PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2005-07-27 INTERCHANGE_FORMAT = ASCII NOTE = "Explanation of time-related issues in MER archives." END_OBJECT = TEXT END MER Time Tag Issues Boris Semenov, NAIF July 29, 2004 Revised January 21, 2005 and July 27, 2005 MER Time Tag Issues =========================================================================== This document summarizes a number of issues related to various types of time tags used in the archived MER science data products. Error in On-board Clock Correlation -------------------------------------------------------- The official rover on-board clock correlation to UTC used in operations and for science archive preparation is incorrect in an absolute sense by as much as 20 seconds during some periods. The error stems from a combinations of reasons: the rover generated too few time correlation packets, the specifics of the process used to derive the correlation function, and the requirement for backward compatibility between correlation versions imposed by the ground system tools. Until an improved version of the on-board clock correlation becomes available and science product labels are regenerated using it, these tables providing on-board clock time, the corresponding UTC computed using the official correlation, and the correction, in seconds, to be added to the UTC computed using the official correlation, can be used to compensate for the error in the UTC time tags attached to the science data products. Linear interpolation can be used to estimate a correction value between the table points. MER-1 On-board Clock Correlation Correction table: SCLK UTC Correction ------------- ------------------- ------------ 122714000.000 2003-11-21T19:12:10 0.00000000 128538000.000 2004-01-28T04:58:47 0.35045579 128539000.000 2004-01-28T05:15:27 0.00000000 129171000.000 2004-02-04T12:48:47 6.03954345 129172000.000 2004-02-04T13:05:33 0.00000000 129521000.000 2004-02-08T14:02:17 -0.07783715 129522000.000 2004-02-08T14:18:56 0.00000000 129881000.000 2004-02-12T18:02:20 -0.74451666 129882000.000 2004-02-12T18:18:59 0.00000000 131747000.000 2004-03-05T08:22:30 -2.78524545 131748000.000 2004-03-05T08:39:07 0.00000000 137232000.000 2004-05-07T19:59:27 -21.78271598 137233000.000 2004-05-07T20:15:45 0.00000000 143539000.000 2004-07-19T19:55:34 -23.00951022 143540000.000 2004-07-19T20:11:51 0.00000000 147623000.000 2004-09-05T02:21:28 3.86354187 147624000.000 2004-09-05T02:38:12 0.00000000 154017000.000 2004-11-18T02:27:44 20.39812636 154018000.000 2004-11-18T02:44:45 0.00000000 154195000.000 2004-11-20T03:54:45 2.34983149 154196000.000 2004-11-20T04:11:27 0.00000000 161118000.000 2005-02-08T06:58:25 -18.98187968 161119000.000 2005-02-08T07:14:46 0.00000000 166353000.000 2005-04-09T21:08:06 8.60842806 166354000.000 2005-04-09T21:24:54 0.00000000 166709000.000 2005-04-14T00:01:35 0.57460395 166710000.000 2005-04-14T00:18:16 0.00000000 MER-2 On-board Clock Correlation Correction table: SCLK UTC Correction ------------- ------------------- ------------ 126091000.000 2003-12-30T21:15:07 0.00000000 127259000.000 2004-01-13T09:41:45 5.31779034 127260000.000 2004-01-13T09:58:31 0.00000000 129032000.000 2004-02-02T22:12:09 -3.41549060 129033000.000 2004-02-02T22:28:46 0.00000000 130349000.000 2004-02-18T04:02:16 -0.90037587 130350000.000 2004-02-18T04:18:56 0.00000000 130793000.000 2004-02-23T07:22:19 -1.16488062 130794000.000 2004-02-23T07:38:58 0.00000000 133470000.000 2004-03-25T06:59:09 -4.57467157 133471000.000 2004-03-25T07:15:45 0.00000000 134539000.000 2004-04-06T15:55:47 -2.56826568 134540000.000 2004-04-06T16:12:24 0.00000000 137118000.000 2004-05-06T12:19:03 -7.99848235 