Paper SSC03-XI-6. Autonomous Telemetry Collection for Single-Processor Small Satellites

Transcription

1 Paper SSC03-XI-6 Autonomous Telemetry Collection for Single-Processor Small Satellites Dave Speer Northrop Grumman Electronic Systems Space Technology & Services 4276 Forbes Blvd. Lanham, MD On site at NASA Goddard Space Flight Center Flight Electronics Branch, Code August 2003

2 Introduction For many spacecraft, the down-linked health and status telemetry includes information derived from analog quantities: Voltages, currents, temperatures, pressures, etc., from all over vehicle. Each analog-to-digital conversion requires a series of micro-operations: signal selection, wait for slew & settle, and wait for A-to-D conversion. To sample N analog signals, this series must be repeated N times. If spacecraft has a distributed data system, with relatively lightlyloaded processors embedded in various subsystems, then might be able to do the A-to-D conversion micro-operations in software. But if there is only one processor, or if the locally embedded processors are heavily loaded, or if periodic samples of analog signals must be made while the processor is busy doing something else, then a software implementation of the micro-operations will likely be complex, difficult or even impossible. D. Speer 14 August

3 Overview Autonomous analog telemetry collection concept described here is being applied on the Space Technology 5 (ST-5) mission. Under development at Goddard Space Flight Center for NASA s New Millennium Program (NMP). ST-5 focus is on small and low power: D = 60cm, h = 30cm, 22 watts. For ST-5, a single microprocessor is required to do many tasks: Interface to all other subsystems and New Millennium technologies. Do all real-time processing associated with attitude control, up-link and down-link communications, science and technology validation data acquisition, solid-state recorder management, power switching and battery charge management, health and status data collection, etc. To off-load very busy ST-5 processor, a hardware-based sequencer and Direct Memory Access (DMA) controller will be used to collect and digitize all 71 of the analog health and status telemetry signals. D. Speer 14 August

4 Block Diagram of ST-5 Subsystems & Interfaces Battery Solar Arrays Power Subsystem Electronics Magnetometer Sun Sensor Variable Emittance Controller #1 Variable Emittance Controller #2 Command and Data Handling (C&DH) Subsystem Cold Gas Micro-Thruster Tank Pressure Sensor Thermistors X-band Transponder D. Speer 14 August

5 Analog Telemetry Collection Details (Part 1) Each analog telemetry signal, regardless of signal type, has a unique 7-bit Analog Channel Number (ACN) assigned to it. Design is based on use of tables stored in memory shared between processor and hardware-based sequencer through DMA controller. Flight software first loads Collection Control Table to define number of analog signals to digitize, and order to digitize them in (series of ACNs). Collection Control Table(s) can be loaded only once at power on, or can be re-loaded to reconfigure as necessary in real-time. VERY FLEXIBLE To initiate collection process, software writes to collection start address. Hardware sequencer then reads each ACN from Collection Control Table, selects the corresponding analog signal, waits for signal to slew & settle, starts the A-to-D converter, and waits for A-to-D conversion to complete. Sequencer then stores A-to-D converter output in a Digitized Data Table. D. Speer 14 August

6 Analog Telemetry Collection Details (Part 2) Collection Control Tables and Digitized Data Tables are arranged in memory as matched pairs that are mirror images of each other. Software has read/write access to both tables, but normally writes to Collection Control Table(s) and later reads from Digitized Data Table(s). Hardware sequencer does DMA read cycles from the Collection Control Table(s) and then does DMA write cycles to the Digitized Data Table(s). Lengths of tables and number of matched table pairs almost unlimited. ACNs written to Collection Control Table can be in any order, with values repeated or not present, so easy to do sampling patterns & frequencies. Processor can either poll a status bit in a readable register or be interrupted when the entire collection process has completed. Flight software can then read out all of the digitized telemetry at once, and then request the start of the next collection process at any time. D. Speer 14 August

7 Matched (Mirror Image) Pairs of Tables in Memory Collection Control Table (written by software) Number of ACNs to do First ACN Second ACN Third ACN Digitized Data Table (read out by software) Number of ACNs done Data from first ACN Data from second ACN Data from third ACN 127th ACN Data from 127th ACN D. Speer 14 August

8 Analog Telemetry Collection Details (Part 3) Current implementation for the ST-5 mission has these properties: Analog signal switching network funnels down to the input of a single A-to-D converter. Length of all Collection Control and Digitized Data tables = 128 entries, so maximum number of ACNs in a single collection process = 127. Number of matched Collection Control/Digitized Data table pairs = 4. Sampling and digitization of analog signals on 1-millisecond boundaries. Collection process can start immediately or after a programmable time delay, and software will be able to perform other real-time tasks while the analog telemetry collection process runs autonomously. Hardware sequencer and DMA controller logic in part of an Actel FPGA. Only one Collection Control Table can be running at any given time because there is only one analog-to-digital converter in the system. D. Speer 14 August

9 Analog Signal Switching Network PSE Processor (and Software) C&DH MAG VEC 1 VEC 2 DSS XPDR Analog-to-Digital Converter (ADC) ADC ADC Controls A D D R C N T L FPGA with DMA Controller D A T A ADDR CNTL DATA SRAM PROP Switch Controls to select a single analog signal and ACN Decoder D. Speer 14 August

10 Conclusion In order to off-load a very busy processor on the New Millennium ST-5 spacecraft, the real-time collection and digitization of all analog health & status telemetry will be done autonomously by an FPGA-based sequencer and DMA controller. Matched pairs of Collection Control Tables & Digitized Data Tables in a shared memory allow for easy top-level management of analog telemetry collection with a minimum of flight software overhead. Very flexible design in terms of the number and order of channels sampled, frequency and interleaving of samples, time phasing of samples, options for handshake with software at completion of collection, and availability of multiple Collection Control Tables which could correspond to multiple spacecraft operating modes or multiple spacecraft downlink rates. Sequencer and DMA controller logic in fraction of rad-hard FPGA. D. Speer 14 August