Survey Methods (Static/RTK/PPK)

Overview#

The choice of GNSS survey method depends on the required accuracy, the available time, the baseline length, and the project's real-time requirements. At one end of the spectrum, static surveying provides sub-centimeter accuracy through long occupations and post-processing. At the other end, RTK delivers centimeter-level positions in real time with occupation times measured in seconds. Between these extremes lie rapid-static, PPK, and Network RTK methods, each with distinct trade-offs.

No single method is optimal for all situations. A surveyor establishing primary geodetic control will use static methods. A surveyor staking lot corners on a subdivision will use RTK. A surveyor collecting topographic data for a road design may use either RTK or PPK depending on cellular coverage. Understanding the strengths, limitations, and appropriate applications of each method is essential for selecting the right tool for the job.

"The choice of GPS survey method should be driven by the accuracy requirements of the project, not by convenience or habit. Using RTK where static is warranted, or static where RTK is sufficient, is equally poor practice." -- Van Sickle, GPS for Land Surveyors (4th Ed.), Ch. 9, p. 195

Static Surveying#

Principles

Static surveying is the most accurate GNSS method. Two or more receivers simultaneously occupy fixed points for an extended period -- typically 1 to 4 hours or more, depending on baseline length and required accuracy. The data are post-processed to resolve carrier phase integer ambiguities and compute precise baseline vectors.

The strength of static surveying comes from the accumulation of carrier phase data over a long time span. As the satellite geometry changes throughout the observation session, the solution geometry improves, ambiguity resolution becomes more reliable, and the effects of multipath average out (especially if the session spans a significant fraction of the sidereal repeat period).

Observation Guidelines

These are general guidelines; actual occupation times depend on the number of satellites, PDOP, ionospheric conditions, and the processing software's ability to resolve ambiguities. During periods of high ionospheric activity, session lengths should be increased.

Applications

• Establishing and densifying geodetic control networks
• Deformation monitoring (structural, tectonic, landslide)
• Connecting to CORS (Continuously Operating Reference Stations)
• High-precision baseline measurements for engineering projects
• Calibrating local site transformations

"Static GPS is the benchmark against which all other GPS methods are compared. When the highest accuracy is required and time permits, static observation is the preferred method." -- Ghilani & Wolf, Elementary Surveying (13th Ed.), Ch. 14, p. 412

Processing Workflow

1. Download RINEX data from all receivers
2. Obtain base station coordinates (CORS or known control)
3. Process baselines independently (forward and reverse)
4. Evaluate solution quality (fixed vs. float, RMS, ratio test)
5. Perform network adjustment (minimally constrained, then fully constrained)
6. Compare adjusted coordinates against independent checks

Rapid-Static Surveying#

Principles

Rapid-static surveying is a modified static technique that achieves centimeter-level accuracy with significantly shorter occupation times -- typically 5 to 20 minutes per point. The method relies on modern ambiguity resolution algorithms (notably LAMBDA) that can fix integer ambiguities with fewer observations, particularly when dual-frequency or triple-frequency data are available.

Rapid-static requires a reference receiver continuously logging at a base station while a rover receiver occupies each project point for the minimum session duration. The data are post-processed after the field work is complete.

When to Use Rapid-Static

Rapid-static is appropriate when:

• The project requires better accuracy than RTK but does not justify full static occupation times
• Dozens of points must be established in a single day (e.g., densifying project control)
• Real-time communications (radio/cellular) are unavailable or unreliable
• Post-processing provides an opportunity for more rigorous quality control

Limitations

• Occupation times increase in poor satellite geometry or high ionospheric activity
• Ambiguity resolution is less reliable than long static sessions; the surveyor must verify that a fixed solution was achieved in post-processing
• Not suitable for the highest-precision geodetic control work

RTK (Real-Time Kinematic)#

Principles

RTK is the workhorse of modern land surveying. It provides centimeter-level positions in real time by transmitting carrier phase corrections from a base station to a rover via radio or cellular link. The rover combines its own observations with the corrections to resolve integer ambiguities on the fly and compute a position within seconds.

The core requirement is a communication link between base and rover that delivers corrections with minimal latency (ideally < 1 second). The correction format is typically RTCM (Radio Technical Commission for Maritime Services), version 3.x.

