Delta-DOR measurements: accurately determining the ephemeris of Venus
The DDOR measurements pinpoint the location of the Venus Express spacecraft; the location of the spacecraft with respect to the planet centre is known to high accuracy. Combining the two allows the orbit of Venus to be determined with very high accuracy. Modern-day DDOR campaigns are carrying on a tradition started by ancient civilizations - the orbit of Venus has been measured throughout the ages, with increasing accuracy; it is increasingly important in the space age.
The DDOR technique uses two widely separated antennas to simultaneously track the location of a transmitter in space in order to measure the time delay between signals arriving at the two stations. Theoretically, the delay depends only on the positions of the two antennas and the spacecraft. In reality, it is affected by several sources of error: for example, the radio waves travelling through the troposphere, ionosphere and solar plasma, and clock instabilities at the ground station. DDOR corrects these errors by 'tracking' a quasar in a direction close to the spacecraft for calibration. The quasar's direction is already known to very high accuracy by astronomical measurements, typically to better than 50 billionths of a degree (a nanoradian). The quasar is usually within 10 degrees of the spacecraft so that their signal paths through Earth's atmosphere are similar. In principle, the delay time of the quasar is subtracted from that of the spacecraft to provide the DDOR measurement.
(For more information about DDOR, see the ESA Bulletin article "Delta-DOR – A New Technique for ESA's Deep Space Navigation" - link in right-hand column.)