| Date: | 1997-04-08 19:00:00 | |
| Sender: | Andrew Sinclair, Van Husson <[Mailed Andy Sinclair <ats@ast.cam.ac.uk>]> | |
| Subject: | [SLR-Mail] No. 47: CSTG NORMAL POINT FORMAT | |
| Author: | Andrew Sinclair, Van Husson | |
| Content: | ******************************************************************************** SLR Electronic Mail 1997-04-08 19:00:00 UTC Message No. 47 ******************************************************************************** Author: Andrew Sinclair, Van Husson Subject: CSTG NORMAL POINT FORMAT CSTG SLR NORMAL POINT FORMAT CHANGES, AND EXTENSION TO LLR Modifications to the description of the format in order to include some minor changes approved at Bern and Shanghai, and in an appendix, some minor additions in order to include LLR data. Andrew T. Sinclair, Van S. Husson. 1997 March 1. Introduction The CSTG normal point format is described in the SLR Newsletter of the SLR Subcommission of the CSTG, dated April 1990. The format was originally intended for the transmission of quick-look data, but now that full-rate data has been phased out, the site-produced normal points which use this format have become the principal SLR data product. Prior to this transition there had been discussion of the need to revise the format, in order to include additional information that would be lost when full-rate data was no longer available. A proposed revision of the format was put forward by Van Husson at the Laser Ranging Workshop in Canberra in November 1994, and was further discussed at the Eurolas meeting in Munich in March 1995. However at the SLR CSTG Subcommission meeting in Bern in December 1995 it was decided not to make major changes to the format at this time, but just to make some minor revisions to a few of the data items. At the SLR CSTG Subcommission meeting in Shanghai in November 1996 a few further minor revisions were agreed. A major revision of the format will ultimately be needed as more stations move towards operation at single photon return level (e.g., SLR2000, KEYSTONE), and more experience is gained with corrections for satellite signature effects, which are mainly seen at single photon levels. The purpose of this note is to list the changes that have been agreed, and to give the new wording to describe the revised data items in the formal description of the format. This note gives the new wording, and also the old wording, for ease of comparison. Thus it is intended to be an aid to implementing the changes, both for the stations and the data analysts. Subsequently a revised version of the complete format will be produced, and placed in the data centres. 2. Outline description of the changes Changes agreed at Bern: Use of the term RMS (root mean square) throughout instead of a mixture of RMS and standard deviation, A modification to the wavelength field to permit wavelengths greater than 1 micron, A data sequence flag in the data records, to indicate if the data are the original or a replacement set. Changes agreed at Shanghai: Merging the calibration method and calibration shift-type indicators in the header record into a single indicator, in order to free a field, Clarification and implementation of the existing configuration flag, as some systems have alternative operational configurations according to the satellite being tracked (e.g., use of a PMT or APD detector), Inclusion of a system change indicator, which will be incremented each time there is a significant change to items of equipment or software, Decision to put the data sequence flag (agreed at Bern) in the header record instead of the data record (however on consideration we have realised that it is better in the data record (as agreed at Bern), so this is what we have adopted - see note * below.) Inclusion of a format revision flag in the header record. Information describing the various configurations of a system and recording changes made to the system will be maintained in log files in a standard format, and these will be held at the data centres and also at the station. The details are described in a separate note, ”SLR SYSTEM CONFIGURATIONS AND SYSTEM CHANGES”. The changes agreed at Bern were described in the CSTG SLR Newsletter dated May 1996. In this note we have slightly revised the description given in that Newsletter for the wavelength field, which specifies that a wavelength value of 0001 to 2999 is to be interpreted as being in units of 1.0 nm, whereas we restrict this range of values to 1000 to 2999. The original form is not inaccurate, but does permit, for example, a wavelength of 0.532 microns to be specified as 0532 or 5320. (* The header record does not contain the time of day of a pass, and so does not uniquely define a pass. Thus it is not possible by comparing header records and data sequence flags alone to remove passes that have been replaced.) 3. Some remarks 3.1 Trend Function. The original wording of the format (April 1990) uses the expression ´trend function´, and this is continued unchanged in this document. This is a short-hand term for a concept originally described in the recommendations for formation of normal points, proposed at the laser ranging workshop in Herstmonceux in 1984. We repeat here the relevant wording: ´Solve for a set of parameters (orbital parameters preferable) to remove the systematic trends of the prediction residuals, not introducing spurious high-frequency signals into the TREND-FUNCTION´. 3.2 Checksum. It is generally agreed that the checksum is now redundant, and in fact whenever it indicates an error it is invariably due to an error in forming the checksum, rather than a data transmission error. Thus stations can if they wish leave this field blank, but if they continue with it then it should still be the sum of integers in columns 1-52, modulo 100. i.e., the new field in column 55 of the header record should not be included. 