SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
(i) more frequent cooler recycling including the possibility of autonomous recycling under control of
the DPU alone;
(ii) slow chop mode in the event of partial BSM failure;
(iii) open loop BSM control using commanded current to the actuators;
(iv) single axis BSM operation;
(v) slow scanning of FTS mirrors;
(vi) step and look operation of the FTS in conjunction with the BSM;
(vii) open loop operation of the FTS mechanism by commanding the current to the actuator;
(viii) DC operation of photometer calibrator this will allow V-I’s on detectors under different loadings for
calibration;
(ix) selection of smaller numbers of detectors from photometer arrays in event of telemetry bandwidth
problems;
(x) selection of smaller number of spectrometer detectors in event of problems with telemetry
bandwidth and/or loss of spectrometer calibrator.
3.9 Redundancy scheme
The general design philosophy of the instrument, as far as is possible, is that the total failure of a single subsystem
will does not lead to the total loss of instrument operations. In order to achieve this, SPIRE has been
designed with both a prime and a redundant side to the instrument. There is no electrical cross strapping
between these two sides of the instrument, except at the SPIRE/Herschel data interface where both the Prime
and Redundant HSDPU subsystems are each connected to the Prime and the Redundant Herschel
MIL-STD-1553 data buses (See Figure 3-2). Normally, to switch between the prime and redundant sides of
the instrument, the spacecraft sends appropriate commands on the MIL-STD-1553 bus to the HSDPU to
firstly shut down the HSFCU and then to prepare itself for shut down. Once the HSDPU is ready, the two
prime spacecraft level LCLs in the HPDU that power the HSDPU and HSMCU are unlatched and the Prime
side is then shut down. The two redundant LCLs are then latched and the Redundant HSDPU and HSFCU
are powered up. Due to impracticalities, some systems (for example, the 3 He Cooler) are not duplicated. In
these cases, either the Prime or Redundant side of the instrument can control them. Importantly, in the signal
detection subsystems, there is no redundancy in the detectors, the JFETs and the Lock-in Amplifiers. The
specific redundancy scheme adopted for each sub-system is described in Table 3-1. Figure 3-2 illustrates the
redundancy in the Warm Electronics. Figure 3-1 shows the redundancy in the FPU subsystems. It can be
seen that the cryogenic bulkhead connectors J10 and J11 on the CVV wall are harnessed to the prime Filter
Boxes (and from there the prime subsystems) Connectors J12 and J13 are connected to the redundant Filter
Boxes and the redundant subsystems.
3.10 System budgets
A summary of the various budget allocations to SPIRE are summarised below in Table 3-4 .
Table 3-4 - SPIRE Budget allocations
Item Budget Allocation
Focal Plane Mass Budget 57.6kg
SVM Mass Budget 30kg
FPU Stage 0 Thermal Load 10 mW average 100 mW peak
FPU Stage 1 Thermal Load 25 mW average 100 mW peak
JFB Stage 2 Thermal Load 33 mW average 200 mW peak
WE dissipation to SVM 86 W
Data transmission from HSDPU to CMDS 100 kbps
56