GNSS Time Detection

From Android 12, Android can optionally use a Global Navigation Satellite System (GNSS) to suggest UTC times to the time_detector service. This isn't enabled by default in AOSP.

When GNSS time detection is enabled, the gnss_time_update_service passively listens to location updates from GNSS sources and submits GNSS suggestions to the time_detector service. The time_detector service then determines whether to update the system clock to match the suggestion.

Impact on power usage

The AOSP gnss_time_update_service passively listens to location updates. This means that it never actively turns on the GPS or consumes additional power. This also means that unless another app or service in the system is actively requesting location updates, the gnss_time_update_service won't get a location update and suggest a GNSS time.

Implementation

To enable GNSS time detection, device manufacturers must explicitly enable the gnss_time_update_service in the system server.

Both the config_enableGnssTimeUpdateService and config_autoTimeSourcesPriority values in the core/res/res/values/config.xml file must be updated to enable this feature. Set the value for config_enableGnssTimeUpdateService to true and add gnss to the item list for config_autoTimeSourcesPriority. The position of gnss in the priority list determines the priority given to GNSS suggestions with respect to those from other sources.

The following is an example core/res/res/values/config.xml file where GNSS time detection is enabled and gnss is third in the priority list after network and telephony.

<!-- Specifies priority of automatic time sources. Suggestions from higher entries in the list
         take precedence over lower ones.
         See com.android.server.timedetector.TimeDetectorStrategy for available sources. -->
    <string-array name="config_autoTimeSourcesPriority">
        <item>network</item>
        <item>telephony</item>
        <item>gnss</item>
    </string-array>

    <!-- Enables the GnssTimeUpdate service. This is the global switch for enabling Gnss time based
         suggestions to TimeDetector service. See also config_autoTimeSourcesPriority. -->
    <bool name="config_enableGnssTimeUpdateService">true</bool>

Debugging and testing

To test GNSS time detection, you can use adb shell cmd location commands. Use these commands to add test location providers where you can specify a location and the associated GNSS time. The gnss_time_update_service listens to these location updates, and makes suggestions periodically.

The following shows examples of adb shell cmd location commands:

# Enable Master Location Switch in the foreground user (usually user 10 on automotive). If you just flashed, this can be done through setup wizard.
adb shell cmd location set-location-enabled true --user 10

# Add GPS test provider (This usually fails the first time. Will throw a SecurityException with "android from <SOME_UID> not allowed to perform MOCK_LOCATION".)
adb shell cmd location providers add-test-provider gps

# Enable mock location permissions for previous UID
adb shell appops set UID_PRINTED_IN_PREVIOUS_ERROR android:mock_location allow

# Add GPS test provider (Should work with no errors.)
adb shell cmd location providers add-test-provider gps

# Enable GPS test provider
adb shell cmd location providers set-test-provider-enabled gps true

# Set location with time (Time can't be lower than the limit set by the lower bound.)
adb shell cmd location providers set-test-provider-location gps --location LATITUDE,LONGITUDE --time TIME