Mission Specification

topex-in-space.jpgTOPEX/Poseidon (T/P) was an altimetric mission jointly collaborated by NASA and CNES (French space agency).  It launched August 10, 1992 and began data collection at September 25, 1992.  T/P operated until October 18, 2005 and collected 481 cycles of data.  T/P was capable of measuring significant wave height, sigma0, dry and wet troposphere and ionosphere, which can be used to calculate sea surface height and anomalies and total electron content. 

T/P has a repeat period of approximately 10 days with 254 passes per cycle.  A list of cycle start times can be found here.

Sometimes there maybe anomalous or missing data.  Occasionally T/P had to perform maneuvers to maintain orbit.  This may have caused anomalous values so a list of maneuvers is available here.  When the satellite detected something abnormal it went into safehold and turned off all instruments and no data was collected.  A list of safeholds and periods of no data collection can be found here.

Tandem Mission

T/P began a drift maneuver August 15, 2002 cycle 365 pass 111 and ended September 16, 2002 cycle 368 pass 171.  During this time T/P moved to a different orbit so that T/P and Jason-1 (which remained on the original T/P orbit) would survey the Earth in a shorter amount of time and give a higher spatial resolution.

Orbits and Auxilary Data
Semi-major axis 7,714.43 km
Eccentricity 0.000095
Inclination 66.04°
Argument of periapsis 90.0°
Inertial longitude of the ascending node 116.56°
Mean anomaly 253.13°
Reference altitude 1,336 km
Nodal period 6,745.72 sec
Repeat period 9.9156 days
Number of revolutions within a cycle 127
Number of passes within a cycle 254
Equatorial cross track separation 315 km
Ground track control band +1 km
Acute angle at Equator crossings 39.5°
Longitude of Equator crossing of pass 1,
cycles 1-365
Longitude of Equator crossing of pass 1,
cycles 369-onward
Inertial nodal rate -2.08°/day
Orbital speed 7.2 km/s
Ground track speed 5.8 km/s

Additional information about instruments, orbit or other properties of TOPEX/Poseidon can be found in the Handbook.

Known Problems and Issues

Switch Between TOPEX-A and TOPEX-B

TOPEX-A altimeter operated from cycles 1-235. In mid 1997, it was determined that there was a degradation in the point target response which impacted the significant wave height. In February 1999, TOPEX-A was turned off and the redundant backup input altimeter, TOPEX-B altimeter, was turned on. Subsequent calibration to assess the data resulted in keeping TOPEX-B on for cycle 236 and all subsequent cycles. Cycle 235 was the last cycle using TOPEX-A.

Errors in MGDR-B

In the MGDR version B data cycles 1-190 there were some small errors, mostly the incorrect default values were placed in the data, but the flags are correct. A full list of the errors can be found at 

Early Pointing Angle Problems

From cycle 1 through cycle 8 pass 189 (September 23 through December 8, 1992, day of year, DOY, 343), the satellite attitude control system was not properly calibrated. This resulted in pointing the altimeter relatively far from nadir (typically 0.3, but up to 0.6) with a sinusoidal signature over each pass much of the time. Since the pointing angle/sea state corrections are less accurate for angles larger than about 0.3 and the geometric pointing correction cannot be done, these data will be less accurate than later data. The pointing calibration was improved on 1992 DOY 353 and 357 and 1993 DOY 046, but these changes were minor compared to the initial calibration on 1992 DOY 343. The user may want to begin analysis on data beginning with cycle 11.

Sigma0 Calibration

A list of calibrations for the TOPEX backscatter can be found at .

The calibrations are provided to improve the sigma0 values provided by the project that are found in the GDRs.  If you are using the MGDRs, the calibrations have been applied to cycles 1-132, otherwise you need to apply the calibrations to the other cycles yourself.  If you are applying the GPC correction to the GDR, it contains the calibration corrections.