What am I seeing?

You are seeing all the satelites that are visible to a GPS-based timing device in Redondo Beach, California. The outer black circle represents the horizon and the center represents the zenith (directly overhead). North is on top. The time you see is the time that the data was taken.

If you were in Redondo Beach, faced north and looked straight up, if you could see the GPS satellites, this is what you would see.

What do the different colors mean?

There is a legend at the bottom of the map that indicates the various colors and their meanings. Briefly, the Motorola Oncore UT+, upon which the timing device is based, uses a robust algorithm to detect problems with reception from the various satellites. The colors on the map combine information from two sources to give an indication of what the unit thinks of the satellite.

A solid black border indicates that we are currently using the satellite to compute the time. No black border indicates that we are not using that satellite.

What is the number on each satellite?

That is the satellite's pseudo-random noise code (PRN). This number uniquely identifies each GPS satellite's transmissions. PRNs range from 01 to 32.

Why are some satellites weak and some strong?

Generally, a satellite will be weak if it is very low on the horizon. This means the radio signal has further to travel and is more likely to bounce off of obstructions such as buildings. Satellites less than 5 degrees above the horizon are ignored because they are too unreliable.

Where can I read more?

You can read an introduction to computer time synchronization by GPS here. We also have lots of links to GPS information. And of course, you can go back to the live view.