China, UAE and US all sending missions to Mars in 2020 (Summer of L̶o̶v̶e Mars); how far apart are their frequencies?

Question

Gizmodo's Second failure of ExoMars parachute test throws schedule in jeopardy says:

ExoMars 2020 is due to launch during a narrow window open between July 25th to August 13th 2020, during which China and the US will launch their own rovers as well. The three spacecraft will each have their own separate mission.

Update: ExoMars 2020 is no longer; the mission is now planned to launch in 2022.

Question: How far apart will the frequencies be for these three missions? Conceivably there could be three ground stations on Earth simultaneously transmitting towards these three missions with overlapping beams, or three ground stations listening at the same time. All receiver front-ends will have some ability to reject out of pass-band signals, but the farther the frequencies are away from each other, the easier it will be to reject the strongest signal and pick up the intended weakest signal.

• China Orbiter & Rover: Mars Global Remote Sensing Orbiter and Small Rover
• ESA/Roscosmos Lander & Rover: ExoMars/Rosalind Franklin (rover)/Kazachok
• UAE Orbiter: Hope Mars Mission (also emiratesmarsmission.ae)
• USA Rover: Mars 2020

July/August 2020 will be the "Summer of Love Mars"

Answer 1

There is an international standard for space communications https://ccsds.org/ Therefore it is no more difficult than WiFI

Today, leading space communications experts from 27 nations collaborate in developing the most well-engineered space communications and data handling standards in the world.

The table below lists missions known to be using CCSDS-recommended protocols. For the missions listed, CCSDS protocol use ranges from CCSDS Version 1 Transfer Frames for telemetry (early missions) to the full suite of conventional and/or Advanced Orbiting Systems (AOS) telemetry and telecommand protocols. Many of these missions also follow CCSDS Recommendations for data archiving, Space Link Extension (SLE) services, Time Code Formats, and Lossless Data Compression; the majority conform to CCSDS Recommendations for Radio Frequency and Modulation Systems. https://public.ccsds.org/implementations/missions.aspx

• BEAGLE-2
• DS-2
• EXOMARS ROVER
• Mars Netlander
• Mars Odyssey
• MARS-1-ZX
• Mars Reconnaissance Orbiter
• Mars Pathfinder
• Spirit etc

Open System Interconnection:

P.S. X-band (8025-8400 MHz)

RADIO FREQUENCY AND MODULATION SYSTEMS—PART 1 EARTH STATIONS AND SPACECRAFT

Under the SFCG (Recommendation 23-1) 12 MHz bandwidth limitation for 8 GHz band non-Mars missions on a non-interfering basis, the maximum telemetry symbol rate using GMSK BTS=0.5 is 9.3 Ms/s. For 8 GHz band Mars missions and non-Mars missions which interfere with Mars missions, the maximum telemetry symbol rate using GMSK BTS=0.5 is 6.2 Ms/s.

Making history at Mars: Proximity-1, key to Mars communications

This successful partnership between ISO and the CCSDS is also evidenced at Mars, where all spacecraft have implemented standard data communications protocols developed by the CCSDS and accepted by ISO on their long-haul links back to Earth. Last month, during the CCSDS bi-annual meeting being held in Toulouse, ISO TC 20/SC 13 considered CCSDS’s latest newsmaker at Mars, a specialized communications protocol called Proximity-1. During demonstrations sponsored by NASA and the European Space Agency (ESA) in February 2004, Proximity-1 was instrumental in establishing the first in-orbit communication between NASA and ESA spacecraft, as well as the first working international communications network around a planet other than Earth.

Prior to the development of Proximity-1, earlier missions like Mars Pathfinder had to transmit data directly from the Martian surface millions of miles to Earth. Because of the great distance between the two planets, as well as the rover’s limited transmitter, transmission signals using this communications path were weak and data reliability was limited.

