WIRE Instrument to Spacecraft Computer System ICD
WIRE-ICD-001
January 2, 1996
Version 2.0
Signature Page
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Prepared by: Mary Baxter, SMEX Software Systems Engineer, Date
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Approved by: Dave Everett, WIRE Systems Engineer, Date
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Approved by: David Henderson, JPL, Date
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Tim Conrow, IPAC, Date
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Walter Gibbons, SDL, Date
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Approved by: Quang Nguyen, WIRE C&DH Lead, Date
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Approved by: Mike Blau, WIRE C&DH Software Lead, Date
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Approved by: Mike Fennel, WIRE ACS Lead, Date
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Approved by: Mark Anderson, WIRE ACS S/W Lead, Date
Table of Contents
1 Introduction 1
2 Instrument Control Interface 1
2.1 WIRE Observation Sequence 1
2.2 Commanding Strategy 2
2.3 Telemetry Strategy 6
3 On-Board Data Interface 9
3.1 High Speed Serial Interface 9
3.2 MIL-STD-1553 Data Bus 11
3.3 Space to Ground Interface 12
4 Electrical Interface 13
4.1 High Speed Serial Interface Electrical Signal Specifications 13
4.2 TBD 13
5 Acronyms 14
1 Introduction
This document describes the following:
1) The instrument control capability provided for the Instrument by the SCS.
2) The data interface between the spacecraft and the WIRE Instrument. The MIL-STD-1553 data interface described here also applies between the SCS and other spacecraft subsystems.
3) The data interface between the spacecraft and the ground, which is managed by the SCS. This interface is utilized by all on-board subsytems.
4) Hardware box-to-box electrical interface between the Instrument and the SCS.
2 Instrument Control Interface
2.1 WIRE Observation Sequence
Three types of observations are essential to the mission -- science survey, calibration and stim flashes. Additional optional observation types are not being addressed at this time.
MethodSubframe Time# of CoaddsDither Step SizeScience Survey412SmallCalibrators41-2MediumStim Flashes42,4NoneI. Survey Operation, 50% of the time:A. Slew - Typically 60 degrees 1. Stimulator Flash and Frame Readout (2 colors) x128x128x(16 bit pixels) 2. Slew Initiation 3. Set Instrument Parameters 4. SettleB. Stare - Perform the following 8 times 1. Co-Add x Frames (approx. 12 frames at 4 seconds integration each) 2. Subframe Readout (2 colors) x128x128x(16 bit pixels) 3. Dither 4. SettleII. Calibration Operation, once per orbit (may drop to less frequent as mission progresses):
A. Slew - Typically 45 degrees
1. Slew Initiation
2. Stimulator Flash and Frame Readout
(2 colors) x128x128x(16 bit pixels)
3. Set Instrument Parameters
4. Settle
B. Stare - Perform the following 9 times
1. Integrate TBD seconds (1/2 second?)
2. Subframe Readout
(2 colors) x128x128x(16 bit pixels)
3. Dither
4. Settle
III. Stimulator Flashes - At least once per field (10-15 minute integration)
1. 2 4-second sky exposures
2. 2 4-second "stim" exposures
Note that the 4 second integration time is a current best estimate, which is subject to change depending on the length of dithers. A dither is a very small slew of approximately 1 arcminute. Any data taken during a slew will be discarded.
Instrumentors may define additional modes which are transparent to the C&DH logic. To take observations in different modes the WIE will simply provide a different set of parameters to the C&DH.
2.2 Commanding Strategy
The spacecraft will provide instrument control capability for the WIRE Instrument since the WIRE science data collection is controlled by a state machine. This instrument control capability is currently provided to the SWAS mission by the Instrument Controller (IC) C&DH software in the SWAS SCS. This IC software will be modified and reused for WIRE. [Note that the WIRE mission differs from the TRACE mission in that TRACE provides its own instrument control capability in the Image Processor in the Instrument]. The WIE will retain all co-add and integration functions, however the timing of these functions will be controlled by the SCS.
The WIRE C&DH will use three Commands to perform all of a WIRE
observation segment; the first is a command to the ACS to specify
observation location and dither pattern; the second is to the WIRE
instrument controller to initiate the observation; the third is to the WIRE
instrument controller and will be executed about 20 seconds before the end
of an observation segment and will perform whatever finish/stim flash
operations are required. The logic of these Commands is detailed below.
