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I don't really get why there is no constellation around Mars already. That's literally the first thing you do when sending a mission to another planet - establi

I don't really get why there is no constellation around Mars already. That's literally the first thing you do when sending a mission to another planet - establishing a simple 3 sat comm network.

ceejayoz 2y ago
We have a comm network there - several satellites serve as comm relays. https://www.nasa.gov/centers-and-facilities/jpl/the-mars-relay-network-connects-us-to-nasas-martian-explorers/ https://www.nasa.gov/centers-and-facilities/jpl/the-mars-rel... They just aren't GPS satellites. At some point we'll likely get some there, but it's a) a good amount of payload and b) not something we've really needed there so far.
michaelt 2y ago
GPS uses a lot more than three satellites (you need at least four a fair bit above the horizon to be able to navigate), and they have to be in different orbits. Earthbound GPS also uses a network of base stations at known locations to figure out the satellites' locations.
mandevil 2y ago
A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satellites in very different orbits, they are not substitutes for each other. Every NASA science mission to Mars orbit for over 20 years has had a communications relay on it, for relaying messages from rovers back to Earth, but the one in the best orbit for communications (Mars Odyssey) is also the oldest and most likely to fail. The other NASA science orbiters (MRO, ME (ESA), MAVEN) all also have relays, but the orbits all leave something to be desired for communications purposes (they are in the right orbits for answering their scientific questions, e.g. MAVEN is in a highly elliptical orbit for studying the Martian atmosphere). There was a proposed Mars Telecommunications Orbiter- a satellite whose primary mission would have been beaming back information from rovers on the surface- proposed back around 2005, but it was canceled in a budget crunch, when actual science producing satellites were prioritized. That would have been in the right orbit. Building a positioning satellite network around another body is going to be significantly harder, incidentally. Even something like TRANSIT (aka NAVSAT) (1) is going to be significantly harder on another body because we haven't mapped their gravity fields due to density fluctuations as well, the upper atmosphere is not as well studied for drag effects, and we don't have fixed locations that can determine orbital parameters very precisely by either visual or radar observations after every orbital maintenance burn. Small uncertainty in orbital position lead to gigantic uncertainty in your position, and none of the techniques we use here on earth to remove that uncertainty would work around Mars. 1: Instead of the "see multiple atomic clocks and use triangulation and the speed of light to determine distance to each of them, then our location from their known locations" which is how all modern satnav systems work, TRANSIT used a single satellite passing nicely overhead every so often. When it was right above you, you could listen to the Doppler shift and know when it reached exactly the zenith above you. If you knew it's orbit very precisely you could tell where it was in space when it reached that zenith, and therefore where you were.
- skissane 2y ago
  > A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satellites in very different orbits, they are not substitutes for each other. Not necessarily. Starlink can be used for positioning – not with the same accuracy as GPS can, but it definitely can be used. According to [0] a research group was involved with discussions with SpaceX about officially using Starlink as a positioning system, but the discussions were terminated because Musk didn't like the idea. And then the researchers went ahead and did it anyway – even without the ability to decrypt the signal, the unencrypted synchronisation data embedded in it is sufficient to get a position fix to within 30 metres. If SpaceX was actually cooperating and using data in the encrypted signal for this purpose, it could likely be made even more accurate. I suppose Musk's point is that however accurate it is, GPS is going to be even more accurate, and having Starlink offer a "just like GPS but less accurate" service has minimal commercial value and great potential for bad PR ("look how inaccurate SpaceX's GPS alternative is!") SpaceX is proposing to build a "Marslink" constellation around Mars [1]. Likely SpaceX wouldn't have the same opposition to using it for positioning as they do for Earth Starlink, given there is no existing GPS to compete with. > because we haven't mapped their gravity fields due to density fluctuations as well, the upper atmosphere is not as well studied for drag effects, and we don't have fixed locations that can determine orbital parameters very precisely by either visual or radar observations after every orbital maintenance burn. Small uncertainty in orbital position lead to gigantic uncertainty in your position, and none of the techniques we use here on earth to remove that uncertainty would work around Mars. This is all true, but anything is better than nothing – a Martian GPS with significantly less accuracy than Earth GPS will still be superior to no Martian GPS. And it will be a step towards building the necessary research infrastructure to answer many of those questions about variations in Martian gravity and atmospheric drag. So the accuracy of the system is likely to improve over time. [0] https://www.technologyreview.com/2022/10/21/1062001/spacex-starlink-signals-reverse-engineered-gps/ https://www.technologyreview.com/2022/10/21/1062001/spacex-s... [1] https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a-version-of-starlink-for-the-red-planet https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a...
  - mandevil 2y ago
zokier 2y ago
We are barely only planning a pnt constellation on Moon[1]. Setting up Martian constellation is significantly more complex simply due the distances involved; pnt is not fire-and-forget system, you need constantly track and tweak the satellites to make sure they know when and where they are. [1] https://tempo.gsfc.nasa.gov/projects/LCRNS https://tempo.gsfc.nasa.gov/projects/LCRNS

