The Extended Standard Product 3 Orbit Format (SP3-c)

                          (12 February 2007)



                             Steve Hilla
                      National Geodetic Survey
                    National Ocean Service, NOAA
                  Silver Spring, MD 20910-6233, USA


                             INTRODUCTION

     The original Standard Product 3 format (SP3-a) was proposed in
(Remondi, 1989) and modified and adopted in (Remondi, 1991). The SP3 format
is similar to the original NGS Standard Product 1 format described in
(Remondi, 1989 and 1985) but includes additional information:  satellite
clock corrections, orbit accuracy exponents, comment lines, the GPS week and
seconds of week associated with the first epoch, and a more flexible header
structure (Spofford and Remondi, 1994).

In 1998, W. Gurtner and M. Rothacher defined an SP3-b format to allow for
the combination of GPS orbits and GLONASS orbits (see IGEX Mail 0042, 27-
Oct-1998).  All of these SP3-b modifications were backwards compatible with
SP3-a, with the exception of the satellite identification labels -- which
were changed from an I3 field to an A1,I2 field to accommodate both GPS and
GLONASS identifiers in a manner similar to RINEX files (Gurtner, 2000).

At the 2000 International GPS Service (IGS) Analysis Center Workshop held at
the U.S. Naval Observatory, it was suggested to modify the SP3 format still
further so that orbit files distributed by the IGS could include some type
of clock accuracy information, and so that separate accuracy information
would be available for the observed versus predicted parts of the IGS ultra-
rapid orbit files.  Rather than just putting two sets of accuracy exponents
in the header (one for the observed part and one for the predicted part) it
was decided instead to put accuracy information at each epoch (for X, Y, Z,
and satellite clock correction).  Since the original SP3-a and SP3-b formats
were only 60 columns wide, this additional information was easily added
using columns 61 through 80 in each Position and Clock Record.  This was
done in such a way as to remain mostly backwards compatible with SP3-a.  As
comments were collected from the IGS (orbit) Analysis Centers, additional
ideas emerged.  It was suggested that a clock event flag be added in column
75 (as currently done by some groups) to denote events like a clock swap on
a satellite.  It was also suggested to add an orbit event flag, to denote
cases where it was known that a satellite went through some type of orbit
maneuver.  It was also suggested that clock prediction and orbit prediction
flags be added.  Finally, for the purposes of computing user range errors
more accurately for each satellite, it was suggested to add correlation
information between the satellite coordinates and the satellite clock
correction.  These ideas were discussed further at the 2002 IGS Network,
Data, and Analysis Center Workshop held in Ottawa (Hilla, 2002).

All of these suggestions have been incorporated in this new SP3-c format. 
As a result, there can now be as many as three different sets of satellite
position accuracy indicators in an SP3-c file.  The first set, the original
accuracy exponents in the header, have been kept in SP3-c to maintain
backwards compatibility with the SP3-a format and with existing GPS
processing software.  These exponents are interpreted as 2**nn millimeters.
A zero exponent means the accuracy is unknown. The quoted orbit error should
represent one standard deviation and be based on the orbital error in the
entire file for the respective satellite.  The second set, in columns 62
through 69 of the Position and Clock Record, represent the standard
deviation of each position component (X, Y, and Z) at that epoch, in
millimeters.  These are also exponents but use a floating-point base (for
example, 1.25**nn) to achieve better resolution for the standard deviations.
A zero exponent here represents a 1 millimeter standard deviation. Blank
spaces mean the standard deviation is unknown.  These first two sets of
standard deviations are expected to be present in all SP3-c files.

A third set of standard deviations can be found in the optional Position and
Clock Correlation Record (the EP record).  This optional record was added to
provide high precision users with the correlation coefficients between the
X, Y, Z, and satellite clock correction values.  In order to be able to
construct the full 4-by-4 covariance matrix for a satellite at an epoch,
without any loss of precision, the standard deviations for the X, Y, and Z
position components are given here to full millimeter precision using a
range of 1 to 9999 millimeters.  Blank spaces mean a standard deviation or a
correlation coefficient is unknown (blanks would probably only occur for the
clock information, unless the EP records were being used to store only
standard deviations, or only correlation coefficients).

In this document the format fields for the SP3-c format are defined.  These
include all of the changes made for SP3-b.  The fields that are defined as
"blanks" are reserved fields which must remain blank.  All times referred to
in an SP3-c file are in the SAME time system, even when they are represented 
as Gregorian Dates or Modifed Julian Dates. In SP3-c, the Time System code 
for the entire file (GPS, GLO, GAL, TAI, or UTC) is now specified in the
header on line thirteen. The information to convert between GPS Time and
Coordinated Universal Time (i.e., the leap seconds and the fractional error
in GPS Time) is not provided as part of the SP3-c format.  The basic format
of an SP3 file is a Header, followed by a series of epoch times each with 
a set of Position and Clock Records listed for each satellite. A second,
optional record contains satellite velocities and the clock correction
rate-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, for each satellite, an additional satellite velocity and
clock rate-of-change has been computed.  SP3-c adds two more optional
records: a Position and Clock Correlation Record (EP record), and a 
Velocity and Clock Rate-of-Change Correlation Record (EV record).

