|Communications With Earth
How the DSN helps engineers navigate the spacecraft during cruise
During cruise, the Deep Space Network antennas pick up signals from the spacecraft that tell navigators where the spacecraft are located. Engineers cannot physically see the spacecraft with the naked eye or a telescope, and they rely on radio "tracking" to know where the spacecraft are at any given time. Like a game of "Marco-Polo," the DSN listens for signals from outer space and can detect where the spacecraft is from where the sound comes from.
This navigation service is called "tracking coverage" and it includes Doppler, ranging and delta differential one-way ranging, or "Delta DOR."
In order to calculate the speed that a spacecraft is flying, engineers use Doppler data to plot velocity along the line of sight between Earth and the spacecraft.
Most people are familiar with the phenomenon of a car horn or train whistle changing its frequency as it moves towards or away from them. Electromagnetic radiation (e.g. light waves or radio signals) also experience this effect. The size of the frequency shift, or "Doppler shift," depends on how fast the light source is moving relative to the observer . Astronomers
often refer to the "redshift" and "blueshift" of visible light, where the light from an object coming towards us is shifted to the blue end of the spectrum (higher frequencies), and light from an object moving away is shifted towards the red (lower frequencies).
The twin Mars Exploration rover spacecraft commmunicate with controllers on the ground by radio signals. Ground controllers know the frequency of the signal that is emitted from the spacecraft. However, since the spacecraft is moving away from (or towards) us, this frequency is being Doppler shifted to a different frequency. So, engineers (or, more accurately, computers) compare the received frequency with the emitted frequency to get the Doppler shift. It´s then straightforward to find the velocity that would cause the resulting Doppler shift.
Ranging is sending a code to the spacecraft, having the spacecraft receive that code and immediately send it back out the spacecraft´s own antenna, and finally receiving that code back on Earth. The time between sending the code and receiving the code, minus the delay in turning the signal around on the spacecraft, is twice the light time to the spacecraft. So that time is divided by two and multiplied by the speed of light is the distance from the DSN station to the spacecraft. This distance is accurate to about five to ten meters (16-33 feet), even though the spacecraft may be 200 billion meters away!
Delta DOR is similar to ranging, but it also takes in a third signal from a naturally occurring radio source in space, such as a quasar, and this additional source helps scientists and engineers gain a more accurate location of the spacecraft.
Quasars are a few billion light years away and a few billion years in the past. Quasars are used as extremely well known positions in the sky to provide a calibration for the same measurements made within a few tens of minutes of each other on a spacecraft. Being able to do quasar and spacecraft ranging near the same time and subtracting the answers cancels a lot of errors that are the same in both measurements from the atmosphere and the equipment.
The "ranging" is not really ranging, but differenced ranging. What is measured is the difference in the distance to the source between two complexes on Earth (for example, Goldstone and Madrid or Goldstone and Canberra). From that an angle in the sky can be determined relative to the stations. The angle for the quasar is subtracted from the angle of the spacecraft, giving the angular separation of the quasar and the spacecraft. That angle is accurate to about five to ten nanoradians, which means when the spacecraft is near Mars, say 200 million kilometers away, it can determine the position of the spacecraft to within one kilometer (.6 miles).