The National Geodetic Survey Standard GPS Format SP3 Paul R. Spofford National Geodetic Survey National Ocean Service, NOAA Silver Spring, MD 20910-3282, USA and Benjamin W. Remondi, PhD P.O. Box 37 Dickerson, Maryland 20842, USA INTRODUCTION Why do we need standardized orbit formats? Standard orbit formats provide many advantages, the most obvious being orbit exchange. ASCII and binary formats both satisfy this function, but ASCII does it with greater generality because binary formats are computer operating system dependent. The NGS standard GPS orbit format SP1 was introduced in Remondi (1985). After a few years of use, it was realized that enhancements would eventually be required. The "orbit type," the coordinate system, and the GPS week associated with the first epoch of the ephemeris file were added in a manner that did not impact the formats and existing software. A more serious omission of the initial NGS orbit format was the satellite clock corrections. This omission reflected an earlier belief that all geodetic applications could be accomplished in differential mode. Today we realize that standard formats serve a wider community and include those who find it inconvenient to operate in a differential mode. A user can operate in single-receiver or navigation mode based on the broadcast message. However, the user can get more accurate (post-processed) results if the precise orbital data and the associated satellite clock corrections, which were determined simultaneously with those precise orbits, are available. This becomes even more valuable when the broadcast orbit and clock information are intentionally degraded. Thus, a new NGS orbit format was proposed in Remondi (1989), and modified and adopted in Remondi (1991). This format is similar to the original NGS orbit format, but comprises positional and satellite clock correction data. Furthermore, other changes were proposed which allow more flexibility with regard to enhancements. This summary document combines the specifications and enhancements discussed in Remondi (1989) and Remondi (1991), with examples for the benefit of the user. SP3 In this document the NGS orbital format SP3 (Standard Product # 3) for Global Positioning System (GPS) satellites is discussed (see examples 1 and 2). The major addition to earlier formats is the satellite clock correction information which is computed simultaneously with the orbits. The basic format is a position and clock record; a second, optional, record contains velocities and clock rates-of-change. The Position Record Flag, P, in line one indicates that no velocities are included. The Velocity Record Flag, V, in line one indicates that at each epoch, and for each satellite, a satellite velocity and clock rate-of-change has been computed. The SP3 format has been designed such that satellites other than GPS could be described as well. Note: All times referred to in this document are GPS times, even when they are represented as Gregorian or Modified Julian Dates. Thus, information for conversion of GPS time to Coordinated Universal Time (UTC) is not provided as part of the SP3 format. ! Standard Product #3 ASCII SP3 Format Version "a" (refer to example given in figure 1) SP3 First Line Columns 1-2 Version Symbol #a Column 3 Pos or Vel Flag P or V Columns 4-7 Year Start 1994 Column 8 Unused _ Columns 9-10 Month Start 12 Column 11 Unused _ Columns 12-13 Day of Month St 17 Column 14 Unused _ Columns 15-16 Hour Start _0 Column 17 Unused _ Columns 18-19 Minute Start _0 Column 20 Unused _ Columns 21-31 Second Start _0.00000000 Column 32 Unused _ Columns 33-39 Number of Epochs _____96 Column 40 Unused _ Columns 41-45 Data Used ____d Column 46 Unused _ Columns 47-51 Coordinate