Monitoring and Measurement Technologies

3.1 The Importance of Sea Level Monitoring

Accurate and continuous monitoring of sea level is fundamental to understanding climate change impacts, validating scientific models, and informing adaptation decisions. The ability to measure sea level changes with millimeter precision from space and ground-based networks represents one of the great achievements of modern Earth observation science. These measurements provide the empirical foundation upon which all projections and policies must be built.

The challenge of measuring sea level is deceptively complex. The ocean surface is constantly in motion due to waves, tides, currents, and atmospheric pressure variations. Distinguishing the signal of long-term sea level rise from this "noise" requires sophisticated measurement systems, careful data processing, and continuous validation against multiple independent data sources. Modern sea level monitoring employs a diverse array of technologies, each with unique strengths and limitations.

The WIA-SLR-001 standard establishes protocols for sea level monitoring that ensure data quality, interoperability, and accessibility. By adhering to these standards, monitoring networks worldwide can contribute to a coherent global picture of sea level change. This chapter explores the key technologies employed in sea level monitoring, from traditional tide gauges to cutting-edge satellite systems.

3.2 Satellite Altimetry

Satellite altimetry has revolutionized our ability to measure global sea level changes. By emitting radar pulses from space and precisely measuring the time for those pulses to reflect off the ocean surface and return to the satellite, altimeters can determine sea surface height with remarkable accuracy. The continuous global coverage provided by satellite altimetry complements and extends the capabilities of ground-based tide gauge networks.

The fundamental principle of radar altimetry is straightforward: the two-way travel time of a radar pulse, multiplied by the speed of light and divided by two, gives the range from the satellite to the sea surface. However, achieving the millimeter-level precision required for climate studies requires numerous corrections for atmospheric delays, electromagnetic bias, tidal effects, and satellite orbital uncertainties. The development of these correction techniques represents decades of scientific and engineering advancement.

3.2.1 The Satellite Altimetry Reference Series

The modern era of satellite sea level measurement began with the TOPEX/Poseidon mission launched in 1992, a joint project of NASA and the French space agency CNES. This mission established the foundation for a continuous climate-quality record that has been maintained through successive missions including Jason-1, Jason-2 (OSTM), Jason-3, and most recently Sentinel-6 Michael Freilich. Each new mission is carefully calibrated against its predecessor to ensure continuity of the record.

3.2.2 Measurement Corrections

The raw range measurement from a satellite altimeter must undergo numerous corrections before it can be used for climate applications. The ionosphere delays radar signals in proportion to electron density along the path; dual-frequency altimeters measure this delay directly. The troposphere introduces delays due to water vapor and dry air density that must be corrected using radiometer measurements and atmospheric models. Ocean tides, which can raise or lower sea level by meters in some locations, must be removed using tidal models.

Electromagnetic bias corrections account for the tendency of radar to preferentially reflect from wave troughs rather than crests, and for variations in sea state. Inverse barometer corrections remove the response of sea level to atmospheric pressure variations. Finally, precise orbit determination using GPS and ground-based laser ranging ensures that the satellite position is known to within a few centimeters—essential for centimeter-level sea level accuracy.

3.3 Tide Gauge Networks

Tide gauges have provided the longest records of sea level change, with some stations operating continuously for over 200 years. These instruments measure the height of the sea surface relative to a local benchmark on land, providing valuable information about relative sea level changes experienced at specific coastal locations. The global tide gauge network, coordinated through the Global Sea Level Observing System (GLOSS), comprises over 2,000 stations distributed around the world's coastlines.

Traditional tide gauges used mechanical float systems within stilling wells to filter out short-period waves and record sea level on paper charts. Modern stations employ a variety of technologies including pressure sensors, acoustic ranging, and radar, often with multiple sensors for redundancy. Data are transmitted in real-time via satellite or telecommunications links to data centers where they undergo quality control and distribution.

3.3.1 Tide Gauge Technology Types

Several technologies are employed in modern tide gauge installations. Pressure sensors measure the weight of the water column above them, with corrections applied for atmospheric pressure and water density variations. Acoustic gauges measure the time for sound pulses to travel to the water surface and back within a protective tube. Radar gauges use similar ranging principles with electromagnetic pulses, offering advantages for maintenance and reliability.

