science

GNSS reflectometry

GNSS reflectometry is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand GNSS reflectometry rather than just read about it. In short: GNSS reflectometry (or GNSS-R) involves making measurements from the reflections from the Earth of navigation signals from Global Navigation Satellite Systems such as GPS. The idea of using reflected GNSS signals for earth observation was first proposed in 1993 by Martin-Neira.

Key takeaways

GNSS reflectometry belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect GNSS reflectometry to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of GNSS reflectometry from memory before moving on to harder problems.

Reference excerpt

GNSS reflectometry (or GNSS-R) involves making measurements from the reflections from the Earth of navigation signals from Global Navigation Satellite Systems such as GPS. The idea of using reflected GNSS signals for earth observation was first proposed in 1993 by Martin-Neira. It was also investigated by researchers at NASA Langley Research Center and is also known as GPS reflectometry. Research applications of space-based GNSS-R are focused in altimetry, oceanography (wave height and wind speed), cryosphere monitoring, and soil moisture monitoring.

Principles GNSS reflectometry is passive sensing that takes advantage of and relies on multiple active sources - with the satellites generating the navigation signals. For this, the GNSS receiver measures the signal delay from the satellite (the pseudorange measurement) and the rate of change of the range between satellite and observer (the Doppler measurement). The surface area of the reflected GNSS signal also provides the two parameters time delay and frequency change. As a result, the Delay Doppler Map (DDM) can be obtained as GNSS-R observable. The shape and power distribution of the signal within the DDM is dictated by two reflecting surface conditions: its dielectric properties and its roughness state. Further derivation of geophysical information rely on these measurements. GNSS reflectometry is a bi-static radar, where transmitter and receiver are separated by a significant distance. Since in GNSS reflectometry one receiver simultaneously can track multiple transmitters (i.e. GNSS satellites), the system also has the nature of multi-static radar. The receiver of the reflected GNSS signal can be of different kinds: Ground stations, ship measurements, airplanes or satellites, like the UK-DMC satellite, part of the Disaster Monitoring Constellation built by Surrey Satellite Technology Ltd. It carried a secondary reflectometry payload that has demonstrated the feasibility of receiving and measuring GPS signals reflected from the surface of the Earth's oceans from its track in low Earth orbit to determine wave motion and windspeed.

Space missions CYGNSS, satellite constellation by NASA using GNSS-R for improving hurricane forecasting, launched in 2016 TechDemoSat-1, technology demonstration small satellite by ESA, launched in 2019 PRETTY, technology demonstration CubeSat by ESA measuring sea state, sea ice, and ocean currents, launched in 2023 HydroGNSS, 2 identical small satellites by ESA for monitoring Essential Climate Variables related to the hydrological cycle, launched in November 2025

GNSS Interferometric Reflectometry GNSS Interferometric Reflectometry (or GNSS-IR) is a specialized case of GNSS-R. Here the receiving instrument is on the surface of the Earth. In this technique the interference of the direct and reflected signals is used rather than a Delay Doppler Map or measuring the two signals separately. In the example shown, a GNSS antenna is ~2.5 meters above a planar surface. Both direct (blue) and reflected (red) GNSS signals are shown. As a GNSS satellite rises or sets, the elevation angle changes; the direct and reflected signals will generate an interference pattern. The frequency of this interference pattern can be used to extract the height of the antenna above the planar surface, the reflector height. Changes in reflector height can be directly used to measure water surfaces and the height of snow.

References

Further reading Zavorotny, Valery U.; Gleason, Scott; Cardellach, Estel; Camps, Adriano (2014). "Tutorial on Remote Sensing Using GNSS Bistatic Radar of Opportunity". IEEE Geoscience and Remote Sensing Magazine. Vol. 2, no. 4. pp. 8–45. doi:10.1109/MGRS.2014.2374220. ISSN 2168-6831. Larson, Kristine M.; Small, Eric E.; Braun, John; Zavorotny, Valery (2014). "Environmental Sensing: A Revolution in GNSS Applications". InsideGNSS. Vol. 9, no. 4. pp. 36–46. ISSN 1559-503X. Archived from the original on 15 March 2016. Retrieved 15 March 2016. Cardellach, Estel (2015): E-GEM – GNSS-R Earth Monitoring; State of the Art Description Document Archived 2020-11-28 at the Wayback Machine. Emery, William and Camps, Adriano (2017): Introduction to Satellite Remote Sensing 1st Edition Atmosphere, Ocean, Land and Cryosphere Applications, Chapter 6: Remote Sensing Using Global Navigation Satellite System Signals of Opportunity, Elsevier, 20 September 2017, Paperback ISBN 9780128092545, eBook ISBN 9780128092590 A complete list of references maintained by the GNSS-R Community can be found at: https://www.ice.csic.es/personal/rius/gnss_r_bibliography/index.html

External links Reflecting on the future Archived 2007-05-14 at the Wayback Machine, The Engineer Online, 28 November 2006. GNSS Applications and Methods, Artech House, September 2009.

Illustrations

Worked examples

Example 1 — a first encounter with GNSS reflectometry

Start with the simplest possible case. Write down what GNSS reflectometry claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to GNSS reflectometry before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about GNSS reflectometry ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of GNSS reflectometry

In research: GNSS reflectometry appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses GNSS reflectometry in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: GNSS reflectometry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Remote sensing, Satellite navigation, so understanding it makes those chapters shorter.
In everyday life: Look for GNSS reflectometry outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study GNSS reflectometry in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what GNSS reflectometry means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain GNSS reflectometry out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is GNSS reflectometry in simple terms?

Why does GNSS reflectometry matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study GNSS reflectometry?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on GNSS reflectometry.