Note: On 27 September 2006, this file was updated slightly to add more
options for the Time System Indicator.  The original version of this file,
which was dated 5 September 2002, had only "GPS" and "UTC" as options for
the Time System Indicator.  This new version adds codes "GLO", "GAL", and
"TAI".  Also, clarifications were made regarding the EP and EV records,
and the order of the satellites and records at each epoch.

Note: On 12 February 2007, this file was modified to include a reference
to a new list of LEO satellites available at CDDIS (see the documentation
below regarding the satellite identifiers found in the third through
seventh lines).



     Standard Product #3 ASCII SP3 Format Version "c".

     Columns             Description        Example             Format
     -----------------   -----------------  -----------------   ---------

     SP3 First Line

     Columns 1-2         Version Symbol     #c                  A2
     Column  3           Pos or Vel Flag    P or V              A1
     Columns 4-7         Year Start         2001                I4          
     Column  8           Unused             _                   blank
     Columns 9-10        Month Start        _8                  I2
     Column  11          Unused             _                   blank 
     Columns 12-13       Day of Month St    _8                  I2
     Column  14          Unused             _                   blank
     Columns 15-16       Hour Start         _0                  I2
     Column  17          Unused             _                   blank
     Columns 18-19       Minute Start       _0                  I2
     Column  20          Unused             _                   blank 
     Columns 21-31       Second Start       _0.00000000         F11.8
     Column  32          Unused             _                   blank
     Columns 33-39       Number of Epochs   ____192             I7
     Column  40          Unused             _                   blank
     Columns 41-45       Data Used          ____d               A5
     Column  46          Unused             _                   blank
     Columns 47-51       Coordinate Sys     ITR97               A5
     Column  52          Unused             _                   blank
     Columns 53-55       Orbit Type         FIT                 A3
     Column  56          Unused             _                   blank
     Columns 57-60       Agency             _NGS                A4


     SP3 Line Two

     Columns 1-2         Symbols            ##                  A2
     Column  3           Unused             _                   blank
     Columns 4-7         GPS Week           1126                I4
     Column  8           Unused             _                   blank
     Columns 9-23        Seconds of Week    259200.00000000     F15.8
     Column  24          Unused             _                   blank
     Columns 25-38       Epoch Interval     __900.00000000      F14.8
     Column  39          Unused             _                   blank
     Columns 40-44       Mod Jul Day St     52129               I5
     Column  45          Unused             _                   blank
     Columns 46-60       Fractional Day     0.0000000000000     F15.13


     SP3 Line Three

     Columns 1-2         Symbols            +_                  A2
     Column  3-4         Unused             __                  2 blanks
     Columns 5-6         Number of Sats     26                  I2
     Column  7-9         Unused             ___                 3 blanks
     Columns 10-12       Sat #1 Id          G01                 A1,I2.2
     Column  13-15       Sat #2 Id          G02                 A1,I2.2
            *
            *
            *
     Columns 58-60       Sat #17 Id         G21                 A1,I2.2
               



     SP3 Line Four
    
     Columns 1-2         Symbols            +_                  A2
     Columns 3-9         Unused             _______             7 blanks
     Columns 10-12       Sat #18 Id         G23                 A1,I2.2
     Columns 13-15       Sat #19 Id         G24                 A1,I2.2
            *
            *
            *
     Columns 58-60       Sat #34 Id         __0                 A1,I2.2


     SP3 Line Five

     Columns 1-2         Symbols            +_                  A2
     Columns 3-9         Unused             _______             7 blanks
     Columns 10-12       Sat #35 Id         __0                 A1,I2.2
     Columns 13-15       Sat #36 Id         __0                 A1,I2.2
            *
            *
            *
     Columns 58-60       Sat #51 Id         __0                 A1,I2.2


     SP3 Line Six

     Columns 1-2         Symbols            +_                  A2
     Columns 3-9         Unused             _______             7 blanks
     Columns 10-12       Sat #52 Id         __0                 A1,I2.2
     Columns 13-15       Sat #53 Id         __0                 A1,I2.2
            *
            *
            *
     Columns 58-60       Sat #68 Id         __0                 A1,I2.2


     SP3 Line Seven

     Columns 1-2         Symbols            +_                  A2
     Columns 3-9         Unused             _______             7 blanks
     Columns 10-12       Sat #69 Id         __0                 A1,I2.2
     Columns 13-15       Sat #70 Id         __0                 A1,I2.2
            *
            *
            *
     Columns 58-60       Sat #85 Id         __0                 A1,I2.2


     SP3 Line Eight

     Columns 1-2         Symbols            ++                  A2
     Columns 3-9         Unused             _______             7 blanks
     Columns 10-12       Sat #1 Accuracy    __7                 I3
     Columns 13-15       Sat #2 Accuracy    __8                 I3
            *
            *
            *
     Columns 58-60       Sat #17 Accuracy   __9                 I3


     SP3 Line Nine

     Columns 1-2         Symbols            ++                  A2
     Columns 3-9         Unused             _______             7 blanks
     Columns 10-12       Sat #18 Accuracy   __9                 I3           
     Columns 13-15       Sat #19 Accuracy   __8                 I3