Digital Electronics Module

The Digital Electronics Module (DEM) consists of two identical units, Prime and Redundant, mounted separately from the telescope unit on the spacecraft bulkhead. Both unit consists of a single piece aluminium box structure into which a series of electronic modules are inserted. The two open ends of the box have covers over them. Electrical Interface with the telescope unit as well as the spacecraft occurs through the front cover of each DEM where five "D-style" connectors protude. A cable harness interconnects the two OM units with the telescope unit as well as the spacecraft.

Each unit is bolted to the spacecraft bulkhead through six mounting lugs, machined as part of the primary box structure. The units are independently removable. A preferred orientation for the two units, relative to each other, is to place them so that their rear covers are facing each other, in order to minimize radiation shielding requirements on these surfaces. Additional radiation shielding is selectively provided on the top and front surfaces of the units as well as internally.

Within each unit, 19 individual electronics "modules" are housed. Each module consists of a multilayer printed circuit board, 180 mm x 92 mm x 1.5 mm thick, mounted in a frame, made either of aluminium or Ryton R-4 thermoplastic. Components are primarily mounted on a single side of the PC board. Assembled modules are 187 mm x 94 mm with thickness varying according to component height requirements. Modules are individually inserted into the box where they interface with a single backplane or motherboard. This type of packaging scheme, using the same module size and configuration, has been successfully used on satellites packages at SNL for over fifteen years (ULYSSES, ALEXIS, GPS).

Each DEM contains an Instrumental Control Unit (ICU) and a Digital Processing Unit (DPU). The ICU commands the XMM-OM and handless communications between the XMM-OM and the spacecraft. The DPU is an image processing computer that digests the raw data from the instrument and applies a non-destructive compression algorithm before the data are telemetered to the ground via the ICU. The DPU supports two main science data collection modes, which can be used simultaneously. The DPU autonomously selects up to 10 guide stars from the full OM image and monitors their position in detector coordinates at intervals that are typically set in the range 10-20 seconds, referred to as a tracking frame. These data provide a record of the drift of the spacecraft during the observation accurate to ~ 0.1 arc second.