Sys ITR92 Column 52 Unused _ Columns 53-55 Orbit Type FIT Column 56 Unused _ Columns 57-60 Agency _NGS SP3 Line Two Columns 1-2 Symbols ## Column 3 Unused _ Columns 4-7 GPS Week _779 Column 8 Unused _ Columns 9-23 Seconds of Week 518400.00000000 Column 24 Unused _ Columns 25-38 Epoch Interval __900.00000000 Column 39 Unused _ Columns 40-44 Mod Jul Day St 49703 Column 45 Unused _ Columns 46-60 Fractional Day 0.0000000000000 SP3 Line Three Columns 1-2 Symbols +_ Column 3-4 Unused __ Columns 5-6 Number of Sats 25 Column 7-9 Unused __ Columns 10-12 Sat #1 Id __1 Column 13-15 Sat #2 Id __2 * * * Columns 58-60 Sat #17 Id _22 SP3 Line Four Columns 1-2 Symbols +_ Columns 3-9 Unused _______ Columns 10-12 Sat #18 Id _23 Columns 13-15 Sat #19 Id _24 * * * Columns 58-60 Sat #34 Id __0 SP3 Line Five Columns 1-2 Symbols +_ Columns 3-9 Unused _______ Columns 10-12 Sat #35 Id __0 Columns 13-15 Sat #36 Id __0 * * * Columns 58-60 Sat #51 Id __0 SP3 Line Six Columns 1-2 Symbols +_ Columns 3-9 Unused _______ Columns 10-12 Sat #52 Id __0 Columns 13-15 Sat #53 Id __0 * * * Columns 58-60 Sat #68 Id __0 SP3 Line Seven Columns 1-2 Symbols +_ Columns 3-9 Unused _______ Columns 10-12 Sat #69 Id __0 Columns 13-15 Sat #70 Id __0 * * * Columns 58-60 Sat #85 Id __0 SP3 Line Eight Columns 1-2 Symbols ++ Columns 3-9 Unused _______ Columns 10-12 Sat #1 Accuracy __7 Columns 13-15 Sat #2 Accuracy __6 * * * Columns 58-60 Sat #17 Accuracy __5 SP3 Line Nine Columns 1-2 Symbols ++ Columns 3-9 Unused _______ Columns 10-12 Sat #18 Accuracy __5 Columns 13-15 Sat #19 Accuracy __5 * * * Columns 58-60 Sat #34 Accuracy __0 SP3 Line Ten Columns 1-2 Symbols ++ Columns 3-9 Unused _______ Columns 10-12 Sat #35 Accuracy __0 Columns 13-15 Sat #36 Accuracy __0 * * * Columns 58-60 Sat #51 Accuracy __0 SP3 Line Eleven Columns 1-2 Symbols ++ Columns 3-9 Unused _______ Columns 10-12 Sat #52 Accuracy __0 Columns 13-15 Sat #53 Accuracy __0 * * * Columns 58-60 Sat #68 Accuracy __0 SP3 Line Twelve Columns 1-2 Symbols ++ Columns 3-9 Unused _______ Columns 10-12 Sat #69 Accuracy __0 Columns 13-15 Sat #70 Accuracy __0 * * * Columns 58-60 Sat #85 Accuracy __0 SP3 Lines Thirteen and Fourteen Columns 1-2 Symbols %c Column 3 Unused _ Columns 4-5 2 characters cc Column 6 Unused _ Columns 7-8 2 characters cc Column 9 Unused _ Columns 10-12 3 characters ccc Column 13 Unused _ Columns 14-16 3 characters ccc Column 17 Unused _ Columns 18-21 4 characters cccc Column 22 Unused _ Columns 23-26 4 characters cccc Column 27 Unused _ Columns 28-31 4 characters cccc Column 32 Unused _ Columns 33-36 4 characters cccc Column 37 Unused _ Columns 38-42 5 characters ccccc Column 43 Unused _ Columns 44-48 5 characters ccccc Column 49 Unused _ Columns 50-54 5 characters ccccc Column 55 Unused _ Columns 56-60 5 characters ccccc SP3 Lines Fifteen and Sixteen Columns 1-2 Symbols %f Column 3 Unused _ Columns 4-13 10-column float _0.0000000 Column 14 Unused _ Columns 15-26 12-column float _0.000000000 Column 27 Unused _ Columns 28-41 14-column float _0.00000000000 Column 42 Unused _ Columns 43-60 18-column float _0.000000000000000 SP3 Lines Seventeen and Eighteen Columns 1-2 Symbols %i Column 3 Unused _ Columns 4-7 4-column int ___0 Column 8 Unused _ Columns 9-12 4-column int ___0 Column 13 Unused _ Columns 14-17 4-column int ___0 Column 18 Unused _ Columns 19-22 4-column int ___0 Column 23 Unused _ Columns 24-29 6-column int _____0 Column 30 Unused _ Columns 31-36 6-column int _____0 Column 37 Unused _ Columns 38-43 6-column int _____0 Column 44 Unused _ Columns 45-50 6-column int _____0 Column 51 Unused _ Columns 52-60 9-column int ________0 SP3 Lines Nineteen to Twenty two Columns 1-2 Symbols /* Column 3 Unused _ Columns 4-60 Comment CC...CC SP3 Line Twenty