The Doris instrument onboard Jason-1 provides real-time location and 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.

Function

Real-time location: Diode

The Diode onboard navigator locates the satellite on orbit in real time. This information is essential for providing altimetry data in real time or near-real time.

Every 10 seconds, Diode runs a program that:

• acknowledges commands;
• predicts the satellite's position using a model of its motion;
• corrects the predicted position on the basis of Doris measurements (when the satellite is within view of a ground beacon);
• and lastly, delivers the calculated position to users.

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 on the altimeter's radar signal.
This function complements the dual-frequency altimeter function.

Principle

Doris orbitography beacons transmit signals at two separate frequencies (2036.25 MHz and 401.25 MHz) to the satellite. The receiver onboard the satellite analyzes 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 centimeters on the radial component.