[FOSS-GPS] GPS in space?
martin.vermeer at tkk.fi
Thu Nov 6 06:17:57 EST 2008
On Wed, 05 Nov 2008 18:56:16 +0200
"Risto H. Kurppa" <risto at kurppa.fi> wrote:
> Does anyone know how well GPS works in the space? How high is it still
> reliable, what causes the possible errors and so on? If you take a GPS
> device beyond the geostationary level (where the satellites are?),
> what happens? I suppose some customization is required but a normal
> handheld GPS wouldn't handle it properly.
Actually an important application of GPS in orbit is the precise
positioning of altimetric satellites, that measure the precise sea
level using a radar beam aimed vertically downward. Such satellites are
Topex-Poseidon and Jason-1.
Here is an article that uses these altimetric observations -- which
cover the whole global ocean within their inclination bounds, contrary
to old-fashioned tide gauges which are only on coasts an islands -- to
extract the details of various natural sea level variation spatial modes
(like El Nino) over the 12 years that these satellites have
operated, and use this knowledge to correct the older tide gauge
measurements for these effects. The method used is Principal Components
Analysis / Empirical Orthogonal Functions (PCA/EOF).
Then there is the CHAMP mission for mapping the Earth's gravitational
field, where precise GPS is used to determine the satellite's
gravitational acceleration. It is in free fall so senses the field
directly -- any nongravitational accelerations by atmospheric drag etc.
are directly sensed by an on-board accelerometer.
All these are "low" orbits.
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