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LOGGING.md

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Data Output and Logging

GPSTest allows you to output raw data about GNSS/GPS to files as well as the Android system log, which can be used to visualize the data using tools like Google's GPS Measurement Tools project shown below. The following sections discuss how to access these logs, and details about the data that is output.

image

Logging data to files

You can enable file logging in GPSTest via the Settings. By default file logging is turned off for all data to avoid unnecessarily filling up storage space.

Steps to log and view data:

  1. In the GPSTest app, go to "Settings", scroll down, tap on "Logging and Output" and under "File Output" make sure the box is checked for each data output type you'd like to see (see next sections).
  2. You can share CSV and JSON log files via the "Share" button in the top action bar of the app, or by copying them off internal memory or SD card over USB.

Data output - CSV

Here are the details of file logging:

  • CSV format - Same file format as the Google GPS Measurement Tools (GNSS Logger) project so you can use MATLAB tools under that project to analyze the data.
  • Files are saved to your Android device storage under the gnss_log directory.
  • After you've enabled logging via Settings, each time you start the app a new file will be created named with the date and time (e.g., gnss_log_2019_09_11_13_09_50.csv). If you end the app (e.g., hit back button) and restart it, another file gets created.
  • Each row of the file is prefixed with a string designating the data type:
    • Raw - Raw GNSS measurements
    • Fix - Location fix information
    • Nav - Navigation message
    • NMEA - NMEA sentences

The header of the CSV file explains the format for each data type:

# Raw GNSS measurements format (Android 7.0 and higher on supported devices):
#   Raw,ElapsedRealtimeMillis,TimeNanos,LeapSecond,TimeUncertaintyNanos,FullBiasNanos,BiasNanos,BiasUncertaintyNanos,DriftNanosPerSecond,DriftUncertaintyNanosPerSecond,HardwareClockDiscontinuityCount,Svid,TimeOffsetNanos,State,ReceivedSvTimeNanos,ReceivedSvTimeUncertaintyNanos,Cn0DbHz,PseudorangeRateMetersPerSecond,PseudorangeRateUncertaintyMetersPerSecond,AccumulatedDeltaRangeState,AccumulatedDeltaRangeMeters,AccumulatedDeltaRangeUncertaintyMeters,CarrierFrequencyHz,CarrierCycles,CarrierPhase,CarrierPhaseUncertainty,MultipathIndicator,SnrInDb,ConstellationType,AgcDb,CarrierFrequencyHz
# 
# Location fix format (all devices):
#   Fix,Provider,Latitude,Longitude,Altitude,Speed,Accuracy,(UTC)TimeInMs
# 
# Navigation message format (Android 7.0 and higher on supported devices):
#   Nav,Svid,Type,Status,MessageId,Sub-messageId,Data(Bytes)
# 
# NMEA format (Android 2.3 and higher) (for [NMEA sentence] format see https://www.gpsinformation.org/dale/nmea.htm):
#   NMEA,[NMEA sentence],timestamp

Sample data looks like:

NMEA,$GNGSA,A,2,66,81,87,,,,,,,,,,1.3,1.0,0.9,2*3F,1568222348217
NMEA,$GNVTG,,T,,M,0.0,N,0.0,K,A*3D,1568222348218
NMEA,$GNGGA,171859.00,2804.281311,N,08225.605044,W,1,07,1.0,39.1,M,-24.8,M,,*75,1568222348220
NMEA,$GNRMC,171859.00,A,2804.281311,N,08225.605044,W,0.0,,110919,3.1,W,A,V*77,1568222348220
Raw,1257164406,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,2,0.0,207,73139369397323,271,22.8,-556.3143920898438,10.200000762939453,0,0.0,0.0,,,,,0,,3,,
Raw,1257164408,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,23,0.0,207,73139369842860,1523,19.3,182.0135955810547,10.510000228881836,0,0.0,0.0,,,,,0,,3,,
Raw,1257164411,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,17,0.0,207,73139361386059,1192,21.2,-592.7639770507812,10.278000831604004,0,0.0,0.0,,,,,0,,3,,
Raw,1257164412,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,5,0.0,15,321557370216944,1041,20.3,208.6379852294922,9.641500473022461,0,0.0,0.0,,,,,0,,1,,
Raw,1257164413,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,6,0.0,15,321557356784894,926,21.9,394.1838073730469,9.560500144958496,0,0.0,0.0,,,,,0,,1,,
Raw,1257164413,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,12,0.0,15,321557361694004,825,22.2,557.084716796875,9.458000183105469,0,0.0,0.0,,,,,0,,1,,
Raw,1257164414,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,13,0.0,15,321557363597446,1504,17.2,-508.64544677734375,9.940999984741211,0,0.0,0.0,,,,,0,,1,,
Raw,1257164415,126692640000000,,,-1252130864797923510,0.9759163856506348,571.2438141927123,74.13293543922987,35.96258578603258,729,15,0.0,15,321557358793297,826,22.0,-689.0819702148438,9.483500480651855,0,0.0,0.0,,,,,0,,1,,
Fix,gps,28.071355,-82.426751,14.320496,0.000000,38.592003,1568222340000

