Command & Data Handling for the Magnetospheric Multiscale Mission

The Magnetospheric Multiscale (MMS) mission is the fourth mission of the Solar Terrestrial Probes (STP) program of the National Aeronautics and Space Administration (NASA). The MMS mission, consisting of four identically instrumented spacecraft, will use Earth's magnetosphere as a laboratory to study magnetic reconnection, a fundamental plasma-physical process that taps the energy stored in a magnetic field and converts it - typically explosively - into heat and kinetic energy in the form of charged particle acceleration and large-scale flows of matter. The Command & Data Handling (C&DH) subsystem is responsible for the spacecraft command and telemetry, time management and distribution, analog data acquisition and also hosts the flight software. Other mission specific functions include interfacing to the Instrument Suite (IS) through the Central Instrument Data Processor (CIDP) and other hardware components such as the S-Band Transponder, the Accelerometer, the Digital Sun Sensor (DSS), and the Star Sensor. The C&DH subsystem consists of one box with two of each card (Backplane, Low Voltage and Power Services (LVPS) Card, Communication Card, Processor Card and Analog Card). One side is designated as Side A or the primary side and the other as Side B or the redundant side. Only one side (primary side) is active (fully powered on) at a given time. However, the other side (redundant side) has a warm front end where the interface to the Radio Frequency (RF) Communication subsystem (Communication Card) is on for command decoding and is also capable of downlinking data. The computing part of the redundant side is powered off. One of the technical novelties on this mission is the use of SpaceWire (SpW) as the primary spacecraft bus. The SpW network provides communication paths for commands and telemetry, including science data, between: The primary and redundant C&DH and the primary and redundant CIDP units Each C&DH and its Navigator (NA V) Each C&DH and its Power Subsystem Electronics (PSE) Each C&DH and its Engine Valve Driver (EVD) The Processor Card, Communication Card and Analog Card modules within each C&DH The network is a cold spare, redundant configuration with the exception that both Communication Cards are always powered. Also, the NA V, PSE, Processor Card, Analog Card and EVD are block redundant. Two SpW Routers reside on the C&DH, one on the Processor Card and the other on the Communication Card. A SpW node core resides on the Analog Card. They are both compatible with the European Cooperation for Space Standardization (ECSS) Space Engineering SpaceWire - Links, Nodes, Routers and Networks (ECSS-E-50-12A) standard in addition to the Router incorporating several Goddard Space Flight Center (GSFC) defined enhancements. The interfaces to the Attitude Control System (ACS) components (Accelerometer, DSS, Star Sensor) as well as the S-Band Transponder are primarily RS-422 with the exception of the DSS which is a single-ended interface. The ACS FSW (hosted by the Processor Card) receives Accelerometer, Star Sensor, and EVD data at 4 Hertz (Hz), and DSS data once per spacecraft revolution. The ACS FSW generates thruster firing commands which are sent over SpW to the EVD hardware. This paper will provide a detailed description into the implementation of the command and data handling functions from uplink command and data processing to data downlink. The hardware design will be highlighted in order to show how (1) requirements are met and (2) the burden on flight software is lessened.