Each technology has strengths and limitations. Pressure sensors can be affected by fouling and require careful density corrections but are robust in harsh environments. Acoustic gauges require protection from waves but offer excellent precision. Radar gauges are increasingly popular due to their contactless operation and low maintenance requirements. The WIA-SLR-001 standard provides guidance on sensor selection, installation, and calibration for different operational environments.

3.3.2 Connecting Tide Gauges to Global Reference Frames

A fundamental challenge with tide gauge data is that they measure relative sea level—the height of the sea surface relative to the land. If the land upon which a tide gauge sits is rising or sinking, this motion will be recorded as an apparent change in sea level. Separating true ocean changes from vertical land motion requires connecting tide gauges to stable global reference frames using geodetic techniques.

GNSS (Global Navigation Satellite Systems) receivers co-located with tide gauges allow precise measurement of vertical land motion at each site. When combined with tide gauge records, this enables calculation of "geocentric" or absolute sea level changes referenced to Earth's center of mass. The WIA-SLR-001 standard requires GNSS co-location at climate-quality tide gauge installations and specifies protocols for combining these measurements.

3.4 GNSS and Vertical Land Motion

Global Navigation Satellite Systems provide essential information about vertical land motion that affects relative sea level experienced at coastlines. In addition to co-location with tide gauges, networks of GNSS stations provide broader spatial coverage of land motion patterns. Understanding vertical land motion is crucial for interpreting tide gauge records, validating satellite altimetry in coastal regions, and projecting future relative sea level change at specific locations.

Vertical land motion arises from multiple processes operating on different spatial and temporal scales. Glacial Isostatic Adjustment (GIA)—the ongoing response of Earth to the melting of ice age glaciers—causes some regions to rise and others to subside at rates of several millimeters per year. Tectonic processes cause both gradual deformation and sudden changes during earthquakes. Sediment compaction and fluid extraction cause subsidence in many deltas and coastal cities.

3.4.1 GNSS Data Processing

Achieving the millimeter-level precision required for climate applications demands sophisticated GNSS data processing. Raw satellite observations must be corrected for atmospheric delays, satellite clock errors, antenna phase center variations, and numerous other effects. The most precise results come from processing techniques that use carrier phase observations and resolve integer ambiguities—the number of wavelengths between satellite and receiver.

Data from global GNSS networks are processed by analysis centers worldwide and combined into authoritative products by the International GNSS Service (IGS). These products include precise satellite orbits and clocks, reference frame realizations, and position time series for thousands of stations. The WIA-SLR-001 standard specifies requirements for GNSS data quality, processing standards, and integration with tide gauge and altimetry data.

3.5 Gravity Measurements and Ice Mass Balance

The GRACE (Gravity Recovery and Climate Experiment) and GRACE-FO (GRACE Follow-On) satellite missions have provided a revolutionary view of mass redistribution within Earth's system, including ice sheet and glacier mass loss. These missions measure tiny variations in the distance between twin satellites caused by variations in Earth's gravitational field, enabling detection of mass changes at the surface with unprecedented sensitivity.

The ability to directly measure ice mass changes independent of surface elevation changes has been transformative for understanding ice sheet contributions to sea level rise. GRACE/GRACE-FO data have confirmed and refined estimates of mass loss from Greenland and Antarctica, revealed patterns of groundwater depletion worldwide, and provided crucial closure of the global sea level budget. The continuity of this measurement capability is essential for ongoing climate monitoring.

3.5.1 Applications for Sea Level Science

GRACE/GRACE-FO measurements have numerous applications for sea level science. Direct measurement of ice sheet mass balance provides one component of the sea level budget independent of altimetry and tide gauges. Measurement of ocean mass changes (barystatic sea level) complements steric changes measured by Argo floats. Detection of groundwater depletion quantifies land water contributions to sea level rise. Together, these measurements enable closure of the sea level budget with unprecedented accuracy.

