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42.7 SDF Clock Errors

HSP has encountered several error reports which seem to indicate that the HSP line counts were out of order. Actually, however, the line counts were in order and the time annotation of the line start by the Control & Data Handler (C&DH) appears to be incorrect. This problem appears to have two signatures. One is that the clock insertion into the packet seems to be interrupted such that the second word of the clock is set prior to an update and the low order clock value after a 125 millisecond step. The second signature is that the C&DH maintained clock value seems to have counted more than 60 seconds of vehicle counts.

The problem can be detected by looking at the packet sequence. Sorting by packet count gives a time-tag inversion, sorting by SDF packet time, gives a line count inversion. Figure 42.3 is an excerpt from a listing of packet headers. The time column is in seconds, the delta is the delta-time from the previous packet. Packet 4112 comes before 4111, and the delta-time is 0.000977 seconds, which is exactly 1000 clock ticks at 1.024 MHz.

Figure 42.3: Listing of Packet Headers


pkx obs owner pfc spp wpp ppf fcount lcount time delta

1 39 HSP 0140 1 34 52941 19 4099 90919501.904297 0.000000
2 39 HSP 0140 1 34 52941 19 4100 90919501.913086 0.008789
3 39 HSP 0140 1 34 52941 19 4101 90919501.921143 0.008057
4 39 HSP 0140 1 34 52941 19 4102 90919501.929932 0.008789
5 39 HSP 0140 1 34 52941 19 4103 90919501.938232 0.008301
6 39 HSP 0140 1 34 52941 19 4104 90919501.947144 0.008911
7 39 HSP 0140 1 34 52941 19 4105 90919501.955200 0.008057
8 39 HSP 0140 1 34 52941 19 4106 90919501.964111 0.008911
9 39 HSP 0140 1 34 52941 19 4107 90919501.972168 0.008057
10 39 HSP 0140 1 34 52941 19 4108 90919501.981201 0.009033
11 39 HSP 0140 1 34 52941 19 4109 90919501.989258 0.008057
12 39 HSP 0140 1 34 52941 19 4110 90919501.998901 0.009644
13 39 HSP 0140 1 34 52941 19 4112 90919502.005371 0.006470
14 39 HSP 0140 1 34 52941 19 4111 90919502.006348 0.000977
15 39 HSP 0140 1 34 52941 19 4113 90919502.013672 0.007324
16 39 HSP 0140 1 34 52941 19 4114 90919502.023315 0.009644
17 39 HSP 0140 1 34 52941 19 4115 90919502.030762 0.007446
18 39 HSP 0140 1 34 52941 19 4116 90919502.039307 0.008545
19 39 HSP 0140 1 34 52941 19 4117 90919502.047852 0.008545
20 39 HSP 0140 1 34 52941 19 4118 90919502.056396 0.008545
21
There is no conclusive proof as to the frequency or randomness of these signatures. The HSP team has requested a change in the sort order to specify lines first and time second when processing the data.



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