GNSS sky view

Four navigation systems

Global navigation satellite systems (GNSS) work by timing signals from several satellites at once. The American GPS system has about 31 working satellites, Europe's Galileo about 25, Russia's GLONASS about 24 and China's BeiDou over 40, all in orbits around 19,000 to 23,000 km up. Most modern phones and receivers use several systems together, which is why they get a fix faster and more reliably than older GPS-only receivers.

What PDOP means

Your receiver works out its position from the distances to at least four satellites. If those satellites are spread all around the sky, small timing errors cancel out and the position is precise. If they're bunched together, the same errors produce a much bigger position error. Position dilution of precision (PDOP) measures that effect: below 2 is excellent, 2 to 4 good, and above 6 poor. The figure here is calculated from the satellites' positions alone, so it shows the best your receiver could do with a clear view of the sky above the elevation mask.

Buildings, trees and hills block low satellites, which is why GPS is less accurate in city streets and deep valleys. Raise the elevation mask to see how many satellites remain when the low ones are blocked.

Using it

Surveyors, drone pilots and anyone using a GPS-disciplined oscillator (GPSDO) as a frequency reference for their radio can use the time slider to find periods with the most satellites and the lowest PDOP. Radio amateurs use GPS for accurate time on FT8 and WSPR, and as a frequency standard for microwave and QO-100 stations. Positions come from openly published orbit data from CelesTrak, worked out in your browser; this page doesn't receive any satellite signals.

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