How to Learn Geodesy, GNSS and Earth Reference Frames: From Coordinates to Millimetre-Scale Measurement of a Moving Earth

Wait, What? A Coordinate Is Incomplete Without Saying Which Earth You Measured

Someone gives you a latitude, longitude and height to many decimal places.

It looks precise.

But three questions are still missing:

Which mathematical Earth model?
Which reference frame?
At what epoch?

Why should time matter to a location?

Because Earth moves.

Tectonic plates shift. Ground subsides. Earthquakes displace stations. The planet’s rotation axis moves relative to the crust. The gravity field changes as water, ice and atmosphere move.

position = coordinate + reference system + epoch + uncertainty

That is the doorway into geodesy.

The One-Sentence Answer

Learn geodesy by separating Earth’s geometric shape from its gravity field, then connect coordinates to a time-dependent reference frame before using GNSS and other space-geodetic techniques to measure how the real Earth moves.

Stage 1: Geodesy Measures Earth as a Physical, Moving Object

Geodesy studies Earth’s:

size and shape;
orientation in space;
gravity field;
surface motion;
reference frames.

Surveying applies geodetic foundations locally. Navigation uses them operationally. Earth science uses them to detect change.

Stage 2: Earth Is Not a Sphere

A sphere is useful for first approximations.

Earth is better represented geometrically by an oblate ellipsoid because rotation makes the equatorial radius larger than the polar radius.

But even an ellipsoid is not the physical surface.

It is a mathematical reference surface.

Stage 3: Geodetic Latitude Is Defined by the Ellipsoid

Geodetic latitude is measured relative to the normal of the reference ellipsoid.

Geocentric latitude is measured from Earth’s centre.

They are not identical except at special locations.

“Latitude” therefore carries a model underneath it.

Stage 4: Longitude Needs a Reference Meridian and Frame

Longitude describes angular position around Earth.

Modern geodetic longitude is embedded in a global terrestrial reference frame, not merely an old line painted conceptually through Greenwich.

The frame determines how coordinates relate to the moving crust.

Stage 5: The Geoid Is a Gravity-Based Surface

The geoid is an equipotential surface of Earth’s gravity field that broadly corresponds to mean sea level extended beneath continents.

It is irregular because Earth’s mass distribution is irregular.

Mountains, mantle structure and density differences perturb the gravity field.

Stage 6: Ellipsoid and Geoid Solve Different Problems

The ellipsoid is mathematically smooth and ideal for coordinates.

The geoid is physically tied to gravity and the direction water tends to flow.

A professional height system must know which surface is being used.

Stage 7: GNSS Gives Ellipsoidal Height, Not Automatically “Height Above Sea Level”

A GNSS receiver can estimate an ellipsoidal height h.

Surveying often wants an orthometric height H, related to the gravity-based geoid.

A common relationship is:

h = H + N

where N is geoid height relative to the ellipsoid.

Therefore a precise GNSS height is not automatically a precise physical elevation.

Stage 8: “Mean Sea Level” Is Not One Perfect Global Surface

Real ocean surfaces are affected by:

currents;
temperature;
salinity;
atmospheric pressure;
wind.

Local mean sea level can differ from the geoid because the ocean is dynamic.

Physical height therefore requires more care than simply measuring to the nearest coastline.

Stage 9: A Datum Connects Coordinates to a Defined Reference

A geodetic datum specifies how coordinates are realised relative to Earth.

Older datums could be strongly regional.

Modern global systems are built from space-geodetic observations.

Mixing coordinates from incompatible datums can create metre-scale or larger errors even if every number has many decimal places.

Stage 10: A Reference Frame Is the Realised Coordinate System

A reference system defines principles.

A reference frame realises them through actual station coordinates, velocities and models.

For modern global geodesy, the International Terrestrial Reference Frame, or ITRF, is foundational.

Stage 11: The ITRF Has an Origin, Scale and Orientation

A global frame must define:

origin;
scale;
orientation;
their stability through time.

ITRF2020 uses multiple space-geodetic techniques to realise these quantities.

The origin is tied strongly to satellite laser ranging and Earth’s centre of mass; scale is constrained by techniques including VLBI and SLR; orientation is maintained consistently with earlier frames.

