GPS

Testing GPS

Unlike WiFi, WiMAX, or cell-phone communications subsystems, GPS is not a receiver/transmitter technology. GPS ICs enable receive-only devices that look for at least four of 24 active GPS satellite signals and do some straightforward triangulation and error correction to give users very accurate position information.

To do so, the GPS-subsystem must be able to receive very low-level satellite signals (minimum signal strength of -130 dBm) and accurately capture the pseudo random noise (PRN) ranging code, ephemeris, and almanac data. With that information captured, the device’s internal circuitry can do the location calculations and correlate them with standard map-coordinate information (i.e. WGS-84).

There are two key things to test in a GPS device. One is to confirm that the device is sensitive enough, and can accurately capture data superimposed (i.e. modulated) onto the satellites’ carrier signals. The other test is to verify that the device calculates the proper positioning coordinates.

One can expose the device to actual satellite signals and verify that it provides the correct location fix. The problem with this approach is that actual satellite signals can vary based on varying atmospheric conditions making it difficult to achieve repeatability of initial test conditions. One could also simulate multiple satellites sending their individual signal data and verify that the device calculates the proper positioning coordinates. The problem with this approach is that multiple-channel testing has been an expensive and time-consuming proposition.

To resolve these dilemmas, LitePoint created IQnav, a multi-channel tester that provides L1 signals with adjustable power levels and capable of producing emulated satellite data. This signal-generator-based tester produces low-level signals that could be used for both SNR (e.g. calibration) and location-fix testing. In addition it produces a high-level CW signal that enables accurate set-up attenuation calculation that is the same at -130 dBm levels. Using software-defined radio design, LitePoint is able to provide a six-channel solution at far lower cost than a conventional multi-channel design. For more detail about GPS testing, read Keeping Test Costs Down as GPS Goes Everywhere [LINK].

Some devices have a combination of wireless standards, such as WiFi, Bluetooth, GPS, and FM. LitePoint makes the world’s only Multicom tester – IQ2010 – designed for testing multiple-radio devices where each standard is tested, faster, and in parallel.