Reply to post: Re: Not quite

Re: Not quite

To quote from the somewhat light-weight website you included the URL of:

"there are four unknowns in the pseudo-range equations: the three components of the receiver’s position, and the offset in the receiver’s clock. To determine its location it then must solve the four equations for the four unknowns."

and

"If the receiver “knows” one of the three position components, then measurements from only three satellites are required to solve for the other two components and the receiver clock offset (3 equations and 3 unknowns). If, for example, the altitude is known perfectly, this is equivalent to tracking a satellite directly above (or below) the user, and having a perfect pseudo-range measurement to it. Since a real fourth satellite will never provide a perfect pseudo-range, the three-satellite solution will be better than the four-satellite solution (assuming similar geometry quality, i.e., PDOP). If on the other hand the assumed altitude is in error, then the two sides of the equations will be equal only if a compensating error is added to the horizontal position components and clock offset. "

or. to put it another way, the receiver can only calculate an accurate position from three satellites if it accurately knows its position or its clock offset already - and if it already knows (ie., has been told) its position accurately, then why bother at all?