Delta-DOR

The Delta - DOR method (English: Delta Differential One -way Ranging) for interplanetary navigation is based on a simple but effective idea. Two widely spaced receive antennas simultaneously track a spacecraft and thereby determine the time difference between the time instants at which the signal arrives at the two stations. The art to determine this difference is one-way differential range (DOR). Theoretically, this difference depends only upon the positions of the two antennas and the observed spacecraft. However, there are a number of confounding factors, they must, for example, the radio waves through the troposphere and the ionosphere, they are disturbed by the plasma in the solar wind and the clocks of the base stations are inaccurate. The Delta - DOR method corrects this error by changing the direction of a quasar is used for correction (within 10 ° ) near the spacecraft.

The direction of the quasar is by prior measurements very accurately known with an accuracy of only 50 billionths of a degree. The distance to the spacecraft is calculated by measuring the time it takes for a radio signal to the probe and to the earth. By measuring the Doppler shift of the signal, the relative velocity along the line of sight is determined. In order to control the spacecraft, however, the real speed is needed in the room. So far, the movement of the probe against the sky background was observed to several days. In principle, the temperature measured by the radio signal of the time difference is subtracted quasar ( Delta ) of the time difference of the spacecraft. The position can be accurately determined with the delta - DOR method, since the error in the measurement of the spacecraft position can be corrected by the similar error in the Quasarposition.

This procedure was applied in the NASA space probe Phoenix and the two Mars Exploration Rovers and the ESA probe Venus Express and Rosetta for use.