The basic orbits are quite exact but just to make
things perfect the GPS satellites are constantly monitored by the Department of Defense.
They use very precise radar to check each satellite's exact altitude, position and
speed.
The errors they're checking for are called "ephemeris errors" because they
affect the satellite's orbit or "ephemeris." These errors are caused by
gravitational pulls from the moon and sun and by the pressure of solar radiation on the
satellites.
The errors are usually very slight but if you want great accuracy they must be taken
into account.
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