FS Exam Preparation

Comprehensive preparation for the Fundamentals of Surveying (FS) exam. 7 modules covering all 7 exam domains with 60 in-depth topics.

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Lesson 3

GNSS/GPS Surveying Methods

Learning Objectives

After completing this topic, you should be able to:

Describe the segments of GNSS and how satellite positioning works
Distinguish between code-based and carrier-phase positioning
Explain the major GNSS observation methods (static, rapid-static, RTK, PPP)
Identify factors that affect GNSS accuracy
Plan GNSS observations considering satellite geometry and DOP
Understand the coordinate systems used in GNSS (WGS 84, ITRF, NAD 83)

Overview

Global Navigation Satellite Systems (GNSS) have transformed surveying by providing three-dimensional positioning anywhere on earth, at any time, without requiring line of sight between points. The term GNSS encompasses all satellite navigation systems: GPS (United States), GLONASS (Russia), Galileo (European Union), and BeiDou (China).

For the FS exam, you must understand the fundamental principles of satellite positioning, the different observation methods, and the factors that affect accuracy. GNSS is now used in virtually every branch of surveying, from control networks to machine guidance.

Key Concepts

Figure FS.1.3 — How satellite positioning works: code vs. carrier, observation methods, and the 4-satellite requirement

Figure FS.1.3b — GNSS Observation Types

The Three Segments of GPS

Space segment:

31+ operational GPS satellites (as of 2025) in six orbital planes
Orbit altitude: approximately 20,200 km (12,550 miles)
Orbital period: approximately 11 hours 58 minutes (half a sidereal day)
Each satellite transmits on L1 (1575.42 MHz) and L2 (1227.60 MHz) frequencies; modernized satellites also transmit L5 (1176.45 MHz)

Control segment:

Master control station (Schriever Space Force Base, formerly Schriever AFB, Colorado) plus monitor stations worldwide
Tracks satellites, computes ephemerides and clock corrections
Uploads corrected data to satellites

User segment:

All GPS receivers on or near earth
Receivers measure the signals from satellites and compute positions

How Satellite Positioning Works

Figure FS.1.3f — Code (~3-5 m) vs carrier-phase (~10-20 mm) GNSS measurement

Code-based positioning (pseudorange):

The receiver correlates the satellite's transmitted code with a locally generated replica
The time offset between the two codes, multiplied by the speed of light, gives the pseudorange (approximate distance to the satellite)
With four or more satellites, the receiver solves for X, Y, Z, and clock bias
Accuracy: 3 to 5 meters (autonomous); sub-meter with differential corrections

Carrier-phase positioning:

The receiver measures the phase of the carrier wave itself (L1, L2, or L5)
The wavelength of L1 is approximately 19 cm -- allowing measurements at the millimeter level
The challenge is ambiguity resolution: determining the integer number of whole wavelengths between the receiver and satellite
Once ambiguities are resolved, positioning accuracy reaches 10-20 mm horizontally and 15-30 mm vertically

GNSS Observation Methods

Method	Duration	Accuracy	Baseline Length	Application
Static	1-4+ hours	3-5 mm + 0.5 ppm	Any (up to 1000+ km)	Control networks, geodetic surveys
Rapid Static	10-30 min	5-10 mm + 1 ppm	Up to 20 km	Densification of control
RTK	Seconds (init)	10-20 mm + 1 ppm	Up to 10-15 km from base	Topographic, boundary, construction
PPK	Seconds (post)	10-20 mm + 1 ppm	Up to 30 km from base	Aerial surveys, remote areas
PPP	20+ min convergence	20-50 mm	N/A (no base needed)	Remote areas, approximate control

Accuracy ranges are from Van Sickle, GPS for Land Surveyors (4th Ed.), Ch. 5, consistent with NGS guidance for static and kinematic GNSS surveys.

Static surveys are the gold standard for control work. Two or more receivers collect data simultaneously for an extended period. The data is post-processed to compute precise baseline vectors.

Figure FS.1.3c — Static Surveys: the gold standard for control work, with field setup, post-processing workflow, and accuracy specs

Real-Time Kinematic (RTK) provides centimeter-level positions in real time by transmitting corrections from a base station to a rover via radio link. The rover resolves carrier-phase ambiguities on the fly (OTF). RTK requires:

A base station on a known point (or connection to a network)
Radio or cellular communication link
Good satellite geometry
Initialization (ambiguity resolution) before measurements

Figure FS.1.3d — Real-Time Kinematic (RTK): base/rover with radio corrections, requirements, and ambiguity-resolution timeline

Satellite Geometry and DOP

Figure FS.1.3g — DOP from satellite geometry (good vs poor sky distribution)

Dilution of Precision (DOP) describes how satellite geometry affects positioning accuracy. A lower DOP value indicates better geometry.

