3/17/2005:
This README file covers the data in the "space_based" directory for JPL_GPS_TEC.

There are currently two types of data in this directory: 
1) vertical TEC from below the JASON satellite, obtained from an altitude of 1336 km.
2) vertical TEC estimates from above the CHAMP satellite, orbiting at ~400 km altitude.

NOTE ABOUT TIME UNITS:
----------------------

At JPL, we often label times as "seconds past J2000". This could be
confusing, but it is easy to interpret if you know the seconds-past
value for the day you are interested in (e.g. Nov 20, 2003). That
information is provided here for your convenience. This number can be
positive or negative.

Nov 20, 2003 0 UT: 122558400.0 (seconds past J2000, 1200UT, Jan 1 2000)

JASON data
----------
File name format:
gtcYYMMDDk81hdbent where YYMMDD is the date.

Columns:
1. J2000sec. Seconds past the J2000 epoch (1200 UT Jan 1, 2000)
2. latitude in degrees, geographic
3. longitude in degrees, geographic
4. jason_tec, TECU units
5. jason_sigma, TECU units
6. gim, value from GIM
7. gim_sig, formal error of GIM (ignore for now)
8. IRI, vertical TEC from IRI, approximate
9. bent, vertical TEC from Bent, approximate

To derive the local time of these data (constant for a given day, the
precession is slow), convert from UT and longitude.

CHAMP data
----------
The dated sub-directories contain the CHAMP data. There is one file in each
directory.

The files CHAMP_Day_031120.jpg and CHAMP_Night_031120.jpg contain
ground-track plots for day and nighttime passes. The UT at the
equatorial crossing is indicated. This should allow rapid selection of
the correct UT for further inspection. The local times are roughly
fixed for one day, so reading the times from one point in the file is
sufficient for the day (the local time varies, of course, with
latitude).

For reference, on Nov 20, 2003, the equatorial local times for CHAMP are:
Day:   ~1100 hours
Night: ~2300 hours

File name format:
chamYYDOY.tec.7
where YY is the year, and DOY is the day of year (e.g. 02108 for April 18, 2002).

The columns in each file are as follows:

1. sec past qmepoch (ignore, col 16 is more useful)
2. hr past ref epoch (ignore, col 16 is more useful)
3. CHAMP latitude (geographic degrees, ground track)
4. CHAMP longitude (geographic degrees, ground track)
5. CHAMP local time (hours)
6. azimuth to GPS (degrees)
7. elevation angle to GPS (degrees)
8. obliquity factor used to estimate vertical TEC (>1)
9. slant tec  (TECU, biases removed)
10. vertical tec (estimated, TECU)
11. loss of lock indicator
12. 0
13. svn (GPS satellite vehicle number being tracked by CHAMP)
14. phase epoch (seconds past qmepoch)
15. tec sigma (standard deviation of TEC, TECU)
16. sec past j2000 (Seconds past the epoch of 1200 UT, Jan 1, 2000)
17 CHAMP magnetic latitude (dipole)
18 slant tec (from pseudorange data, TECU, noisier than col 9)
19 vertical tec estimate (from pseudorange)
20 CHAMP magnetic local time (hours)
21-23 CHAMP Earth-fixed x,y, and z relative to geocenter (km)
24-26 GPS satellite gps xyz (km)