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As of 2016 there are multiple operational missions in orbit around Mars. There are possibly more non-operational missions in orbit around Mars, simply because we do not know the final fate of all missions to Mars. Each mission carries at least one radio link designed to communicate data back to Earth, otherwise the mission would not be very helpful.

On Earth, satellites are essentially moving in the sky or geosynchronous (which really means not moving in the sky from the position of an observer on the ground).

For a geosynchronous satellite, RF spectrum is generally coordinated as a position in the sky and a range of frequencies in use. Even if multiple satellites occupy the same position in sky, each one can use a unique range of frequencies to avoid interference.

For satellites that move across the sky the situation is a bit more complex, but there is still some coordination amongst operators. For example, no one would put a satellite into orbit with a transponder on 121.5 MHz because it is the aircraft emergency frequency.

But a satellite in orbit on Mars is a significantly different situation. The satellite is generally oriented so that it's high gain antenna is pointing at the Earth at all possible times. From the perspective of an observer on Earth, Mars is just a point in the sky. It doesn't matter how accurate the positioning data you have is, there is no way to point a receiver at a specific object in orbit around Mars.

So basically every single receive site on Earth is pointing at the same spot in the sky. If two satellites in orbit around Mars were attempting to use the same frequency, the receiver on Earth would get interference between them. How is RF spectrum usage coordinated for transmitting from Mars orbit back to Earth?

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    $\begingroup$ It's not a full answer, but I think you want to start by looking at the International Telecommunication Union's (ITU's) Space Services Department, and perhaps their Space Plans in particular. $\endgroup$ – a CVn Oct 25 '16 at 15:30
  • $\begingroup$ @MichaelKjörling Those are quite interesting - but opaque to me at least. I hope someone asks the question "What (actually) are ITU's Space Plans" at some point! $\endgroup$ – uhoh Oct 25 '16 at 23:34
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I went through the ITU's Space Plans for information on this. They are the internationally agreed authoritative body (some exceptions, perhaps, but most spacefaring countries have signed up) The wording is actually very simple (despite there being extensive documentation - 430 pages!) as it is developed from the ITU's more general documentation on radio frequency usage on land, sea and air so I've pulled out some of the most relevant:

Article 4: Assignment and use of frequencies

4.3 Any new assignment or any change of frequency or other basic characteristic of an existing assignment (see Appendix 4) shall be made in such a way as to avoid causing harmful interference to services rendered by stations using frequencies assigned in accordance with the Table of Frequency Allocations in this Chapter and the other provisions of these Regulations, the characteristics of which assignments are recorded in the Master International Frequency Register.

4.4 Administrations of the Member States shall not assign to a station any frequency in derogation of either the Table of Frequency Allocations in this Chapter or the other provisions of these Regulations, except on the express condition that such a station, when using such a frequency assignment, shall not cause harmful interference to, and shall not claim protection from harmful interference caused by, a station operating in accordance with the provisions of the Constitution, the Convention and these Regulations.

4.5 The frequency assigned to a station of a given service shall be separated from the limits of the band allocated to this service in such a way that, taking account of the frequency band assigned to a station, no harmful interference is caused to services to which frequency bands immediately adjoining are allocated.

So allocation of frequencies is the main control here to avoid collisions. Section 1 deals with interference and there are further provisions, for example:

15.4 a) locations of transmitting stations and, where the nature of the service permits, locations of receiving stations shall be selected with particular care;

15.5 b) radiation in and reception from unnecessary directions shall be minimized by taking the maximum practical advantage of the properties of directional antennas whenever the nature of the service permits;

For specific frequencies it's worth getting a copy and reading The Table of Frequency Allocations, Regional Allocations, and the full range of frequencies detailed in Chapter II.

Countries and regions are given allocations, and can decide where in their ranges they want to transmit. They need to tell the ITU their choice, so it can be incorporated in the allocation table.

If they need to use a previously allocated frequency, this can be arbitrated through the ITU, but each country is expected to stay within their allocated ranges.

Interestingly though, in emergency, 4.9 comes into play:

4.9 No provision of these Regulations prevents the use by a station in distress, or by a station providing assistance to it, of any means of radiocommunication at its disposal to attract attention, make known the condition and location of the station in distress, and obtain or provide assistance.

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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – Rory Alsop Dec 21 '16 at 16:10
  • $\begingroup$ These are just regulations/guidelines, which at least some people may find difficult to read through. They don't describe how coordination is actually done which is the kind of answer this question deserves. Here is one example of actual procedures followed to coordinate frequency usage. NASA Radio Frequency (RF) Spectrum Management Manual Chapter 3 RF Allocation and Assignment Process and Procedures If you could add a simple description of how coordination is actually done that would help alot! $\endgroup$ – uhoh Dec 22 '16 at 1:14
  • $\begingroup$ Is frequency allocation done exclusively by the ITU, or do individual countries and governments and agencies also have some allocation authority and responsibility? If a space agency has multiple current missions, how is that coordinated? How can we all get a copies of "The Table of Frequency Allocations, Regional Allocations" so that we can all see "the full range of frequencies detailed in Chapter II."? $\endgroup$ – uhoh Dec 22 '16 at 1:35
  • $\begingroup$ Does SpaceX have a frequency allocation for their planned unmanned Mars landing test? Do commercial companies like SpaceX apply to the appropriate government, or agency, or to ITU directly? $\endgroup$ – uhoh Dec 22 '16 at 3:46
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    $\begingroup$ The ITU does spectrum allocation for satellites in Earth orbit and for ground stations. I get the impression they don't do spectrum allocation for Mars orbit, although there is some overlap (ground stations). $\endgroup$ – Hobbes Dec 22 '16 at 12:31
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The procedure for coordination is specified in the links provided by Rory Alsop. To recap:

  1. A space organization submits a request to the Radiocommunications Bureau of the ITU.
  2. The RB examines the request, makes sure there are no collisions with current frequency allocations.
  3. On approval, the space organization is free to use this frequency. The Master Register (list of allocated frequencies) is updated and published.

When an administration intends to bring into use its assignment(s) in the appropriate Regional Plan or Regions 1 & 3 List, it notifies this to the Bureau. The procedure of Article 5 of Appendices 30 and 30A is then applied. The Bureau examines the submission to assure that the information received is complete, that the data elements are in conformity with Appendix 4, and that the notified characteristics comply with those of the entries in the Plans or Lists.

At first glance, this applies to Earth-orbiting satellites only. I'm looking for more details.

The ITU has reserved several slices of the radio spectrum for deep-space use. In the US, use of these slices is coordinated by JPL.

Through international agreements, the International Telecommunications Union (ITU) allocates and regulates portions of the frequency spectrum for both commercial and government use. The primary objective of the ITU is to establish regulatory procedures for the coordinated use of frequencies by those agencies permitted to operate in the allocated bands. The ITU has allocated certain bands to deep space (Category B) research. In some cases, the deep space missions may be required to conditionally share a frequency band between multiple users in the same band.

The Consultative Committee for Space Data Systems (CCSDS) is an international organization for space agencies interested in mutually developing standard transmission and data handling techniques to support space research, including space science and applications. As a member of the CCSDS, NASA has submitted recommendations for various space systems applications.

The National Telecommunications and Information Administration (NTIA), an agency of the U.S. Department of Commerce, is the Executive Branch's principal authority on domestic and international telecommunications and information technology issues. During the planning phase of all missions using the DSN, the proposed operating frequencies and other operating parameters are reviewed by the NTIA for approval through the System Review process. The NTIA evaluations are based upon the technical and regulatory criteria for the efficient and coordinated use of the frequency spectrum by NASA missions.

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