64 lines
2.9 KiB
Kotlin
64 lines
2.9 KiB
Kotlin
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package com.hawhamburg.micr0bu.data
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import android.os.SystemClock
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import android.util.Log
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import com.hawhamburg.micr0bu.domain.asn1.ItsTime
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import java.time.DateTimeException
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/**
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* Puts the timestamps this phone transmits on GNSS time instead of its own wall clock.
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*
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* ## Why
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* Every CAM carries a generationDeltaTime and every GeoNetworking header a TST, and receivers use
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* them to judge how fresh a message is and in what order messages came. Both used to come straight
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* from `System.currentTimeMillis()`, so they were only as good as the phone's clock setting. On
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* 2026-09-10 the bench phone was 24 minutes fast: automatic time had no source (no SIM, and the
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* lab Wi-Fi has no internet time), so it had not set the clock once in 69 hours, and every CAM
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* went out stamped 24 minutes in the future. A bike-mounted phone on the road is in exactly that
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* position. GNSS time depends on none of it.
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*
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* ## How
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* [SystemClock.currentGnssTimeClock] (API 29, this app's minSdk) is a UTC clock the platform keeps
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* synchronised from GNSS fixes. One reading of it taken alongside the wall clock gives the wall
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* clock's error, which is then applied to the fix's own timestamp. When GNSS time is unavailable,
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* typically indoors before any satellite fix since boot, the wall clock is used unchanged.
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*
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* Which clock is in use is logged whenever it changes, with the measured error, so a capture shows
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* where a given run's timestamps came from.
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*
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* Only the transmit path uses this. Everything else in the app stays on the wall clock, because
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* received messages, sensor samples and trip records are all stamped with it and must stay
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* comparable with one another.
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*/
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object GnssTimeSource {
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private const val TAG = "GnssTimeSource"
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/** Whether the last correction used GNSS time; null before the first. For change-only logging. */
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@Volatile private var lastUsedGnss: Boolean? = null
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/** [systemMs], a wall-clock reading, moved onto GNSS time where GNSS time is available. */
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fun correct(systemMs: Long): Long {
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val systemNow = System.currentTimeMillis()
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val gnssNow = try {
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SystemClock.currentGnssTimeClock().millis()
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} catch (e: DateTimeException) {
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null
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}
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noteSource(gnssNow, systemNow)
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return ItsTime.onGnssTime(systemMs, gnssNow, systemNow)
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}
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private fun noteSource(gnssNow: Long?, systemNow: Long) {
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val usingGnss = gnssNow != null
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if (lastUsedGnss == usingGnss) return
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lastUsedGnss = usingGnss
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if (gnssNow != null) {
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Log.i(TAG, "transmit timestamps now on GNSS time; phone clock is " +
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"${"%+.1f".format((systemNow - gnssNow) / 1000.0)} s off")
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} else {
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Log.w(TAG, "GNSS time unavailable, transmit timestamps fall back to the phone clock, " +
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"which has no automatic time source without a SIM or internet")
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}
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}
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}
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