315 or 225 degrees, depending on whether XBS1 or XBS2 is selected.
• To add a corrective quantity to the SRP which will be established after inflight calibration of the principal axis tilt and sensor misalignment. This quantity is sent by telecommands as an 8 bit with sign; therefore a range of plus or minus 127, with the weight of the least significant bit equal to 1/8192 of the spin period (maximum correction 5.5 degrees with a resolution of 0.044 degrees).
• In case of failure when only 0 or 1 pulse is present and 2 are expected, or 0, 1, 2, or 3 are present and 4 are expected, the SPIN program will generate a pseudo-SRP with a period equal to the last computed period.
• When new valid sun datation pulses are received, the pseudo-SRP generation will be stopped and the program will resume its normal path.
• When in the double-crossing mode, a failure detection automatically switches the algorithm to single-crossing mode.

 

The SPIN program maintains a spin counter register that is incremented every time that an SRP is issued. The value of the counter (recycles every 256 counts) is transmitted also in the telemetry to help the correlation between the spin data and the spin revolution.

 

The SRP pulse which the data handling delivers to the users is given by the time x(n-1) computed on the previous loop and is therefore delayed by one spin period. This allows the generation of the Spin Segment Clock of 16384 pulses per spin period with an accuracy of plus or minus 6 pulses.

 

The values of x(n-1), the actual period Q, and the spin counter are transmitted in the telemetry.

 

 

Next: Chapter 4.7.5 Onboard Generation of the Spin Segment Clock