The ability to precisely determine a satellite’s position on orbit is a key factor in the quality of altimetry data. Besides measurements acquired by the location systems onboard the satellites, which are cross-calibrated, we now rely on increasingly accurate orbit determination models.

Different products require different levels of accuracy. Data generated within three hours are based on a preliminary orbit from the Diode onboard navigator (DORIS). On the other hand, data generated 30 days post acquisition require the most accurate orbit possible and therefore demand more orbit data and more time for calculations. Expected accuracy on the radial orbit component is 20 cm rms for three-hour data, 2.5 cm rms for three-day data, and 1.5 cm for 30-day data. The ultimate aim is to achieve centimetre accuracy.

To achieve the goal of an orbit error of just one centimetre, we need a detailed knowledge of the satellite and its variations – due to manoeuvres, fuel consumption, solar panel orientation and so on – so that we can precisely model the forces acting on it (attraction, atmospheric drag, etc.). We also need to determine the gravity field very precisely. The geodesy missions (Champ, Grace, and Goce) will help us to improve our understanding of these factors.

 

Orbit choice

For dedicated missions like Topex/Poseidon and Jason-1, orbit precision is the most important criteria. A high-altitude orbit is chosen to attenuate the effects of the Earth’s gravity potential, which are not known with great accuracy for lower orbits. The orbit must not be too close to the poles, because the Earth’s gravity potential is also less well understood there. A high revisit capability is in order, to enable ocean signals to be observed. And being able to overfly any absolute calibration sites is also an advantage.
For multi-instrument missions like ERS and Envisat, the compromise must take into account any other intended observations (ice, in particular).
Conversely, the higher the orbit, the more power will be needed for the radar emission, in order to get back a strong enough signal, and the more vulnerable it will be to solar winds and cosmic particles. Lastly, to observe as much ocean as possible, an orbit close to the poles will be required.

The inclination of a satellite’s orbit is the angular distance of the orbital plane with respect to the Earth’s equatorial plane. An inclination of, e.g., 90 degrees indicates a polar orbit (this is the case with ESA’s Earth-observing Envisat satellite), in which the satellite passes nearly above both poles of the planet on each revolution. An inclination of 66 degrees, like the CNES/NASA Topex/Poseidon satellite, implies that the orbits sample from 66° North to 66° South, so as to cover most of the world’s oceans. There are different types of orbits: sun-synchronous, geosynchronous, geostationary, prograde, retrograde, etc. A sun-synchronous orbit (also called a heliosynchronous orbit) such as Envisat’s combines altitude and inclination in such a way that the satellite passes over any given point of the Earth’s surface at the same local solar time. Envisat, for instance, crosses the equator fourteen times a day, always at 10:00 local time. This is achieved by having the orbital plane of the satellite’s orbit precess (rotate) approximately one degree eastward each day, to keep pace with the Earth’s revolution around the sun. With a sun-synchronous orbit, observation of the ground is improved as the surface is always illuminated at the same Sun angle when viewed from the satellite. Sun-synchronous satellite orbits are retrograde (they orbit the Earth in an opposite direction to the Earth’s spin rotation). However, as 24 hours is the period of some tidal constituents, these will thus always be observed at the same stage of their cycle. Topex/Poseidon, in contrast, has a non-sun-synchronous and prograde orbit and a repeat period of 9.916 days (i.e. the satellite passes vertically over the same location, to within 1 km, every ten days). Envisat’s repeat cycle is 35 days. A satellite can be accurately tracked in a number of ways. The DORIS system on board Topex/Poseidon and Envisat, for instance, uses a worldwide network of ground beacons, transmitting to the satellite. It was developed by CNES. DORIS uses the Doppler shift on the beacon signals to accurately determine the velocity of the satellite on its orbit, and dynamic orbitography models to deduce the satellite’s trajectory relative to Earth.

Orbit selection

The factors to be considered for selecting the orbit for each instrument include:

 

Requirement Influence Factors Orbital Parameter
observation frequency swath width; revisit time altitude
global access maximum latitude; spacing between ground-tracks inclination, altitude
regular ground pattern synchronous or drifting orbit altitude
regular illumination conditions sun-synchronism inclination and altitude
aliasing of solar tides sun-synchronism inclination and altitude
aliasing of all tides repeat period altitude
accessibility of celestial sphere orbital precession inclination and altitude
discontinuities in orbit orbit maintenance frequency altitude
mission lifetime orbital decay gross altitude
instrument spatial resolution / radar transmitter power gross altitude
radar PRF (pulse repetition frequency) altitude range
Permanent cold radiator surfaces sun-synchronism inclination and altitude

 

Some of these are fundamental and have an impact on the overall design of the system. In particular, the selection of a sun- or non-sun-synchronous orbit is of primary importance. The total altitude range is also critical to the design. After that, there is a certain degree of freedom in the choice of parameters. Previous mission orbits may also determine the choice of orbit (e.g. Jason-1 on the tracks of Topex/Poseidon, Envisat on those of ERS-2).

Orbit maintenance

The orbit maintenance requirements for altimetry missions are usually that the deviation of the actual ground track from the nominal one is kept below 1 km and that the mean local nodal crossing time matches the nominal one to better than to within five minutes. The orbit maintenance strategy aims for minimum disturbance of the payload operation. In-plane manoeuvres are used for altitude adjustment to compensate for the effects of air-drag. This altitude decay control affects the ground-track repeatability, mainly in the equatorial regions. The frequency of these manoeuvres is determined by the rate of orbital decay, which in turn is determined by the air density, and this is a function of solar activity. The nominal rate for these in-plane manoeuvres is nominally twice a month. They will do not interrupt the operations of most sensors. Out-of-plane corrections are used to correct rectify the steady drift of inclination mainly caused by solar and lunar gravity perturbations. The solar wind also influences inclination, but its contribution is typically an order of magnitude smaller than the one given made by solar and lunar gravity.
Inclination drift degrades ground-track maintenance at high latitudes. The drift rate does not depend on air density and corrections are required every few months. As they are out-of-plane they require a 90-degree rotation of the spacecraft, to align the thrusters with the required thrust direction, so these manoeuvres will be performed in during the eclipse to avoid the risk of optical sensors viewing the sun.