Differential GPS - Understanding GPS: Principles and Applications - page 454

IGS products come in three varieties, with progressively greater latencies and
accuracies. The ultrarapid products are 48 hours in length. The first 24 hours are
observed, and the remaining 24 hours are predicted. Ultrarapid products are pro-
duced twice a day, so that one can always utilize the early part of the prediction
interval. Accuracies range from 5 to 10 cm (orbit) and 0.2 to 5 ns (clock). The rapid
products have 17-hour latency and have better than 5-cm orbit accuracy with 0.1-ns
clock accuracy. The final combination products have an approximate 13-day
latency with slightly better orbit and clock accuracies.
The IGS products are organized by GPS week number and are available for FTP
access through any browser. For example, ftp://igscb.jpl.nasa.gov/igscb/product/1261
contains precise orbit, clock, and EOP products for the week of March 7, 2004 (GPS
week 1261). The nomenclature is described at http://igscb.jpl.nasa.gov/components/
dcnav/igscb_product_wwww.html. So, for example, “igr12610.sp3.Z” refers to the
Rapid product, week 1,261, day 0 (Sunday, March 7, 2004) orbit in the SP3 format,
compressed with a UNIX-compatible algorithm. Note that slightly different directory
trees are used at the four sites that archive IGS products.
IGS also archives and disseminates GPS CORS data globally. The master index,
http://igscb.jpl.nasa.gov/components/data.html, shows CORS data grouped by
update intervals and archive sites. Directory paths vary between archive sites. For
example, SOPAC data are mapped at http://igscb.jpl.nasa.gov/components/dcnav/
sopac_rinex.html. Continuing the example, GPS observation CORS data for
Sunday, March 7, 2004, can be found at ftp://garner.ucsd.edu/pub/rinex/2004/067.
The “2004” refers to the year; the “067” refers to the day number for March 7. The
file name, “ajac0670.04o.Z,” refers to AJAC, a site in Corsica, day number 067, a
daily (not hourly) file, year 2004, GPS receiver data, compressed with a UNIX-com-
patible algorithm. Not all CORS sites are found at all the IGS servers.
An overview map of the IGS international cooperative GPS tracking network is
found at http://igscb.jpl.nasa.gov/network/maps/allmaps.html. One can immedi-
ately note the global distribution of the sites. Some sites upload their data hourly,
while others do so daily. The Web page at http://itrf.ensg.ign.fr/ITRF_solutions/
2000/ITRF2000.php contains authoritative ITRF00 Cartesian coordinates and
velocities for the IGS sites. Weekly network solutions are also available.
The IGS site has a rich set of resources, as would be expected from an interna-
tional scientific operation. In addition to the items already mentioned, one may also
find products for tropospheric zenith path delay and global grids of ionospheric
TEC. Data are available from GPS sensors in LEO satellites. Publications are at
http://igscb.jpl.nasa.gov/overview/pubs.html; mail archives are at http://igscb.jpl.
nasa.gov/mail/mailindex.html; and analysis conventions are at http://maia.usno.
navy.mil/conv2003.html.

References

[1] Olynik, M., et al., “Temporal Variability of GPS Error Sources and Their Effect on Relative

Positioning Accuracy,” Proc. of The Institute of Navigation National Technical Meeting ,
January 2002.

[2] Department of Defense, Global Positioning System Standard Positioning Service Signal

Specification , 2nd ed., U.S. Department of Defense, Washington, D.C., June 2, 1995.