How accurate is GPS? GPS accuracy from metres to millimetre-level RTK
Key takeaways
- Standalone GPS gives metre-level positions; centimetre accuracy needs carrier-phase corrections such as RTK.
- Accuracy is closeness to the true position; precision is how tightly repeated measurements cluster.
- Atmospheric delay, orbit and clock errors, multipath and satellite geometry are the main error sources.
- RTK accuracy is written as a fixed part plus parts per million of the baseline, so it degrades slowly with distance from the base.
- Tracking several constellations and frequencies improves availability and reliability more than any single setting.
How accurate is GPS?
GPS accuracy ranges from several metres for a phone to a few millimetres for a geodetic receiver, depending on the signals tracked and the corrections applied. The satellites are the same for everyone; the difference lies in the receiver, antenna and processing.
The Global Positioning System is the US satellite navigation system, one of several global navigation satellite systems (GNSS) alongside GLONASS, Galileo and BeiDou. A modern survey receiver uses all of them, which is why surveyors often say GNSS rather than GPS; our article on GNSS vs GPS explains the difference.
For a typical smartphone under open sky, horizontal accuracy is usually around 3–5 m. Near tall buildings or under trees it can degrade to tens of metres, because signals are blocked or reflected.
GPS accuracy levels compared: from metres to millimetres
Each step up in positioning method removes more error sources. The table shows typical horizontal accuracies under good conditions; actual results vary with environment and equipment.
| Method | How it works | Typical horizontal accuracy | Typical users |
|---|---|---|---|
| Standalone GPS / GNSS | Code ranges from satellites only | 3–5 m | Phones, car navigation |
| SBAS | Wide-area corrections from geostationary satellites | 1–2 m | Aviation, basic mapping |
| Differential GPS (DGPS) | Code corrections from a reference station | 0.3–1 m (sub-metre) | GIS, marine, asset mapping |
| PPP | Precise orbits and clocks, carrier phase, single receiver | Decimetres to a few cm after convergence | Offshore, remote areas, agriculture |
| RTK | Carrier-phase corrections from a nearby base or network | About 1 cm + 1 ppm | Surveying, construction, machine control |
| Static post-processing | Hours of carrier-phase data, processed later | A few mm | Control networks, geodesy |
Sub meter GPS is enough for mapping trees, poles or land parcels for inventory. Centimeter accurate GPS is needed for boundaries, design surfaces and stakeout. See handheld GPS for surveying and GIS for how these grades map to equipment.
GPS precision vs accuracy: what is the difference?
Accuracy is how close a measured position is to the true position; precision is how closely repeated measurements agree with each other. A receiver can be precise but inaccurate if all its readings cluster tightly around the wrong point.
This matters when reading specifications. A receiver set to the wrong datum or with an incorrect antenna height can give beautifully repeatable, consistently wrong coordinates. Checking against a known control point is the only way to prove accuracy.
Specifications also use different statistics. RMS (root mean square) describes about 63–68 % of results depending on dimension, CEP (circular error probable) contains 50 %, and a 95 % figure is roughly twice RMS. Comparing a CEP value with a 95 % value can make one receiver look twice as good as another when it is not.
What limits global positioning system accuracy?
Global positioning system accuracy is limited by delays and errors that affect the signal between satellite and antenna. Corrections exist for most of them; multipath and obstruction remain the hardest to remove.
- Ionospheric delay: charged particles in the upper atmosphere slow the signal. Dual- and triple-frequency receivers measure and remove most of it.
- Tropospheric delay: water vapour and air pressure in the lower atmosphere add delay, modelled by the receiver and largely cancelled by RTK over short baselines.
- Orbit and clock errors: small errors in broadcast satellite positions and clocks, removed by differential corrections or precise products.
- Multipath: signals reflected off buildings, vehicles or water reach the antenna by a longer path. Good antennas and open sites reduce it.
- Satellite geometry: measured as dilution of precision (DOP). Satellites spread across the sky give low DOP and better positions.
- Receiver noise: carrier-phase measurements are around a hundred times less noisy than code measurements, which is why phase-based methods reach centimetres.
Differential GPS vs RTK: how corrections improve accuracy
Both differential GPS and RTK compare measurements at a known reference station with those at a roving receiver to cancel shared errors. The difference is what they measure: DGPS corrects code ranges, while RTK uses the carrier phase of the signal.
| Aspect | DGPS | RTK |
|---|---|---|
| Measurement used | Code pseudorange | Carrier phase plus code |
| Typical accuracy | Sub-metre | 1–2 cm |
| Typical baseline range | Up to hundreds of km | Up to about 10–30 km from base, or within a network |
| Initialisation | None | Seconds, to resolve carrier ambiguities |
| Receiver cost | Lower | Higher |
In the DGPS vs RTK choice, RTK wins wherever centimetres matter, but needs a correction link by radio or internet. Corrections can come from your own base station or from a CORS network over NTRIP, as described in NTRIP and CORS network RTK. The full method is covered in what is RTK GPS.
