|
Communications SystemHow does NASA communicate with spacecraft? |
Lag time is directly related to the position of the spacecraft, which can be tracked using its radio beacon. The radio beacon signal is used for tracking the spacecraft and for carrying out some types of science experiments. Tracking takes advantage of the Doppler effect to see how position is changing. The Doppler effect is routinely observed in the frequency of the signals received by ground receiving stations when tracking spacecraft. If two widely-separated tracking stations on Earth observe a single spacecraft in orbit about another planet, they will each have a slightly different view, and there will be a slight difference in the amount of Doppler shift observed by each station. For example, if one station has a view exactly edge-on to the spacecraft's orbital plane, the other station would have a view slightly to one side of that plane. The differencing of the two received signals can be used to describe the spacecraft's arc through space in three dimensions. This data type, called differenced Doppler, is very accurate as it is based upon measurements taken from the frequency of the downlink.
In order to get any useful information from the Doppler shift, however, an extremely stable downlink frequency is required so that Doppler shifts on the order of fractions of a Hertz may be detected out of many GHz over periods of hours. It would be impossible for any spacecraft to carry the massive equipment on board required to generate and maintain such high stability. The solution is to have the spacecraft generate a downlink based on the uplink it receives.
Down in the basement of each DSN Signal Processing Center, there looms a hydrogen maser-based standard frequency generator in an environmentally controlled room. This is used as a reference for generating an extremely stable uplink frequency for the spacecraft to use in generating its downlink. The resulting spacecraft downlink has the same extraordinarily high frequency stability as the massive hydrogen maser-based system in its controlled environment in the DSN basement. It is almost as if the signal is reflected off of the spacecraft. This signal can be used for precisely tracking the spacecraft and for carrying out science experiments.
How much power is available for the communications system?