DOP Type	Measures
GDOP	Overall 3D position + time
PDOP	3D position (X, Y, Z)
HDOP	Horizontal position only
VDOP	Vertical position only

Good geometry means satellites are spread across the sky -- not clustered together. A PDOP below 3 is considered good; above 6 is poor.

Factors that degrade geometry:

Obstructions (buildings, trees, canyon walls) blocking satellite signals
Low satellite count above the elevation mask
Satellites clustered in one part of the sky

Coordinate Systems in GNSS

Figure FS.1.3e — Coordinate systems in GNSS: ECEF Cartesian → geodetic; reference frames (WGS 84, NAD 83, ITRF); ellipsoid vs orthometric height with geoid correction

GNSS positions are computed in earth-centered, earth-fixed (ECEF) Cartesian coordinates and typically converted to geodetic coordinates (latitude, longitude, ellipsoid height).

WGS 84 -- the native datum of GPS; a global reference frame
NAD 83 -- the North American datum; practically coincident with WGS 84 at the meter level, but differs by 1-2 meters
ITRF -- the International Terrestrial Reference Frame; the most accurate global reference frame; WGS 84 is aligned to ITRF at the centimeter level

Ellipsoid height vs. orthometric height:

GNSS measures ellipsoid height (height above the reference ellipsoid)
Surveyors and engineers need orthometric height (height above the geoid, approximating mean sea level)
The difference is the geoid height (N): Orthometric Height = Ellipsoid Height - Geoid Height
A geoid model (e.g., GEOID18) is required for this conversion

Common wrong path — reporting ellipsoid height as elevation. GNSS receivers natively output ellipsoid height (h). Elevation on a topographic or construction plan is almost always orthometric height (H), measured from the geoid (approximately mean sea level). The difference — geoid height N — can be 20 to 35 meters in the continental U.S. and varies spatially. A student who reads the raw GNSS ellipsoid height and reports it as "elevation" will be off by tens of meters — obviously wrong for a stakeout benchmark, but easy to overlook if the receiver output isn't labeled. The conversion: H = h − N, where N comes from a geoid model like GEOID18. Exam questions set this up by giving you a GNSS height and asking for an orthometric elevation; if you skip the geoid correction, you'll pick the wrong answer.

Quick retrieval check — try before reading on.

▶A GNSS receiver gives an ellipsoid height of +145.62 m at a control point. The geoid model reports N = −26.87 m at that location (typical for the continental U.S., where the geoid is below the ellipsoid). What is the orthometric elevation?

$H = h - N = 145.62 - (-26.87) = 145.62 + 26.87 = \mathbf{172.49\text{ m}}$ . When N is negative (geoid below ellipsoid), subtracting N adds its magnitude — the orthometric height is larger than the ellipsoid height. If you used H = h + N mechanically with N treated as positive (26.87), you'd get 172.49 — correct by accident in this case. But if N were positive (rare in CONUS, common elsewhere), H = h − N would give the right sign and H = h + N would be wrong. Always apply the sign: H = h − N, taking N as reported by the geoid model including its sign.

Exam Tips

GNSS measures ellipsoid heights, not orthometric heights -- you must apply a geoid model to convert
RTK requires a minimum of 5 satellites for reliable ambiguity resolution (though 4 is the theoretical minimum for a 3D position)
PDOP below 3 is good; above 6 is poor -- lower is better
Static surveys are the most accurate GNSS method; RTK trades some accuracy for speed
Know the difference between code (pseudorange) and carrier-phase measurements
The L1 carrier wavelength is about 19 cm; the L2 wavelength is about 24 cm
Multipath is a major GNSS error source in urban and reflective environments
The FS exam may ask about observation planning (satellite availability, DOP, elevation masks)

FS Exam Preparation

Module 1: Surveying Processes & Methods

Module 2: Mapping Processes & Methods

Module 3: Boundary Law & Real Property

Module 4: Surveying Principles & Geodesy

Module 5: Survey Computations

Module 6: Business Concepts

Module 7: Applied Math & Statistics