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That’s actually a pretty hard problem when you don’t have a GPS constellation to assist you. Accelerometers, gyros, and magnetometers are used on Earth but all

That’s actually a pretty hard problem when you don’t have a GPS constellation to assist you. Accelerometers, gyros, and magnetometers are used on Earth but all of them need calibration and drift compensation. We don’t really have much available for sensors that can directly detect position or velocity, it’s all estimated by double integrating acceleration data (which has noise and bias). On Earth most UAVs use GPS as a coarse position sensor and can use that to correct for accelerometer error but if you don’t have that constellation a camera is probably your best bet.

bri3d 2y ago
The latest UAVs also use ToF LIDAR and SLAM for GPS-rejected navigation / inertial dead reckoning correction. I'm not sure that would work well on Mars either, though, as the environment might be too featureless for that approach also. I think the best approach would probably be to equip the "base station" / lander with an RF beacon. Ideally, you could drop some RF beacons throughout the environment as you went, but even a single beacon with some directional receivers on the drone should work pretty well (here, the featureless environment becomes a benefit as you have to contend much less with radio reflections).
- tonyarkles 2y ago
  SLAM is likely exactly what they were doing on Mars (it falls apart if there’s no image features to register against on the ground) and ToF lidar works great for vertical position and velocity but doesn’t help much for horizontal position or velocity if there isn’t much terrain variability for the lidar to reflect off of nearby. 100% agree on the RF beacons. You might not even need the directional antennas. Using things like RSSI might be enough to augment the local sensors. Alternatively having a digitally-controlled phased array could probably result in quite excellent positioning.
  - bri3d 2y ago
    > SLAM is likely exactly what they were doing on Mars I think the system on Ingenuity just used feature detection based 2D optical flow rather than a more full-scale 3D environment-reconstruction type SLAM setup. Which is fine since neither would work in a blank environment like that.
    - tonyarkles 2y ago
      Ah yeah that’s true. I’d forgotten about that step in the middle. I suppose too… very valuable lesson learned there for the team: you’ll need to come up with something to augment the optical nav system in that environment!
- deleted 2y ago
  [deleted]
wafflemaker 2y ago
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.
magicalhippo 2y ago
Since it's communicating with the rover, I was thinking it could use latency and/or Doppler effect as an input to narrow down the position, assuming the rover knew where it was, potentially by getting told by the satellites we have there. However I assume that would require special radio software, and they were using commercial Zigbee modules. In addition, I guess perhaps the helicopter and potentially rover wouldn't have accurate enough oscillators for this to be viable in any case.
f33d5173 2y ago
You can use radar or similar technologies to determine velocity. Point it straight down and you can see how fast you're falling. Point it at an angle, subtract vertical velocity, and you have horizontal velocity.
- adriancr 2y ago
  alright, radar at angle, unknown surface but all looks the same, no landmarks to track against, unknow radar properties of surface, how do you translate that into directional speed?
  - zokier 2y ago
    doppler effect.
    - deleted 2y ago
      [deleted]
    - nomel 2y ago
      A doppler shifted reflection requires a feature, significant in size to the wavelength, to reflect off of, somewhat perpendicular to the direction of movement. For RADAR, I don't think a smooth sand field would have such a thing. Doppler LIDAR could probably detect it, but I naively assume lidar is hard in dusty environments, without oodles of large moving optics required to penetrate a dusty, optically clear, window.
      - touisteur 2y ago
        To get a Doppler-shifted reflection you can also... move. Ground-based surface radars can work (somehow) with just no Doppler info, cataloguing, mapping fixed reflectors and very often just building ground maps. If your radar has multiple vertically and horizontally spaced receptors (eg. phased-array) or you can aim your beams, you can also somehow build a 3D map or at least map surface features/masks. This is before anything moves.

bri3d 2y ago

The latest UAVs also use ToF LIDAR and SLAM for GPS-rejected navigation / inertial dead reckoning correction. I'm not sure that would work well on Mars either, though, as the environment might be too featureless for that approach also. I think the best approach would probably be to equip the "base station" / lander with an RF beacon. Ideally, you could drop some RF beacons throughout the environment as you went, but even a single beacon with some directional receivers on the drone should work pretty well (here, the featureless environment becomes a benefit as you have to contend much less with radio reflections).

tonyarkles 2y ago
SLAM is likely exactly what they were doing on Mars (it falls apart if there’s no image features to register against on the ground) and ToF lidar works great for vertical position and velocity but doesn’t help much for horizontal position or velocity if there isn’t much terrain variability for the lidar to reflect off of nearby. 100% agree on the RF beacons. You might not even need the directional antennas. Using things like RSSI might be enough to augment the local sensors. Alternatively having a digitally-controlled phased array could probably result in quite excellent positioning.
- bri3d 2y ago
  > SLAM is likely exactly what they were doing on Mars I think the system on Ingenuity just used feature detection based 2D optical flow rather than a more full-scale 3D environment-reconstruction type SLAM setup. Which is fine since neither would work in a blank environment like that.
  - tonyarkles 2y ago
    Ah yeah that’s true. I’d forgotten about that step in the middle. I suppose too… very valuable lesson learned there for the team: you’ll need to come up with something to augment the optical nav system in that environment!
deleted 2y ago
[deleted]

wafflemaker 2y ago

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.

magicalhippo 2y ago

Since it's communicating with the rover, I was thinking it could use latency and/or Doppler effect as an input to narrow down the position, assuming the rover knew where it was, potentially by getting told by the satellites we have there. However I assume that would require special radio software, and they were using commercial Zigbee modules. In addition, I guess perhaps the helicopter and potentially rover wouldn't have accurate enough oscillators for this to be viable in any case.

f33d5173 2y ago

You can use radar or similar technologies to determine velocity. Point it straight down and you can see how fast you're falling. Point it at an angle, subtract vertical velocity, and you have horizontal velocity.

adriancr 2y ago
alright, radar at angle, unknown surface but all looks the same, no landmarks to track against, unknow radar properties of surface, how do you translate that into directional speed?
- zokier 2y ago
  doppler effect.
  - deleted 2y ago
    [deleted]
  - nomel 2y ago
    A doppler shifted reflection requires a feature, significant in size to the wavelength, to reflect off of, somewhat perpendicular to the direction of movement. For RADAR, I don't think a smooth sand field would have such a thing. Doppler LIDAR could probably detect it, but I naively assume lidar is hard in dusty environments, without oodles of large moving optics required to penetrate a dusty, optically clear, window.
    - touisteur 2y ago
      To get a Doppler-shifted reflection you can also... move. Ground-based surface radars can work (somehow) with just no Doppler info, cataloguing, mapping fixed reflectors and very often just building ground maps. If your radar has multiple vertically and horizontally spaced receptors (eg. phased-array) or you can aim your beams, you can also somehow build a 3D map or at least map surface features/masks. This is before anything moves.