What Is GNSS? Global Navigation Satellite System

Every time a phone provides driving directions, a surveyor measures a property boundary, or a scientist tracks the movement of a tectonic plate, satellites thousands of kilometers above Earth may be involved. The technology behind these measurements is called GNSS, or Global Navigation Satellite System.

GNSS is actually a broader term than GPS. GPS, operated by the United States, is one GNSS constellation. Other systems include Europe's Galileo, China's BeiDou, and Russia's GLONASS. Modern receivers can often communicate with several constellations, providing more satellites and improving positioning reliability.

How Does GNSS Find Your Location?

GNSS satellites continuously transmit radio signals containing extremely precise timing and orbital information.

A receiver—such as your phone—records when a signal arrives. Because radio waves travel at approximately the speed of light, the receiver can estimate its distance from a satellite using:

Distance ≈ speed of light × signal travel time

Knowing your distance from one satellite is not enough to determine your location. Instead, a GNSS receiver simultaneously measures signals from multiple satellites.

At least four satellites are normally needed to solve for four unknowns: your three-dimensional position—latitude, longitude, and altitude—and the timing error in the receiver's clock.

This process is commonly called trilateration.

Why Isn't Your Phone Perfectly Accurate?

GNSS positioning sounds straightforward, but the signals encounter several problems while traveling to Earth's surface.

The ionosphere and troposphere can delay radio signals. Satellite orbit and clock uncertainties introduce additional errors. Buildings, mountains, and other objects can reflect signals before they reach a receiver, producing an effect called multipath.

This is why a smartphone might place you on the wrong side of a street even though GNSS itself can achieve much greater precision.

Professional geodetic receivers use sophisticated techniques to correct these errors. Differential GNSS compares observations from receivers at known locations, while Real-Time Kinematic (RTK) positioning uses measurements of the signal's carrier wave to achieve centimeter-level positioning under favorable conditions.

GNSS Can Measure a Moving Earth

GNSS becomes particularly interesting when it is used for geodesy.

Permanent GNSS stations installed on stable ground continuously record their positions. By comparing measurements across months or years, scientists can detect extremely small movements of Earth's crust.

For example, tectonic plates may move only a few centimeters each year. That movement sounds insignificant, but GNSS stations can measure it. Networks of receivers therefore allow geodesists to map plate motion and study how strain accumulates around faults.

After major earthquakes, GNSS can also reveal how far the surface moved.

Measuring Glaciers, Volcanoes, and Sea Level

GNSS has applications far beyond navigation.

Researchers can install receivers on glaciers to measure how quickly the ice flows. Stations around volcanoes can detect ground inflation or deflation caused by magma moving underground. GNSS measurements are also combined with tide gauges to separate changes in ocean height from movement of the land itself.

Even Earth's atmosphere can be studied using GNSS. Scientists analyze how satellite signals are delayed as they travel through atmospheric water vapor, providing information useful for meteorology.

More Than a Navigation System

GNSS has effectively created a continuously available global measurement framework.

A technology most people encounter as the blue location dot on their phones can measure processes occurring across an extraordinary range of scales—from tracking a moving vehicle to detecting the slow deformation of an entire continent.

For geodesy, that is what makes GNSS so powerful. It doesn't simply tell us where something is. By repeatedly measuring position with extreme precision, it tells us how Earth itself is moving and changing over time.

Earth System Evolution Dynamics

Earth today—with blue oceans, oxygen-rich air, continents, glaciers, and abundant life—is only one version of our planet. Over its roughly 4.5-billion-year history, Earth has continuously evolved as its atmosphere, oceans, rocks, ice, and living organisms interacted. Understanding these connections is the focus of Earth system science, which treats the planet not as separate environments but as one enormous dynamic system.

Earth can be divided into several interconnected components: the geosphere (rocks and Earth's interior), hydrosphere (water), atmosphere (air), cryosphere (ice), and biosphere (life). A change in one can trigger changes throughout the others.

A Planet Built by Feedback Loops

One of the most important concepts in Earth system dynamics is the feedback loop.

Consider ice. Snow and glaciers have high albedo, meaning they reflect large amounts of incoming sunlight. If global temperatures decrease, ice can expand. More ice increases Earth's reflectivity, causing less solar energy to be absorbed and encouraging additional cooling.