Data output - JSON

GnssAntennaInfo logging is available on supported devices (e.g., Pixel 5) with Android 11 and is logged in the JSON format. GNSS antenna(s) characteristics, such as phase center offset (PCO) coordinates, phase center variation (PCV) corrections, and signal gain corrections can be applied to the raw measurements to improve accuracy.

Logging works similar to the CSV file process, with a file name like gnss_log_2019_09_11_13_09_50.json. Here's example data from a Pixel 5:

[
   {
      "carrierFrequencyMHz":1575.42,
      "phaseCenterOffset":{
         "xoffsetMm":1.2,
         "xoffsetUncertaintyMm":0.1,
         "yoffsetMm":3.4,
         "yoffsetUncertaintyMm":0.2,
         "zoffsetMm":5.6,
         "zoffsetUncertaintyMm":0.3
      },
      "phaseCenterVariationCorrections":{
         "correctionUncertaintiesArray":[
            [ 0.1, 0.2, 0.3, 0.4 ],
            [ 1.1, 1.2, 1.3, 1.4 ],
            [ 2.1, 2.2, 2.3, 2.4 ]
         ],
         "correctionsArray":[
            [ 11.22, 33.44, 55.66, 77.88 ],
            [ 10.2, 30.4, 50.6, 70.8 ],
            [ 12.2, 34.4, 56.6, 78.8 ]
         ],
         "deltaPhi":60.0,
         "deltaTheta":120.0
      },
      "signalGainCorrections":{
         "correctionUncertaintiesArray":[
            [ 0.11, 0.22, 0.33, 0.44 ],
            [ 0.55, 0.66, 0.77, 0.88 ],
            [ 0.91, 0.92, 0.93, 0.94 ]
         ],
         "correctionsArray":[
            [ 9.8, 8.7, 7.6, 6.5 ],
            [ 5.4, 4.3, 3.2, 2.1 ],
            [ 1.3, 2.4, 3.5, 4.6 ]
         ],
         "deltaPhi":60.0,
         "deltaTheta":120.0
      }
   }
   ...
]

Data Analysis

Use the CSV file output from GPSTest along with the Google GPS Measurement Tools project to analyze the data (as of late October 2020 supported for GnssAntennaInfo doesn't seem to exist).

For details see the Android documentation:

Here's a screenshot from the GPS Measurement Tools desktop software:

image

Accessing the system log via Android Studio

You can view the data output from GPSTest by using Android Monitor, which is included with Android Studio.

Steps to view data:

  1. Install Android Studio
  2. Enable USB debugging on your device
  3. In Android Monitor, in the drop down box on far right side, select No Filters.
  4. In Android Monitor, in the search box with magnifying glass, enter GpsOutput to filter out all other system output.
  5. In the GPSTest app, go to "Settings", scroll down, tap on "Logging and Output" and under "Android Monitor Output" make sure the box is checked for each data output type you'd like to see (see next section).

Data output

Android 2.3 and higher:

  • NMEA output - NMEA strings. Enabled by default for Android Studio. To show only this output in Android Monitor, use the search box text GpsOutputNmea.

Android 7.0 and higher:

  • GNSS Measurements - Raw GNSS satellite measurements observed by the GNSS subsystem. Disabled by default for Android Studio. To show only this output in Android Monitor, use the search box text GpsOutputMeasure.
  • GNSS Navigation Message - Navigation messages observed by the GNSS subsystem. Disabled by default for Android Studio. To show only this output in Android Monitor, use the search box text GpsOutputNav.

What devices support pseudorange measurements and navigation messages?

Check out the FAQ for known device information.