Measurements are accurate to a billionth of a second

Engineers at BAE Systems' Great Baddow site in Essex, with help from the European Space Agency (ESA) and the University of Rome, have developed an incredibly accurate measurement system which was used for the first time in November 2005, to guide the Venus Express spacecraft into orbit insertion. The measurement system - dubbed 'Delta-DOR' - is part of BAE Systems' 'IFMS' (Intermediate Frequency Modem System) and plays a crucial part in ensuring that the spacecraft is slowed down by exactly the right amount so that it correctly enters Venus orbit. "This is the very first time that ESA used our equipment with its newly developed and especially accurate Delta-DOR technique," said Andy Baslington, programme manager at BAE Systems. "This works by measuring the difference in the time it takes signals from the spacecraft to reach two different receiving stations and how this varies over a 30-minute period. We can do this to an accuracy of better than a billionth of a second." The first part of the delta-DOR ranging process is to synchronise the clocks at the two ground stations, located in Spain and Australia. This is achieved by getting them to look at the signal received from an astronomical object called a quasar, compact, distant radio objects that have well known positions in the sky. The noisy, 'thermal' radio signal which these emit cannot normally be detected by the ESA ground stations since they are much smaller (less than 1/100) than the thermal noise generated by the stations themselves. But, when there are two ground stations whose clocks are correctly synchronised and the Earth's rotation is taken into account (and its motion around the Sun), the signal from the quasar will be the same in the two stations, although the noise from each station is usually different. By capturing the received signal using the IFMS equipment and seeing what time difference between the two stations is needed to maximise the similarity of the signals received, their clocks can be correctly synchronised. Once this has been done, the time difference between the spacecraft signals being received at the two ground stations can be used to accurately position it, in a way similar to the normal triangulation method.