Index of /hsa/legacy/operations/telemetry

Name	Last modified	Size

Parent Directory		-
tmfiles/	2016-11-14 16:54	-

Herschel Telemetry Files

This directory contains telemetry data from Herschel for both science and housekeeping.

The data was originally received from the Data Disposition System (DDS). It has since been cleaned up by sorting, removing duplicates and filling gaps where possible.

The files are normal DDS (GDDS_BINARY) telemetry files but the data has been organised as described below for use with the HCSS "tmfiles" Java package. This can return a stream of packets for a specified APID and time range. It can also retrieve packets for given observation ID, using the index files in the 'meta' directory.

Format/ICD

Telemetry and Telecommand Packet Utilisation Standard, ECSS-E-70/41, ECSS, 1999.
Herschel/Planck Packet Structure ICD, SCI-PT-ICD-07527, issue 5.0, 2004-07-20.
Herschel-Planck Mission Control System ICD: Data Disposition System, PT-CMOC-MDS-3108-OPS-GDS, issue 1.9, 2010-03-04.

Data Structure

The telemetry is organized as one directory per APID. Within these, the data is organized as a set of files, each covering an interval of 16384 seconds. The file names are of the form "0018_106236016066560.TLM", where 0018 is the APID and 106236016066560 is the onboard (CUC) time of the start of the period. This allows the correct file for any given APID and time to be located very quickly. If a time interval contains no packets, the file is omitted, rather than including an empty file.

Each file contains a sequence of binary DDS packets. A DDS packet is a binary PUS TM packet with an additional 18-byte DDS header that includes the SCET time, source sequence count (SSC) and packet size. The packets are ordered by CUC time. Packets with the same CUC time are further ordered by SSC, taking wrap-around into account.

The directory "meta" contains some index files that are used to support the search process:

The file OBS_INDEX is a table that allows the time-range of an observation to be looked up, so that packets can be retrieved by OBSID. It contains the scheduled start and end times for each Herschel observation. In the case of manual commanding, the times are the SCET times of the first and last packet received. The file also contains a list of the APIDs used for each observation. This avoids the need to read the files for unused APIDs. It also includes the Operational Day (OD) and instrument.
File APID_INDEX contains the times of the first and last packets for each APID.

Known Issues

The OBS_INDEX gives the scheduled observation times in seconds since the TAI epoch of 1 January 1958. The TM files are indexed by onboard (CUC) time which is in units of 1/65536 seconds. Using the index to retrieve TM data requires correcting for the onboard clock drift and initial offset of -34 seconds with respect to TAI. This is taken care of in the "tmfiles" software.