Relative-Time-Sequences will be used to store multiple commands to the WIE
box. The first 1 or 2 commands in each instrument-control RTS will have 0 time delays, while the rest will have 1 second delays. This will ensure that all commands to the instrument FIFO go out sometime before they are needed, but not necessarily all at once.
Certain WIRE instrument commands will be placed into a FIFO in the WIE (WIRE Instrument Electronics). The FIFO will be 512 bytes in depth. FIFO overflow prevention will be implemented via use of FIFO full flags.
2.2.1 Observation Logic in the SCS - Commands
This is a "first cut" at the basic Commands that would be required onboard the spacecraft to perform the WIRE observing sequences. These Commands will be executed via ATSs and performed by the C&DH IC software in the SCS, which will transmit commands to the Instrument via RTSs and commands to the ACS via the 1553, in order to implement the necessary handshaking:
1) SLEW -
t1 SLEW: target and dither info
Send command to ACS to Slew, with specific target and dither parameters, at a specific absolute time.
2) OBSERVE -
t1+ TBD OBSERVE: gain, offset, bias, RTS1, RTS2
This Command will be kicked off at TBD seconds after t1.
Gain, offset and bias parameters will be sent to the instrument immediately.
[The mechanism for this was not discussed at the meeting].
Wait to receive the settle flag from the ACS, then execute RTS1.
After last command of RTS1 executes in Instrument,
send Dither command to the ACS, if end-of-sequence flag and dither flag is set. For end-of-sequence flag with no dither flag, proceed to next step without the dither.
[The instrument will notify the SCS when it is finished executing RTS1 by setting the end-of-sequence flag and will set the dither flag if a dither is required before RTS2]
Repeat until FINISH command says to stop:
Wait to receive the settle flag from the ACS, then execute RTS2.
After last command of RTS2 executes in Instrument,
send Dither command to the ACS, if end-of-sequence flag and dither flag is set. For end-of-sequence flag with no dither flag, repeat without the dither.
[The instrument will notify the SCS when it is finished executing RTS2 by setting the end-of-sequence flag and will set the dither flag if a dither is required before repeating].
3) FINISH -
t2-n FINISH: RTS3
This Command will be kicked off a TBD number of seconds (20 seconds has been proposed) before the next slew will occur, so that the instrument knows that it is time to complete the processing of the current observation.
The SCS executes RTS3.
2.2.2 Observation Logic in the WIE - RTSs
The following RTSs will be executed by the SCS, and will send commands to the WIE. The detailed contents of these RTSs has not been established, but a first cut at their possible cOontents is described here.
RTS1
STIM FLASH
Initial Fame RTS
RTS2
Middle Frame RTS
RTS3
Clear FIFO, FINISH co-adds, transfer the data
STIM SEQUENCE
NOCOLLECT
This scheme allows for great flexibility in the definition of alternative observation schemes. For example, different versions of RTS1 and RTS2 will be defined for survey and calibration, since they each have slightly different dither patterns. Or, by redefining the contents of RTS1, RTS2 and RTS3, test sequences can be performed during IOC without dithers, if necessary, and the bias at changed at times other than slew times.
2.2.2.1 Commands to the WIE Box
The following WIE commands for immediate execution have been defined to date:
1) RESET -
Sent after a SAA passage. Reverts the instrument to power-up default mode and starts again with the first spin and first subframe.
2) SYNCHRONIZE -
Sent once per orbit. Reset the sampling of the FPA back to the beginning in sync with TBD time. The instrument begins with the first spin and the first subframe.
3) TIME SYNC -
WIE restarts subsecond timer in response to 1553 message from SCS.
All remaining WIE commands are executed at the next natural boundary (end of a co-add). Additional WIE commands which have been defined to date are:
1) COLLECT - This command will put the instrument into a mode where it is reading out the FPA and transmitting data to the spacecraft. This command will be inserted as a part of the sequencing mechanism and will be an explicit command to the WIE.
2) NOCOLLECT - This command is a means of overcoming the first (useless) readout of the array; we expect the first command after a slew or dither to be a
"nocollect" command which will read out the data and not transmit it to the
spacecraft. This command will be inserted as a part of the sequencing
mechanism and will be an explicit command to the WIE.
