Each satellite in the GPS constellation broadcasts two navigation signals: the L1 and L2. L1 is not encoded and broadcast at 1575.42MHz and L2 is encoded and broadcast at 1227.6MHz. Therefore, L2 signals provide a more accurate time and location measurement. Because of its high accuracy, the L2 signal is only available to authorized users such as the military and government agencies. The L1 signal is accurate enough for a consumer-grade GPS receiver and there is a large and growing market for other devices that use the L1 signals such as multi-radio devices such as smartphones.

One question that always comes up in consumer-grade GPS receivers is: how accurate is the measurement of location in these devices?

With the competition existing in today’s market, it is essential that the makers of these devices guarantee as good accuracy as possible. This boils down to accurate testing at the manufacturing floor. An excellent test to run in this scenario is the Distance Accuracy test.

How do I measure accuracy with Distance Accuracy Test?

LitePoint offers a couple of products for testing GPS devices – IQnav and IQ2010. Either of these testers can be used to perform the distance accuracy test.

Using either of these test systems to measure GPS distance accuracy is as easy as a plug and play—you simply load and play a scenario file, which provides the exact coordinates of a location and compare the values with the measured value from the receiver.

Here is a sample of the data location information:

To run the test using the LitePoint IQnav or IQ2010 system:

1.     Make sure the IQnav GPS software is connected to the test system.

2.     Using the Operation Mode drop-down menu, select Scenario Mode.

3.     Click the Open Folder icon .

4.     Select an XML data file that is provided in the IQnav GPS application.

5.     Click the Play icon . The test system starts transmitting the signals that correspond to the loaded data and displays the information next to the channel indicator.

Once the GPS device obtains a position fix, record the position of the fix. Since the scenario files are stationary, the tester is simulating a fixed point over a period of time. By opening up the scenario .xml file, the exact co-ordinates of the location can be determined, and by comparing this with the measured value from the receiver, distance accuracy can be derived.  

Distance accuracy is typically reported in meters. To make the conversion from a difference in latitude and longitude to meters, you can use the Haversine formula to calculate the shortest distance between two points on a sphere. This will yield the distance in meters.