Delta-DOR

Delta-Differential One-Way Ranging (or Delta-DOR, Δ-DOR for short) is an interplanetary radio-tracking and navigation technique.

==Procedure== As with standard DOR (Differential One-Way Ranging), radio signals from a given spacecraft are received by two widely separated deep space ground stations on Earth and the difference in the times of signal arrival is precisely measured and used to calculate a bearing, while the Doppler shift is used to calculate the velocity of the spacecraft. Due to Earth's atmosphere altering the speed and frequency of the incoming transmission, some error is generated. This error is corrected using information about the current delays and signal variance due to Earth's atmosphere, obtained by simultaneously tracking radio signals from a well known quasar (preferably within 10 degrees of the spacecraft from the station's perspective) from each ground location and calculating the difference between the expected values and actual values, thus gaining the ability to determine accurate atmospheric delay live from both sites. This method is somewhat similar to the laser beacons used on optical telescopes such as the Extremely Large Telescope to actively adjust for the distortion caused by the atmosphere.