I don't really get why there is no constellation around Mars already. That's literally the first thing you do when sending a mission to another planet - establishing a simple 3 sat comm network.

ceejayoz 2y ago

We have a comm network there - several satellites serve as comm relays. https://www.nasa.gov/centers-and-facilities/jpl/the-mars-relay-network-connects-us-to-nasas-martian-explorers/ https://www.nasa.gov/centers-and-facilities/jpl/the-mars-rel... They just aren't GPS satellites. At some point we'll likely get some there, but it's a) a good amount of payload and b) not something we've really needed there so far.

michaelt 2y ago

GPS uses a lot more than three satellites (you need at least four a fair bit above the horizon to be able to navigate), and they have to be in different orbits. Earthbound GPS also uses a network of base stations at known locations to figure out the satellites' locations.

mandevil 2y ago

A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satellites in very different orbits, they are not substitutes for each other. Every NASA science mission to Mars orbit for over 20 years has had a communications relay on it, for relaying messages from rovers back to Earth, but the one in the best orbit for communications (Mars Odyssey) is also the oldest and most likely to fail. The other NASA science orbiters (MRO, ME (ESA), MAVEN) all also have relays, but the orbits all leave something to be desired for communications purposes (they are in the right orbits for answering their scientific questions, e.g. MAVEN is in a highly elliptical orbit for studying the Martian atmosphere). There was a proposed Mars Telecommunications Orbiter- a satellite whose primary mission would have been beaming back information from rovers on the surface- proposed back around 2005, but it was canceled in a budget crunch, when actual science producing satellites were prioritized. That would have been in the right orbit. Building a positioning satellite network around another body is going to be significantly harder, incidentally. Even something like TRANSIT (aka NAVSAT) (1) is going to be significantly harder on another body because we haven't mapped their gravity fields due to density fluctuations as well, the upper atmosphere is not as well studied for drag effects, and we don't have fixed locations that can determine orbital parameters very precisely by either visual or radar observations after every orbital maintenance burn. Small uncertainty in orbital position lead to gigantic uncertainty in your position, and none of the techniques we use here on earth to remove that uncertainty would work around Mars. 1: Instead of the "see multiple atomic clocks and use triangulation and the speed of light to determine distance to each of them, then our location from their known locations" which is how all modern satnav systems work, TRANSIT used a single satellite passing nicely overhead every so often. When it was right above you, you could listen to the Doppler shift and know when it reached exactly the zenith above you. If you knew it's orbit very precisely you could tell where it was in space when it reached that zenith, and therefore where you were.

skissane 2y ago
> A comm sat network is very very different from a positioning satellite network. You'll notice that here on earth those missions are done by very different satellites in very different orbits, they are not substitutes for each other. Not necessarily. Starlink can be used for positioning – not with the same accuracy as GPS can, but it definitely can be used. According to [0] a research group was involved with discussions with SpaceX about officially using Starlink as a positioning system, but the discussions were terminated because Musk didn't like the idea. And then the researchers went ahead and did it anyway – even without the ability to decrypt the signal, the unencrypted synchronisation data embedded in it is sufficient to get a position fix to within 30 metres. If SpaceX was actually cooperating and using data in the encrypted signal for this purpose, it could likely be made even more accurate. I suppose Musk's point is that however accurate it is, GPS is going to be even more accurate, and having Starlink offer a "just like GPS but less accurate" service has minimal commercial value and great potential for bad PR ("look how inaccurate SpaceX's GPS alternative is!") SpaceX is proposing to build a "Marslink" constellation around Mars [1]. Likely SpaceX wouldn't have the same opposition to using it for positioning as they do for Earth Starlink, given there is no existing GPS to compete with. > because we haven't mapped their gravity fields due to density fluctuations as well, the upper atmosphere is not as well studied for drag effects, and we don't have fixed locations that can determine orbital parameters very precisely by either visual or radar observations after every orbital maintenance burn. Small uncertainty in orbital position lead to gigantic uncertainty in your position, and none of the techniques we use here on earth to remove that uncertainty would work around Mars. This is all true, but anything is better than nothing – a Martian GPS with significantly less accuracy than Earth GPS will still be superior to no Martian GPS. And it will be a step towards building the necessary research infrastructure to answer many of those questions about variations in Martian gravity and atmospheric drag. So the accuracy of the system is likely to improve over time. [0] https://www.technologyreview.com/2022/10/21/1062001/spacex-starlink-signals-reverse-engineered-gps/ https://www.technologyreview.com/2022/10/21/1062001/spacex-s... [1] https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a-version-of-starlink-for-the-red-planet https://www.pcmag.com/news/spacex-pitches-nasa-on-marslink-a...
- mandevil 2y ago

zokier 2y ago

We are barely only planning a pnt constellation on Moon[1]. Setting up Martian constellation is significantly more complex simply due the distances involved; pnt is not fire-and-forget system, you need constantly track and tweak the satellites to make sure they know when and where they are. [1] https://tempo.gsfc.nasa.gov/projects/LCRNS https://tempo.gsfc.nasa.gov/projects/LCRNS