The same process can operate in reverse. Warming melts ice, exposing darker ocean or land underneath. Those surfaces absorb more solar energy, potentially causing additional warming. This is known as ice-albedo feedback.

Other feedbacks involve vegetation, clouds, atmospheric water vapor, oceans, and the carbon cycle. Some amplify an initial change, while others counteract it.

The Continents Are Constantly Moving

Earth's solid surface is also dynamic.

Plate tectonics continuously moves continents, creates mountains, opens and closes oceans, produces volcanoes, and recycles crust into Earth's mantle. Over hundreds of millions of years, these processes can completely reorganize the planet.

Continental position can even influence climate. Moving land toward the poles can create environments where large ice sheets develop. Mountain building alters atmospheric circulation and rainfall. Meanwhile, volcanic eruptions transfer carbon dioxide from Earth's interior into the atmosphere.

Plate tectonics also interacts with the long-term carbon cycle. Carbon dioxide dissolves in rainwater and participates in chemical weathering of rocks. Some of that carbon eventually reaches the oceans, where it can become incorporated into carbonate minerals and sediments. Tectonic processes eventually recycle some of this material into Earth's interior.

This creates a carbon cycle operating over millions of years.

Life Changed the Planet

Perhaps the most extraordinary example of Earth system evolution is that life itself transformed Earth.

Early Earth's atmosphere contained extremely little free oxygen. Beginning billions of years ago, photosynthetic microorganisms released oxygen as a byproduct. Eventually, oxygen began accumulating in the atmosphere during what scientists call the Great Oxidation Event.

This atmospheric transformation changed ocean chemistry, mineral formation, and biological evolution.

Life has continued modifying the planet ever since. Plants influence atmospheric carbon dioxide, forests affect rainfall and evaporation, marine organisms contribute to carbon storage, and microorganisms drive chemical cycles involving carbon, nitrogen, and sulfur.

From Greenhouse Worlds to Snowball Earth

Earth's climate has never been constant.

At different points in geological history, the planet experienced extremely warm greenhouse climates with little permanent polar ice. At other times, glaciers expanded dramatically. During several episodes hundreds of millions of years ago, ice may have reached tropical latitudes, producing conditions commonly associated with Snowball Earth.

These transitions demonstrate that Earth's systems can cross important thresholds. A relatively small initial forcing can sometimes trigger much larger changes through feedback mechanisms.

A Planet That Never Reaches a Final State

Modern satellites, ocean sensors, GNSS stations, ice cores, sediment cores, fossils, and computer models allow scientists to investigate these interactions across timescales ranging from seconds to billions of years.

The central lesson of Earth system evolution is that nothing operates completely independently. Tectonics affects climate. Climate influences glaciers. Glaciers reshape landscapes. Oceans redistribute heat. Life alters the atmosphere, and the atmosphere influences life.

Earth is therefore less like a static collection of environments and more like a continuously evolving network. Understanding those connections helps explain not only how the planet reached its present state, but also how today's changes could shape the Earth of the future.

Tidally Locked Moon

No matter when you look at the Moon from Earth, you see essentially the same collection of craters, mountains, and dark volcanic plains. We never naturally get a direct view of the opposite hemisphere from Earth's surface. This happens because the Moon is tidally locked to Earth, one of the most interesting consequences of gravity in our solar system.

Tidal locking does not mean the Moon has stopped rotating. In fact, the opposite is true: the Moon rotates once on its axis in approximately 27.3 days, almost exactly the same amount of time it takes to complete one orbit around Earth. Because these two periods match, the same hemisphere continually faces us.

How Did the Moon Become Tidally Locked?

The Moon probably did not begin this way.

Early in its history, the Moon likely rotated much faster. Earth's gravity, however, does not pull equally strongly on every part of the Moon. The side facing Earth experiences a slightly stronger gravitational attraction than the opposite side.

This difference produces a tidal force.

When the young Moon rotated more rapidly, Earth's tidal forces slightly distorted its shape. Because the Moon was rotating, this tidal bulge was not always perfectly aligned with Earth. Earth's gravity pulled on the misaligned bulge, creating a torque that gradually slowed the Moon's rotation.

