RTK GPS Survey FAQ: Accuracy, Setup, and Troubleshooting

How RTK Works

Standard GPS (autonomous) achieves 3-5 meter accuracy because atmospheric delays and satellite orbit errors corrupt the ranging measurements. RTK corrects these errors in real time using a base station receiver at a precisely known coordinate. The base computes the error at its location and transmits a correction message to the rover via radio or cellular link. The rover applies the correction to its own measurements, resolving position to centimeter accuracy. This correction process is called differential GNSS; the "kinematic" part means it works while the rover is moving.

The key step in RTK initialization is resolving the integer ambiguity in the carrier phase measurement — called "fixing the ambiguity." Until the ambiguity is fixed, the receiver is in "float" mode with degraded accuracy. Fixed RTK typically initializes within 30 seconds to 2 minutes in good conditions. Always wait for a fixed solution before recording measurements or beginning layout work.

Frequently Asked Questions

What accuracy does RTK GPS achieve?

Manufacturer-specified RTK accuracy is typically 8-10mm horizontal + 1ppm and 15-20mm vertical + 1ppm. In real-world construction conditions, expect 10-25mm horizontal and 15-35mm vertical accounting for satellite geometry, multipath, and atmospheric conditions. RTK is appropriate for earthwork stakeout, road alignment, and utility layout. For structural layout requiring 3-6mm accuracy, use a total station.

What is the difference between GPS, GNSS, and RTK?

GPS refers specifically to the United States satellite navigation system. GNSS (Global Navigation Satellite System) is the umbrella term for all satellite systems: GPS (US), GLONASS (Russia), Galileo (EU), and BeiDou (China). Modern survey receivers are multi-constellation GNSS devices. RTK describes the real-time differential correction technique applied to GNSS data — it is not a satellite system itself, but a method for achieving centimeter accuracy from GNSS measurements.

How do I set up an RTK base station?

Set the base receiver on a tripod directly over a known control point. Measure the antenna height from the control point monument to the antenna reference point (ARP) precisely — height errors translate directly to vertical errors in all rover measurements. Enter the control point coordinates in the base software. Set up the radio or cellular correction link. Confirm the base is transmitting corrections before moving to the rover. Never disturb the base during a survey session — any shift resets the initialization.

How far can I work from my base station?

RTK accuracy degrades with distance from base. Most receivers specify 1ppm distance-dependent error — approximately 1mm per km of base-to-rover separation. Practical limits for survey-grade RTK work are 5-10 km. Beyond 10 km, consider a network RTK service or move the base station closer to your work area. For machine control, tighter limits apply because vertical accuracy is more critical.

What is network RTK and how is it different from using my own base station?

Network RTK uses a regional network of continuously operating reference stations (CORS) to generate a virtual reference station (VRS) correction near your rover's location. Corrections are delivered over cellular data. The advantages: no base station setup, no radio link required, no base station disturbance risk, and consistent accuracy across the network coverage area. The disadvantages: requires cellular coverage at the job site, subscription cost ($50-150/month per rover), and no fallback if cellular data drops.

What is PDOP and how does it affect RTK accuracy?

PDOP (Position Dilution of Precision) measures the geometric spread of satellites in the sky — lower PDOP means better satellite geometry and better accuracy. A PDOP below 2.0 is excellent; below 3.0 is acceptable for most survey work; above 4.0 degrades accuracy significantly. Avoid critical measurements during high PDOP events (typically caused by satellites clustered in one part of the sky). Modern multi-constellation receivers rarely exceed PDOP 2.0 in open conditions.

Why won't my RTK fix initialize?

Common initialization failures: no correction signal from base (radio link down, cellular data off, wrong NTRIP credentials), too few satellites visible (under tree canopy or near structures), high PDOP, multipath interference, or a base station set on an incorrect control point. Check the correction age display — correction age above 5-10 seconds indicates the link is degraded or down. Move to a more open location and verify the correction link before attempting to initialize.

What is multipath and how does it degrade RTK?