RTK GPS accuracy: what "8 mm + 1 ppm" means
RTK GPS accuracy is quoted as a fixed error plus a distance-dependent error in parts per million (ppm) of the baseline. One ppm equals 1 mm for every kilometre between rover and base.
Take a horizontal specification of 8 mm + 1 ppm, which is the published value for our P2 handheld GIS RTK rover:
- At 2 km from the base: 8 + 2 = 10 mm
- At 10 km: 8 + 10 = 18 mm
- At 20 km: 8 + 20 = 28 mm
Vertical RTK accuracy is typically about twice the horizontal figure, because satellites are only above the antenna, never below. These are RMS values under good sky view with a fixed solution. A float solution, where ambiguities are not yet resolved, is typically decimetre-level, so always check that the controller shows FIXED. You can test baseline scenarios with our RTK accuracy calculator.
How to improve location accuracy in the field
Most accuracy problems in the field come from site conditions and setup, not the receiver. These steps typically give the largest gains.
- Use multi-constellation, multi-frequency tracking. More satellites and signals mean better geometry and faster, more reliable fixes.
- Keep baselines short. Place the base close to the work or use a dense CORS network.
- Set an elevation mask of about 10–15° to reject low, noisy satellites.
- Avoid multipath: keep away from metal walls, vehicles and fences where possible.
- Check DOP and solution status before storing a point.
- Enter antenna height correctly, including the measurement reference.
- Occupy longer and average for control points, and re-measure them at a different time of day.
- Check a known point at the start and end of each session.
Choosing a high precision GNSS receiver for surveying
A high precision GPS receiver for surveying should deliver a reliable RTK fix with all constellations, support your correction source and survive field conditions. Match the receiver class to the accuracy your deliverables require.
For base and rover work, the E1 GNSS RTK receiver tracks full-constellation signals with 1408 channels, adds IMU tilt up to 60° and runs up to 20 hours on its battery. Where a CORS network is available, the 409 g Mini Network RTK Rover works as a pocket rover. For GIS mapping, the 199 g P2 handheld offers 8 mm + 1 ppm horizontal accuracy over network RTK.
Mars Geosystems ships with an MOQ of one unit and offers OEM branding from 10 units. To discuss accuracy needs for your projects, contact us through the quote page.
Frequently asked questions
How accurate is GPS on a phone?
Under open sky, smartphone GPS is typically accurate to about 3–5 m horizontally. Accuracy drops near tall buildings, under dense trees and indoors, where signals are blocked or reflected, and errors of tens of metres are possible. Some newer phones track dual-frequency signals, which helps, but they still cannot match survey-grade RTK receivers.
What is the difference between DGPS and RTK?
DGPS corrects code-based ranges using a reference station and typically achieves sub-metre accuracy over long distances. RTK uses the much more precise carrier phase of the signal, resolves its whole-cycle ambiguities and achieves around 1–2 cm, but needs a base or network within roughly 10–30 km and a continuous correction link.
How accurate is RTK GPS?
RTK GPS is typically accurate to about 8–10 mm plus 1 ppm horizontally and 15–20 mm plus 1 ppm vertically with a fixed solution and good sky view. One ppm adds 1 mm per kilometre of distance from the base. Obstructions, multipath and long baselines reduce accuracy, and float solutions are only decimetre-level.
What is sub meter GPS used for?
Sub-metre GPS, typically from SBAS or DGPS corrections, is used where features must be located to within about a metre: GIS asset inventories, utility pole and valve mapping, forestry, environmental sampling and agricultural field mapping. It is not suitable for boundaries, construction layout or design surveys, which need centimetre-level RTK accuracy.
Why is GPS height less accurate than horizontal position?
All satellites are above the antenna, so their geometry constrains height less well than horizontal position. Atmospheric delay also correlates with height. As a result, vertical GNSS accuracy is typically about one and a half to two times worse than horizontal. Surveyors check heights against benchmarks or use levelling for tight vertical tolerances.
What is the most accurate GPS for surveying?
The most accurate GPS for surveying is a multi-frequency, multi-constellation GNSS receiver using RTK for real-time work or static post-processing for control. RTK typically gives about 1 cm horizontal; static sessions of an hour or more on short baselines can reach a few millimetres. A geodetic-grade antenna further reduces multipath for base stations.
Need equipment for this work?
Tell us the job, the country and the quantity. An engineer replies with a configuration and price within one business day. MOQ is one unit; OEM is available.
