GNSS in Modern Surveying

GNSS has become the primary positioning tool for professional surveyors worldwide. Where traditional optical methods require line-of-sight between instruments and targets, GNSS receivers deliver centimetre-level coordinates independently at each point — enabling faster data collection across larger areas with fewer personnel.

Modern multi-frequency, multi-constellation receivers track GPS, GLONASS, Galileo, and BeiDou simultaneously, providing robust satellite geometry even in partially obstructed environments. Combined with RTK (Real-Time Kinematic) corrections, these receivers achieve horizontal accuracies of 8 mm + 1 ppm and vertical accuracies of 15 mm + 1 ppm — well within the requirements for boundary surveys, topographic mapping, construction stakeout, and as-built verification.

The shift from single-frequency to multi-frequency receivers has also expanded the practical RTK baseline length from 10-15 km to 40+ km, reducing the density of base stations needed and making GNSS survey practical in areas that previously required network RTK or post-processing.

RTK Survey Workflows

An RTK survey uses real-time correction data from a known reference position to resolve the carrier phase ambiguities in the rover receiver's measurements. The result is centimetre-level positioning updated at rates of 1-20 Hz.

Base-Rover Operation

The classic RTK workflow uses a base station (a receiver set up over a known point or allowed to self-survey) transmitting corrections to one or more rovers via radio or cellular link. The rover combines its own satellite observations with the base corrections to compute a fixed RTK solution.

Key parameters that affect RTK survey quality:

  • Baseline length — Accuracy degrades roughly 1 ppm per kilometre of baseline. At 10 km, this adds about 10 mm to the horizontal uncertainty. Multi-frequency receivers maintain fix reliability at baselines up to 40 km.
  • Occupation time — A single RTK observation takes 1-5 seconds once the receiver has a fixed solution. For higher confidence, surveyors may average 5-15 seconds per point.
  • Initialization time — Modern multi-frequency receivers typically achieve a fixed RTK solution within 10-30 seconds of receiving corrections. Re-initialization after a brief signal outage is usually faster.
  • PDOP threshold — Most survey software rejects observations when PDOP exceeds 4-6, ensuring adequate satellite geometry for reliable results.

Stakeout & Layout

For construction stakeout, the RTK rover guides the surveyor to pre-computed design coordinates. The receiver displays the real-time offset (north, east, elevation) from the target point, allowing the operator to mark the ground when the offset falls within tolerance — typically ±2 cm horizontal for building layout and ±1 cm vertical for grade work.

Base Station Configuration

Setting up a reliable base station is critical to RTK survey quality. The base receiver must maintain a stable, uninterrupted view of the sky throughout the survey session, and its coordinates must be accurately known.

Self-Survey vs. Known Point

  • Known control point — The base is set up over a monumented control point with published coordinates. This provides the highest absolute accuracy because the base position error directly transfers to every rover observation.
  • Self-survey (autonomous) — The base averages its autonomous position over a period (typically 5-20 minutes) and uses that as its reference coordinate. Relative accuracy between rover points is preserved, but the absolute position of the entire survey may be offset by 1-3 metres unless corrected later.
  • UHF/VHF radio — Dedicated radio modems (e.g., SATEL EASy Pro) provide reliable, low-latency correction delivery without cellular coverage dependencies. Range varies from 5-35 km depending on power, antenna height, and terrain.
  • NTRIP over cellular — Network RTK services deliver corrections via the internet, eliminating the need for a physical base station. This requires cellular data coverage at the rover location.

PPK Post-Processing

Post-Processed Kinematic (PPK) survey collects raw satellite observations at both the base and rover, then processes them together after the fact using desktop software. PPK offers several advantages over RTK:

  • No real-time radio link required — The rover operates independently, logging raw data. This is valuable in areas with radio interference or very long baselines.
  • Backward-forward processing — PPK software processes the data in both directions (forward and reverse in time), then combines the solutions. This resolves ambiguities that the real-time RTK engine could not fix, improving the percentage of fixed solutions.
  • Longer baselines — PPK can produce reliable results at baselines of 50+ km where real-time RTK would struggle with fix reliability.
  • UAV survey — PPK is the standard processing mode for drone-based survey, where the UAV logs raw GNSS data alongside camera triggers, and the base station logs simultaneously on the ground.

