As GPS technology becomes more common, GPS receiver manufacturers, OEM integrators, and contract manufacturers struggle to determine the appropriate standard tests to verify GPS receiver performance. Verification procedures require a controlled environment that facilitates precise repeatability. In most cases, using actual GPS satellite signals received through an antenna does not provide such an environment. This excerpt from Agilent's GPS Receiver Testing white paper introduces a real-time GPS signal simulation technique capable of generating the required GPS signals for a repeatable and flexible test environment.
GPS Test Requirements
The GPS user experience for commercial applications is affected by several factors. GPS devices which provide an enhanced user experience will sell better, so manufacturers are looking for factors to differentiate their receivers. Typical factors which determine the outcome of the user experience include the following factors:
- When a GPS device is turned on, how long is it until the position of the receiver is determined?
- When a weak or poor signal area is encountered, can the receiver still determine its position?
- If the signal is interrupted and then restored, how long does it take for the receiver to recover and resume calculating its position?
- Accuracy of the calculated location.
There are of course other factors such as cost, user interface, turn-by- turn navigation, spoken directions, and so forth that are important to users, but these are not so dependent on the GPS receiver performance.For commercial or military applications, there may be many other kinds of GPS conditions that are important, such as:
GPS receiver basics
- How accurately can a position or time be determined?
- How repeatable is the solution?
- How sensitive is the receiver to interference or jamming?
- How rapidly can the receiver report its position (if the receiver is moving rapidly – such as in an airplane)?
From a high level perspective, the GPS unit appears as an antenna which senses the navigation signals, and has an output of some kind which reports status, positions, and time (via some I/O port or a screen).
A typical block diagram for GPS receivers includes an antenna, an RF front end/down converter, a baseband processing element, and a computation engine. In some cases these elements may be integrated in a single module which can be quite small – on the order of 1 square cm. See Figure below.
Figure: GPS receiver block diagram.