Stage 12: Earth Stations Need Velocities as Well as Coordinates

A station on a moving tectonic plate does not keep one permanent coordinate.

A reference-frame solution therefore includes position and velocity.

To compare surveys years apart, the coordinate epoch matters.

This is why a professional position can be expressed as:

position at reference epoch + velocity model + event corrections

Stage 13: Earthquakes Break Simple Linear Motion

Before an earthquake, a station may follow an approximately steady plate velocity.

During the event, it can jump.

Afterward, the crust can relax nonlinearly.

Reference frames therefore include post-seismic deformation models for affected stations.

Stage 14: ITRF Is Now Updated More Regularly

ITRF2020-u2024 extends the observational record through 2025 while retaining ITRF2020’s origin, scale and orientation to avoid a datum discontinuity for users.

The update uses extended time series from the four major space-geodetic technique centres.

This reflects a deep principle:

the reference frame must evolve because the measured Earth evolves

Stage 15: Four Space-Geodetic Techniques Support the Global Frame

The major techniques are:

GNSS — Global Navigation Satellite Systems;
SLR — Satellite Laser Ranging;
VLBI — Very Long Baseline Interferometry;
DORIS — Doppler Orbitography and Radiopositioning Integrated by Satellite.

They do not measure the same thing in the same way.

Their complementarity strengthens the frame.

Stage 16: GNSS Measures Signal Travel From Satellites

GNSS satellites broadcast time-tagged signals and orbital information.

A receiver compares signal timing with its own clock.

The apparent range is called a pseudorange because it contains receiver-clock and propagation errors as well as geometric distance.

Stage 17: GNSS Is Trilateration, Not Ordinary Triangulation

The receiver estimates distances to several satellites.

Intersecting those range constraints gives position while also solving for receiver clock offset.

Angles are not the primary measured quantity.

Therefore the stronger word is trilateration.

Stage 18: Four Unknowns Explain the Four-Satellite Minimum Model

A basic three-dimensional GNSS solution has four unknowns:

x;
y;
z;
receiver clock bias.

At least four satellite observations are therefore required in the simplest instantaneous model.

Real receivers use many more for redundancy and accuracy.

Stage 19: Satellite Orbits and Clocks Must Be Known

Your position cannot be more accurate than the geometry and timing model that connects you to the satellites.

Precise geodesy therefore depends on:

precise orbit determination;
satellite clocks;
Earth orientation;
relativity corrections.

GNSS is a global timing-and-orbit system before it is a map dot.

Stage 20: The Ionosphere Delays GNSS Signals

Free electrons in the ionosphere change radio-wave propagation.

The effect depends on frequency.

Multi-frequency GNSS observations can therefore estimate and remove much of the ionospheric delay.

A propagation error becomes measurable because its frequency dependence is known.

Stage 21: The Troposphere Creates a Different Delay

The neutral atmosphere delays GNSS radio signals too.

This delay depends on:

pressure;
water vapour;
elevation angle.

Tropospheric delay is not removed by the same frequency combination used for the ionosphere.

Different physics requires different correction models.

Stage 22: Multipath Is a Local Geometry Error

Signals can reflect from:

buildings;
ground;
vehicles;
metal structures.

The receiver then combines direct and reflected paths.

Urban positioning errors can therefore arise even when the satellites themselves are functioning perfectly.

Stage 23: Carrier Phase Enables Much Higher Precision

GNSS receivers can track the phase of the carrier wave, not only the code timing.

Carrier wavelength is short, making phase extremely precise.

But the receiver initially does not know the whole number of wavelengths between satellite and antenna.

This creates the integer ambiguity problem.

Stage 24: Differential GNSS Cancels Shared Errors

A reference receiver at a known position observes the same satellites as a nearby user.

Many errors are correlated across the two receivers.

Corrections from the reference station improve the user solution.

Common-mode error becomes a resource.

Stage 25: RTK Uses Carrier Phase in Real Time

Real-Time Kinematic positioning uses reference data and carrier-phase ambiguity resolution to achieve centimetre-scale positioning under suitable conditions.

