acquisition time

Acquisition time is the elapsed duration from when a GPS receiver powers on or restarts until it successfully locks onto enough satellite signals to compute an initial position fix. This is not the ongoing update rate of your location; it is the startup delay. A cold start, where the receiver has no prior knowledge of its location or the satellite constellation, typically requires 30 seconds to several minutes. A warm start, where the receiver retains some orbital data but has been off for hours or days, might complete in 10 to 30 seconds. A hot start, performed within minutes of the last fix with cached ephemeris data still valid, can achieve lock in under 10 seconds.

The receiver must collect at least four satellite signals to calculate a three-dimensional position plus time correction. Each satellite transmits its ephemeris (orbital position), almanac (constellation status), and timing signal on 1575.42 MHz, the L1 frequency. The receiver's processor performs a two-dimensional search across possible Doppler shifts and code phases to detect each satellite's spread-spectrum signal, which occupies only about 2 MHz bandwidth despite carrying data at 50 bits per second. Weak signals and poor geometry, such as urban canyon shadowing or a receiver under dense foliage, extend acquisition time substantially, sometimes to several minutes or result in no fix at all.

Assisted GPS (A-GPS) and Assisted GNSS (A-GNSS) systems reduce acquisition time by downloading predicted ephemeris data and reference time over a network connection before attempting satellite lock. Military and survey receivers using dual-frequency L1/L5 or L1/L2C signals can acquire faster than single-frequency civilian units because they resolve ionospheric delay more quickly. Software-defined receivers with large coherent integration windows can detect weaker signals but at the cost of longer computation, trading off speed for sensitivity.

The term reflects the telecommunications sense of acquisition, meaning to establish or lock onto a signal. In radar and communications, acquisition also refers to target detection or carrier lock. GPS receivers must acquire each satellite independently before the solution engine can compute position, so total acquisition time often depends on whichever satellite is hardest to detect, not an average. Specifications differ between manufacturers and chipsets; a consumer smartphone receiver and a professional survey-grade receiver solve the same physical problem but with vastly different sensitivities and coherence strategies.