The orbit

TOPEX/POSEIDON altimeter tracks  The orbit configuration was chosen to optimize the study of the temporal large scale oceanic variability and avoid aliasing different tidal constituents to the same frequency.  A high orbit altitude was also selected to minimise atmospheric drag and gravity forces acting on the satellite, and make orbit determination easier, more accurate.

TOPEX/POSEIDON orbit characteristics  Main characteristics  Semi-major axis 7 714.4278 km Eccentricity 0.000095 Inclination (not 1-synchronous) 66.039 degrees Argument of periapsis 90.0 degrees Inertial longitude of the ascending node 116.5574 degrees Mean anomaly 253.13 degrees Auxiliary data  Reference (Equatorial) altitude 1 336 km Nodal period 6 745.72 seconds   (112' 42" or 1H52') Repeat period (10-day cycle) 9.9156 days Number of revolutions within a cycle 127 Equatorial cross-track separation 315 km Ground track control band +/- 1 km Acute angle at Equator crossings 39.5 degrees Longitude of Equator crossing of pass 1 99.9242 degrees Inertial nodal rate -2.0791 degrees / day Orbital speed 7.2 km / second Ground track speed 5.8 km / second Get the correspondance between the date and the cycle/pass number.

The exact altitude that allows the orbit to satisfy these constraints and fly over 2 verification sites located in the Mediterranean Sea (Lampedusa Island, Italy) and at the Harvest Platform (California, USA) is 1336 km. An inclination of 66° was selected - ie the inclined orbit samples from 66° north to 66° south - so as to cover most of the world's ocean (~ 90%). The orbit is not 1-synchronous and is prograde. A repeat period of 9.916 days has been chosen (ie; the satellite passes vertically over the same location, to within 1km, every ten days) as a better compromise best between spatial and temporal resolutions. The distance between successive tracks is on the order of 315 km at the Equator. The local time of successive passes shifts by nearly two hours per cycle.