FIGURES
FIGURE 2-1. TRACE SPACECRAFT COMPUTER HARDWARE CONFIGURATION
FIGURE 3-1 TRACE C&DH SOFTWARE INTERFACE DIAGRAM
The flight software requirements specified within this document are bounded by the spacecraft hardware configuration and the intended operational environment. Figure 2-1 shows the Spacecraft Computer System (SCS), the TRACE insrument, the Spacecraft Power Electronics (SPE), the transponder, and the Attitude Control Electronics (ACE) hardware subsystems of the TRACE spacecraft and the additional subsystems they provide interfaces for. The flight software is comprised of two configuration items (CSCI), C&DH and ACS software. Both of these CSCIs will reside and execute on the SCS processor.
FIGURE 2-1. TRACE SPACECRAFT COMPUTER HARDWARE CONFIGURATION
The hardware components of the SCS consist of one 80386 based processor card, 88 megabytes of bulk memory, a power converter, an I/O card, an uplink/downlink card and a backplane. The cards of the SCS communicate with each other over the backplane. A MIL-STD-1553B bus provides an external communications link between the SCS and the instrument, ACE, and SPE subsystems. The SCS is responsible for the execution of both ACS and C&DH software.
The SCS processor card contains the 80386 processor along with 64 kilobytes of bootstrap EEPROM, 512 kilobytes of EEPROM, 1 megabyte of SRAM, 128 kilobytes of shared RAM (shared between a UMTC 1553B protocol chip and the processor), an 80387 math coprocessor, an 82380 DMA controller, and a 8251 serial communications device.
The SCS processor card serves as the bus controller of the 1553B bus network that connects to the TRACE instrument, the SPE, and the ACE. The processor card also supports all command and data handling functions. A hardware watchdog timer, resets the C&DH card if the flight software fails. A Universal Asynchronous Receiver and Transmitter (UART) channel is available to support flight software loads and software debugging during ground tests. This interface is not available for flight operations.
The Up/Down card interfaces to the flight software (via the customized backplane) and the RF transponder. The uplink portion interfaces to the RF transponder command receiver for receipt of ground commands at a 2 kbps rate. The downlink portion interfaces to the RF transponder telemetry transmitter for downlink of the spacecraft housekeeping and science data. Downlink rates are selectable at 23.4375 kbps, 1.125 Mbps, or 2.25 Mbps.
This card also provides a hardware command decoder which decodes critical ground commands and directly distributes the decoded commands (pulses and/or bi-level signals) to selected subsystems via hardlines. These commands will bypass the command verification of the processor. These critical commands will be used when ground commands are not accepted by the processor because of the failure of the flight software or processor hardware.
The high speed serial interface for receiving data from the instrument is part of this card as well. The interface permits 900 kbps data transfers from the instrument to the SCS where the data is either stored or downlinked.
A bulk memory card contains a total of 88 Mbytes of RAM for the storage of engineering data and the science data between ground passes. The Error Detection and Correction (EDAC) function is located on the SCS processor card corrects single bit errors, detects multi-bit errors, and stores the error address. The bulk memory is accessed from the processor via the backplane.
The I/O card provides an interface to several onboard subsystems and receives the housekeeping status and data from the RF transponder, subsystem thermistors and the instrument thermistors. It also sends bi-level commands and analog signals to subsystems as needed. It provides the pyro control functions to the Pyro Control Unit in the SPE. Additionally, the I/O card provides the telemetry and command interface between the SCS and the Pegasus launch vehicle for a communications link prior to payload separation.
The I/O card maintains the MET (Mission Elapsed Time) and a one Hertz clock, and distributes them to on-board subsystems. The one Hertz clock is used to provide the spacecraft timing synchronization for the other subsystems including the instrument and ACS software.
The Power Converter is used to convert the spacecraft unregulated +28V power into +5.0V and +/- 15V DC for the SCS subsystem.
The SCS backplane provides for internal card-to-card communications. The backplane supports full 32-bit addressing, 8-bit, 16-bit, and 32-bit data transfers, 7 interrupt levels from external I/O cards, EDAC requirements, and provides +5VDC, power return, and signal return lines to the SCS cards.