But performance depends on:

satellite geometry;
communications;
atmosphere;
multipath;
ambiguity fixing.

“Centimetre GNSS” is a system state, not an unconditional device specification.

Stage 26: Singapore’s SiReNT Is National Geodetic Infrastructure

The Singapore Land Authority’s Singapore Satellite Positioning Reference Network, SiReNT, supports high-precision positioning, navigation and tracking.

SLA’s current public information states that the network consists of nine GNSS reference stations operating 24/7, supporting constellations including GPS, GLONASS, QZSS, BeiDou and Galileo.

For Singapore students, this is a direct local example of abstract geodesy becoming national infrastructure.

Stage 27: PPP Solves Precision Without a Nearby Local Base

Precise Point Positioning uses precise satellite orbit and clock products together with advanced modelling of atmosphere and antenna effects.

It can reach high accuracy without a nearby base station, but convergence time and correction quality matter.

RTK and PPP solve related positioning problems with different architectures.

Stage 28: The International GNSS Service Anchors Precise Products

The IGS combines a global network and analysis centres to provide precise GNSS products.

Its current reference frame is IGc20, used from 11 January 2026 onward and extracted from ITRF2020-u2024.

The reference frame used by a precision GNSS product is therefore explicit and versioned.

Stage 29: Satellite Laser Ranging Measures Distance With Light Pulses

Ground stations fire laser pulses toward satellites carrying retroreflectors.

Measure the round-trip time.

Infer range.

SLR provides powerful information about:

geocentre;
orbit scale;
Earth gravity parameters.

Stage 30: VLBI Uses Quasars to Measure Earth Geometry and Rotation

Radio telescopes separated by thousands of kilometres observe the same distant quasar.

Differences in arrival time constrain the baseline between stations and Earth’s orientation in inertial space.

Because quasars are extremely distant, they act as nearly fixed celestial reference points.

Stage 31: DORIS Adds Another Independent Space-Geodetic Network

DORIS uses Doppler shifts of radio signals from ground beacons to satellites.

It contributes to orbit determination and reference-frame realisation.

No single technique owns the global frame.

Stage 32: Earth Orientation Parameters Connect Terrestrial and Celestial Frames

Earth’s rotation is not perfectly uniform.

Geodesists measure:

UT1 rotation angle;
polar motion;
precession–nutation corrections.

A telescope or satellite orbit needs to know how Earth is oriented at the observation time.

Stage 33: Polar Motion Means the Rotation Axis Moves Relative to the Crust

The instantaneous rotation axis shifts by metres relative to Earth’s surface.

Mass redistribution in oceans, atmosphere and hydrosphere contributes.

Therefore “the North Pole” has several meanings depending on whether you mean geographic convention, instantaneous rotation axis or magnetic pole.

Stage 34: Gravity Is Part of Geodesy, Not a Separate Decoration

Physical heights depend on gravitational potential.

Geoid models combine:

terrestrial gravity;
airborne and marine observations;
satellite gravity;
topography.

That allows ellipsoidal GNSS heights to connect to physically meaningful elevations.

Stage 35: Satellite Gravimetry Measures Mass Redistribution

Missions such as GRACE and GRACE-FO detect changes in Earth’s gravity field caused by moving mass.

Scientists can infer changes in:

ice sheets;
groundwater;
ocean mass;
large hydrological systems.

Gravity becomes an Earth-system sensor.

Stage 36: Sea-Level Science Needs Both Ocean Height and Land Motion

A tide gauge measures sea level relative to the land on which the gauge sits.

If the land subsides, relative sea level rises even if the ocean surface does not.

Co-located GNSS helps separate:

ocean change;
vertical land motion.

This is why reference-frame stability matters to climate science.

Stage 37: Optical Clocks Open Relativistic Geodesy

General relativity predicts that clocks at different gravitational potentials tick at slightly different rates.

Near Earth’s surface, a height difference of about one centimetre corresponds to a fractional frequency shift of roughly 10⁻¹⁸.

NIST notes that modern optical clocks can in principle sense sub-centimetre geopotential differences, although practical field deployment remains difficult.

