The Physics of GPS Clocks

The GPS satellites transmit inside the microwave range (1.5 Ghz) using a spread-spectrum encoding mechanism. This transmission requires a line of sight between the receiver and the satellite. In fact, any bounced signals would just cause erroneous readings as the signal will be delayed by the additional distance it has to travel, and the delay will throw off subsequent computations.

In order for a GPS system to work correctly, it must have as many satellites in view as it has pieces of information to determine. In the general case, there are four variables -- X position, Y position, Z position, and time. For general-purpose 3D navigation, we need to have four satellites in view.

If some variables are known, we can make do with fewer satellites. For example, a boat is always at sea-level, so the Z position will not vary significantly (in fact, if the Earth were a perfect spheroid and were equally dense everywhere, it would always be zero). Some GPS systems will make an effort to supply degraded readings when fewer satellites are in view by making assumptions which reduce the number of variables, for example, by assuming that the course or altitude has not changed recently.

If we are only interested in extracting the time signal, we only have one variable. Once the GPS receiver has determined its precise X, Y, and Z positions, all that's left is the time. A well-designed GPS clock can remain extremely accurate with just one satellite in view. This capability is often referred to as 'position hold' mode.

Though the GPS system was designed so that there would be six satellites in view from any point on Earth at any time, the ability to work well with fewer satellites is important for locations where a full view of the sky is impractical. While the receiver is unlikely to work well with the antenna in the middle of a room, it may work perfectly if it's in a window.

Unfortunately, not all GPS receivers are designed to be used as time sources. Some will create time glitches when they change which satellites they are using. Some are unable to keep a time lock with fewer then three satellites in view. Some consider their serial output a 'low priority' task and can delay time code outputs by randomly varying amounts if they are busy computing.

If you are serious about accurate time, it is important to use a GPS unit designed for this purpose.

Sources

We offer the GPSClock 200, a GPS receiver that is designed for time reference use from the ground up. It weighs about .8 pound and has threads on it to allow it be easily mounted to a PVC pole.

Here are more details.

Further Reading
The Next Level
The Capture Effect