3) NOCOLLECT WITH FLAG - This command is a variation of the NOCOLLECT - the end command after completing any frame (when the S/C is to be slewing or dithering) will be a NOCOLLECT WITH FLAG. This command will put the instrument into a mode where it is reading out the FPA, then discarding the data, but the end-of-sequence flag will be copied into a C&DH-visible register inside the WIE to tell the C&DH the last command of the current RTS has been completed. Anytime the end-of-sequence flag is set, the dither flag should also be set if a dither is required. When the C&DH Instrument Controller task polls the WIE and notices that the end-of-sequence flag is set, then the C&DH will know that it's OK to dither the spacecraft, if the dither flag is also set. This command will be inserted as a part of the sequencing mechanism and will be an explicit command to the WIE.
4) STIM FLASH - This command will put the instrument into a mode where it is reading out stimulator flash data and transmitting it to the spacecraft. This command can be inserted as a part of the sequencing mechanism or can be initiated via a 1553 command.
5) FINISH -
Invoked by RTS3. Premature termination of a frame at the next co-add boundary, empty FIFO.
2.2.2.2 Default Modes
The last command read from the FIFO is used as the default mode for the instrument to stay in when it finishes the current collection sequence without receiving a new mode command.
2.2.3 Command Uplink
Command loads to be uplinked daily from the ground consist of Instrument commands needed to survey the targets planned for the next 24 hour period, and associated guide star information. 12 targets/orbit would be included in a typical observing day.
The number of commands per uplink currently allowed by SWAS is 400. This will be increased to TBD.
2.2.4 Rationale for Selection of Asynchronous Commanding Approach
Early in the WIRE requirements definition, there was a debate as to whether the SCS should do synchronous or asynchronous commanding of the Instrument. There are three times in the observation sequence which are difficult to predict, which create an obstacle to synchronous commanding of the Instrument by the SCS:
- slew time
- dither time
- instrument clock drift [can be reduced if the two oscillators are synchronized]
If the slew and dither times are crudely estimated, observing efficiency will suffer. For example, if we wait 10 seconds for a dither when it was completed in 7 seconds, observation time has been lost. Over the life of the mission, these seconds can add up to quite a bit of lost observing time.
Asynchronous processing is more in line with reuse of the SWAS software and provides for higher observing efficiency. Asynchronous command processing uses flags from the ACS to indicate that a slew or dither has settled. Observations can resume as soon as these flags are received. It was agreed that the SCS would provide asynchronous commanding of the Instrument and that the Instrument would be designed according to this assumption.
2.3 Telemetry Strategy
2.3.1 Science Data
The following units of science data are produced by WIRE:
Spin - One cycle thru full array, with or without resets
Subframe - On-chip integration interval between resets
Frame - Output of co-adder which sums N (16 max.) subframes
Could contain as few as one subframe
Frames are the unit of data for transmission of science data from the WIE to the SCS. In some modes, specific subframes of data should be discarded. It has been agreed that, as a general strategy, all discarding of subframes will be performed by the WIE and that the SCS will process all subframes of science data that it receives.
One ApId will be assigned for all science data packets. The ApIDs of all WIRE packets will be defined in WIRE-SPEC-005, SMEX/WIRE Telemetry and Command Handbook, Volume II, Database.
2.3.2 Ancillary and Housekeeping Data
There is a set of status registers in the Instrument, separate from the housekeeping data, that reflect the current state of the instrument, i.e., spin rate, spin factor, bias, offset, gain, mode, actual co-add count, cryostat and FPA temperatures, etc., that can be sampled by the SCS anytime. This "ancillary" data, to be used in the interpretation of the science data, will be collected by the SCS just after the FINISH command is sent. It will also be sampled by the SCS every half second and the most recent values will be used if no FINISH command is sent for the current frame. This data will be used by the SCS for the following purposes:
1) as input for building the secondary header of the image data packet
2) as ancillary data in the application data portion of the image data packets
3) as instrument housekeeping.
Instrument housekeeping data will be grouped by sampling rate into 1553 packets. The C&DH has the capability of filtering this data (for example, so that only every 10th sample is stored). The filter rates are maintained via one of the onboard tables, which can be modified post-launch.
