A-GPS (Assisted GPS) speeds up position fixes by downloading satellite orbit data over the mobile network instead of waiting to receive it from the satellites themselves.
A GPS receiver starting from scratch has a slow problem to solve. Before it can calculate a position it needs to know where the satellites are, and that information — the ephemeris — is broadcast by the satellites themselves at 50 bits per second. Receiving it takes 30 to 60 seconds per satellite under perfect conditions, and you need at least four.
A cold start can therefore take several minutes, and any interruption to the signal restarts the download. Under trees, or in a moving vehicle, it may never complete.
A-GPS solves this by fetching the same data over the mobile network in a fraction of a second.
| Start type | What the receiver already knows | Time to fix |
|---|---|---|
| Hot | Valid ephemeris, recent position, accurate time | 1–5 s |
| Warm | Almanac and rough position, stale ephemeris | 20–40 s |
| Cold | Nothing useful | 1–5 min, sometimes never |
| A-GPS | Everything, downloaded over the network | 1–5 s |
The difference between a device that reports its position seconds after powering on and one that sits blank for four minutes is almost always A-GPS.
Three practical consequences.
Battery. Keeping the GPS receiver powered while it hunts for satellites is expensive. A tracker that fixes in 3 seconds instead of 90 uses a fraction of the energy per report. On a battery-powered tracker reporting every few minutes, this is the difference between weeks and days of runtime.
First fix after a gap. A tracker that has been in a garage overnight, or a phone that had location disabled, is doing a cold start. Without assistance, the first report after the gap may be minutes late — precisely when you most want to know where something is.
Indoor and marginal conditions. A-GPS supplies the ephemeris so the receiver only needs to measure signals rather than decode them, which it can do at much weaker signal strengths. This is why phones sometimes get a position indoors that a standalone receiver cannot.
A-GPS needs a working data connection. On a hardware tracker that means the SIM must have data — not just SMS — and the APN must be configured correctly.
This is a common failure mode on budget trackers. The SIM works, SMS commands are accepted, the LEDs look right, but the APN was never set. The device falls back to unassisted GPS and takes minutes to fix, or falls back further to cell tower positioning and reports a position accurate to several hundred metres. From the outside this looks like “the GPS is broken” when it is really “there is no data connection”.
If a tracker reports slowly and inaccurately, check the APN before concluding the hardware is faulty.
Worth being precise about this, because it is widely misunderstood.
A-GPS makes fixes faster and lets them happen in weaker signal conditions. Once a receiver has a proper satellite fix, the accuracy is the same whether it was assisted or not — the satellites and the maths are identical.
What A-GPS does improve indirectly is effective accuracy over time. A device that fixes quickly reports a real satellite position; a device that fixes slowly may report a cell-tower estimate in the meantime, or nothing at all. So assistance produces better data in practice, not because each fix is better but because more of them are genuine fixes. For what determines the quality of an individual fix, see GPS accuracy.
A related distinction. GPS is the American satellite constellation. GNSS is the general term covering GPS plus GLONASS (Russia), Galileo (Europe), and BeiDou (China).
A receiver using several constellations sees far more satellites, which improves geometry and helps considerably in cities where buildings block much of the sky. Most phones and mid-range trackers are multi-constellation now; the cheapest 2G units are often GPS-only, which is one reason they perform worse in urban environments.
Last reviewed 7 August 2026.
GPS accuracy is how close a reported position is to the true location, usually expressed as a radius within which the real position probably falls.
LBS positioning estimates a device's location from nearby mobile network towers rather than satellites — far less accurate than GPS, but it works indoors and needs no fix.
GPS drift is the apparent movement of a stationary device caused by errors in its reported position rather than by any real change in location.
An APN (Access Point Name) is the setting that tells a SIM-equipped device which gateway to use for mobile data — and the most common reason a GPS tracker never reports.
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