3.3 Lunar Laser Ranging data. It is proposed to use this CSTG normal point format for LLR data also, which can be accomplished by just a few additional flags, and corresponding changes to the interpretation of some data fields. These changes are described in an appendix to this note. 3.4 Replacement data releases. A flag has been included to permit indication of replacement data releases, should they be necessary. Stations should be aware that replacement data causes problems for data centers and analysts, and should make strong efforts to get the data right first time. --------------------------------------------------------------- 4. Revised wordings of descriptions of items that have been changed HEADER RECORD: cols 21-24: Wavelength of the laser. The user of the data should interpret the value given as follows: Value in range 3000 to 9999: units are 0.1 nm Value in range 1000 to 2999: units are 1.0 nm For the station generating the data, the rule is: Wavelength in range 0.3000 to 0.9999 microns: unit 0.1 nm Wavelength in range 1.000 to 2.999 microns: unit 1.0 nm cols 39-42: RMS of raw system delay values from the mean. Two-way value in picosec. If pre- and post-pass calibrations are made, use the mean of the two RMS values, or the RMS of the combined data set. col 45: System calibration method and delay shift indicator. Indicates the type of calibration, and the type of calibration shift given in cols 33-38. pre- to post-pass minimum to maximum calibration shift calibration shift external cal 0 5 internal cal 1 6 burst cal 2 7 some other cal 3 8 not used 4 9 col 46: System CHange indicator (SCH) - a flag to increment for every major change to the system (hardware or software). After the value ´9´ return to ´0´, and then continue incrementing. The station and data centres should keep a log in a standard format of the value used, the date of the change, and a description of the change. col 47: System Configuration Indicator (SCI). A flag used to indicate alternative modes of operation for a system (e.g., choice of alternative timers or detectors, or use of a different mode of operation for high satellites). Each value of the flag indicates a particular configuration, which is described in a log file held at the station and at the data centres. If only a single configuration is used then use a fixed value. If a new configuration is introduced then use next higher flag value. If value exceeds ´9´ then return to ´0´, overwriting a previous configuration flag (it is not likely that a station will have 10 current possible configurations). cols 48-51: Pass RMS from the mean of raw range values minus the trend function, for accepted ranges. Two-way value in picosec. col 55: Format revision number indicator. Value ´1´ for this 1997 revision. Implied value ´0´ or ´space´ for original 1990 release. DATA RECORD: cols 25-31: Bin RMS from the mean of raw range values minus the trend function, for accepted ranges. Two-way value in picosec. If point is a single raw data point then use pass RMS. col 48: A flag to indicate the data release: 0 = first release of the data 1 = first replacement release of the data, 2 = second replacement release, etc. -------------------------------------------------------------- 5. Original descriptions of the items that have been changed For ease of comparison, the original wordings are given below: HEADER RECORD: cols 21-24: Wavelength of the laser in units of 0.1 nanometer cols 39-42: Standard deviation of system delay value. Two-way value in picosec. col 45: System calibration method indicator 0 = external calibration 1 = internal calibration 2 = burst calibration (3-minute bursts) 3 = override calibration col 46: Calibration shift indicator - specifies the type of delay shift given in columns 33-38 0 = pre- to post-shift 1 = minimum to maximum shift col 47: System configuration flag indicator - a flag to increment for every major system configuration change. The same value that is used for full-rate data in MERIT II format. cols 48-51: Pass rms of accepted raw ranges. Two-way value in picosec. col 55: Unused (blank or space) DATA RECORD: cols 25-31: Standard deviation of two-way time-of-flight, in picosec. Calculated from Herstmonceux formula for a normal point, or use pass rms for a raw data point. col 48: Unused (zero-filled) ___________________________________________________________ APPENDIX Changes to CSTG Normal Point Format in order to accommodate LLR data. Randall L. Ricklefs, Van S. Husson, Andrew T. Sinclair, Peter J. Shelus 1997 March At the Laser Ranging Subcommission Meeting in Shanghai it was recommended that Lunar Laser Ranging data should be set in the same format as SLR normal points, in order to make the data more easily available to a wider analysis community, and to encourage more SLR stations to attempt ranging to the Moon. Only a few fairly minor changes are needed to the SLR format in order to accommodate LLR data, and these are listed in this note. LLR normalpoint data does not now explicitly record values of calibration, calibration standard deviation, and calibration shift, so these fields may be zero filled. Individual stations may choose to include this information if available. ------------------------------------------------------- HEADER RECORD Col 43: For SLR data: normal point window indicator (an integer from 0 to 9) For LLR data: the integer ´2´ (this value is not used in practice by SLR) Col 52: For SLR data: not in general use, but is used by NASA stations as an indicator of data quality. For LLR data: data quality assessment indicator (this could be adopted by all SLR stations). 0 Undefined or no comment. 1 Clear, easily filtered data, with little or no noise. 2 Clear data with some noise; filtering is slightly compromised by noise level. 3 Clear data with a significant amount of noise, or weak data with little noise. Data are certainly present, but filtering is difficult. 4 Un-clear data; data appear marginally to be present, but are very difficult to separate from noise during filtering. Signal to noise ratio can be less than 1:1. 5 No data apparent. DATA RECORD Col 49: For SLR data: not used For LLR data: integer seconds of the two-way time of flight (cols 13-24 contain the fractional part). Col 50: For SLR data: not used For LLR data: Normal point window indicator - indicates the time span of the normal point (can be variable from point to point). 1 <= 5 minutes 2 = 10 minutes 3 = 15 minutes ... 9 >= 50 minutes Cols 51-52: For SLR data: not used For LLR data: Signal to noise ratio, in units of 0.1 e.g. 00 No information 01 Signal/noise = 0.1 99 Signal/noise = 9.9 or greater [Mailed From: Andy Sinclair ******************************************************************************** |