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    Re: Celestial Navigation on the surface of Mars
    From: Frank Reed
    Date: 2018 Nov 29, 19:20 -0800

    Right now... as we speak... there is a navigation challenge on Mars. They don't quite know where the InSight spacecraft landed. Quoting from sciencemag.org:

    The biggest mystery for the lander team right now is figuring out exactly where it is. A Mars orbiter set to image the center of the landing zone on Thursday will miss the lander, because it missed the center slightly. An instrument on InSight called the inertial measurement unit has pinned the location to within a 5-kilometer-wide circle. InSight’s entry, descent, and landing team will refine that estimate down to a kilometer or less. “But they haven’t done that yet because they were so happy to have landed safely that we don’t know what they did last night,” Golombek says with a smile. “And they have not yet shown up today.”

    There is one more technique that could help: InSight’s third primary experiment, called the Rotation and Interior Structure Experiment (RISE). The main purpose of RISE’s two sensitive listening antennas is to detect wobbles in the martian core. But the InSight team can also use them to map the lander’s latitude and longitude by using the radio signals of passing orbiters. That has given the geologists a location to within about 100 meters or so.

    Now, a friendly competition is on. Golombek and his peers hope to beat the satellites to fixing InSight’s location. 

    So there are at the moment two distinct forms of navigation in play which we have discussed before:

    • the "air traffic control" solution where the lander is simply observed by a high-power telescope on a satellite flying overhead,
    • and passive satellite navigation listening to normal radio signals from several satellites in orbit.

    Why nothing celestial? Why not observe the position of Phobos in the sky, as I recently suggested? The principal reason seems to be that the main camera isn't up and running yet and won't be for a while. It's also quite possible that the camera on the InSight lander isn't up to the task since this geology misson was not designed to produce much in terms of photography. Within a week or two they'll get a photo of it from orbit, and the game will be over. For the foreseeable future, that omniscient view will always win for navigation on the surface of Mars.

    Frank Reed

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