Gaia
GAIA.ASU.SP.ESM.00007
Issue 2.00
Page 16 of 79
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GAIA.ASU.SP.ESM.00007 (Avionics SCOE Req Iss 2).doc
3.5.4 SCET Correlation
The Gaia CDMU transmits Spacecraft Elapsed Time (SCET) using a SVM 1553 broadcast message at
subaddress SA 29 (refer to Section 9.5 of [AD05]).
The CDMU (Telemetry Frame Generator) also inserts SCET in VC0 of each Telemetry Frame for
transmission to the Ground System.
As the Gaia Central Checkout System (CCS) is the acquisition and archive media for Telemetry, it must
correlate Telemetry reception with T
REF
.
As the receptor of SVM 1553 BUS data and PPS, the Avionics SCOE will provide the means for the CCS to
correlate SCET with T
REF
.
AVI-1654/CDMU-836,CDMU-837/T
The Avionics SCOE shall receive 1Hz (PPS) synchronisation signals from the CDMU (both prime and
redundant), in accordance with the following specification: [AD06] Section 3.5.5.1.3.1 (LVDS)
The SCET value broadcast by the CDMU is valid at the occurrence of the next rising edge of the CDMU 1Hz
(PPS) signal.
AVI-2845/CREATED/T
The Avionics SCOE shall detect the 1553 SCET broadcast message and extract the SCET time.
AVI-2846/CREATED/T
On the occurrence of the next PPS, the Avionics SCOE shall latch the value of SCET and T
REF
.
AVI-3371/CREATED/T
From (latched) T
REF
the Avionics SCOE shall calculate the value of H
REF
, where H
REF
is 'the number of
microseconds since the beginning of the current year' expressed as a 64 bit integer.
AVI-3392/CREATED/T
The value of T
REF
, shall be accurate to <10us referenced to the rising edge of PPS.
AVI-2847/CREATED/T
When connected to the CCS, at intervals of PPS, the SCET/H
REF
pair shall be transmitted to the CCS
over the EGSE LAN using the format defined in Section 3.7.
AVI-3036/CREATED/T
The default CDMU PPS signal to be used for correlation shall be the Prime.
AVI-3038/CREATED/T
Selection between Prime and Redundant CDMU PPS correlation signals shall be available from the
MMI or under the control of the CCS (Test Procedure).
AVI-3037/CREATED/T
In the case of loss of the selected CDMU PPS correlation signal, the Avionics SCOE shall
automatically switch to the other.
AVI-3040/CREATED/T
In the case of reacquisition of the selected CDMU PPS correlation signal, the Avionics SCOE shall
switch to it.
AVI-4229/CREATED/T
All acquisitions or re-acquisitions of the CDMU PPS shall be achieved within 2s (2 cycles of PPS).
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