66 GPS 3

Question 1

Satellite selection

Answer 1

Initial start-up uses almanac data and/or operator entered position, velocity, and time (incl. date)(PVT)
Chooses the best four satellites within its horizon
If the receiver does not have PVT or almanac data it goes into a searching the sky mode

Question 2

Searching the sky mode

Answer 2

using a preprogrammed sequence the receiver picks a particular satellite and begins searching for its unique C/A code
The inability to find a satellite also precludes other satellites due to constellation geometry
Once a particular satellite is found the memory will tell it what others are available
In multi – channel receivers the GPS looks for many satellites at the same time and chooses the best based on geometry

Question 3

Satellite Acquisition

Answer 3

The receiver now knows what C/A codes it will search
Estimates the phase shift required for the time difference
Estimates the Doppler shift required for the relative speeds
The code and carrier tracking loops now start to lock onto the signals

Question 4

Tracknig Loops

Answer 4

A signal generator within the receiver that attempts to match its signal and the satellites signal
Compensates for:
Phase shift caused by time differences: code tracking loop
Doppler shifts caused by relative speed changes: carrier tracking loop

Question 5

Code Tracking Loop

Answer 5

C/A code search commences at the phase shifted point allowing for the time lag from satellite to receiver
Allows reception of weak signal and helps to offset gain limits from a relatively small antenna
Code lock is when the two signals match and is also the point of highest signal to noise ratio. This point is maintained for the remainder of the flight

Question 6

Carrier-Tracking Loop

Answer 6

The receiver uses a crystal oscillator to develop a frequency which it adds to the incoming frequency to compensate for Doppler shifts due to motion
The receiver can now extract the navigation message from the carrier
Any further Doppler shifts equate to relative motion changes

Question 7

Navigation Solution

Answer 7

the receiver now has code and carrier lock on to all viable satellites. Any changes will be as a result of the receiver moment
the carrier tracking loop now assists the refining of position by providing velocity measurements to the code tracking loop to help it maintain lock on
Gathers the navigation message from each satellite
Receiver begins to refine the pseudo-ranges it originally calculated
Uses a fourth pseudo-range to offset the clock error from its clock to the more accurate GPS system time. Calculates and carries a clock bias
Commences navigating because it has the required satellites, clock bias, and nav message