"RTK GPS has fundamentally changed the practice of land surveying. Tasks that once required hours of conventional field work -- stakeout, topographic data collection, boundary location -- can now be accomplished in minutes with centimeter accuracy." -- Van Sickle, GPS for Land Surveyors (4th Ed.), Ch. 10, p. 225

RTK Setup

A basic RTK system consists of:

1. Base station. A GNSS receiver set up over a known point (or a point whose coordinates will be determined). It continuously tracks all available satellites and transmits raw observations or corrections to the rover.
2. Communication link. UHF radio (typical range 5--10 km, line-of-sight dependent) or cellular data connection (NTRIP -- Networked Transport of RTCM via Internet Protocol).
3. Rover. A GNSS receiver mounted on a survey pole. The rover receives corrections, resolves ambiguities, and displays the real-time position on a data collector.

Initialization

Before RTK can deliver centimeter-level positions, the receiver must resolve the integer ambiguities -- a process called initialization. Modern receivers typically achieve initialization (a "fixed" solution) within 10--30 seconds under good conditions. The initialization is lost if the rover loses lock on satellites (e.g., passing under dense tree canopy or a bridge), requiring re-initialization.

Accuracy and Limitations

The "1 ppm" component means accuracy degrades by 1 mm per kilometer of baseline length. At 10 km from the base, the ppm component contributes 10 mm to the horizontal error budget, doubling the total error.

RTK Best Practices

• Verify initialization. Always check the RTK solution on a known point before and after each work session. If the check exceeds tolerance, all work collected during that session is suspect.
• Monitor solution status. Only collect data when the solution shows "Fixed." Float solutions are not survey-grade for boundary or control work.
• Redundant observations. Occupy critical points twice, preferably with different satellite geometry (e.g., at different times of day). Compare the two positions; if they agree within tolerance, confidence is high.
• Base station QC. If using a project base (not a CORS network), ensure the base coordinates are correct. An error in the base coordinates propagates directly to every rover position.

PPK (Post-Processed Kinematic)#

Principles

PPK is conceptually identical to RTK except that corrections are not transmitted in real time. Both base and rover log raw observations simultaneously, and the data are processed after returning to the office. The rover can be operated in a kinematic mode (continuous data collection while moving) or a stop-and-go mode (pausing at each point of interest).

Advantages over RTK

• No communication link required. Eliminates radio/cellular infrastructure, simplifying logistics in remote areas.
• Forward and reverse processing. PPK software can process the data in both the forward and reverse time directions, providing two independent solutions that can be compared as a quality check. This is particularly valuable for detecting and repairing cycle slips.
• Use of precise ephemerides. Post-processing allows the use of IGS rapid or final orbit and clock products, improving accuracy for longer baselines.
• No real-time initialization dependency. Even if the rover experiences frequent cycle slips in the field (e.g., working in partial tree canopy), the software can resolve ambiguities after the fact using data from multiple epochs.

Disadvantages

• No real-time position feedback in the field -- the surveyor does not know the precise coordinates until post-processing
• Not suitable for stakeout or any task requiring real-time position information
• Additional office processing time

Applications

PPK is commonly used for UAV (drone) photogrammetry ground control, corridor surveys (pipelines, transmission lines), and any application where real-time communication is impractical but kinematic data collection is needed.

Network RTK#

Principles

Network RTK extends the single-base RTK concept by using a network of permanently installed reference stations (typically CORS) to generate corrections that are valid over a wide area. Instead of relying on a single base station -- whose corrections degrade with distance -- the network models the spatial variation of atmospheric and orbital errors across the region and provides the rover with corrections appropriate to its specific location.

"Network RTK effectively removes the baseline-length limitation of single-base RTK by modeling the spatial structure of the distance-dependent errors -- primarily ionospheric and tropospheric delays -- across the reference network." -- GEOG 862, GPS and GNSS for Geospatial Professionals, Penn State, Lesson 8

Common Network RTK Methods

All methods achieve similar accuracy. The primary differences are in bandwidth requirements, rover intelligence, and how the corrections are parameterized.

Accuracy and Coverage

Network RTK provides accuracy comparable to single-base RTK (1--2 cm horizontal, 2--3 cm vertical) but over distances of 50--70 km or more from the nearest reference station. The key requirement is that the rover must be within the network coverage area, and a reliable cellular data connection must be available.

Advantages

• No user-deployed base station required -- reduced setup time and equipment
• Consistent accuracy over the entire network coverage area
• Coordinates are on the network datum (typically NAD 83), eliminating the need for site calibration
• Automatic quality monitoring by the network operator

Limitations

• Requires cellular data coverage
• Subscription fees for commercial networks
• Dependent on reference station availability and health
• Quality degrades near the edges of the network or during ionospheric storms

PPP (Precise Point Positioning)#

Principles

Precise Point Positioning uses a single dual-frequency GNSS receiver with precise satellite orbit and clock products (from IGS or commercial services) to achieve centimeter to decimeter-level accuracy without a base station. Unlike differential methods, PPP does not rely on spatial correlation of errors -- it models and corrects each error source explicitly.