137119000.000 2004-05-06T12:35:35 0.00000000 138620000.000 2004-05-23T21:32:10 -7.62354127 138621000.000 2004-05-23T21:48:42 0.00000000 144743000.000 2004-08-02T18:21:11 -18.77159578 144744000.000 2004-08-02T18:37:32 0.00000000 161963000.000 2005-02-18T01:37:36 12.00370592 161964000.000 2005-02-18T01:54:28 0.00000000 163650000.000 2005-03-09T14:14:11 6.30416778 163651000.000 2005-03-09T14:30:58 0.00000000 166225000.000 2005-04-08T09:30:42 26.56784931 166226000.000 2005-04-08T09:47:48 0.00000000 167734000.000 2005-04-25T20:41:15 2.39826447 167735000.000 2005-04-25T20:57:57 0.00000000 True Local Solar Time vs. Hybrid Solar Local Time -------------------------------------------------------- For surface operations the MER project adopted a Hybrid Local Solar Time (HLST) time system as the Mars local time system to be used in sequencing and planning. HLST was introduced as a replacement of the True Local Solar Time (TLST) because a time system with a constant duration of a second was needed. The HLST is essentially a linear approximation of the true local time with the rate set equal to the mean Mars local second duration and the initial reference point set to make sure that HLST deviates from TLST by no more that 15 local minutes during the primary mission. This approximation continued to deviate from the TLST as the surface operations went on, reaching over 50 local minutes difference by the end of the extended mission. The tables below illustrate the difference between TLST and HLST. MER-1 TLST vs. HLST table True LST Hybrid LST ---------------- ---------------- SOL-001-12:00:00 SOL-001-12:11:04 SOL-010-12:00:00 SOL-010-12:09:46 SOL-020-12:00:00 SOL-020-12:07:49 SOL-030-12:00:00 SOL-030-12:05:23 SOL-040-12:00:00 SOL-040-12:02:36 SOL-050-12:00:00 SOL-050-11:59:30 SOL-060-12:00:00 SOL-060-11:56:10 SOL-070-12:00:00 SOL-070-11:52:39 SOL-080-12:00:00 SOL-080-11:49:01 SOL-090-12:00:00 SOL-090-11:45:20 SOL-100-12:00:00 SOL-100-11:41:39 SOL-110-12:00:00 SOL-110-11:38:00 SOL-120-12:00:00 SOL-120-11:34:26 SOL-130-12:00:00 SOL-130-11:31:00 SOL-140-12:00:00 SOL-140-11:27:41 SOL-150-12:00:00 SOL-150-11:24:34 SOL-160-12:00:00 SOL-160-11:21:37 SOL-170-12:00:00 SOL-170-11:18:54 SOL-180-12:00:00 SOL-180-11:16:23 SOL-190-12:00:00 SOL-190-11:14:05 SOL-200-12:00:00 SOL-200-11:11:58 SOL-210-12:00:00 SOL-210-11:10:04 SOL-220-12:00:00 SOL-220-11:08:18 SOL-230-12:00:00 SOL-230-11:06:43 SOL-240-12:00:00 SOL-240-11:05:13 SOL-250-12:00:00 SOL-250-11:03:50 SOL-260-12:00:00 SOL-260-11:02:29 SOL-270-12:00:00 SOL-270-11:01:09 SOL-280-12:00:00 SOL-280-10:59:50 SOL-290-12:00:00 SOL-290-10:58:29 SOL-300-12:00:00 SOL-300-10:57:06 SOL-310-12:00:00 SOL-310-10:55:39 SOL-320-12:00:00 SOL-320-10:54:10 SOL-330-12:00:00 SOL-330-10:52:36 SOL-340-12:00:00 SOL-340-10:51:01 SOL-350-12:00:00 SOL-350-10:49:24 SOL-360-12:00:00 SOL-360-10:47:49 SOL-370-12:00:00 SOL-370-10:46:15 SOL-380-12:00:00 SOL-380-10:44:48 SOL-390-12:00:00 SOL-390-10:43:29 SOL-400-12:00:00 SOL-400-10:42:24 SOL-410-12:00:00 SOL-410-10:41:34 SOL-420-12:00:00 SOL-420-10:41:06 SOL-430-12:00:00 SOL-430-10:41:02 SOL-440-12:00:00 SOL-440-10:41:29 SOL-450-12:00:00 SOL-450-10:42:29 SOL-460-12:00:00 SOL-460-10:44:09 SOL-470-12:00:00 SOL-470-10:46:31 SOL-480-12:00:00 SOL-480-10:49:39 SOL-490-12:00:00 SOL-490-10:53:34 SOL-500-12:00:00 SOL-500-10:58:14 SOL-510-12:00:00 SOL-510-11:03:40 SOL-520-12:00:00 SOL-520-11:09:43 SOL-530-12:00:00 SOL-530-11:16:19 SOL-540-12:00:00 SOL-540-11:23:16 SOL-550-12:00:00 SOL-550-11:30:21 SOL-560-12:00:00 SOL-560-11:37:24 SOL-570-12:00:00 SOL-570-11:44:08 SOL-580-12:00:00 SOL-580-11:50:24 SOL-590-12:00:00 SOL-590-11:56:01 SOL-600-12:00:00 SOL-600-12:00:52 MER-2 TLST vs. HLST table: True LST Hybrid LST ---------------- ---------------- SOL-001-12:00:00 SOL-001-12:09:17 SOL-010-12:00:00 SOL-010-12:09:11 SOL-020-12:00:00 SOL-020-12:08:23 SOL-030-12:00:00 SOL-030-12:07:00 SOL-040-12:00:00 SOL-040-12:05:03 SOL-050-12:00:00 SOL-050-12:02:40 SOL-060-12:00:00 SOL-060-11:59:53 SOL-070-12:00:00 SOL-070-11:56:48 SOL-080-12:00:00 SOL-080-11:53:28 SOL-090-12:00:00 SOL-090-11:49:58 SOL-100-12:00:00 SOL-100-11:46:21 SOL-110-12:00:00 SOL-110-11:42:40 SOL-120-12:00:00 SOL-120-11:38:59 SOL-130-12:00:00 SOL-130-11:35:19 SOL-140-12:00:00 SOL-140-11:31:46 SOL-150-12:00:00 SOL-150-11:28:18 SOL-160-12:00:00 SOL-160-11:25:00 SOL-170-12:00:00 SOL-170-11:21:51 SOL-180-12:00:00 SOL-180-11:18:55 SOL-190-12:00:00 SOL-190-11:16:10 SOL-200-12:00:00 SOL-200-11:13:39 SOL-210-12:00:00 SOL-210-11:11:20 SOL-220-12:00:00 SOL-220-11:09:13 SOL-230-12:00:00 SOL-230-11:07:17 SOL-240-12:00:00 SOL-240-11:05:32 SOL-250-12:00:00 SOL-250-11:03:55 SOL-260-12:00:00 SOL-260-11:02:26 SOL-270-12:00:00 SOL-270-11:01:02 SOL-280-12:00:00 SOL-280-10:59:41 SOL-290-12:00:00 SOL-290-10:58:22 SOL-300-12:00:00 SOL-300-10:57:03 SOL-310-12:00:00 SOL-310-10:55:42 SOL-320-12:00:00 SOL-320-10:54:19 SOL-330-12:00:00 SOL-330-10:52:53 SOL-340-12:00:00 SOL-340-10:51:23 SOL-350-12:00:00 SOL-350-10:49:50 SOL-360-12:00:00 SOL-360-10:48:14 SOL-370-12:00:00 SOL-370-10:46:38 SOL-380-12:00:00 SOL-380-10:45:01 SOL-390-12:00:00 SOL-390-10:43:29 SOL-400-12:00:00 SOL-400-10:42:01 SOL-410-12:00:00 SOL-410-10:40:42 SOL-420-12:00:00 SOL-420-10:39:35 SOL-430-12:00:00 SOL-430-10:38:45 SOL-440-12:00:00 SOL-440-10:38:15 SOL-450-12:00:00 SOL-450-10:38:10 SOL-460-12:00:00 SOL-460-10:38:35 SOL-470-12:00:00 SOL-470-10:39:34 SOL-480-12:00:00 SOL-480-10:41:11 SOL-490-12:00:00 SOL-490-10:43:32 SOL-500-12:00:00 SOL-500-10:46:36 SOL-510-12:00:00 SOL-510-10:50:29 SOL-520-12:00:00 SOL-520-10:55:08 SOL-530-12:00:00 SOL-530-11:00:31 SOL-540-12:00:00 SOL-540-11:06:33 SOL-550-12:00:00 SOL-550-11:13:07 SOL-560-12:00:00 SOL-560-11:20:03 SOL-570-12:00:00 SOL-570-11:27:09 SOL-580-12:00:00 SOL-580-11:34:11 SOL-590-12:00:00 SOL-590-11:40:57 SOL-600-12:00:00 SOL-600-11:47:14 Fractional Part of the On-board Clock Strings -------------------------------------------------------- In MER science product labels and accompanying documentation, and in supporting data products, the on-board clock (SCLK) times are represented by a string in the "SSSSSSSSS.FFF" format. In this string "SSSSSSSSS" represents integer SCLK seconds while "FFF" represents fractional SCLK seconds. Unfortunately the meaning of "FFF" varies across different data sets. In some cases it means a true decimal fraction (i.e. milliseconds) while in other cases it stands for a count of 1/256th of a second (which stems from the fact that the fractional part was represented by a single byte in the original telemetry stream.) When comparing SCLK times or converting them to other time systems, it is important to know which of the two representations the SCLK string follows. Normally the documentation accompanying a particular data set -- such as the product SIS, or other label content specification document -- defines which type of SCLK fractional part was used in that data set. The SPICE system SCLK conversion routines expect the fractional part to be the count of 1/256th of a second. To convert an SCLK tag with milliseconds to that format one could use the following simple approach: starting with an SCLK with milliseconds (for example "129882123.725"), multiply the fractional part by 256 (0.725 * 256 = 185.6), round to the nearest integer (186), and replace the milliseconds portion of the string with it ("129882123.725" becomes "129882123.186".)