Eventually, the rotation period matched its orbital period. At that point, the tidal bulge remained approximately aligned with Earth, producing the stable configuration we observe today.

This state is called synchronous rotation.

The “Dark Side” Isn't Actually Dark

The hemisphere facing Earth is called the near side, while the opposite hemisphere is the far side.

Calling the far side the "dark side of the Moon" is misleading. It receives approximately the same amount of sunlight as the near side. During a new moon, for example, the far side is largely illuminated while the near side faces away from the Sun.

Humanity did not see the far side until the Soviet Luna 3 spacecraft photographed it in 1959.

Those images revealed something surprising: the two sides look dramatically different. The near side contains large dark regions called maria, created by ancient lava flows. The far side contains far fewer maria and is dominated by heavily cratered highlands.

Scientists continue investigating why the Moon developed this geological asymmetry.

Is Exactly 50% of the Moon Visible?

Interestingly, tidal locking doesn't restrict us to exactly half of the lunar surface.

The Moon's orbit is slightly elliptical, and its orbital speed changes as it travels around Earth. Meanwhile, its rotation remains relatively steady. Combined with the Moon's tilted orbit and our changing viewing position, this produces an apparent rocking motion called libration.

Over time, libration allows observers on Earth to see about 59% of the Moon's surface, although only roughly half is visible at any single moment.

Tidal Locking Happens Across the Solar System

Our Moon is far from unique. Many large moons in the solar system are tidally locked to their planets, including Europa around Jupiter, Titan around Saturn, and Triton around Neptune.

The same physics may even affect exoplanets. Planets orbiting close to small red-dwarf stars can experience strong tidal forces and may eventually become tidally locked, potentially producing permanent daysides and nightsides.

Earth and the Moon are still exchanging angular momentum today. Tidal interactions cause the Moon to move away from Earth by about 3.8 centimeters per year, a rate measured using lasers reflected from mirrors left on the lunar surface.

The familiar face we see in the night sky is therefore not a coincidence. It is the result of billions of years of gravitational interaction, slowly synchronizing the Moon's rotation with its journey around our planet.

Analyzing Exoplanet Cryospheres

When scientists discuss the cryosphere, they usually mean Earth's glaciers, ice sheets, sea ice, permafrost, and seasonal snow. But the same idea can be extended far beyond our solar system. Thousands of confirmed exoplanets orbit other stars, and some may contain enormous quantities of frozen water or other ices. Studying these hypothetical exoplanet cryospheres could help scientists understand planetary climates—and determine which distant worlds might be capable of supporting liquid water.

The challenge is obvious: we cannot currently photograph glaciers on an Earth-sized planet dozens of light-years away. Instead, scientists have to combine astronomical observations with climate models, spectroscopy, and planetary physics.

Where Would Ice Form?

The first clue comes from a planet's orbit and its star.

A planet receiving little stellar radiation could maintain extensive surface ice. But distance alone does not determine whether a planet freezes. Atmospheric composition, pressure, rotation, ocean circulation, cloud cover, and surface reflectivity can dramatically change temperature.

Researchers can feed these variables into general circulation models (GCMs)—similar to models used to simulate Earth's climate. These simulations divide a planet's atmosphere and surface into a three-dimensional grid and calculate how energy, air, and sometimes oceans move between regions.

Scientists can then ask fascinating questions: Would glaciers develop near the poles? Could an entire ocean freeze? Would an equatorial region remain ice-free?

The Snowball Planet Problem

One important process is ice-albedo feedback.

Ice and snow reflect a large percentage of incoming radiation. If a planet begins cooling and its ice expands, the brighter surface reflects more energy back into space. That causes additional cooling, potentially producing even more ice.

Under certain conditions, models suggest an Earth-like exoplanet could enter a "snowball" state, with ice covering most or all of its oceans. Earth itself appears to have experienced extreme global glaciations hundreds of millions of years ago, making our planet an important comparison for these simulations.

But a frozen surface does not necessarily mean a completely frozen planet.

A thick ice layer could insulate liquid water underneath, especially if heat continues escaping from the planet's interior. Similar environments already exist within our own solar system on icy moons such as Europa and Enceladus.

