WEC meeting TED status and WEC timing

WEC timing TED status - and future plans v v2.4.4 Includes support for the new additional ground station IDs used for the extended mission coverage v3.0 Substantial rewrite. Now includes microsecond timing corrections for DWP, but many new bugs Compilation bugs and microsecond instability have been fixed (by CETP and Sheffield respectively) v3.1 Fix remaining bugs in version 3.0. Unclear if this will also include support for RDM time corrections (TCOR files) In the event that termination of PPARC funding for Sussex is confirmed and they are unable to continue TED maintenance, then Sheffield will take over. If TED status is still not satisfactory at this time, we will review v2.4.4 and v3.1 and release a new version based on the one that offers the easiest path to the required functions without bugs.

WEC timing Questions Who is using TED and which versions? Are there any other issues (apart from microsecond timing) that need addressing? asking for comments sent by Ian Bates on 23 December. Only STAFF has replied.

WEC timing Overview of WEC timing WEC timing is co-ordinated on board the spacecraft using the VC0 reset pulse. The UTC of this pulse is provided as the SCET (spacecraft event time) in the HK packet headers. WEC data is timed relative to this pulse by counting the number of cycles of the DWP 900Hz master clock. This clock count is included in the science packet headers, together with a modulo 8 count of the reset pulse, so the correct HK packet can be identified. WEC time = SCET + MC count / 900 See reference [3] for more details.

WEC timing Accuracy of WEC time tags The Spacecraft event time (SCET) given in the raw data (RDM) may have timing errors up to 2ms due to the drift between the on board time (OBT) and UTC. This term is called DIFF (ref [1] and [2]). Additionally, when data is recorded to the solid state recorder (SSR), the OBT is not precisely correlated with the VC0 reset pulse, but may be offset by up to +/– 360 µs. This is called OFFSET and may change at each telemetry mode change. For most WEC data the final accuracy is limited to ms by the resolution of the DWP master clock. EFW and STAFF waveform (MWF) sampling is synchronised to the DWP 900Hz master clock, so higher accuracy is possible provided the frequency and phase of this clock is accurately known.

WEC timing More accurate timing - TCOR files Correct errors in SCET. The time correction (TCOR) files specify DIFF and OFFSET using information from ESOC and Wideband. Produced by the DWP team in text, binary, and CEF formats. Data for first quarter of 2004 is loaded in the CAA. Also available at: Data for the rest of 2004 and up to July 2005 is in production and will be distributed when available. For earlier dates we are waiting on the availability of source data from ESOC and Wideband. Current data is organised according to the intervals between ESOC time correlations. For ease of application we propose that new data will be organised by quarter years.

WEC timing

WEC timing

WEC timing More accurate timing - 900Hz clock phase The HK parameter EW5SSOFF measures the time between the VC0 reset pulse and the following 900Hz clock pulse. All Cluster TED implementations include this in the time calculation. However, the measurement can be subject to large errors (100’s of μs) when particle correlator or STAFF MWF sampling interrupt the process. A new TED module HKTIM has been developed which identifies these erroneous values and replaces them by extrapolation. Averaging of successive values is also possible to further reduce errors. This module is included in TED v3.0, but does not work correctly. Fix available, will be in TED v3.1.

WEC timing More accurate timing - 900Hz clock frequency The actual frequencies of the 900Hz clock are now: SC1: Hz SC2: Hz SC3: Hz SC4: Hz These have been stable to within Hz since the start of the mission. Using the nominal rather than the actual frequency can result in timing errors up to 120 μs. Versions of TED prior to 3.0 use only the nominal frequency. The actual frequency can be computed from successive phase measurements (EW5SSOFF), and this is done by TED 3.0.

WEC timing Implementation of accurate timing Correcting the SCET using the TCOR files. This may be done at any stage in the data processing chain. Eg. by pre-processing the raw data, by TED (if implemented in new version), or by post-processing the calibrated data. Where ever it is done the OFFSET and DIFF from the TCOR file are added to the time tag of the data. 900Hz clock phase and frequency. This can only be done in TED. It may be important for data users to know what time corrections have been applied - TCOR not always available. Where the SCET is corrected by pre-processing the raw data, it is proposed to add 4 to the 'Time Quality' field of the DDS header (byte 14, bits 0..3), making it 4, 5, or 6 (all presently unused). For files of Decommutated Science Data produced by the TED ‘levelone’ software, it is proposed to define two new bits in the diagnostic word of the header. These will indicate if the SCET and 900Hz master clock corrections have been applied.

WEC timing Events marked by WEC time tags For STAFF MWF: One master clock cycle (approx μs) after the sample time for the first waveform sample in the packet. In other words to get the time for the first sample 1111 μs should be subtracted from the time tag (see ref. [4]). The time of the other samples in the packet may be obtained using the frequency of the sample clock (25 or 450Hz nominal). The frequency of this clock will be subject to proportionally the same frequency errors as the DWP 900Hz master clock. For EFW: the active transition of the WEC sample clock, following the transmission of the ‘sync byte’ from DWP to EFW, that precedes transmission of the data packet(s) from EFW to DWP. To obtain sample times for EFW data, the delays within EFW must be accounted for. The sample clock (25 or 450Hz) and AD clock (36kHz) supplied by DWP to EFW will be subject to proportionally the same frequency errors as the 900Hz master clock.

WEC timing Reference documents [1] CL-OPS-RP-1002-OPS-OFC, “TIME CORRELATION TEST REPORT:”, David Fornarelli, 18 Feb 2004 [2] CLU2-TN-240, “Time stamping of Science Data, B. Fairclough (Anite), 15 Jan 2003 [3] CL-WEC-UM-002, WEC User Manual, Chapter 1 [4] Verification of STAFF MWF and EFW time tagging in DWP, Keith Yearby, 30 June 2005