Jason-1 ground tracks, before February 2009
Choice of orbit
Jason-1’s high altitude (1,336 kilometres) reduces interactions with the Earth’s atmosphere and gravity field to a minimum, thus making orbit determination easier and more precise. The orbit inclination of 66 degrees North and South enables the satellite to cover most of the globe’s unfrozen oceans. The orbit’s repeat cycle is just under 10 days (9.9156 days to be precise, i.e. 10 days minus two hours) – in other words, the satellite passes over the same point on the Earth’s surface (to within one kilometre) every ten days. This cycle is a trade-off between spatial and temporal resolution designed for the study of large-scale ocean variability. The fact that the orbit is prograde and not sun-synchronous also avoids aliasing of different tide components at the same frequency.
Furthermore, using the same orbit as Topex/Poseidon ensures better intercalibration and data continuity. The orbit is also designed to pass over two dedicated ground calibration sites: Cap Senetosa in Corsica and the Harvest oil rig platform in California, USA.
Jason-1 has undergone an orbit change at the end of OSTM/Jason-2’s calibration phase in February 2009. Currently, Jason-2 is located on the former orbit of Topex/Poseidon (before 2002) and Jason-1 (before February 2009). The Jason-1’s orbit is shifted midway between its original ground tracks (corresponding to those of Topex/Poseidon after 2002). Jason-1 and OSTM/Jason-2 also have a time lag of 5 days.
A satellite’s orbit parameters tend to change over time as a result of atmospheric drag. In the long term, more or less periodic variations also occur due to instabilities in the Earth’s gravity field, solar radiation pressure and other forces of smaller magnitude.
|Jason-1 orbit Heavens Above|
Orbit manoeuvres are performed every 40 to 200 days. Intervals between manoeuvres depend chiefly on solar flux and each manoeuvre lasts from 20 to 60 minutes. Wherever possible, they are performed at the end of the orbit cycle, and above the solid earth, so that lost data acquisition time is reduced to a minimum.
|Semi-major axis||7714.4278 km|
|Reference altitude (equatorial)||1,336 km|
|Nodal period||6,745.72 seconds
(112’42” or 1h52′)
|Repeat cycle||9.9156 days|
|Number of passes per cycle||254|
|Ground track separation at Equator||315 km|
|Acute angle at Equator crossings||39.5°|
|Orbital velocity||7.2 km/s|
|Ground scanning velocity||5.8 km/s|