SLR IRV Format

The reference frame used for the IRVs is a pseudo-body-fixed system, in which
the x-axis points towards the Greenwich meridian, and the z-axis to the true-of
-date rotation axis (i.e., not the CIO pole). The velocities are relative to
this rotating reference frame, and are therefore not inertial. The
reconstruction software converts the velocities to an inertial frame before
integrating, using a value for the Earth rotation rate of ( 7.2921151463E-05 +
ddrate*1.E-14 ) radians/second, where the quantity 'ddrate' is given in the
format.

An estimated value of UT1-UTC is used when the IRVs are generated, in order to
refer them to the Greenwich meridian. If the IRVs are predicted for a long time
ahead then it might be necessary to apply a correction to the final predicted
positions in order to allow for an error of the estimated UT1-UTC. In order to
do so it is necessary to obtain separate information from the supplier if the
IRVs of the estimated values used for UT1-UTC.

The values of the polar motion components, 'jxpole' and 'jypole', are estimated
values, and are provided for convenience only, for users who want to convert
the final predicted positions from the true rotation axis to the CIO pole.
Unlike the UT1-UTC value they are not incorporated in any way into the IRVs.
Some suppliers of IRVs simply leave these values as zero, and leave it for the
users to obtain current values if they want to include this small correction.

A complete description of the IRV Force Model and Reference System is given in
the CSTG SLR Newsletter, June 1995, pp 33-36.

A set of IRVs for a single day consists of a header line, followed by three
lines of data. Usually a set of IRVs will cover several weeks, and usually the
same header line is repeated for each set of IRVs. At present the header line
is used by IRV- generating agencies to identify the agency, and in some cases
for additional information.

The format of the header line has recently changed from 80 characters of text
to 22 characters, specifying the agency. This information is followed by an
integer defining the number of multiple sets of IRVs follwing. This is a method
of overcoming deficiencies of the IRV force model for very low and very high
satellites, and is in use for the IRVs of the GPS satellites formed by RGO from
the daily predicted orbits from CODE, Bern.

General Form:

Contents
00000000011111111112222222222333333333344444444445555555555666666666677777777778
Line 12345678901234567890123456789012345678901234567890123456789012345678901234567890
1  IRV ID................ no
2   year mm dd hh mn ss.sxxxxxxxxxxx.xxxxxxyyyyyy.yyyyyyyyyyyzzzzzzzzzzz.zzzzzz
3   satt   eph	 nno	   llllllll.lllllllllmmmmmmmm.mmmmmmmmmnnnnnnnn.nnnnnnnnn
4   jxpole jypole ddrate bbbbbbbbbbbbbbbb.bccccccccccc.ccccccdddddddd.ddddddddd

Explanation:

Above, lowercase letters represent variables; UPPERCASE letters are literals.
Given in parentheses below, is the FORTRAN format used for reading each
variable.

1  IRV ID  literal string identifying start of IRV format, (A22)
1  no	   number of multiple sets of IRVs (I2), optional
2  year	   year (I4)
2  mm	   month of year (I2)
2  dd	   day of month (I2)
2  hh	   hour of day, UTC (I2)
2  mn	   minutes of day (I2)
2  ss.s	   seconds (F4.1)
2  xxx...  x-position component in meters (F18.6)
2  yyy...  y-position component in meters (F18.6)
2  zzz...  z-position component in meters (F18.6)
3  satt	   satellite number or support identification code (SIC) (I4)
3  eph	   ephemeris identification number (I3)
3  nno	   IRV sequence number (I3)
3  lll...  x-velocity component in meters/second (F18.9)
3  mmm...  y-velocity component in meters/second (F18.9)
3  nnn...  z-velocity component in meters/second (F18.9)
4  jxpole  x-polar motion component in milli-arcseconds (I6)
4  jypole  y-polar motion component in milli-arcseconds (I6)
4  ddrate  change in Earth rotation rate in E-14 radians/seconds (I6)
4  bbb...  checksum of all numerical IRV values excluding position and
velocity components (F18.1)
4  ccc...  checksum of position components (F18.6)
4  ddd...  checksum of velocity components (F18.9)

Updated: 11 November 2001

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Last modified date: Thursday, December 29, 2005
Author: Andrew T. Sinclair
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