Going through the working of GPS technology, I came to know that at least 4 satellites must be visible to solve for the location with considerable accuracy. Going through most of the resources, I can visualize how three coordinates of the location are determined.
But there is an error in this calculated location due to the timing offset. So this calculation is invalid. In order to make the calculation valid, the timing offset must be considered. And this is the propose of the 4th satellite.
How exactly does the 4th satellite know the timing offset?
If we talk about the differential GPS, the actual base station's location is compared against the GPS determined location and the timing offset is calculated and transmitted to the receiver within certain coverage area. But if we talk about direct communication between the satellite and my mobile phone, the actual position is to be determined unlike the base station where it is already known.
Hoping for a clear and more visual answer (rather than mathematical) from this community as always. Thank you!!
Best Answer
The satellite doesn't know your timing offset. GPS is a one-way street: the satellites basically only transmit their own time. Your receiver needs to do the rest, as the satellites can't know you even exist.
So, you really need to re-visit what you've learned about GPS so far!
Regarding the timing offset estimation: Don't consider the timing as a separate thing from the position. Instead, consider the 3 dimensions of your location and the time as one estimation problem with 4 unknowns.
All you get from your satellites is the time it was when they transmitted a signal. Your receiver hence only sees a time difference of arrival between these four. Only after solving the equations that gives all four unknowns by inserting all four time differences can you answer any of the questions of "how far North am I? How far west am I? How high am I? What is the time?", and you can answer all of them. So, it's either "all" or "none", not "location separately from time".