TAUVEX Data Reception
From Tauwiki
| Author | Jayant Murthy |
| Date Created | 27th January, 2005 |
| Last Modified | 19th March, 2005 |
| Version | IIA_TAU_data_reception_V1.0 |
Document Scope: This document addresses the transfer and
conversion of data from the spacecraft to a format suitable for
ingest into the TAUVEX pipeline.
Contents |
[edit] Overview
TAUVEX is an astronomy mission intended to survey the ultraviolet sky in 3 bands. TAUVEX will launch with the GSAT-4 platform in mid-2006 and will obtain approximately 5 GB of data per day including engineering data. The TAUVEX pipeline software will produce data products usable by the general scientific community from the Level 0 data which comes to the ground station at MCF in Hasan. This document is intended to address the transfer of data and information from the spacecraft to IIA.
[edit] Observing Modes
The nominal mode of operation will be to observe for 53 minutes through one of the filters in HKI (House Keeping Inhibit) mode when the solar panel motion is disabled followed by 7 minutes of observation during HK (House Keeping) mode when the solar panel position is adjusted for maximum power. There may be a significant impact on the TAUVEX pointing while the solar panels are in motion and we have chosen these points to be "breaks" in the TAUVEX observation cycle. Thus, for the purposes of the data pipeline, we define a TAUVEX observation as a 53 minute observation of the sky through a science filter with a fixed MDP (Mounting Deck Platform) position. The HK data will be processed and archived separately. Note that this definition of an observation is independent of the actual operation of the instrument; that is, TAUVEX, itself, may continue to observe the sky during HK mode. The information about the mode of operation and other TAUVEX events may come from either the TAUVEX event log or from the s/c telemetry.
[edit] TAUVEX Data Transfer
The raw data from the instrument is transmitted through the spacecraft telemetry system to the receiving station at MCF in Hasan and will constitute the Level 0 data. The spacecraft checkout group (SCHEMACS) will merge the TAUVEX data with the s/c telemetry to form the Level 1 data. The filler data and the sync frame can be stripped out at MCF reducing the burden on transmission and storage. In addition, the data can be compressed further using standard algorithms such as gzip or zip. These data will be transferred to IIA through a mechanism yet to be determined. The current mode of transfer is by DVD once per week but the possibility of transferring over the internet will be explored. A QLF for the TAUVEX data will be designed and built by SCHEMACS based on our input. A low (128 kbps) line will be available for direct transmission of data to IIA and will be used for monitoring of key parameters and monitoring of the health of the instrument.
The TAUVEX science data will be transmitted from the spacecraft at a
constant rate of 1 Mb/s, including science data, engineering
parameters and filler for a total size of about 80 Gb/day. We
anticipate that after processing at MCF and taking into account the
duty cycle of the instrument, the effective data rate for the Level 1
data will be less than 250 kbps.
[edit] TAUVEX Data Format
Transmission of the data begins when TAUVEX is in either Imaging or CLBR mode and ceases when a STOP_IMAGING command is sent to TAUVEX . Data transmission begins with 200 kbits of filler data consisting of 0101. This is followed by the Start Log Data consisting of N major frames. After the end of the science data transmission comes the End Log Data. The format of the tables is in the Appendix with the last two bits of each segment being 01. All GSAT-4 commands relating to TAUVEX are logged in the Event Log including MDP movement, thruster firing and other events. The science data processing will have to take account of these events.
The science data will also be in the form of major frames with the only difference being that the last two bits of each segment being 10. Because the science data may not fill the 1 Mbps data stream, the data are padded with filler containing only 01. Data and filler will not be mixed in a major frame; that is, a frame will only contain data if there are enough data to fill the entire frame. Thus the last data frame may be discarded if there are not enough data to fill the frame.
The last two bits of each data segment will be 10 and the first 2 will be:
00 for time tag 01 for telescope #1 02 for telescope #2 03 for telescope #3
In the case of time tag data, the last 20 bits of each segment will be the time in units of 128 ms where the MSB is bit 20 and LSB is bit 1 . The telescope data always consists of 36 bits with w, s, and z being 12 bits each. If the TAUVEX mode is CLBR, w0, s0, and z0 will be read instead. If we define:
W = w - w0 S = S - s0 Z = z - z0
then
x = W/(W + S + Z)*2048 - x0 y = S/(W + S + Z)*2048 - y0
assuming a pixelization of 1024 x 1024.
x0 and y0 will be determined during the instrument calibration and will not be changed during the mission. We reject pulses if W+S+Z is either too low or too high.
[edit] Data required from Spacecraft Telemetry
- Data sent by TAUVEX on 1553 bus (64 bytes every 64 seconds).
- Attitude information.
- HK/HKI times.
- Thruster firings.
- Time tags.
We also require the mission timeline.
[edit] Processing required at MCF
The TAUVEX science data must be stripped of filler and sync and output as a continuous stream. Additional information from the spacecraft telemetry should be stripped and written as a separate file. Parameters which must be monitored continuously will be listed separately and may require graphical interfaces with alerts for critical parameters.
[edit] Pipeline Software: Ingest
We assume that we will get two files from MCF: the TAUVEX science data and the s/c telemetry file which contains selected parameters. The TAUVEX data will be in the format defined above with no filler and no synch data. The telemetry data is in a TBD format.
