GPS: why first azimuth and elevation?

Question

When I start the GPS (in USB), I look at the data comming and after a while, first reasonable data is $GPGSV sentence (see http://aprs.gids.nl/nmea/#gsa), that contains azimuth and elevation of satellites. Only after some time, real coordinates start to come in another sentence.

I thought first, it may be some world coordinates in $GPGSV sentence, but the values are in the ranges expected for azimuths (0-360) and elevations (0-90).

How could my GPS know satellite's elevations (and azimuths) before knowing its own actual location?

Do I miss something? Is the satellite signal similar to a VOR signal?

Answer 1

The numbers you see for azimuth and elevation are related to the last position the receiver was on and tracking satellites. It is assuming it is still in that position or somewhere close, so it starts to look for satellites that are supposed to be there. It calculates the angles based on the stored almanac and the last stored position. If you move to a different location (more than 100km), it will start looking for satellites that are supposed to be visible in the previous location but after a short while, it won't find said satellites and will start searching for satellite signals. Once it has locked on a satellite, more will follow and it will be able to calculate its position. This is referred to as a cold start, which takes more time than a warm start which happens when the receiver is switched on near the location it was last working on.

Answer 2

Your GPS receiver uses the signals from multiple satellites to get it's positional fix. Because, it uses multiple satellites it needs to know its relationship to each satellite individually before it can calculate the combined relationship and derive its own location. Additionally, if you are only receiving a small number of satellites their geometry can be important in how accurate your location results are (see signal timing link).

Edit to address comments below: It is likely that the GPS is recording your position and calculating azimuth, elevation, and other attributes before it actually collects the point. This is why it's generally standard procedure to stay in one place for a specified length of time before starting to take readings. So, it already does know it's location, it just hasn't been asked to record it yet. Time To First Fix (TTFF) is the measurement of this time. This is reflected in the order in which it reports the data in the NMEA sentences.

As for the difference between GPS and VOR; VOR stations have a known earthbound location and transmit a signal that varies phase according to the angle from the station at which it is being transmitted. This means that the receiver knows the location of the station and its relative angle. As far as I can tell there is no distance information discernible from the transmission and position is only known as some unknown point on a line (the line created by the receiver and base station on the angle noted by the base station). GPS uses signal timing to determine the distance from a satellite and uses triangulation to get an exact location envelope (receiver is exactly xyz +- error).