GPS/GNSS Receiver FAQ for Construction Contractors

GPS vs GNSS: What the Terms Mean

GPS (Global Positioning System) is the US satellite constellation. GNSS (Global Navigation Satellite System) is the umbrella term for all satellite constellations: GPS (US), GLONASS (Russia), Galileo (EU), BeiDou (China), and QZSS (Japan). Modern construction receivers track all available constellations simultaneously — more satellites means faster initialization, better accuracy, and improved performance under tree cover or near buildings.

When a manufacturer says "GNSS receiver," they mean a receiver capable of tracking multiple constellations. All professional construction receivers sold today are multi-constellation GNSS, not GPS-only.

RTK Corrections and Network Options

RTK (Real-Time Kinematic) is the correction technology that takes GNSS from 1-3 meter autonomous accuracy to centimeter-level accuracy. RTK corrections can come from a base station you set up on a known point, or from a network of reference stations (VRS network) accessed via cellular internet using the NTRIP protocol. Network RTK removes the need to set up and maintain a base station but requires a cellular data subscription. See the RTK GPS guide for a full explanation.

Frequently Asked Questions

What is the accuracy of a construction GPS receiver?

RTK GNSS receivers achieve 8-15mm (0.03-0.06 ft) horizontal and 15-20mm (0.05-0.08 ft) vertical accuracy in good conditions with a valid RTK fix. Autonomous (no correction) accuracy is 1-3 meters. SBAS-corrected accuracy (WAAS/EGNOS) is 0.3-1 meter. Only RTK provides construction-grade accuracy. See the GPS accuracy FAQ for application-specific tolerances.

What is the difference between a base station and network RTK?

A base station is a second GNSS receiver you set up on a known control point on your project. It broadcasts corrections to your rover via UHF radio. Network RTK (VRS) uses a regional network of continuously operating reference stations and delivers corrections via cellular data — no base station required. Network RTK costs a monthly subscription ($50-150/month per rover) but eliminates setup time and base station equipment costs.

What is a GNSS rover?

The rover is the field receiver carried by the operator. It receives satellite signals plus RTK corrections (from base or network), computes its position in real time, and displays coordinates, cut/fill, and stakeout directions on a connected data collector. The rover is the instrument you use to locate points and collect data in the field.

How long does it take for a GPS receiver to get an RTK fix?

Initialization (acquiring an RTK fixed solution) typically takes 30 seconds to 3 minutes depending on satellite geometry, number of available signals, and correction latency. Multi-constellation receivers (tracking GPS+GLONASS+Galileo+BeiDou) initialize faster than GPS-only receivers. If initialization takes more than 5 minutes, check your correction source and satellite count.

What is multipath error in GNSS?

Multipath occurs when satellite signals reflect off buildings, vehicles, or terrain before reaching the antenna, causing position errors. Multipath is the main cause of poor GPS accuracy near structures. Modern receivers use signal processing and antenna design to mitigate multipath, but no receiver is immune. When working near buildings, use a total station for the precision work close to the structure and GPS for open-area layout.

What satellite constellations should my receiver track?

For construction use, a receiver tracking GPS + GLONASS + Galileo is the minimum. BeiDou support adds additional satellites that improve performance in weak signal environments and at low elevations. The Trimble R12i, Topcon HiPer VR, and Leica GS18 all track all major constellations and are the current standard for construction layout.

What is NTRIP and how do I use it?

NTRIP (Networked Transport of RTCM via Internet Protocol) is the standard protocol for delivering RTK corrections over the internet. Your data collector connects to an NTRIP caster (server) via cellular data, selects your nearest reference station or VRS mountpoint, and streams corrections to the rover. You need a cellular data plan for the data collector and an account with an NTRIP service provider.

Can I use GNSS inside buildings or in tunnels?

No — GNSS requires a clear view of the sky. Inside buildings, tunnels, or anywhere satellite signals are blocked, GNSS does not work. Use a total station or optical layout for interior work. Some systems combine GNSS with inertial measurement units (IMU) to maintain position briefly during signal loss, but these are not substitutes for a clear sky view.

What is tilt compensation in a GNSS rover?

Tilt compensation uses an IMU (inertial measurement unit) inside the receiver to correct for rod tilt. Without tilt compensation, the pole must be held perfectly vertical to get an accurate measurement — difficult in the field. With tilt compensation active (Trimble R12i, Topcon HiPer VR, Leica GS18 T), you can collect points with the pole tilted up to 30-60 degrees from vertical and still achieve centimeter-level accuracy.

What is the difference between RTK and PPK?

RTK (Real-Time Kinematic) computes corrected positions in the field in real time. PPK (Post-Processed Kinematic) logs raw data in the field and computes corrections in the office after the fact. RTK is used for layout and stakeout where you need positions immediately. PPK is used for drone mapping and survey where the data is processed later — it can achieve higher accuracy by selecting the best correction data after the fact.

How do I set up a GNSS base station on a construction site?

Set up the base receiver on a tripod over a known control point, enter the point coordinates, and power on. The base begins broadcasting corrections to rovers via UHF radio. Ensure the base has a clear view of the sky (no overhanging structures or trees within 15 degrees of the horizon) and a stable power supply. For sites larger than 5 km diameter, consider network RTK instead of a single base station.

What is the maximum range from a GNSS base station?

UHF radios on standard construction GNSS systems reach 5-15 km line-of-sight. In hilly terrain or with obstructions, effective range drops to 2-5 km. For larger sites or areas with terrain obstruction, network RTK via cellular eliminates range limitations. Higher-power radios are available for extended base-to-rover range.

What causes a GPS receiver to lose its RTK fix?

Common causes: moving into an area with obstructed sky view (under trees, near buildings), the base station losing power or connection, cellular data interruption for network RTK, or severe multipath interference. Most receivers reinitialize in 30-60 seconds when a clear sky view is restored. If fixes are unreliable, check your correction source and satellite count before collecting critical data.

What is the elevation mask setting on a GNSS receiver?

The elevation mask is the minimum satellite elevation angle (above the horizon) that the receiver uses. A 10-15 degree mask is standard — satellites below this angle have longer signal paths through the atmosphere and are more prone to multipath. Lowering the mask below 10 degrees can add more satellites but often degrades accuracy; raising it above 20 degrees reduces the satellite count unnecessarily.

How do I verify GNSS accuracy on a construction site?

Check your GNSS receiver against at least two known control points at the start of each day. The difference between your measured position and the known coordinates should be within your required tolerance (typically 0.05-0.1 ft for construction layout). If errors are larger, check your correction source, initialization status, and control point coordinates before beginning work.

What data collector do I need with a GNSS receiver?

Most professional GNSS receivers use the manufacturer's data collector with matching field software: Trimble TSC7 with Trimble Access, Topcon FC-6000 with Magnet Field, or Leica CS20 with Leica Captivate. Third-party data collectors with compatible Bluetooth connections and GNSS software (Survey Master, FieldGenius) can also work with most receivers via standard NMEA output.