GPS functionality test

The %XRFTEST GPS SNR test starts Global Positioning System (GPS) Signal-to-Noise Ratio (SNR) measurement. By default, the test expects the Carrier Wave (CW) in the signal generator to be at 1575.750 MHz, which means a 330 kHz offset from the GPS center frequency of 1575.42 MHz. The duration of the measurement is 1 ms.

The test is automatically disabled when it is finished. OFF <operation> is not needed due to automatic stop.

The recommended signal level for GPS testing is -90 dBm. The expected SNR with this signal level is normally more than 16 dB. For details on setting the correct signal level parameter for the %XRFTEST command, see GPS mode.

AFC correction can be performed to increase the accuracy of the SNR measurement. This can compensate the frequency error between nRF9160 and the test equipment used. AFC measurement and setting is supported by modem firmware v1.2.x and later.

Note: The reported SNR is very sensitive to external RF disturbance that may be present in the test environment. An RF shielded chamber is recommended as a stable test environment. A known device, such as a golden unit, can be used as a reference to confirm the validity of the test result.

The following command example executes a GPS SNR test with AFC correction enabled, default receiver frequency, and external Low-Noise Amplifier (LNA) gain of 16 dB:

%XRFTEST: 361, -18758 // Result: SNR 361/16 = 22.56dB, RSSI -18758/256
= -73.3dBm OK  

The GPS functionality test has one input parameter that is described in the following table:

Table 1. Input parameter of GPS functionality test
Parameter name Parameter description Range Value description
<param0> Signal level at antenna –127 to –25 Signal level at the nRF9160 antenna input (dBm)
<param1> AFC correction 0–1 AFC correction control in measurement
<param2> Frequency 1565.42–1585.42 Manual setting of RF frequency

nRF9160 responds to the %XRFTEST GPS SNR test with the measured GPS SNR and RSSI values. The SNR is given in the q4 (dB) format. Dividing the result by 24 = 16 gives the dB value of the antenna power.

The RSSI signal level at the antenna is given in the q8 (dBm) format. Dividing the result by 28 =256 gives the dBm value of the RSSI.

Note: The GPS signal level parameter in the %XRFTEST command refers to the signal level at the nRF9160 GPS antenna input pin. Consider external LNA gain with this setting.

For more information on GPS testing with the %XRFTEST AT command, see GPS SNR test %XRFTEST in nRF91 AT Commands Reference Guide.