PS Exam Preparation

Comprehensive preparation for the NCEES Principles and Practice of Surveying (PS) exam. 5 modules covering all 5 exam domains with 50 in-depth topics.

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Lesson 4

GPS/GNSS Methods (Static, RTK, PPP)

Learning Objectives

After completing this topic, you should be able to:

Identify the major GNSS constellations and their characteristics
Explain the principles of satellite positioning including trilateration and signal structure
Compare static, rapid static, RTK, PPP, and network RTK methods
Describe CORS and VRS concepts and their application in surveying
Identify error sources in GNSS observations and their mitigation strategies
Select the appropriate GNSS method for a given project type and accuracy requirement
Understand the role of carrier phase versus code-based positioning

Overview

Global Navigation Satellite Systems (GNSS) have fundamentally transformed professional surveying. Where conventional methods require line of sight between points, GNSS provides three-dimensional positioning from satellites orbiting approximately 20,200 kilometers above the earth. Understanding GNSS methods, their capabilities, their limitations, and the error sources that affect them is essential for every professional surveyor.

The term GNSS encompasses all satellite navigation systems. GPS (Global Positioning System) is the United States system, but modern survey receivers can track signals from multiple constellations simultaneously, improving accuracy, reliability, and availability.

Key Concepts

GNSS Constellations

Figure PS.2.23 — Four GNSS constellations: GPS, GLONASS, Galileo, BeiDou

Constellation	Country/Region	Nominal Satellites	Orbital Planes	Orbit Altitude (km)	Orbital Period
GPS (NAVSTAR)	United States	31 (24 baseline)	6	20,200	11 hr 58 min
GLONASS	Russia	24	3	19,100	11 hr 15 min
Galileo	European Union	30 (planned)	3	23,222	14 hr 7 min
BeiDou (BDS)	China	35 (planned)	3 + GEO/IGSO	21,528 (MEO)	12 hr 53 min
QZSS	Japan	4 (regional)	3	32,000-36,000	~24 hr
NavIC (IRNSS)	India	7 (regional)	--	36,000 (GEO/GSO)	~24 hr

Multi-constellation receivers track signals from two or more constellations simultaneously. Benefits include:

More satellites available, improving geometry (lower PDOP)
Better performance in obstructed environments (urban canyons, tree canopy)
Faster initialization times for RTK
Greater redundancy for quality control

Satellite Positioning Principles

Signal Structure

Each GNSS satellite transmits signals on multiple frequencies. For GPS:

Signal	Frequency (MHz)	Wavelength (cm)	Primary Use
L1 C/A	1575.42	19.0	Navigation, code positioning
L1C	1575.42	19.0	Civil interoperable signal
L2C	1227.60	24.4	Civil code, dual-frequency
L5	1176.45	25.5	Safety of life, high accuracy

Each signal carries two types of information:

Pseudorange (code) measurement -- the receiver correlates incoming code with internally generated replica to determine satellite-to-receiver range. Accuracy: approximately 1-3 meters for standard code, better for modern signals.

Carrier phase measurement -- the receiver measures the phase of the carrier wave, providing a much more precise range measurement. The carrier wavelength (approximately 19 cm for L1) determines the fundamental measurement precision. Accuracy: a few millimeters, but requires resolving the integer ambiguity (the unknown number of full wavelengths between satellite and receiver).

Positioning Geometry

GNSS positioning requires simultaneous observations to at least four satellites:

Three satellites determine a three-dimensional position (trilateration)
A fourth satellite is needed to solve for receiver clock error
Additional satellites provide redundancy and improve accuracy

Dilution of Precision (DOP) quantifies the effect of satellite geometry on positioning accuracy:

DOP Measure	Description
GDOP	Geometric DOP (3D position + time)
PDOP	Position DOP (3D position only)
HDOP	Horizontal DOP
VDOP	Vertical DOP
TDOP	Time DOP

Lower DOP values indicate better geometry. A PDOP below 3 is generally considered good. PDOP above 6 indicates poor geometry, and observations should be postponed if possible.

Figure PS.2.25 — Good vs poor satellite geometry (GDOP)

Survey Methods

Figure PS.2.24 — Static / RTK / PPK comparison

Static GPS

Application: Highest accuracy control surveys, geodetic networks, deformation monitoring.

Procedure: Two or more receivers occupy known and unknown stations simultaneously for an extended period (typically 1-4 hours depending on baseline length and accuracy requirements). Data is post-processed to determine precise baseline vectors.

