GPS Test Detailed Explanation

    GPS is a very common global positioning system. The general civilian GPS uses the L1 carrier of the GPS system, with a frequency of 1575.42MHz. Generally, GPS satellites are 12,000 kilometers above the ground, and 24 satellites orbit the earth in a 12-hour cycle, so that at any time, at any point on the ground, more than 4 satellites can be observed at the same time. In this way, GPS can effectively perform 2D (flat map) and 3D (stereo map) positioning.

 

    So the performance of GPS actually depends on its ability to receive signals, that is, the accuracy of its positioning time. The more satellites it searches for and the shorter the time, the more accurate the positioning. Therefore, there is a complete testing process for GPS in the industry.

 

 

    For GPS, the tests include positioning accuracy, TTFF of cold start, hot start and warm start, tracking time and capture time. In addition, there is A-GPS performance test. However, this is mostly a standard for chip processing and computing capabilities. For a simple RF link, the main thing is the signal-to-noise ratio C/N0, where N0 is the noise power spectrum density. This is also a test of the NF of the receiver. Previous projects have used -130dBm as a reference value. According to the thermal noise at normal temperature, it is -174dBm/HZ, so the input SNR should be 44dB. Now mobile phone GPS solutions are generally filter + LNA + filter solutions. According to normal projects, the insertion loss of the previous filter should be around 0.8dB, and the noise coefficient of the LNA should be around 1. The gain of LNA is generally above 10, so the NF contributed by the subsequent filter is relatively small, so the NF of the entire RF path is about 2dB, so the output SNR should be 41-42dB (this requires special software to read). In addition, if there is no GPS satellite signal generator, you can input a single-point frequency signal of 1575.42MHz for testing. The principle is the same as above, but the required value should be adjusted appropriately according to the RBW setting of the spectrum analyzer. Since the noise floor of a general spectrum analyzer is 145dBm/Hz, the input signal should be set higher, such as around -80 to -100dBm, but not too high, after all, the compression characteristics of the receiver should be taken into consideration. In addition, the output is not the noise power spectral density but the noise, so the result must take into account the RBW setting. For example, if the input is -100, the RBW is set to 1K, and the NF is 2, then the output SNR is -100dBm-(-174dBm/Hz+10LOG1000)-2=42. Here we distinguish between C/N0 and S/N, otherwise it will be confusing to see the same result with different input powers.

 

GPS test items are as follows:

    GPS first positioning time (2D positioning and 3D positioning time when first started)

    Radio frequency signal interference

    Cold start and warm start positioning time

    The time it takes for the receiver to reacquire the GPS signal

    Static positioning accuracy

    Receiver sensitivity

    Dynamic positioning time

 

 

    During the test, the GPS first positioning time is the primary condition for detecting its performance. It mainly tests the GPS positioning time and the number of search satellites under normal conditions without interference signals.

    Next I will introduce the GPS first fix time test.

    The equipment used for the test includes: signal source (E4438C), interference source (E8572D), power divider, attenuator, and RF line.

 

    Note: When we test, we must build the test environment in a shielded room, otherwise the signal received by the GPS will not be the signal given by the signal source, and the test result will not be the desired result.

 

Specific test steps:

   GPS First Fix Time

   Turn on the signal source E4438C, enter the Mode option and click the last button on the screen to turn the page. After entering the second page, select the GPS option.

   When entering the GPS option, there will be two lines of display on the screen. At this time, we select the Real Time MSGPS option.

 

   When entering the interface shown in Figure 1 below, we need to set the maximum number of satellites and then enter the Scenar io option.

    Then enter the Goto Row option and select the required map, find the "select Scenario" option and click this option, and you will enter the required map.

    After selecting the map, there will be a Real Time MSGPS Off/ON option in the upper right corner of the screen. This option is to turn on or off the GPS signal. We press the button to turn on the GPS signal. (When pressing the option button, please note that you should wait for a few seconds to see if the signal is turned on. If not, repeat the operation)

Enter the Amplitude option in E4438C to set the signal strength. Do not set the signal strength too high because the signal received by GPS is very small (the signal is about -100dBm). If the output signal is too large, GPS will not be able to locate.

 

    Start the GPS positioning software and fill in the following table to test the GPS 2D positioning time and GPS 3D positioning time in turn. (GPS must be restarted when testing different signal strengths)

 

Signal strength (dBm)	GPS 2D positioning time	GPS 3D positioning time	Remark
-128	120 seconds	10 points
-105	85 seconds	7 points
-99	76 seconds	6 points

 

                                                                                    Table 1

Radio Frequency Interference Test

The test diagram is as shown in Figure 2

Build the system according to the schematic diagram in Figure 2.

    First, put the GPS into search mode and turn on the GPS signal source.

    Confirm that GPS has valid 3D positioning and record the signal strength at this time.

    When the analog signal remains unchanged, the interference signal is set to the same frequency and power, and then the interference signal is gradually increased by 1dBm.

    Detect the strength of the interference signal at what point the GPS loses navigation positioning.

    Record the signal strength of the interfering signal source.

 

 

    Here we only briefly introduce the test of GPS products. If you want to know more test information, please contact the nearest MORLAB.