Can We Actually Detect Exoplanet Ice?

Direct detection remains extremely difficult, but future observations may provide clues.

When an exoplanet passes in front of its star, telescopes can analyze how different wavelengths of light interact with its atmosphere. Spectroscopy can reveal molecules such as water vapor, carbon dioxide, and methane, helping researchers constrain possible surface conditions.

Reflected light could eventually provide another clue. Snow, ocean water, rock, and vegetation reflect different wavelengths differently. A sufficiently powerful future telescope might detect spectral patterns consistent with extensive surface ice.

Astronomers must be careful, however. Clouds can also make planets highly reflective, creating a major challenge when trying to distinguish an icy surface from a cloudy atmosphere.

Glaciers on Tidally Locked Worlds

Some especially interesting possibilities involve planets orbiting red dwarf stars.

Planets close to these relatively cool stars may become tidally locked, meaning one hemisphere permanently faces the star while the other remains in darkness. Climate simulations suggest this could produce unusual cryospheres. Water might freeze on the permanent nightside while remaining liquid near the warmer region facing the star.

Ice could therefore physically redistribute a planet's water over geological time, potentially creating enormous glaciers unlike anything found on modern Earth.

Using Earth's Ice to Understand Alien Worlds

This is where glaciology, astronomy, geophysics, and computer science increasingly overlap. Researchers can use what we know about ice flow, albedo, atmospheric circulation, geothermal heating, and glacier mass balance to simulate environments we cannot directly visit.

Exoplanet cryosphere research ultimately reverses the usual way we think about glaciers. Instead of only asking what ice tells us about Earth's climate, scientists can ask a much bigger question:

What can ice tell us about whether an entirely different planet could be habitable?

Modeling the Carbon Cycle

Carbon is constantly moving around our planet. A carbon atom released from a forest fire might enter the atmosphere as carbon dioxide, dissolve into the ocean years later, become incorporated into marine life, and eventually settle into sediments on the seafloor. This enormous network of exchanges is known as the carbon cycle, and computer science is becoming increasingly important for understanding how it is changing.

The carbon cycle can be thought of as a giant computational system. Scientists divide Earth into major carbon reservoirs, including the atmosphere, oceans, vegetation, soils, sediments, and rocks. Carbon then moves between these reservoirs through processes such as photosynthesis, respiration, decomposition, ocean-atmosphere exchange, weathering, and volcanic activity.

The challenge is that these processes operate on dramatically different spatial and temporal scales. Photosynthesis can change atmospheric carbon concentrations seasonally, while geological carbon storage can operate over millions of years. This is where computational modeling becomes essential.

Turning Earth Into a Mathematical Model

At its simplest, a carbon-cycle model can represent each reservoir as a variable.

Imagine a basic model containing only the atmosphere, land, and ocean. Every simulated year, the computer calculates how much carbon moves between them. Plants remove CO₂ through photosynthesis, respiration returns some carbon to the atmosphere, and the ocean simultaneously absorbs and releases CO₂.

The model updates each reservoir repeatedly:

New carbon = existing carbon + carbon entering − carbon leaving

Real Earth-system models perform versions of this calculation across millions of grid cells while also considering temperature, rainfall, ocean circulation, vegetation, soil chemistry, and human emissions.

That requires enormous computing power.

Satellites Create a Data Problem

Computer science is also transforming where carbon-cycle models get their information.

NASA's Orbiting Carbon Observatory-2 (OCO-2) measures atmospheric carbon dioxide from space, while other Earth-observing missions provide information about vegetation, fires, ocean conditions, temperature, and land cover. Together, these instruments generate enormous datasets that would be difficult to analyze manually.

Algorithms can combine these observations with ground measurements and models to estimate where carbon is being emitted and absorbed.

This produces what scientists sometimes describe as carbon "sources" and "sinks." A forest that absorbs more carbon than it releases acts as a sink, while fossil-fuel combustion is a major source.

Machine Learning Enters the Carbon Cycle

One emerging area involves machine learning.

Instead of explicitly programming every environmental relationship, researchers can train algorithms on observations and simulations to recognize patterns. Machine-learning models can help estimate forest carbon storage, predict ecosystem productivity, detect land-use changes, and approximate computationally expensive components of climate models.