Stage 38: 2026 Optical-Clock Comparisons Show the Measurement Frontier

On 10 April 2026, NIST reported optical-clock frequency-ratio measurements among Al⁺, Yb and Sr systems with total fractional uncertainties at or below 3.2 × 10⁻¹⁸.

That result was aimed strongly at the future redefinition of the second, but the same precision scale illustrates why optical clocks are increasingly relevant to geodesy.

Stage 39: A Coordinate Without Uncertainty Is Incomplete

Every geodetic result should carry uncertainty.

Error sources can include:

observation noise;
atmospheric modelling;
multipath;
antenna calibration;
reference-frame uncertainty;
local monument motion.

Printing more decimal places does not reduce any of those errors.

Stage 40: Professional Geodesy Is a Global Inverse Problem

Millions of observations from satellites, lasers, radio telescopes, gravity sensors and clocks are combined to estimate parameters that cannot all be observed directly.

The professional question becomes:

Which reference frame, physical Earth model, propagation corrections and time-dependent station motions are required so that observations made by different instruments and on different days refer to the same Earth?

Evidence: How Do We Know Earth’s Surface Is Moving?

Continuous GNSS station time series directly show tectonic plate motion, earthquake offsets, subsidence and seasonal deformation.

SLR, VLBI, DORIS and GNSS converge on a coherent global reference frame. Independent gravity and sea-level measurements then provide physical cross-checks.

Misconceptions Worth Hunting

Earth is a sphere for precise positioning.
The geoid and ellipsoid are the same surface.
GNSS height automatically means height above mean sea level.
A coordinate is timeless.
GPS determines position by triangulation from angles.
Four satellites guarantee high accuracy.
More decimal places mean greater accuracy.
Reference stations never move.
A tide gauge measures global sea-level change independently of land motion.
Relativity matters only in theoretical physics, not positioning.

Transfer Check

A GNSS receiver reports an ellipsoidal height of 40 m while the geoid height is 15 m. Using h = H + N, what is the approximate orthometric height? 25 m.

Now repeat a precise survey ten years later on a station moving 25 mm/year. Can you compare the raw coordinates directly without epoch information? No.

Next, place a receiver between glass towers. Satellite geometry is excellent but reflected signals are strong. Can accuracy still be poor? Yes—multipath is local.

Finally, a tide gauge shows +4 mm/year while nearby GNSS shows the land subsiding 2 mm/year. How much of the relative rise could be land motion? About half of the observed rate under that simplified model.

How We Know the Learning Has Held

A learner should be able to distinguish sphere, ellipsoid and geoid; distinguish ellipsoidal and orthometric height; explain datum/reference-frame/epoch differences; explain GNSS pseudorange and carrier phase; identify ionospheric, tropospheric and multipath errors; distinguish RTK and PPP conceptually; explain ITRF and IGS roles; identify SLR, VLBI and DORIS; explain Earth orientation parameters; connect gravity to physical height; and explain how clocks can sense geopotential.

Model Limits

Ellipsoids smooth the real Earth. Plate velocities are not perfectly constant. Atmospheric corrections are imperfect. Reference-frame stations can move locally. Geoid models have finite spatial resolution. Optical-clock geodesy remains technologically demanding. Professional geodesy therefore keeps frame + epoch + technique + physical model + uncertainty visible together.

Teaching Guide

Teach in this order: sphere → ellipsoid → geoid → coordinates → datum/frame → epoch → GNSS pseudorange → errors → carrier phase → differential methods → ITRF → space geodesy → gravity → sea level → optical clocks.

Begin with: “If I give you latitude, longitude and height to ten decimal places, do you know exactly where I am?” The correct answer is no until the frame and epoch are known.

At advanced level compare a GNSS coordinate time series, a geoid map and a tide-gauge record. Ask which sees crust motion, which represents gravity potential and which measures sea level relative to local land.

Connect This to the eduKate Learning Estate

Research Foundations and Further Learning

The Quiet Ending

The beginner asks, “Where am I?” The developing geodesist asks, “Relative to which ellipsoid and datum?” The advanced learner asks, “At what epoch, in which reference frame, with what velocity and uncertainty?”

Which combination of geometry, gravity, time and reference-frame realisation makes measurements from different instruments describe the same moving Earth?