Multipath occurs when satellite signals reflect off buildings, equipment, or terrain before reaching the antenna — the reflected signal interferes with the direct signal and introduces ranging errors. Multipath is the dominant source of RTK error in urban and industrial environments. Symptoms: fixed RTK solutions that drift or jump, inability to initialize near structures, and large residuals in the solution. Mitigation: use a choke ring antenna or survey antenna with ground plane, move away from reflective surfaces, and avoid measuring near large metal objects.

What is the difference between L1-only and L1/L2 dual-frequency receivers?

L1/L2 dual-frequency receivers measure two carrier frequencies simultaneously, allowing them to model and eliminate ionospheric delay — a major error source, especially over longer baselines. Dual-frequency receivers initialize faster (often under 30 seconds vs several minutes for L1-only), maintain fix longer in challenging conditions, and provide better accuracy at distances beyond 10 km. L1-only receivers are adequate for short-baseline RTK (under 10 km, open sky) at significantly lower cost. For professional survey and machine control, dual-frequency is the standard.

What is the antenna height and why is it critical?

The antenna height is the vertical distance from the survey point (nail, pin, or monument) to the antenna reference point (ARP) of the GNSS antenna. The receiver subtracts the antenna height to compute the ground point elevation. A 10mm error in measured antenna height translates directly to 10mm of vertical error in every measurement. Always measure and re-enter antenna height after moving the pole or changing antenna configurations. For fixed-height poles, use the same pole extension consistently.

What is a CORS network and can I use it for free?

CORS (Continuously Operating Reference Stations) networks are arrays of permanently mounted GNSS receivers operated by government agencies (NGS, state DOTs, USGS) and commercial providers. Many state DOT networks provide free or low-cost RTK corrections to in-state users. The NGS OPUS service uses CORS data for post-processing (not real-time RTK). Check your state DOT website for available free network RTK services before purchasing a commercial subscription.

How do I check my RTK solution is correct?

After setting up, measure a second known control point with the rover and compare the measured coordinates to the known values. Horizontal discrepancy within 0.04 feet (12mm) is typically acceptable for construction survey. Vertical discrepancy within 0.05 feet (15mm) is typical. If discrepancies are larger, check the base station setup, antenna height entry, and site calibration before proceeding with measurements.

What GNSS equipment do I need for RTK survey?

A complete RTK kit includes: two receivers (base and rover) with GNSS antennas, a UHF radio set or cellular modems for corrections, a data collector with field software, tripod and tribrach for the base, and a survey pole with bipod for the rover. For network RTK, you need only the rover receiver and a cellular data connection — no base station is required. See the GNSS receiver FAQ for receiver selection guidance.

What is NTRIP and how do I connect to a correction network?

NTRIP (Networked Transport of RTCM via Internet Protocol) is the standard protocol for delivering RTK corrections over cellular data. To connect, you need: the NTRIP caster address (URL or IP), port number, and login credentials from your correction service provider. Enter these in your rover's field software correction settings. The rover connects over cellular, selects the nearest virtual reference station, and begins receiving corrections. Most modern GNSS field software (Trimble Access, Topcon Magnet Field) includes built-in NTRIP client configuration.

What is the difference between RTK and post-processed GNSS?

RTK provides real-time centimeter positioning — you see corrected coordinates in the field instantly. Post-processed GNSS collects raw GNSS data in the field and processes it back in the office against CORS or base station data using software (Trimble Business Center, Topcon Magnet Office). Post-processing can achieve higher accuracy than RTK by using longer observation times and redundant data. It is used for control point establishment, boundary surveys, and projects requiring highest accuracy — not practical for stakeout or machine control where real-time positions are required.

What is the typical battery life of an RTK rover?

Professional RTK rovers (Trimble R10, Topcon HiPer VR, Leica GS18) provide 6-10 hours of continuous operation on a single battery charge. Running the cellular modem continuously consumes significantly more power than radio link operation. Carry at least one spare battery and a charger for full-day field work. In cold weather, battery capacity drops — keep batteries warm and plan for shorter intervals between swaps.