However, AI does not eliminate the need for physical science. A model might find a statistical relationship that works under present conditions but fails as Earth's climate moves into conditions absent from its training data. Combining physics-based models with machine learning is therefore an important research direction.

Debugging a Planet

Perhaps the most interesting connection between computer science and carbon-cycle science is uncertainty.

Scientists cannot simply run one simulation and assume it represents Earth's future. They may run models thousands of times while changing emissions, climate sensitivity, ecosystem responses, or ocean behavior.

In a sense, researchers are continuously debugging a digital version of Earth—comparing its output against satellites, atmospheric stations, ocean measurements, and historical records.

The better these computational models become, the better we can understand where human-produced carbon goes, how long it remains there, and how Earth's oceans and ecosystems may respond in the future.

SpaceX Reusable Rockets

For most of spaceflight history, launching a rocket has meant throwing away a remarkably expensive machine after only a few minutes of use. SpaceX is attempting to change that model with Starship, an enormous launch system designed around one central idea: make essentially the entire rocket reusable.

Starship consists of two stages. The lower Super Heavy booster provides the enormous thrust required to escape the lower atmosphere, while the upper-stage spacecraft—also called Starship—continues toward orbit. SpaceX's newer Version 3 system uses Raptor 3 engines, which burn liquid methane and liquid oxygen. On Starship's 13th flight test in July 2026, Super Heavy ignited all 33 of its engines for liftoff. 

Why Methane?

Starship's choice of methane (CH₄) is particularly interesting for long-duration space exploration.

Methane engines can be designed for reuse while producing less soot than kerosene-burning engines. Even more importantly, methane could potentially be manufactured away from Earth. Mars has an atmosphere dominated by carbon dioxide, while water ice exists beneath and around its surface. Through chemical processes such as the Sabatier reaction, hydrogen derived from water can react with carbon dioxide to produce methane.

A future Mars mission could therefore theoretically manufacture part of its return propellant on Mars rather than carrying all of it from Earth.

Catching a Rocket Instead of Landing It

One of Starship's strangest engineering features is its recovery system. Rather than always landing Super Heavy on conventional landing legs, SpaceX has demonstrated catching boosters with enormous mechanical arms attached to its launch tower.

The concept removes the mass of traditional landing legs and could eventually allow a recovered booster to be rapidly inspected, refueled and flown again. SpaceX sees rapid reuse as essential to making very high launch rates practical.

This is where Starship differs fundamentally from most previous rockets. A reusable rocket isn't truly revolutionary if rebuilding and inspecting it takes months. SpaceX is ultimately trying to make rocket operations behave more like aircraft operations, where the vehicle lands, is serviced and flies again.

Refueling a Spacecraft in Space

Reusability alone isn't enough for Starship's most ambitious missions. Traveling to the Moon or Mars requires enormous amounts of energy, so SpaceX plans to refuel Starship in Earth orbit.

A tanker Starship would launch propellant into orbit and transfer extremely cold liquid methane and oxygen to another spacecraft. This presents major engineering challenges because cryogenic liquids behave differently in microgravity and continuously absorb heat from their surroundings.

NASA's inspector general has identified large-scale vehicle-to-vehicle cryogenic propellant transfer as one of the most significant technical challenges facing the Starship lunar program.

From Earth Orbit to the Moon

Starship is also connected directly to NASA's future lunar exploration.

NASA is developing a specialized Starship Human Landing System (HLS) with SpaceX. The lunar version is roughly 50 meters tall and is intended to transport astronauts between lunar orbit and the Moon's surface during future Artemis missions. NASA is currently conducting additional testing, including wind-tunnel studies of the Version 3 Super Heavy configuration.

Starship remains an experimental system, and important capabilities—including routine rapid reuse and the full orbital-refueling architecture—still have to be demonstrated. But that experimentation is also what makes the project scientifically and technologically interesting.

If its major engineering goals can be achieved, Starship could change more than how rockets look. Large payload capacity, orbital refueling and rapid reusability could change what kinds of missions are physically and economically possible, from giant space telescopes and lunar laboratories to much more ambitious exploration of Mars.