The PPP observation model uses the ionosphere-free combination of carrier phase and pseudorange observations:

$\Phi_{\text{IF}} = \rho + c \cdot dt_r + d_{\text{tropo}} + \lambda_{\text{IF}} N_{\text{IF}} + \epsilon$

$P_{\text{IF}} = \rho + c \cdot dt_r + d_{\text{tropo}} + \epsilon_P$

where satellite clock errors are removed using precise clock products rather than differencing.

Convergence

The principal limitation of PPP is convergence time -- the time required for the solution to stabilize at its full accuracy. Traditional PPP requires 20--40 minutes of continuous observation for the float solution to converge to centimeter-level accuracy. During convergence, the accuracy progressively improves from meter-level to decimeter and eventually centimeter.

PPP-AR (PPP with Ambiguity Resolution) reduces convergence time by fixing the carrier phase integer ambiguities, but this requires additional correction products (satellite phase biases) from the PPP service provider. With PPP-AR, convergence to centimeter-level can be achieved in 5--15 minutes.

Applications

PPP is particularly useful for:

• Surveys in remote areas far from any CORS or reference network
• Marine and offshore positioning
• Establishing initial coordinates for a project base station
• Single-receiver post-processing of static data

Method Comparison#

Choosing the Right Method#

The selection process should begin with the project's accuracy requirements and work backward to the appropriate method:

1. Sub-centimeter control (geodetic, deformation)? Static surveying with long occupations and precise ephemerides.
2. Centimeter-level control (project control, boundary)? Rapid-static or static with shorter sessions. RTK with redundant observations and independent checks.
3. Centimeter-level production (topography, stakeout)? RTK or Network RTK for real-time work. PPK for post-processed kinematic.
4. Decimeter-level (GIS, mapping)? RTK with relaxed tolerances, or DGNSS. PPP if no base station is available.
5. Remote location, no base station available? PPP or PPP-RTK.

"The surveyor must match the method to the accuracy requirement. Over-specifying the method wastes time and money; under-specifying it produces inadequate results." -- Ghilani & Wolf, Elementary Surveying (13th Ed.), Ch. 14, p. 420

Key Takeaways#

• Static surveying provides the highest GNSS accuracy (sub-centimeter) through long occupations and post-processing. It is the standard for geodetic control and deformation monitoring.
• Rapid-static reduces occupation times to 5--20 minutes while maintaining centimeter-level accuracy. It is efficient for densifying control networks.
• RTK delivers centimeter-level positions in real time using a base-rover radio/cellular link. It is the primary method for boundary surveys, topographic surveys, and construction stakeout.
• PPK provides the same accuracy as RTK but without a real-time communication link. Data are post-processed, enabling forward/reverse processing and cycle slip repair.
• Network RTK (VRS, MAC) uses CORS networks to provide RTK-quality corrections over large areas without a user-deployed base station. It requires cellular data coverage.
• PPP uses a single receiver with precise orbit/clock products. It requires no base station but has a convergence period of 5--40 minutes depending on the variant.
• Always verify RTK initialization on known points before and after work. A fixed solution is mandatory for survey-grade work.
• Redundant observations (multiple occupations, independent checks) are essential for quality assurance regardless of the GNSS method used.

References#

1. Van Sickle, J. GPS for Land Surveyors (4th Ed.). CRC Press, 2015. Chapters 9--11.
2. Ghilani, C.D. & Wolf, P.R. Elementary Surveying: An Introduction to Geomatics (13th Ed.). Pearson, 2012. Chapter 14.
3. GEOG 862: GPS and GNSS for Geospatial Professionals. Penn State College of Earth and Mineral Sciences. Lessons 7--9.
4. Rizos, C. "Network RTK Research and Implementation: A Geodetic Perspective." Journal of Global Positioning Systems, Vol. 1, No. 2, 2002.
5. Zumberge, J.F. et al. "Precise Point Positioning for the Efficient and Robust Analysis of GPS Data from Large Networks." Journal of Geophysical Research, Vol. 102, No. B3, 1997.
6. National Geodetic Survey. "OPUS -- Online Positioning User Service." NOAA/NGS. https://geodesy.noaa.gov/OPUS/