For example Talking to Martians: Communications with Mars Curiosity Rover

In terms of communications capabilities, MRO has transceivers at three different frequency bands:

X-band: 8 GHz Primary communications with Earth during the launch and cruise stage and also while orbitting Mars. The centre frequencies used are 8.439 GHz for transmit (Tx) and 7.183 GHz for receive (Rx). A bandwidth of 50 MHz is allocated. Ka-band: 32 GHz Experimental payload to investigate the performance for space to Earth communications compared to using X-band. Only transmit is used by MRO. The centre frequency is 32.0 GHz. A bandwidth of 500 MHz is allocated for Ka-band. UHF: 400 MHz Used for relaying commands and data from rovers on Mars' surface back to Earth. MRO has 16 preset channels, ranging in frequency from 390 MHz up to 450 MHz. When using half-duplex (transmit or receive, not both), any channel can be chosen, but when using full-duplex Tx channels are chosen from the range 435 to 450 MHz and Rx channels from the range 390 to 405 MHz.

Answer 2

TL;DR

All but the Chinese use the Deep Space Network, so the DSN is in charge of coordination. Spacecraft also need a surprisingly low amount of communication time with the ground, and they have a unique ID to check that the messages they receive are indeed destined to them. Spacecraft operators know the exact frequency of their spacecraft radios, and communicate that to operators.

Ground stations

ExoMars, EMM / Amal / Hope, and Mars 2020 Perseverance all use NASA's Deep Space Network (DSN) for at least part of the mission. The current mission set (link to the public April 2019 Excel file) ExoMars, EMM and Mars 2020 have the launch date, the future critical event and expected end of mission dates (respectively rows 44, 11 and 29 in the April 2019 list).

The DSN is composed of three communication sites: Goldstone, Canberra and Madrid. This allows deep space missions to be in sight of two sites at all times. ESA does have Estrack, but they will still use the DSN for critical events (src):

The NASA Ground Stations & Communication Subnet (DSN), to be considered for “critical phases“ like Safe Mode(s) or Flight Software upload or for “extreme contingencies” like the loss of SCC attitude

Communication requirements

Telecommunication can be tricky for spacecraft, especially scientific missions. Scientists want to collect as much data as possible to perform lots of calculations on the ground. Engineers determine the data bandwidth available to the spacecraft based on the chip rate, the data modulation, the carrier frequency, and the bandwidth reserved for telemetry (metrics used for the state of health of the spacecraft). Moreover, spacecraft navigators must perform tracking measurements with the spacecraft: these require antenna time, but do not necessarily transfer data between the ground and spacecraft.

Summarizing: deep space spacecraft will need antenna time for tracking every few days for 4 to 8 hours (assuming a tracking measurement every 60 seconds, which is the common sample rate for DSN tracking). Then, after tracking, the antenna is reserved for another 30 minutes to 2h (depending on the mission) for data transfer only. Hence, a deep space spacecraft only communicates less than a dozen hours every few days.

Source: me, as a spacecraft navigation engineer on an upcoming Moon mission.

Coordination

Since all missions adhere to the same communication frequency bands. These are specified by the International Telecommunications Union (ITU), and all deep space missions launching from a country which is part of the ITU must obey these rules (summarized on Wikipedia).

Since communication is quite limited (as we saw above), it's the job of the DSN to tell spacecraft operators when they will be communicating with their spacecraft. This is usually frozen two weeks ahead of the tracking and communication pass.

Moreover, there is an international agreement on the data formats for space communication: Consultative Committee for Space Data Systems (CCSDS). One of the (very many) agreements, is that the spacecraft packets must start with a Spacecraft ID, which uniquely identifies a spacecraft. The list of these IDs is searchable here. Typically, spacecraft radios include the spacecraft ID in the decoding and encoding circuits in a way which cannot be overwritten or corrupted by radiation. Just like an ethernet or Wifi network card, the radios are programmed to ignore all packets they receive which do not start with their spacecraft ID (in the case of computer network cards, you can do some low level stuff to still read those packets, but that's a whole different conversation).

