Factory Test

Factory test is a GLL operational mode for production hardware testing.

The factory test is capable of detecting:

  • Higher noise figure caused by misplaced parts, wrong values, etc.
  • Excess frequency error of a TCXO part
  • Sensitivity loss of 1.5 dB or more due to excessive phase noise of a bad TCXO

There are two different factory test modes:

  • Factory tracking test
  • Factory CW test

Factory Tracking Test

To conduct the factory tracking test the signal simulator must be configured to generate the satellite signal at the center frequency. There are two configurations, one is GPS only and other is GNSS (GPS+GLONASS). A GLCT entry, like the example entry below, creates the Factory Test Request. The user should edit the PRN and FCN parameters appropriately. For reliable results, the signal is expected to initially be in the range of -125dBm to -142dBm with the frequency offset in the range of -2 ppm to +2 ppm. Once the signal is found in the specified range, the power level can be reduced to perform an tracking test at lower signal levels.

GPS

 <job id="Factory_GPS_Track_Test">
    <task id="test" repeat="1">
        <startup
            IgnoreOsc="true"
            IgnoreRomAlm="true"
            IgnoreRamAlm="true"
            IgnorePosition="true"
            IgnoreNav="true"
            IgnoreTime="true"
       />
        <req_ftest
            Prn="1"
            FactoryTestItems="GL_FACT_TEST_WER" 
            FactoryTestMode="GL_FACT_TEST_CONT"
            AveragingIntervalSec="10" 
            DurationSec="120"
       />
    </task>
</job> 

GNSS (GPS+GLONASS)

Here the "FCN" number corresponds to a particular GLONASS satellite frequency, which can be found in the table below.

The GPS signal at the specified PRN must be present.

<job id="Factory_GNSS_Track_Test">
    <task id="test" repeat="1">
        <startup
            IgnoreOsc="true"
            IgnoreRomAlm="true"
            IgnoreRamAlm="true"
            IgnorePosition="true"
            IgnoreNav="true"
            IgnoreTime="true"
       />
        <req_ftest
            Prn="1"
            Fcn="0" 
            FactoryTestItems="GL_FACT_TEST_WER" 
            FactoryTestMode="GL_FACT_TEST_CONT"
            AveragingIntervalSec="10" 
            DurationSec="120"
       />
    </task>
</job> 

Factory CW Test

To conduct the factory CW test, the signal simulator must be configured to generate the signal for one GPS satellite or one GLONASS satellite respectively: For GPS the signal should be at +200 kHz offset relative to center frequency i.e., L1+200 kHz = 1575.62 MHz. For GLONASS the signal should be at FCN0(= 1602 MHz) frequency.

  • 4752 has two different parts compared to previous 47511 chips:
    • An FFT-based search engine.
    • A correlator based track engine.
      In 47511 and previous chips, both operations were performed by the same hardware. Because of this design differences, CW operation in 4752 is slightly different from 47511
  • The input power should be between -140 dBm and -120 dBm.
    • The upper limit is a design restriction. Since 4752 is a system designed to work with very weak signals, a CW stronger than that can saturate the receiver.
    • All the processing chain, both analog and digital, is prepared to work with weak signals, not strong.
    • This means that a signal much stronger than what can be possibly seen in a real world scenario will still be detected, but if severe saturation happens the power and / or frequency may not be properly estimated.
    • The lower limit is a test design issue. Aiming for weaker CW signals is quite hard, and needs a lot of work and a lot of acquisition time.
    • The test could be modified for this purposes, but then the runtime would be higher.
    • So in marginal low limits such as -138 ~ -140dBm, sometimes acquisition time can be longer (7~11s)
    • Typical runtime Input signal strength(-138 dBm ~ -120 dBm), typical response time : under 3s
    • Saturated area(over -120dBm) / weak signal area(under -138dBm), response time : 3~11s
  • We recommend to set INPUT power to around -130dBm (receiving at antenna point)
    • This will provide more margin(8dB to -138dBm) to detect Good or bad sample
    • In this higher input power will give short acquisition time and better signal strength report.

An example factory CW test configuration entry for the GLCT is shown below.

GPS

<job id="Factory_GPS_CW_Test">
        <task id="test" repeat="1">
            <startup 
                IgnoreOsc="true"
                IgnoreRomAlm="true"
                IgnoreRamAlm="true"
                IgnorePosition="true"
                IgnoreNav="true"
                IgnoreTime="true"
           />
            <req_ftest 
                FactoryTestItems="GL_FACT_TEST_CW" 
                FactoryTestMode="GL_FACT_TEST_ONCE"
                AveragingIntervalSec="10" 
                DurationSec="60"
           />
        </task>
</job> 

GLONASS

Here the "FCN" number corresponds to a particular GLONASS satellite frequency, which is FCN=0 for CW test.

     <job id="Factory_GLONASS_CW_Test">
        <task id="test" repeat="1">
            <startup 
                IgnoreOsc="true"
                IgnoreRomAlm="true"
                IgnoreRamAlm="true"
                IgnorePosition="true"
                IgnoreNav="true"
                IgnoreTime="true"
           />
            <req_ftest 
                Fcn="0" 
                FactoryTestItems="GL_FACT_TEST_CW" 
                FactoryTestMode="GL_FACT_TEST_ONCE"
                AveragingIntervalSec="10" 
                DurationSec="60"
           />
        </task>
</job>

The req_ftest element should have the following attributes specified:

  • PRN - The GPS Satellite PRN configured in the simulator (255 disables it)
  • FCN - FCN number of the corresponding GLONASS Satellite frequency configured in the simulator (-8 disables it)
                        FCN                Frequency(MHz)               
                        -7                 1598.0625
                        -6                 1598.625
                        -5                 1599.1875
                        -4                 1599.75
                        -3                 1600.3125
                        -2                 1600.875
                        -1                 1601.4375
                         0                 1602.0
                         1                 1602.5625
                         2                 1603.125
                         3                 1603.6875
                         4                 1604.25
                         5                 1604.8125
                         6                 1605.375
  • GL_FACT_TEST_ITEMS - What to test( see ::GL_FACT_TEST_ITEMS )
  • GL_FACT_TEST_MODE - Specifies if test needs to be run once or continously( see ::GL_FACT_TEST_MODE)
  • timeout- Maximum duration of the test.

If you don't use the GLCT your application must create the GlRequest object using the GlRequest::MakeFactTestReq() method.

The output of the factory test is provided through the $PGLOR,3,FTS - Factory Test Status NMEA message.

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