Parameter	Typical Values
Occupation time	1-4 hours (longer for long baselines)
Baseline accuracy	5 mm + 0.5-1 ppm
Baseline length	Up to hundreds of kilometers
Processing	Post-processing required
Satellites required	4 minimum (5+ recommended)
Frequency	Dual-frequency required for long baselines

Advantages: Highest accuracy, works over long baselines, well-established methodology.

Limitations: Long occupation times, requires post-processing, inefficient for many points.

Rapid Static

Application: Control surveys where static accuracy is needed with shorter occupation times.

Procedure: Similar to static, but with occupation times of 5-20 minutes per point. Relies on advanced ambiguity resolution algorithms that can resolve integer ambiguities with shorter data spans.

Parameter	Typical Values
Occupation time	5-20 minutes
Baseline accuracy	5-10 mm + 1 ppm
Baseline length	Generally under 20 km
Processing	Post-processing required

Real-Time Kinematic (RTK)

Application: Boundary surveys, topographic surveys, construction staking, any work requiring centimeter-level accuracy in real time.

Procedure: A base receiver at a known point transmits corrections via radio or cellular data link to a rover receiver. The rover resolves carrier phase ambiguities and computes its position in real time, typically achieving centimeter-level accuracy.

Parameter	Typical Values
Initialization time	Seconds to minutes
Position accuracy	10-20 mm horizontal, 15-30 mm vertical
Baseline length	Generally under 10-15 km from base
Communication	UHF radio, cellular data, or internet
Update rate	1-20 Hz

Critical considerations for RTK:

The base station must be on a known point with reliable coordinates
Communication link reliability directly affects productivity
Accuracy degrades with increasing baseline length
Initialization (ambiguity resolution) must be verified by checking known points
Multipath and signal obstructions can prevent or degrade initialization

Re-initialization check: After every initialization (or re-initialization), the surveyor should observe a known point to verify that the solution is correct. Incorrect initializations can produce positions that appear valid but are wrong by full wavelength multiples (approximately 19 cm or multiples thereof).

Common wrong path — trusting a "Fixed" solution that wasn't verified. RTK receivers report a "Fixed" status when they believe they've resolved integer ambiguities. But a Fixed status is not a guarantee of correctness — under marginal conditions (multipath near buildings, weak satellite geometry, or short initialization), a receiver can return "Fixed" on a false integer solution. The position looks precise (reported σ of 10–20 mm) but is wrong by one or more whole wavelengths — typically 19 cm on L1, sometimes meters after repeated cycle slips. The only defense is to check against a known point after each initialization, comparing the measured coordinates to the published ones. A few-cm agreement validates the fix; a 19-cm discrepancy reveals a false fix and requires re-initialization. Exam questions bait this by describing an RTK survey with "Fixed" status that produces coordinates disagreeing with control — the correct diagnosis is a false fix, not a blunder in the control or a datum issue.

Quick retrieval check — try before reading on.

▶You're running RTK near a two-story building. After initialization, the receiver reports "Fixed" with σ = 15 mm. You check a nearby CORS-tied control monument whose published coordinate is N 1,234,567.42, E 2,345,678.31. Your RTK position reads N 1,234,567.45, E 2,345,678.51. What does this tell you, and what do you do?

ΔN = +0.03 m (3 cm) — fine. ΔE = +0.20 m (20 cm) — close to one L1 wavelength (19 cm). This is a classic signature of a false fix on the east component. The receiver is confident in its solution (Fixed + 15 mm σ), but the solution is off by an integer wavelength in one axis. Do NOT accept this fix. Force a re-initialization (cover/uncover the antenna, or switch to a different base position), then re-check the known point. Repeat until the coordinates agree with published to within a few cm. Multipath from the building is the likely culprit; if re-initialization keeps producing false fixes, relocate the rover or the base station, or switch to a static session to resolve ambiguities with more observations.

Network RTK and Virtual Reference Stations (VRS)

Application: RTK positioning without establishing a local base station, using a network of permanent reference stations.

How VRS works:

A network of continuously operating reference stations (CORS) collects GNSS observations
A processing center models atmospheric and orbital errors across the network
When a rover connects to the network, it sends its approximate position
The processing center generates correction data as if a virtual reference station existed near the rover
The rover applies these corrections to achieve RTK-level accuracy

Parameter	Typical Values
Position accuracy	10-20 mm horizontal, 20-40 mm vertical
Coverage	Within the network polygon
Communication	Cellular data (internet)
Base station required	No (uses CORS network)

Advantages: No base station setup, consistent accuracy across the network, rapid deployment.

Limitations: Requires cellular coverage, dependent on network availability, subscription cost.