three (The Epoch Header Record) Columns 1-2 Symbols *_ Column 3 Unused _ Columns 4-7 Year Start 1994 Column 8 Unused _ Columns 9-10 Month Start 12 Column 11 Unused _ Columns 12-13 Day of Month St 17 Column 14 Unused _ Columns 15-16 Hour Start _0 Column 17 Unused _ Columns 18-19 Minute Start _0 Column 20 Unused _ Columns 21-31 Second Start _0.00000000 SP3 Line Twenty four (The Position and Clock Record) (See example 1) Column 1 Symbol P Columns 2-4 Vehicle Id. __1 Columns 5-18 x-coordinate(km) __16258.524750 Columns 19-32 y-coordinate(km) __-3529.015750 Columns 33-46 z-coordinate(km) _-20611.427050 Columns 47-60 clock (microsec) ____-62.540600 * * * In addition, one could use the Velocity and Clock Rate-of-Change record, V, after the Position and Clock record. The clock rate- of-change units are 10**-4 microseconds/second. SP3 Line Twenty five (See example 2) Column 1 Symbol V Columns 2-4 Vehicle Id. __1 Columns 5-18 x-velocity(dm/s) __16258.524750 Columns 19-32 y-velocity(dm/s) __-3529.015750 Columns 33-46 z-velocity(dm/s) _-20611.427050 Columns 47-60 clock rate-chg ____-62.540600 * * * SP3 Line 22+NUMEPS*(NUMSATS+1)+1 (i.e., The Last Line) Columns 1-3 Symbols EOF !Discussion of the SP3 Format On line one, character two is the format version identification character. The first released version has been designated version 'a'. Subsequent versions will use lower case letters in alphabetical order. The first line comprises the Gregorian date and time of day of the first epoch of the orbit, the number of epochs in the ephemeris file (up to 10 million), the data used descriptor, the orbit type descriptor, and the agency descriptor. The data used descriptor was included for ease in distinguishing between multiple orbital solutions from a single organization. This will have primary use for the agency generating the orbit. A possible convention is given below; this is not considered final and suggestions are welcome. u -- undifferenced carrier phase du -- change in u with time s -- 2-receiver/1-satellite carrier phase ds -- change on s with time d -- 2-receiver/2-satellite carrier phase dd -- change in d with time U -- undifferenced code phase dU -- change in U with time S -- 2-receiver/1-satellite code phase dS -- change in S with time D -- 2-receiver/2-satellite code phase dD -- change in D with time + -- type separator Combinations such as "__u+U" seem reasonable. If the measurements used were complex combinations of standard types, then one could use "mixed" where mixed could be explained on the comment lines. Orbit type is described by a three character descriptor. At this time only three have been defined: FIT (fitted), EXT (extrapolated or predicted), and BCT (broadcast). Naturally, others are possible. The computing agency descriptor allows four characters (e.g. _NGS). The second line has: the GPS week (which will exceed 1000 in the year 1999); the seconds of the GPS Week elapsed at the start of the orbit (0.0 <= seconds of week < 604800.0); the epoch interval (0.0 < epoch interval < 100000.0) in seconds; the modified Julian Day Start (where 44244 represents GPS zero time - - January 6, 1980); and fractional part of the day (0.0 <= fractional < 1.0) at the start of the orbit. The third line to the seventh lines indicate the number of satellites followed by their respective identifiers. The identifiers must use consecutive slots and continue on lines 4-7, if required. The value 0 should only appear after all the identifiers are listed. Satellite identifiers may be listed in any order. However, for ease in reviewing satellites included in the orbit file it is recommended that numerical order be used. The eighth line to the twelfth lines have the orbit accuracy exponents. The