The Doris instrument onboard Topex/Poseidon provided precise orbit determination. Doris measurements are also used for geophysical studies, in particular through the International Doris Service (IDS). Doris is a dual-frequency instrument able to determine atmospheric electron content.

Functions

Precise orbit determination

Doris measurements are used for precise orbit determination (POD).

Ionospheric electron content

By measuring and comparing the path delay of signals transmitted at two separate frequencies, Doris is able to calculate the electron content in the atmosphere. This information is then used to determine perturbations acting on the altimeter's radar signal.
This function complements the Topex dual-frequency altimeter function.

The Doris network of orbitography beacons and their spatial coverage. (Credits Cnes).

Principle

Doris orbitography beacons transmit signals at two separate frequencies (2 036.25 MHz and 401.25 MHz) to the satellite. The receiver on board the satellite analyses the received signal frequencies to calculate its velocity relative to Earth. This velocity is fed into orbit determination models to derive the satellite's position on orbit to within two centimetres on the radial component.