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> It would be like asking for the GPS coordinates of the moon No problem at all, just give the location where the moon is at the Zenith and use the distance as
by echoangle 1y ago
> It would be like asking for the GPS coordinates of the moon
No problem at all, just give the location where the moon is at the Zenith and use the distance as the altitude.
> Reason this works for the ISS is because of the subsatellite point. It can receive GPS signals regardless of altitude above the Earth’s surface.
No, wether the object can actually receive GPS signals is completely irrelevant to wether its location can be described in the GPS coordinate system.
You could describe the location of the Sun in GPS coordinates too, the altitude value would just be very large.
- firesteelrain 1y agoYou can use GPS to describe a point on Earth. To use the moon or sun is kind of weird because of their size to use GPS coordinates for this I was referring to finding your position on the moon using Earth referenced GPS signals.
- echoangle 1y ago> You can use GPS to describe a point on Earth. No, you can describe any point in the universe using GPS coordinates. You just lose some resolution the further away from earth you are because it's basically spherical coordinates (like polar coordinates but for 3D). And the system isn't inertial but earth-fixed, of course, so you would have to give the coordinates together with a time. And if you're describing the location of the moon and the sun, you would probably pick their center of gravity.
- firesteelrain 1y agoI believe this isn’t true otherwise NASA would be doing this - Earth isn’t a universal reference - GPS uses WGS84 - GPS is bound to the Earth’s surface and center - It’s Geodetic - There's no universal “equator” or “prime meridian” beyond Earth - Space uses inertial frames or celestial coordinate systems (right ascension and declination, or galactic coordinates)
- echoangle 1y agoThat's exactly what I said. It isn't very practical for space ops, but you can absolutely give a current GPS position for every object you want.
- firesteelrain 1y agoThat’s conceptually misleading. They are meaningless for things not near Earth because they’re tied to Earth's shape, rotation, and gravity field
- echoangle 1y agoI wouldn't call it meaningless if it can be converted back and forth with a (non-linear) transformation.
- firesteelrain 1y agoYou can do a lot of things…
- gmiller123456 1y agoThere are a handful of Earth centered, geocentric standard reference frames. The most used today is the Geocentric Celestial Reference System (GCRS). It should be obvious that if you want to compute where to point a telescope, a transformation of coordinates will involve a step through such a coordinate system. GPS is it's own system, but there are transformations to and from the GCRS and GPS frames. Which one makes sense depends a lot on your application.
- firesteelrain 1y agoI agree that GCRS and other Earth-centered inertial systems are fundamental for astronomy, tracking, and spaceflight. And yes, you can transform between GPS coordinates and GCRS (or ECI/ECEF), depending on what you’re trying to do. The key distinction I’m making is about contextual meaning and application. GPS (WGS-84) coordinates are geodetic. Once you're transforming positions millions of kilometers away (eg. L2, the Moon, or Mars) into lat/lon/alt, you're applying a system optimized for geodesy to a domain where the altitude is arbitrary, and "longitude" rotates with Earth.
- therealpygon 1y agoI could build a house with my pinkie if I excuse the fact I’ll use a team of laborers to do the work and accept that they are so inaccurate that I would be lucky to end up with a shed… if I only cared about technicality.