TBD (either one or two) ApIds will be assigned for all science housekeeping packets. The ApIDs of all WIRE packets will be defined in WIRE-SPEC-005, SMEX/WIRE Telemetry and Command Handbook, Volume II, Database.
2.3.3 Memory Management Interface
The SCS shall route Image and housekeeping data to the Bulk Memory on a continuous basis, until the memory is full or until playback occurs. Science image data and instrument housekeeping are both stored in the Science partition. Science data and engineering housekeeping data are stored in separate memory partitions. The SCS shall provide the capability to read back a specified partition of memory, and dump it to the ground. Data is not automatically deleted from the Bulk Memory when it is downlinked. WIRE shall have the ability to redump data on request, as needed.
The WIRE memory management scheme will be the same scheme used for the SWAS mission. Reference SWAS-REF-008, "SWAS C&DH Flight Software User's Guide" for a detailed description of the SWAS memory management capabilities.
3 WIRE On-board Data Interface
The two major interfaces between the Spacecraft and the Instrument are the RS-422 high speed serial interface and the MIL-STD-1553 data bus. The RS-422 high speed serial interface is used to transmit science data from the Instrument to the SCS in the Spacecraft. The MIL-STD-1553 data bus is used to communicate between all of the spacecraft subsystems, including the instrument and is controlled by the SCS.
3.1 High Speed Serial Interface
3.1.1 Description of High Speed Serial Interface
The Wire Electronics (WIE) will transmit a burst of 64Kbytes of image data nominally every 48 seconds to the SCS over the RS-422 High Speed Serial I/F. The burst is < 600ms.
The high speed serial interface will transmit the 64K bytes of image data as a set of 64 1Kbyte transfers. The data transfer rate is 900k bits per second. The SCS will build CCSDS packets from the image data. The length of the application data in the image data packets will be 1024 bytes. The SCS will add the required CCSDS header information, place the packets into transfer frames, store them in bulk memory and transmit them to the ground at the next downlink opportunity. The bulk memory will be emptied twice a day via a 1.8 Mb/sec downlink.
3.1.2 Protocol
The handshaking protocol consists of three signals passed between the WIE and the SCS. These signals are Data Ready, Data, and Clock. The serial interface timing specifications of these signals are shown in Figure 3.1.2, WIRE Serial Interface Timing Specification. The sequence of receiving a single block of data from the WIE is as follows:
The SCS will transmit a gated clock, to indicate that it is ready to receive data.
The WIE shall put the first bit of the data block on the data line before asserting the data ready signal. The WIE then asserts the Data Ready signal to begin transmission of the data to the SCS. Upon receipt of the data ready signal, the SCS will sample the first bit of data, then begin sending clocks to the WIE at the 900k rate. The number of clock pulses is equal to the number of bits in the block length (8192). The WIE will change to the next bit of data on the rising edge of the clock, and the SCS will sample the data on the rising edge of the clock. The WIE should de-assert the data ready signal on the rising edge of the clock for the last bit of data in the block. The WIE must not re-assert data ready until at least 2.5 clock periods (approximately 2.777 microseconds) have elapsed.
There are 8192 bits per block and 8192 clocks, but the first data bit is sampled before the clocks begin. Therefore, there will be one extra clock at the end of the data transfer, during the 2.5 clock periods that the WIE has the data ready signal de-asserted. The WIE should ignore this and any other clock pulses that occur while data ready is de-asserted.
t1: 1.5 to 2.5 clock periods t2: 2.5 clock periods minimum (2.777 microseconds)FIGURE 3.1.2. WIRE Serial Interface Timing Specification3.1.3 Handling of Overflow ConditionNormally the time between the WIE asserting data ready and the rising edge of the first clock pulse from the SCS will be between 1.5 and 2.5 clock periods. If the SCS detects a FIFO overflow condition, the clock will be changed to a static low condition, to indicate that the SCS is not ready to receive data. Anytime that the WIE is looking for a clock and can't find it for 8 clock periods, the WIE will time-out and bring the data ready down within one clock period. The SCS will reset the FIFO and send a "reset" message to the WIE over the 1553. The WIE will resume transmission of data at the beginning of the next frame, as soon as this reset message is received from the SCS.