The trade-off is that PPK results are not available in the field — the surveyor cannot verify solution quality until back at the office.

GIS Data Collection

Geographic Information System (GIS) data collection uses GNSS receivers to capture the spatial coordinates of features (points, lines, polygons) along with attribute data. Accuracy requirements vary by application:

  • Utility mapping — Sub-metre to decimetre accuracy for locating underground utilities, poles, and infrastructure.
  • Asset inventory — Metre-level accuracy for cataloging signs, hydrants, trees, and other assets.
  • Cadastral/boundary — Centimetre-level RTK accuracy for legal boundary surveys and property corner monumentation.
  • Environmental monitoring — Decimetre to metre accuracy for wetland delineation, habitat mapping, and sample point locations.

Smart antennas like the S631 and C631 combine a multi-frequency receiver, antenna, and Bluetooth radio in a single unit that pairs directly with field data collection software on a tablet or phone — eliminating external cables and reducing setup time.

Correction Sources & Network RTK

Surveyors today have multiple correction source options beyond a dedicated base station:

Network RTK (VRS/MAC)

Network RTK services operate dense networks of continuously operating reference stations (CORS) and model the spatially-varying errors (ionospheric, tropospheric, orbital) across the network. The rover connects via cellular data (NTRIP protocol) and receives a virtual reference station correction tailored to its approximate location. This eliminates the need for a physical base station and provides consistent accuracy across the network coverage area.

Atlas Corrections (PPP)

Hemisphere GNSS Atlas is a satellite-delivered PPP correction service that provides decimetre to centimetre accuracy without a base station or cellular connection. Atlas corrections are received via L-band satellite, making them available anywhere with a clear sky view. Convergence time to full accuracy is typically 15-30 minutes, compared to the near-instant fix of RTK.

Atlas is particularly valuable for surveyors working in remote areas without cellular coverage or existing CORS infrastructure. The R632 receiver supports both RTK and Atlas corrections, giving surveyors maximum flexibility.

RTK + PPP Hybrid

Some survey workflows combine RTK (for fast, centimetre-level results near a base or network) with Atlas PPP (for operations beyond the RTK correction range). The receiver seamlessly switches between correction sources based on availability and quality.

Receivers

  • R632 GNSS Receiver — Multi-frequency, multi-constellation receiver with RTK and Atlas PPP support. Designed for survey rovers and base stations with support for long baselines and fast initialization.

Smart Antennas

  • Vector V500 GNSS Smart Antenna — High-performance smart antenna with integrated multi-frequency receiver, ideal for RTK survey rovers requiring maximum satellite tracking performance.
  • A631 GNSS Smart Antenna — Compact smart antenna combining receiver, antenna, and communications in a single housing for efficient field survey.
  • C631 GNSS Smart Antenna — Cost-effective smart antenna for GIS data collection and sub-metre to centimetre survey applications.
  • S631 GNSS Smart Antenna — Versatile smart antenna with Bluetooth connectivity for tablet-based GIS workflows.
  • SATEL EASy Pro Radio Modem — Reliable UHF/VHF radio modem for base-to-rover RTK correction delivery at ranges up to 35 km, with no cellular dependency.

Integration Considerations

For survey applications, key selection criteria include:

  • Number of frequencies and constellations — More frequencies and constellations provide faster initialization, better fix reliability in partially obstructed environments, and longer practical baselines.
  • Atlas PPP support — Critical for remote-area survey where cellular or radio correction delivery is impractical.
  • Raw data logging — Required for PPK post-processing workflows, especially UAV survey.
  • Update rate — 1-5 Hz is adequate for static survey; 10-20 Hz may be preferred for mobile mapping or machine control.
  • Communication interfaces — Serial, USB, Bluetooth, and Ethernet options for integration with survey controllers, tablets, and data collectors.