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While in atmosphere, altitude is measured by using pressure gauges - higher the altitude, lower the pressure. How is the altitude measured once we cross the atmosphere?

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    $\begingroup$ Use of GPS receivers on satellites can provide altitude measurements. If the satellite's mission requires altitude knowledge for some time intervals, then the altitude measurements are combined with satellite propagator models to "fuse" those measurements and provide a more complete picture of the satellite's orbit. $\endgroup$ – Manny Nov 29 '20 at 22:12
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    $\begingroup$ @Manny. Thanks Manny, but how do the GPS receivers judge the altitude? Difference in pressure is an indication of altitude, so long as the satellite is in atmosphere. Once above the atmosphere, I could think, only of "time delay" in receiving echo of some wavelength such as sound / light / any other radiation. Just wanted to know about what is actually used, when people declare that a certain object is orbiting at "XX" Km of altitude. How trustworthy and accurate is the method used for this calculation. $\endgroup$ – Niranjan Nov 30 '20 at 9:17
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    $\begingroup$ GPS is the Global Positioning System. A GPS receiver gives you your position in 3D space. Since you know where the Earth is, and the GPS receiver tells you where you are, you know your altitude. $\endgroup$ – Jörg W Mittag Nov 30 '20 at 11:25
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    $\begingroup$ For an other example, see "Lunar Laser Altimeter" , a ruby laser -based system used in the Apollo missions. $\endgroup$ – Carl Witthoft Nov 30 '20 at 13:44
  • $\begingroup$ Doppler interferometry between multiple ground stations is another method. $\endgroup$ – honeste_vivere Feb 22 at 23:28

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