value 0 is interpreted as accuracy unknown. A satellite's accuracy exponent appears in the same slot on lines 8-12 as the identifier on lines 3-7. The accuracy is computed from the exponent as in the following example. If the accuracy exponent is 13, the accuracy is 2**13 mm or 8 m. The quoted orbital error should represent one standard deviation and be based on the orbital error in the entire file for the respective satellite. This may lead to some distortion when orbit files are joined together. Lines 13-18 allow the SP3 ASCII file to be modified, since the SP3 format has been designed so that additional parameters may be added. Lines 19-22 are free form comments. Line 23 is the epoch header date and time. Line 24 is the position and clock line, and the first character is 'P' indicating a position line. The positional values are in kilometers and are precise to 1 mm. A precision of 0.5 mm can be accommodated if rounding is used, i.e., the value shown is never more than 0.5 mm from the computed value. The clock values are in microseconds and are precise to 1 picosecond. Bad or absent positional values are to be set to 0.000000. Bad or absent clock values are to be set to _999999.9999__. The six integer nines are required, whereas the fractional part nines are optional. When the position/velocity mode flag is set to 'V' in line one, each position record for a given satellite is followed by a velocity record for the same satellite. The first character of the velocity record is a "V". The velocity components are given in decimeters/second and have a precision of 10**-4 mm/second. The last column of a velocity record is the rate-of-change of clock correction given in units of 10**-4 microsecond/second. The precision of this parameter is 10**-16 second/second. !Example 1 #aP1994 12 17 0 0 0.00000000 96 d ITR92 FIT NGS ## 779 518400.00000000 900.00000000 49703 0.0000000000000 + 25 1 2 4 5 6 7 9 12 14 15 16 17 18 19 20 21 22 + 23 24 25 26 27 28 29 31 0 0 0 0 0 0 0 0 0 + 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 + 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 + 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ++ 7 6 5 5 5 5 5 5 5 6 5 5 5 5 6 5 5 ++ 5 5 6 5 5 5 5 5 0 0 0 0 0 0 0 0 0 ++ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ++ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ++ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 %c cc cc ccc ccc cccc cccc cccc cccc ccccc ccccc ccccc ccccc %c cc cc ccc ccc cccc cccc cccc cccc ccccc ccccc ccccc ccccc %f 0.0000000 0.000000000 0.00000000000 0.000000000000000 %f 0.0000000 0.000000000 0.00000000000 0.000000000000000 %i 0 0 0 0 0 0 0 0 0 %i 0 0 0 0 0 0 0 0 0 /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC * 1994 12 17 0 0 0.00000000 P 1 16258.524750 -3529.015750 -20611.427050 -62.540600 P 2 -21998.652100 -8922.093550 -12229.824050 -131.326200 P 4 -26019.547600 4809.810900 -2508.578200 3.544600 P 5 7014.950200 21130.960300 -14387.334650 79.692800 * * * P 28 13204.937750 -20485.533400 10794.787000 55.200800 P 29 -1638.431050 -24391.479200 10455.312650 3.690300 P 31 6265.255800 -25687.986950 -753.359000 70.830800 * 1994 12 17 0 15 0.00000000 P 1 15716.820135 -1169.850490 -21281.578766 -62.542746 P 2 -22813.261065 -9927.616864 -9816.490189 -131.328686 * * * P 28 13416.746195 -22186.753441 6248.864499 55.385492 P 29 -2745.269113 -22169.709690 14469.340453 3.718873 P 31 5629.986510 -25241.323751 -5659.769347 71.118497 * 1994 12 17 23 45 0.00000000 P 1 16708.907949 -5150.972262 -19904.291167 -62.727331 P 2 -21321.617042 -8048.187511 -13856.581227 -131.555527 P 4 -26107.382526 5010.736034 -422.963345 3.672587 P 5 7932.078481 21838.230749 -12767.671968 79.888744 * * * P 28 13308.321924 -21306.183480 8935.290694 55.387446 P 29 -2059.774801 -23532.083663 12229.852140 3.719337 P 31 6034.395625 -25605.621951 -2843.783172 71.121661 EOF!Example 2 #aV1994 12 17 0 0 0.00000000 96 d ITR92 FIT NGS ## 779 518400.00000000 900.00000000 49703 0.0000000000000 + 25 1 2 4 5 6 7 9 12 14 15 16 17 18 19 20 21 22 + 23 24 25 26 27 28 29 31 0 0 0 0 0 0 0 0 0 + 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 + 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 + 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ++ 7 6 5 5 5 5 5 5 5 6 5 5 5 5 6 5 5 ++ 5 5 6 5 5 5 5 5 0 0 0 0 0 0 0 0 0 ++ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ++ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ++ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 %c cc cc ccc ccc cccc cccc cccc cccc ccccc ccccc ccccc ccccc %c cc cc ccc ccc cccc cccc cccc cccc ccccc ccccc ccccc ccccc %f 0.0000000 0.000000000 0.00000000000 0.000000000000000 %f 0.0000000 0.000000000 0.00000000000 0.000000000000000 %i 0 0 0 0 0 0 0 0 0 %i 0 0 0 0 0 0 0 0 0 /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC /* CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC * 1994 12 17 0 0 0.00000000 P 1 16258.524750 -3529.015750 -20611.427050 -62.540600 V 1 -6560.373522 25605.954994 -9460.427179 -0.024236 P 2 -21998.652100 -8922.093550 -12229.824050 -131.326200 V 2 -9852.750736 -12435.176313 25738.634180 -0.029422 P 4 -26019.547600 4809.810900 -2508.578200 3.544600 V 4 2559.038002 -3340.527442 -31621.490838 0.016744 * * * P 29 -1638.431050 -24391.479200 10455.312650 3.690300 V 29 5754.005457 -12065.761570 -27707.056273 0.003537 P 31 6265.255800 -25687.986950 -753.359000 70.830800 V 31 3053.344058 -63.091750 31910.454757 0.033749 * 1994 12 17 0 15 0.00000000 P 1 15716.820135 -1169.850490 -21281.578766 -62.542746 V 1 -5439.955846 26738.341429 -5409.793390 -0.023226 P 2 -22813.261065 -9927.616864 -9816.490189 -131.328686 V 2 -8178.974330 -9924.329320 27813.754308 -0.025238 * * * P 31 5629.986510 -25241.323751 -5659.769347 71.118497 V 31 5213.646243 -5585.922919 30831.379942 0.040199 * 1994 12 17 23 45 0.00000000 P 1 16708.907949 -5150.972262 -19904.291167 -62.727331 V 1 -7218.304166 24494.550676 -12283.334526 -0.023824 * * * P 31 6034.395625 -25605.621951 -2843.783172 71.121661 V 31 3831.346050 -2469.229615 31655.436179 0.028935 EOF! REFERENCES 1. Remondi, B. W. 1985: Distribution of Global Positioning System Ephemerides by the National Geodetic Survey, Presented at the First Conference on Civil Applications of GPS - Institute of Navigation, September 12, 1985, 8 pp. 2. Remondi, B. W. 1989: Extending the National Geodetic Survey Geodetic Orbit Formats, NOAA Technical Report 133 NGS 46, 85 pp. 3. Remondi, B. W., 1991: NGS Second Generation ASCII and Binary Orbit Formats and Associated Interpolated Studies, Proceedings of the Twentieth General Assembly, International Union of Geodesy and Geophysics, Vienna, Austria, August 11-24, 1991, 28 pp. ORDERING INFORMATION Geodetic publications of NOAA and the National Geodetic Survey are sold in paper form by the Information Services Division, National Geodetic Survey, NOAA. To obtain a price list or to place an order contact: NOAA, National Geodetic Survey SSMC3, N/CG112 1315 East-West Highway Silver Spring, Maryland 20910-3282 USA Telephone: 301-713-3242 FAX: 301-713-4172. ===================================================================== Paul R. Spofford Internet: pauls@ngs.noaa.gov NOAA, National Geodetic Survey Telephone: 301-713-3205 1315 East-West Highway, N/CG12 FAX: 301-713-4322 Silver Spring, Maryland 20910-3282, USA Home: 301-831-6099 =====================================================================
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