Location is the T-Box's most visible output: it powers fleet tracking, stolen-vehicle recovery, charging-station routing and — most critically — the coordinates an eCall transmits after a crash. Yet satellite positioning in a road vehicle is repeatedly denied by tunnels, parking garages and urban canyons. This article explains how a T-Box turns raw satellite signals into a continuous, trustworthy position: multi-constellation GNSS, assisted start-up, accuracy metrics and inertial dead reckoning that bridges every gap.
A GNSS receiver measures the time-of-flight of signals from orbiting satellites, each carrying its precise orbit (ephemeris) and an atomic-clock timestamp. With distance to four satellites the receiver solves for latitude, longitude, altitude and receiver-clock bias. Automotive modules track four constellations simultaneously — GPS, BeiDou, GLONASS and Galileo — and premium designs receive two frequency bands (L1 and L5) so that ionospheric delay and multipath reflections can be separated out. More visible satellites mean faster fixes and better geometry, expressed as dilution-of-precision (DOP) values.
Time-To-First-Fix depends on what the receiver already knows. From a cold start, with no ephemeris, time or position, an unaided receiver may take 25–45 seconds to download satellite data from the weak space signal. A hot start, with fresh ephemeris and valid time, fixes in 1–2 seconds. Assisted GNSS (A-GNSS) closes the gap: the cellular network delivers ephemeris, almanac, coarse position and precise time over the modem, pulling cold-start fix time down to roughly 2–10 seconds. For an emergency call that must report location within seconds of a crash, A-GNSS is not a luxury but a requirement.
Position accuracy is quoted statistically. CEP50 (circular error probable, 50 %) is the radius within which half of fixes fall; a typical automotive receiver achieves around 2–2.5 m CEP50 in open sky, while dual-band receivers can reach sub-metre figures. Sensitivity — acquisition near −148 dBm and tracking near −160 dBm — determines whether weak, reflected signals in city centres can still be used. Engineers design antennas with right-hand circular polarisation and sufficient sky view, because no algorithm recovers a signal the antenna never sees.
Satellites disappear exactly where users still expect a track: tunnels, underpasses, covered garages and skyscraper corridors. Automotive dead reckoning (DR) fuses the last GNSS fix with independent motion sensors — a three-axis MEMS inertial measurement unit (gyroscope and accelerometer sampled up to 100 Hz), the vehicle's wheel-tick speed pulse and often steering-angle or reverse-gear signals. An estimation filter (typically an extended Kalman filter) integrates heading and distance forward, so the track continues smoothly with no satellite in view. When GNSS returns, the filter uses the new fix to correct accumulated inertial drift.
For an eCall minimum set of data, a crash in an underground road must still be located; for usage-based insurance and V2X, position continuity through urban canyons underpins lane-level decisions. Three properties separate credible automotive DR from consumer phone navigation: sensor calibration that learns gyro bias and wheel-size over time, immunity to magnetic interference (vehicle-grade DR avoids relying on compass heading), and tightly-coupled fusion that uses even partial satellite measurements. Modules also output the 1PPS pulse that aligns sensor, bus and network timestamps to within tens of nanoseconds.
A dead-reckoning track and an emergency fix depend on the T-Box staying alive, however brief the power interruption. Weijiang Power manufactures wide-temperature, long-service-life NiMH and lithium backup cells and custom packs for telematics terminals, delivering stable rail voltage through crash disconnects and cold cranking. Share your module peak current, cabin temperature range and required hold-up time, and our team will specify and build the matching backup-power assembly.