Precise Point Positioning (PPP)

Application: Positioning in remote areas without base stations or network coverage, reconnaissance surveys, GIS data collection requiring sub-decimeter accuracy.

Procedure: A single receiver collects dual-frequency observations. Post-processing uses precise satellite orbits and clocks (from organizations like the International GNSS Service) to achieve high accuracy without a base station.

Parameter	Typical Values
Occupation time	15 minutes to several hours
Position accuracy	10-50 mm (post-processed, convergence dependent)
Base station required	No
Processing	Post-processing with precise products

Convergence is a key concept in PPP. Unlike differential methods where ambiguities are resolved quickly, PPP solutions converge gradually as the receiver collects more data and the processing algorithm refines its estimates. Real-time PPP services are emerging but typically require 15-30 minutes for convergence.

Continuously Operating Reference Stations (CORS)

Figure PS.2.26 — CORS network and NGS OPUS service

CORS are permanent GNSS receivers that operate 24/7 and provide reference data for post-processing or real-time corrections.

CORS Network	Operator	Coverage
NGS CORS	National Geodetic Survey	United States
State CORS networks	Various state agencies	Individual states
Private networks	Commercial providers	Regional/national
IGS stations	International GNSS Service	Global

CORS data is used for:

Post-processing static and rapid static surveys
Providing reference stations for network RTK
Monitoring crustal deformation and plate tectonics
Maintaining national spatial reference systems

GNSS Error Sources

Error Source	Magnitude	Mitigation
Satellite orbit errors	1-5 m (broadcast), cm (precise)	Use precise orbits for post-processing
Satellite clock errors	1-5 m (broadcast), cm (precise)	Differential methods cancel; PPP uses precise clocks
Ionospheric delay	2-50 m (varies with solar activity)	Dual-frequency observations, differential over short baselines
Tropospheric delay	2-25 m (elevation dependent)	Tropospheric models, low elevation mask
Multipath	0.5-5 m (code), cm (carrier)	Site selection, antenna design, long observations
Receiver noise	mm to cm	Averaging, carrier phase methods
Antenna phase center variation	mm	Calibrated antenna models, consistent antenna orientation
Integer ambiguity errors	Multiples of 19 cm	Verification on known points, robust algorithms

Ionospheric Effects

The ionosphere delays GNSS code signals and advances carrier phase signals. The delay is proportional to the total electron content (TEC) along the signal path and inversely proportional to the square of the frequency.

Dual-frequency advantage: Because the ionospheric delay is frequency-dependent, receivers that track two or more frequencies can form an "ionosphere-free" linear combination that virtually eliminates ionospheric effects. This is essential for long baselines and high-accuracy work.

Tropospheric Effects

The troposphere delays GNSS signals due to the atmosphere's refractive index. Unlike ionospheric delay, tropospheric delay is not frequency-dependent and cannot be eliminated by dual-frequency combinations.

Mitigation strategies:

Apply tropospheric models (Saastamoinen, Hopfield, or more complex models)
Set a minimum elevation mask angle (typically 10-15 degrees) to exclude low-elevation satellites where tropospheric delay is greatest
For high-accuracy work, estimate tropospheric delay parameters during processing

Multipath

Multipath occurs when satellite signals reach the antenna via reflected paths in addition to the direct path. Reflections from buildings, vehicles, water surfaces, and the ground cause positioning errors.

Mitigation strategies:

Select observation sites away from reflective surfaces
Use antennas with ground planes or choke rings
Observe for longer periods (multipath has a characteristic period and averages out)
Use modern signal processing techniques (narrow correlator, multipath mitigation)

Planning a GNSS Survey

Effective GNSS survey planning includes:

Mission planning -- predict satellite availability and DOP for the project area and time period
Site reconnaissance -- identify obstructions, multipath sources, and accessibility
Network design -- plan baselines, session schedules, and redundancy
Communication planning (for RTK) -- verify radio or cellular coverage
Control -- identify existing control points for constraining the survey
Quality checks -- plan verification procedures including checks to known points

Exam Tips

Know the difference between code (pseudorange) and carrier phase measurements -- carrier phase is more precise but requires ambiguity resolution
RTK initialization must always be verified by checking a known point
Dual-frequency receivers can eliminate ionospheric delay; single-frequency cannot
Tropospheric delay cannot be eliminated by dual-frequency combinations
PDOP below 3 is good; above 6 is poor
VRS/network RTK eliminates the need for a local base station but requires cellular coverage
PPP does not require a base station but requires long convergence times
Static GPS provides the highest accuracy but requires the longest occupation times
Multipath is reduced by careful site selection and longer observation periods
CORS stations provide continuous reference data for both post-processing and real-time applications