3.6 Ocean Temperature and Salinity: The Argo Network

The Argo program maintains a global array of nearly 4,000 free-drifting profiling floats that measure temperature and salinity in the upper 2,000 meters of the ocean. Initiated in 2000, Argo has revolutionized our understanding of ocean variability and provided essential data for calculating thermosteric sea level changes. The array provides approximately 12,000 temperature-salinity profiles per month, with near-global coverage of the ice-free ocean.

Each Argo float operates autonomously, drifting at a parking depth (typically 1,000 m) and ascending every 10 days to measure a profile of temperature and salinity as it rises to the surface. At the surface, the float transmits its data via satellite before descending to begin another cycle. Floats typically operate for 4-5 years before battery exhaustion, requiring continuous deployment of new floats to maintain the array.

3.6.1 Deep Argo Extension

Recognizing the importance of deep ocean warming for long-term sea level rise, the oceanographic community has developed Deep Argo floats capable of profiling to 6,000 meters depth. The deployment of Deep Argo floats is expanding to provide coverage of the deep ocean below 2,000 meters, where significant warming has been detected using ship-based measurements. This extension will improve estimates of total ocean heat content and thermosteric sea level change.

3.7 Coastal Monitoring Systems

While open-ocean monitoring systems provide essential information about global sea level changes, coastal communities need detailed information about local conditions to plan effectively for rising seas. Coastal monitoring systems combine multiple technologies to track not only sea level but also waves, currents, storm surge, erosion, and subsidence at scales relevant for local decision-making.

High-resolution coastal monitoring often employs arrays of sensors including tide gauges, wave buoys, current meters, and weather stations. Increasingly, remote sensing technologies such as coastal radar and satellite imagery provide spatial coverage of nearshore conditions. Integration of these diverse data streams requires sophisticated data management systems that are addressed by the WIA-SLR-001 standard's coastal monitoring protocols.

3.7.1 InSAR for Coastal Subsidence

Interferometric Synthetic Aperture Radar (InSAR) provides detailed measurements of land surface deformation across wide areas, making it particularly valuable for monitoring subsidence in coastal cities and deltas. By comparing radar images acquired at different times, InSAR can detect millimeter-scale changes in surface elevation over areas spanning hundreds of kilometers. This technology has revealed alarming subsidence rates in many coastal megacities.

InSAR measurements complement point-based GNSS measurements by providing spatial coverage that reveals patterns of differential subsidence within urban areas. This information is essential for understanding local relative sea level rise and identifying areas most vulnerable to flooding. The WIA-SLR-001 standard incorporates InSAR-based subsidence monitoring as a component of comprehensive coastal assessment.

3.8 Data Integration and Quality Control

The diverse measurement systems described in this chapter generate enormous quantities of data that must be integrated, quality-controlled, and made accessible to scientists, engineers, and policymakers. Data integration is challenging due to differences in measurement techniques, spatial and temporal resolution, reference frames, and error characteristics. The WIA-SLR-001 standard establishes protocols for data harmonization and quality assurance that enable robust scientific conclusions.

Quality control procedures identify and flag or remove erroneous measurements that could bias scientific conclusions. Automated quality control algorithms check for physical plausibility, internal consistency, and agreement with nearby measurements. Expert human review addresses cases that automated systems cannot resolve. Metadata describing measurement conditions, processing versions, and known issues must accompany all data products.

3.9 Chapter Summary

This chapter has explored the diverse technologies employed to monitor sea level changes, from space-based radar altimeters to in-situ tide gauges and oceanographic floats. Key points include satellite altimetry providing continuous global sea level monitoring since 1992, with precision approaching 1 mm/year for trend detection. Tide gauge networks provide coastal relative sea level measurements and historical records extending back centuries in some locations. GNSS measurements enable separation of ocean changes from vertical land motion, essential for interpreting tide gauge records.

GRACE/GRACE-FO gravity missions directly measure ice mass loss and ocean mass changes. The Argo float network provides global coverage of ocean temperature and salinity for thermosteric calculations. Integration of diverse measurements enables closure of the sea level budget and validation across systems. The WIA-SLR-001 standard establishes requirements for data quality, accessibility, and interoperability.