Frequencies

Do spacecraft use the same frequencies? No. They all have slightly different frequencies in the same telecommunication band. For spacecraft which will use the DSN, the frequency must be approved by NASA's frequency allocation office (and this can be a long process).

Spacecraft operators and the DSN are aware of the exact transmission frequency of each spacecraft. The DSN can also correct for expected Doppler shift due to the relative speed of the spacecraft compared to the ground station. DSN is also aware of each critical event of the mission.

If the spacecraft faces an anomaly, the DSN will try to schedule a time slot sooner for engineers to debug the problem. It's also important to note that each DSN site is composed of several antennas, and one antenna can transmit on several slightly different frequencies allowing for mutliplexing of communication by antenna. Whether or not DSN dedicates one antenna to a given spacecraft during a communication pass depends on spacecraft radio power and the criticality of the situation. I believe that for the Phobos Grunt anomaly, DSN would dedicate an antenna to tracking and attempting to communicate with the probe.

Answer 3

The Deep Space Network channel center frequencies are defined in the DSN Telecommunications Link Design Handbook, Document 810-005, Module 201, "Frequency and Channel Assignments". The differences between the centers of adjacent channels, as of Revision D, dated 09/04/2020, are

• S Band
  • Uplink 341 kHz (2110-2120 MHz)
  • Downlink 370 kHz (2290-2300 MHz)
• X Band
  • Uplink 1156 kHz (7145-7190 MHz)
  • Downlink 1358 kHz (8400-8450 MHz)
• Ka Band
  • Uplink 5554 kHz (34200 MHz - 34700 MHz)
  • Downlink 5136, 5160, or 5185 kHz (31800 MHz - 32300 MHz)

Other parts of the handbook, and the DSN Services Catalog, define some other aspects of the signal types DSN can use, which vary greatly in how much bandwidth they actually occupy, but additional details of the channel assignments to specific missions are not described. Instead, it says only

Channel selection is also highly dependent on bandwidth considerations. The channel plan was developed to accommodate both low-rate spacecraft operating within a single channel and higher-rate spacecraft requiring one or more adjacent channels on each side of the nominal operating channel. Before selecting operating frequencies or channels for a project, the telecommunication designer should consult the JPL Frequency Spectrum Management Committments Office to avoid frequency interference with other spacecraft, present or planned.

That link leads to the web site of the Interplanetary Network Directorate (IND) Commitments Office, which says its personnel

will assist persons designing new missions who are planning for support from the Deep Space Network (DSN)... will provide potential customers with information about DSN facilities, capabilities, costing, and plans... will review telecommunications link designs to ensure compatibility with the DSN and will assist development flight projects in completing a preliminary DSN Service Agreement.

For those interested in preparing a proposal for a mission that will require DSN services, they host "the online DSN RF Aperture Fee tool" at https://dse.jpl.nasa.gov/ext/ , which you may find an entertaining way to explore the available parameter space.

Other parts of NASA, such as the Spectrum Policy and Planning Division, actually do the negotiation with international partnership organizations such as the ITU and CCSDS mentioned in other answers. The only one that seems to handle precise frequency allocation at the level of this question is the Space Frequency Coordination Group. Their satellite database is only open for members (that is, the people who represent their countries at committee meetings) to access, but the user's guide for that database is publicly accessible, and it contains several screenshots from what the database looked like a few years ago, including center frequencies and bandwidths from a small number of satellites. Because of the way the database is sorted in those examples (by name rather than by frequency), however, they don't help answer the question of how much bandwidth overlap there really is.

Searching on "Deep Space Network Spectrum Management" led me to https://sites.nationalacademies.org/cs/groups/bpasite/documents/webpage/bpa_048958.pdf , which includes the best picture I've been able to find of what I think you really want to see:

If you want a textbook treatment of what they actually model in order to decide which frequency assignments to make, your best bet is back to JPL DESCANSO for Chapter 11, Radio Frequency Selection and Interference Prevention, by Norman F. de Groot, pages 517-555 of Deep Space Telecommunications Systems Engineering, edited by Joseph H. Yuen, 1982.