The TRACE instrument provides the capability of collecting, formatting and sending the mission specific science data to the C&DH flight software for storage and downlink. The instrument communicates over the 1553B bus interface for receiving commands and sending housekeeping data and over the high speed serial RS422 interface to send image data packets to the SCS for storage.
The ACE is an 80C85 processor based subsystem which translates commands from the ACS flight software to direct control of the attitude control devices. Actuator commands to the attitude control and orbit maintenance flight hardware are routed to the ACE via the SCS 1553B bus network. Attitude sensor and control device data are captured by the ACE and routed to the ACS flight software via the C&DH software and the 1553B bus.
The ACE provides a hardware controlled spacecraft safehold capability which will ensure safe pointing of the spacecraft relative to the sun if the spacecraft computer experiences a critical fault.
All spacecraft power controls and sensors are included within the SPE control unit. Commands to spacecraft power devices are routed to the SPE via the SCS 1553 bus. Battery status and related power sensor data are received from the SPE on the SCS 1553B bus.
TRACE shall be launched into orbit from a Pegasus launch vehicle. The SCS will be in normal operations mode during launch. The ACE Box will be turned on shortly after payload separation to initiate analog safehold attitude control.
The spacecraft will have multiple ground contacts each day. Between ground contacts the flight software provides autonomous control of the spacecraft by issuing pre-loaded commands from memory. The flight software monitors the spacecraft data for anomalies and responds to them. It maintains the desired spacecraft attitude and orbit knowledge via processing and commands to the actuators.
Between ground contacts, data is stored for later playback to the ground. The bulk memory provides the storage for the engineering data, processor status, attitude control status, and science data.
The flight software will detect Single Event Upset (SEU) errors within static areas of flight memory and perform warm restarts or cold restarts as appropriate to attempt to recover with the minimum impact to the mission. Bulk memory has Error Detecting and Correcting (EDAC) capability. The entire memory is "scrubbed" at least once per orbit to detect multi-biterrors and correct single bit errors.
All flight code and default software control table values will be stored in EEPROM prior to launch. During boot mode operations the flight software executes from EEPROM. When in normal mode operations, program code is copied from EEPROM to RAM where it executes.
All flight memory may be dumped to the ground. In addition, a memory dwell capability exists in the flight software which will allow the ground to specify particular memory words to be collected by the flight software over time and dumped to the ground for troubleshooting anomalies.
The ground may change normal mode program code and constants by uplinking new instructions and data into flight memory.
Figure 3-1 shows the external hardware and software interfaces to the C&DH flight software. Actual software requirements related to these interfaces are provided within the FLIGHT SOFTWARE REQUIREMENTS. Applicable ICDs (see Applicable Documents) should be referenced for detailed information. In case of conflicts between this document and the ICD, the ICD shall take precedence.
The RF transponder receives ground commands and provides them to the uplink portion of the Uplink/Downlink card. The Uplink hardware function provides command frames (as codeblocks) via a FIFO buffer along with card functional status to the flight software.
All telemetry to the downlink card is formatted as CCSDS transfer frames. The downlink hardware portion of the card is able to output frames at 23.4375 kbps, 1.125 Mbps, or 2.25 Mbps telemetry rate. Telemetry Encoding is commandable for NRZ L, Bi-Phase L, Convolution, and Convolution || Bi-Phase L.
The I/O card consists of the following functions:
All functions of the I/O card are memory-mapped. More detailed information on the I/O card and its interaction to the SCS processor can be found in the TRACE Command and Data Handling (C&DH) Subsystem SCS Input/Output Specification.
The I/O card provides a 1 Hz pulse signal and Mission Elapsed Time (MET) to the SCS processor. The MET is kept in a 28-bit seconds time format. This format provides for slightly more than 8.5 years of elapsed time. The MET cannot be set nor can it be adjusted by the flight software. The MET also possesses a test mode which will allow the most significant bits of the MET to count at a faster rate since test time is limited.
The I/O card provides the barker time tag to the SCS processor. The barker time tag consists of 2 bits of seconds time and 14 bits of subseconds time.
The I/O card maintains a FIFO of analog thermistor data. Thermistor data from the spacecraft structure, spacecraft computer, transponder, ACE box, SPE, TRACE instrument, solar array wings and body panel, and the battery is available to the flight software through this FIFO.
