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The issues would be A) that Mars upper atmosphere and internal mass distribution are not as well mapped as on Earth, so knowing your orbital accuracy is much mo

The issues would be A) that Mars upper atmosphere and internal mass distribution are not as well mapped as on Earth, so knowing your orbital accuracy is much more difficult (1)- and at 8km/s small orbital error bars become giant error bars on the surface and B) Putting a full constellation of 24-30 satellites around Mars is going to be really expensive. That's more than the sum total of all satellites to successfully orbit Mars to this day (18). 1: On Earth we account for that by using ground stations to track the satellite locations, with the ground station locations determined very very precisely using non-GPS techniques (old school surveying techniques). On Mars, that's not going to be possible until we get a lot more done, probably a later human mission would be the first time that could be done.

bluGill 2y ago
We don't need all of mars though. One GPS receiver on your lander will give you enough information for the area you can feasibly reach - and if not just old school survey some location to park another receiver to map things out.
JoeAltmaier 2y ago
All good points. The solutions for Mars will necessarily be unique to that environment. Still, a combination of ground stations and satellites will be inevitably used for location-finding. And, the atmosphere? What atmosphere? It's negligible compared to Earth. Got to be down the list of important variables. We can send a mission to Mars and arrive within a few meters of desired orbit, but it's going to be hard to figure out where a satellite is? My doubt-meter is hitting the pin.
- mandevil 2y ago
  Atmospheric drag on satellites- especially how it changes with solar output levels- is a hard thing to model accurately, and a major contributor to orbital uncertainty here on Earth. The Martian atmosphere is two orders of magnitude thinner, but it is far less than two orders of magnitude understood. And the level of our understanding matters for our ability to correct for it's perturbations. NASA is very good at sending spacecraft through regular space and hitting precise windows (MCO units issues aside), it's in orbit that things get more complicated, because now there are just a lot more potential interactions to deal with. We can use LOS on planetary occultations to give you some data, but it's still a lot of work to get from there to mascon maps, upper atmospheric data, etc.

bluGill 2y ago

We don't need all of mars though. One GPS receiver on your lander will give you enough information for the area you can feasibly reach - and if not just old school survey some location to park another receiver to map things out.

JoeAltmaier 2y ago

All good points. The solutions for Mars will necessarily be unique to that environment. Still, a combination of ground stations and satellites will be inevitably used for location-finding. And, the atmosphere? What atmosphere? It's negligible compared to Earth. Got to be down the list of important variables. We can send a mission to Mars and arrive within a few meters of desired orbit, but it's going to be hard to figure out where a satellite is? My doubt-meter is hitting the pin.

mandevil 2y ago
Atmospheric drag on satellites- especially how it changes with solar output levels- is a hard thing to model accurately, and a major contributor to orbital uncertainty here on Earth. The Martian atmosphere is two orders of magnitude thinner, but it is far less than two orders of magnitude understood. And the level of our understanding matters for our ability to correct for it's perturbations. NASA is very good at sending spacecraft through regular space and hitting precise windows (MCO units issues aside), it's in orbit that things get more complicated, because now there are just a lot more potential interactions to deal with. We can use LOS on planetary occultations to give you some data, but it's still a lot of work to get from there to mascon maps, upper atmospheric data, etc.