# Relativistic effects in space mission communications

How important are relativistic effects in communication with space missions?

I am doing some work on Mars Pathfinder and want to know if effects like gravitational deflection of microwaves matter in the communication with Earth - and if so how the problem is solved.

• Couple hours of radio delay insufficient?
– SF.
Jun 6, 2018 at 15:53
• Relativistic effects of Earth's mass are on the order of one part in a million. Mars is smaller, so even less. My bet is that it is swamped by the other issues such as doppler shift as Earth's orbit and the Martian orbit interact. Jun 6, 2018 at 17:37
• The atomic clocks used within any GPS satellite have been adjusted to the frequency change caused by relativistic effects. All clocks of the GPS system existing in the satellites and ground stations should run as synchronous as possible. Here relativistic effects caused by the Earth and the orbit could not be neglected.
– Uwe
Jun 6, 2018 at 21:15

## Deflection correction:

Microwave beams, even the most tightly focused beam from the large 70 meter dishes of the Deep Space Network are still physically wide when they get to their destination, so relativistic deflection effects would not be big enough to require corrections, although I'm assume these effects are included in the pointing algorithms anyway.

As @DavidHammen notes in this comment and @DuffBeerBaron noted in this answer there is increasing use of Very Long Baseline Interferometry (VLBI) and Delta-DOR to try to obtain angular measurements of spacecraft, and those might be subject to some relativistic deflection.

## Light time correction (non relativistic):

However the light time correction is huge, though not considered relativistic in nature. You have to point in the direction that the spacecraft will be in the future or the past. If for example the spacecraft is out near Jupiter, an incoming signal will come from the direction in space where Jupiter was about 45 minutes ago and if you want to send a signal, you point where it will be about 45 minutes later. However correcting for the finite speed of light does not really count as a relativistic correction.

## Situations where relativistic effects are/will be important::

1) Radio and in the future, optical links to spacecraft usually include navigation signals like delay-doppler for precise tracking as well as data and telemetry channels, and relativistic effects will be much more significant for these. See the questions and their answers:

2) In the future, a navigation satellite constellation may be deployed at Mars, equivalent to GPS/GNSS for Earth. Relativistic effects are absolutely critical when using those signals to get a location fix. See for example the questions and answers at:

3) Missions close to the Sun will experience much larger relativistic effects as well. See the currently nicely but incompletely answered question: