Mission Operations Downlink Processing

<P>An example of the first portion of the engineering information obtained for the project is illustrated by the Viking Lander 1, Sol 1,694 "MK4" output which lists data frames and Deep Space Network performance for the downlink. The standard "link" consists of 30 minutes of spacecraft commanding and low data rate communications between DSN and the lander at 8-1/3 bits/second followed by 30 minutes of engineering and science downlink at 1,000 bits/sec. <P>The downlinked Viking data are stored frame by frame and each frame is either an engineering or science frame for one of the instruments. A 31 bit pseudo-random synchronization word begins every data frame followed by the instrument's Format identifier, ID, which describes the structure and expected length of the frame. In the output below, the "ID" is followed by the type of frame. FMT4 and FMT5 contain engineering data, including our atmospheric pressure, while METE indicates a meteorology frame. Each meteorology frame nominally contains 16 sequential scans of wind and temperature sensor data, and room for a 17th; the 17th is used if the Guidance and Sequencing Computer is too busy to read out the VMIS memory after it sets its "data ready" flag bit. The total memory of VMIS is 5,120 bits. Note, each meteorology frame is surrounded by an engineering frame so we can obtain atmospheric pressure. <P> For more robust extraction under difficult conditions, i.e., low signal/noise, we search for a 15 bit subset of 31 bit synchronization word and, when found, then check the rest of the synch word and the instrument ID to see if it is a valid frame. "NO SYNCH" means that no synch word was found at the beginning of the frame, generally due to missing or corrupted data. These frames have a fair number of errors since the signal/noise ratio, S/N, expressed in decibels, is around 3.0 DB. 3.0 DB means that the signal is only twice as large as the system noise! <pre> *UW DECSET REV 8.01 11/12/87 14:13:01 * * INPUT DATA FILE: IDR>VL1.INPUT>E8197 * TAPE STRUCTURE FILE: DATA.V>DECSET>BYTE1.HIR * DECOM/DECAL FILE: DATA.V>DECSET>DECAL.VL1.REV.8.00.870811 *TIME UTILITY DATA BASE: DATA.V>TIMING>VL1.TABLE.REV.2 * * VIKING LANDER: 1 * EPOCH: 16 * EXTERNAL TAPE ID: E8197 *DEEP SPACE STATION: 43 Canberra Australia, 64 meters diameter parabolic dish antenna * SPACECRAFT: 26 * PASS NUMBER: 1697 * TAPE NUMBER: 01 * FILE TYPE: TLM * DATA TYPE: TLM INT BIT SYNCH.. ..SYMBOL SYNCH * LINK START TIME: 81 119 00:35:00 RUN: 119 0:57:56.30 DEMODULATOR.. . . ..DDA * LINK STOP TIME: 81 119 02:00:00 RUN: 119 1: 1:27.90 RECEIVER.. . . . . ..BLOCK DECODER *----------------------------------- . . . . . . *ID TYPE GCSC TIME REC # SYNCH ERT LLT BLCK . . . . . . S/N BITS * ERR *- ---- ------ --- --- -------- -------- -- - - - - - - --- -- - -- NO SYNCH 270 119 0:57:55.40 797 0 0 0 0 0 0 3.0 43 24 BAD FID 270 1 119 0:57:55.45 797 0 0 0 0 0 0 3.0 0 NO SYNCH 270 119 0:57:55.45 797 0 0 0 0 0 0 3.0 792 25 FMT4 171008 270 2 119 0:57:56.24 1693 20:25: 5.89 797 0 0 0 0 0 0 3.0 792 25 FMT4 540864 270 3 119 0:57:57.04 1693 21:26:44.40 798 0 0 0 0 0 0 3.0 792 26 FMT5 540928 270 4 119 0:57:57.82 1693 21:26:45.04 799 0 0 0 0 0 0 3.0 768 10 BAD FID 270 5 119 0:57:58.59 800 0 0 0 0 0 0 3.0 0 NO SYNCH 270 119 0:57:58.59 800 0 0 0 0 0 0 3.0 1536 26 FMT5 152704 271 6 119 0:58: 0.13 1693 20:22: 2.85 802 0 0 0 0 0 0 3.1 768 etc. NO SYNCH 279 119 0:58:41.86 846 0 0 0 0 0 0 3.1 48 13 25 FMT4 4239424 282 40 119 0:58:55.37 1694 7: 3:34.29 861 0 0 0 0 0 0 3.2 784 7 METE 3742720 282 41 119 0:58:56.15 1694 5:40:47.31 861 0 0 0 0 0 0 3.2 5168 25 FMT4 4609280 284 42 119 0:59: 1.32 1694 8: 5:12.81 867 0 0 0 0 0 0 3.7 792 25 FMT4 4979136 284 43 119 0:59: 2.11 1694 9: 6:51.32 868 0 0 0 0 0 0 3.7 784 7 METE 4388864 284 44 119 0:59: 2.89 1694 7:28:28.67 869 0 0 0 0 0 0 3.7 5168 25 FMT4 5349056 285 45 119 0:59: 8.06 1694 10: 8:30.47 874 0 0 0 0 0 0 3.7 784 etc. 25 FMT4 20883136 310 99 119 1: 1: 3.91 1696 3:58:18.84 998 0 0 0 0 0 0 3.9 784 7 METE 18903040 310 100 119 1: 1: 4.69 1695 23: 7:53.38 999 0 0 0 0 0 0 3.9 5168 NO SYNCH 310 119 1: 1: 4.69 999 0 0 0 0 0 0 3.9 47 13 NO SYNCH 311 119 1: 1: 9.90 1004 0 0 0 0 0 0 4.0 737 26 6 0 1 </pre> Decoding engineering data from this downlink from a few of the above frames produces the below results. These values come from the above MK4 frames with the corresponding "SYNCH" numbers. For example at Lander Local time "1693 20:25: 5.89" below, frame "2", had a digital value of "123" "DN" for AMBPRESS, which converts to atmospheric pressure of "8.2610" millibars at the surface of Mars. "CAMBT" is the temperature in degrees Kelvin, K of the footpad temperature sensor. ( 0° K = Absolute zero, 273° K = 0° Celsius, 373° K = 100° C ) <pre> *---------------------------------------- *PG 1 LLT SYNCH AMBPRESS CAMBT CAMBPOST EP & ID * DN EU DN EU DN EU DN EU *-------- --- --- ------ --- ------ --- ------ --- ---- 1693 20:25: 5.89 2 123 8.2610 229.00 227.06 16 E8197 1693 21:26:44.40 3 124 8.3480 216.67 215.13 16 E8197 1693 21:26:45.04 4 1 0.00000 16 E8197 1693 20:22: 2.85 6 129 8.7830 16 E8197 etc. 1694 7: 3:34.29 40 127 8.6090 198.28 346.72 16 E8197 1694 8: 5:12.81 42 170 12.370 197.26 197.74 16 E8197 1694 9: 6:51.32 43 129 8.7830 196.11 197.74 16 E8197 1694 10: 8:30.47 45 128 8.6960 268.62 197.12 16 E8197 etc. 1696 3:58:18.84 99 125 8.4350 204.02 275.15 16 E8197 1696 6: 1:35.87 102 125 8.4350 200.07 200.58 16 E8197 1696 7: 3:14.38 103 126 8.5220 199.17 200.82 16 E8197 </pre>