The Pegasus launch vehicle sends separation status information to the I/O card which puts the data onto the backplane which is then routed to the flight software hosted on the SCS processor.
The MET is read by the flight software via the interrupt service routine which services the 1 Hz pulse. The software initiates its seconds count by reading the MET. The 1 Hz pulse is used to synchronize this time message. Sub-seconds time is read from the 82380 chip on the microprocessor card and has a resolution of one microsecond.
The barker time tag is read by the flight software in response to an interrupt after receipt of each ground command. This provides synchronization between the SCS processor and the Barker Time Tag. All reads to the barker time tag are 32-bit reads.
The flight software must poll a FIFO buffer via the I/O card to obtain all the 12-bit analog feedback and thermistor data.
Telemetry is output to the Pegasus launch vehicle through the I/O card. The rate of data sent from the I/O card to the Pegasus launch vehicle must match the allocated space in the PCM output frame.
The SPE includes all spacecraft power controls and sensors. Commands to spacecraft power devices are routed to the SPE via the SCS 1553B bus network. Battery status and related power sensor data are received from the SPE on the SCS 1553B bus network. Detailed information on the C&DH-to-SPE interface can be found in the TRACE 1553 Data Bus Implementation.
The flight software reads voltages, currents, passive analog temperatures, and bi-level relay status from the SPE via the 1553B bus . All telemetry and control data transferred between the SPE and the flight software are sent most significant bit first. The flight software reads the muxed data from the SPE by first sending a control message to the SPE to start analog data collection and then reading 64 data values from a FIFO. Bi-level telemetry data is read by the flight software using a simple data request to read four words from the SPE.
All telemetry data is periodically read from the SPE. This data is formatted into data packets along with the time of the data acquisition. Periodically the SPE packet is stored in bulk memory for later playback.
The flight software sends commands to the SPE using the 1553B bus. The SCS processor acts as the bus controller while the SPE acts as a remote terminal. All control commands contain 1 data word. The data word identifies which discrete signal or relay command is to be selected. Relay and discrete commands are sent no faster than once per 50 milliseconds to the SPE. When relay or discrete command is sent to the SPE, it does not respond to any other bus transaction for 50 milliseconds. The telemetry interface between the flight software and the SPE is described in section 3.3.1.
The ACE is an 80C85 processor based subsystem which directly interfaces to all attitude control sensors and actuators and provides a hardware controlled spacecraft safehold capability which will ensure safe pointing of the spacecraft if the spacecraft experiences a critical fault. Data transfers from the ACE to the flight software occur via the SCS 1553B bus. Sensor and control hardware data are used in the real time attitude control loop. ACE housekeeping voltages, currents and temperature monitors of the sensors and actuators are used to determine the health, status and operational condition of the ACE. The ACE safehold packet includes software status and the commands most recently sent to the actuators and ACE diagnostics.
The data packets for the ACE raw sensor data, housekeeping data, and safehold data are sent from the ACE box to the ACS software. More detailed interface information regarding the ACE to ACS CPU interface can be found in the TRACE TBD ICD.
Actuator command packets, and switch set packets are sent to the ACE over the 1553B bus interface. The Actuator Command packet comes from the ACS task, and contains reaction wheel commands and torque rod currents. The Switch Set packet contains various ACE box switch settings.
More detailed interface information regarding the ACS CPU to ACE box interface can be found in the TRACE 1553 Data Bus Implementation.
The interface between the flight software and the TRACE instrument consists of transfers of ground commands and science and housekeeping data. Detailed information on this interface can be found in the TRACE Instrument to Spacecraft Data Format ICD.
Science image data is transmitted across a high speed serial RS422 link to the SCS. The flight software accepts the data in the form of a CCSDS packet and stores it in the bulk memory for later play back to the ground.
The flight software sends commands to the instrument using the 1553B bus interface. These commands originate either from the ground or as stored commands and are reformatted for the 1553B bus transfer.
The following documents provide background, context and specification information relevant to the TRACE flight software and hardware system. (Please note that at time of this document's publication some of the documents listed below were still under development.)