3.1.4 Image Data Format
The WIE shall transmit Image data in transfers exactly 1024 bytes long over the high speed serial interface. If the data being transferred is shorter than 1024 bytes, the WIE will add fill data to the transmission to make it 1024 bytes long. The fill data pattern is zeros. These Image transfers will be packetized into CCSDS packets by the SCS, one packet per transfer. The format of the application data in these packets will be transparent to the spacecraft.
3.1.4.1 Time Stamp
The SCS will place the current spacecraft time into the time stamp in the secondary header of each packet. The time stamp will reflect the time that the packet is received by the SCS. Therefore, each packet in a frame will have a slightly different time than the other packets in the frame. The SCS will also assign a sequence number to each packet.
3.1.4.2 Verifying Data Integrity
The SCS will perform no validity checking on the format of data received from the instrument. All data received from the instrument will be processed as valid.
3.1.5 Reuse from the TRACE mission
The TRACE mission is currently developing a RS-422 high speed serial I/F capability for SMEX, which will be reused for the WIRE mission. The hardware portion of the RS-422 high speed serial I/F should be identical to the TRACE implementation. However, the software implementation will be customized for the transmission of WIRE data.
3.2 1553 Data Bus Implementation
3.2.1 Description of 1553 Interface
One communication path between the Spacecraft processors and the WIRE Instrument is the MIL-STD-1553B Data Bus. All onboard data is transmitted via this data bus, except for image data (and some housekeeping) which is sent via the RS-422. All data sent to the spacecraft on the MIL-STD-1553B Data Bus is routed through the SCS to the appropriate destination, through use of subaddresses.
1553 WIRE instrument data consists of commands, telemetry and messages. Reference WIRE-ICD-002, "WIRE 1553 Data Bus Implementation" for a description of the details of this interface.
3.2.2 Time Management Interface
The SCS shall send a time synchronize command to the WIE at TBD intervals over the 1553 interface. The WIE shall restart its subsecond timer after each time synchronize command.
There will be no hardware interface to the Spacecraft provided for the transmission of time (no 1Hz pulse).
3.3 Space to Ground Data Interface
Reference WIRE-SPEC-005, "SMEX/WIRE Telemetry and Command Handbook" for a description of this interface.
4 Electrical Interface
4.1 High Speed Serial Interface Electrical Signal Specification
The three electrical signals shall conform to the EIA-422 specification. The RS-422 is a balanced two wire differential data transmission. All signals will be differential TTL lines with a line impedance of 130 ohms. The logic levels for the signals will be 0 to 0.4 volts for a low state (logic 0) and 2.4 to 5 volts for a high state (logic 1). The clock will be a 50% duty cycle at 900kHz. The cable will be twisted pair 24 AWG or larger and will not exceed 40 feet in length.
4.2 TBD
5 Acronyms
ACS Attitude Control System
AWG Average Wire Gauge
ATS Absolute Time Sequence
CCSDS Consultative Committee on Space Data Systems
CRC Cyclic Redundancy Check
C&DH Command and Data Handling
DSN Deep Space Network
EOF Experiment Operations Facility (TRACE)
FIFO First In First Out
FLEX Follow On Small Explorer
FPA Focal Point Array
FOT Flight Operations Team
GFSC Goddard Space Flight Center
Hz Hertz
IC Instrument Controller
ICD Interface Control Document
I/F Interface
IOC Initial Operational Checkout
IPAC Infrared Processing Analysis Center
I&T GSE Integration and Test Ground Support Equipment
JPL Jet Propulsion Laboratory
LSB Least Significant Bit
LZP Level Zero Processing
NRZ-L Non-Return to Zero Level
NSSDC National Space Science Data Center
PDMP Project Data Management Plan
PI Principal Investigator
RA Right Ascension
RTS Relative Time Sequence
SAA South Atlantic Anomaly
SCS Spacecraft Computer System (Spacecraft)
SDL Space Dynamics Laboratory
SMEX SMall EXplorer
SOC Science Operations Center
SW Software
SWAS Submillimeter Wave Astronomy Satellite
TBD To Be Determined
TRACE Transition Regional and Coronal Explorer
TTL Transistor-Transistor Logic
WIE Wire Instrument Electronics
WIRE Wide-field Infra-Red Explorer
TRACE Instrument to Spacecraft Data Format ICD
TRACE-ICD-001
DRAFT
WIRE Instrument to Spacecraft Computer System ICD
WIRE-ICD-001
DRAFT