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Chapter IV: Sensors and measuring principles

Satellites are the platform (satellite bus) used to carry meteorological instruments in an orbit around the planet. There is a wide variety of instruments operating as imagers, sounders or scatterometers. There are active and passive instruments flying in low earth or geostationary orbits.

In this chapter, you will find material on the physics of radiation and measuring principles but also an overview on the currently used meteorological satellite instruments assorted into 8 chapters:

Notice: The resources you will find in this chapter are more oriented to the technical side of the bespoken instrument and less on possible applications.

For practical applications of the instrument data, please refer to chapter 6.

Measuring Principles


In this sub-chapter, you will find resources dealing with thermal (infra-red) and short wave (visible) radiation. The physical basics as well as the characteristics of the spectral bands are treated.

Principles of Radiation and Remote Sensing (Webcast, 60 minutes), 2014

The course introduces radiation and remote sensing principles: Questions like "what is remote sensing?", "what is the electromagnetic spectrum?" and "which physical radiation laws describe all this?" are addressed. We then go into the specific application of all this to the earth-atmosphere system. The focus here is on radiation in the infrared and solar spectral range, as this is used in imaging instruments. Processes like absorption, emission, scattering and reflection will be described. In the end, a short outlook to (infra-red) hyperspectral data will be provided.

CO2 and O3 Absorption in IASI (Webcast, 60 minutes), 2012

This lecture on absorption channels and IASI instrument on board MetOp satellite was held by Xavier Calbet from EUMETSAT. MetOp is LEO polar orbiting satellite that is spinning around Earth at a height of approximately 800 km. IASI instrument is scanning atmosphere below him in mostly infra-red spectrum band, therefore it can detect emission/absorption of gases like CO2 and O3 quite good, due to their intrinsic physical properties. In that way you one can track these gases in the atmosphere and mark their spatial and temporal distribution.

Lecture starts with comprehensive physical background of radiation, with topic like absorption/emission of the atmosphere, properties of the black bodies (Sun and Earth) and EM waves, etc. which are crucial for understanding of instrument functioning. After that there is explanation of spectral band of IASI instrument, CO2 and O3 SEVIRI channels, followed by some examples of application areas of these.

Thermal Radiation (Webcast, 60 minutes), 2011

The first block of this session presents the physical principles and algorithms underlying the retrieval of Land Surface Temperature (LST) and emissivity from satellite observations. Both geostationary, GEO (MSG) and low-orbit, LEO (MetOp) satellite advantages and disadvantages are discussed. Emphasis of the presentation is on the methodologies used by LSA SAF for deriving these LST and emissivity products. The presentation also gives focus on the assessment of the uncertainty associated to the retrievals and respective validation is given.

Second part of this presentation is dedicated to algorithms used in the estimation of one of the components of the surface radiation budget - Down-welling Surface Long-wave Fluxes, i.e. DSLF. This product is derived from MSG SEVIRI instrument within LSA SAF. Problematics of first 100m atmosphere long-wave radiation is also well discussed. Presented are examples and validation results that put into evidence the strengths and caveats of this product. Current Status and further developments (CDOP2) will follow.

Solar radiation (Webcast, 60 minutes), 2011

Land surface albedo quantifies the fraction of energy reflected by the surface of the Earth. As a corollary it then also determines the fraction of energy absorbed by the surface and transformed into heat or latent energy. Land surface albedo therefore is a key variable for characterising the energy balance in the coupled surface-atmosphere system and constitutes an indispensable input quantity for soil-vegetation-atmosphere transfer models. On the other hand, the down-welling surface short-wave radiation flux (DSSF) refers to the radiative energy in the wavelength interval [0.3 to 4.0 microns] reaching the Earth\'s surface per time and surface unit. It essentially depends on the solar zenith angle, on cloud coverage, and to a lesser extent, on atmospheric absorption and surface albedo.

First presentation is devoted to an introduction of method retrievals for surface albedo and DSSF products that are implemented in framework of the LSA SAF in using MSG /SEVIRI observations. A second presentation will detail the validation exercise of these two LSA SAF operational products, which is based on inter-comparison with other satellite products, in situ measurements and outputs from NWP models.

Principles of Radiative Transfer (Webcast, 44 minutes), 2011

Satellite instruments measure radiation at different wave lengths. To correctly assess the information provided by these measurements, it is essential to know about laws of radiation as well as special characteristics of atmospheric gases. This lecture leads from physical principles to applications such as IASI measurements.

Sensors and instruments


This sub-chapter gives an overview on satellite sensors and instruments in general.

Satellite-Derived Rain Rate from Microwave Sensors Aboard Polar-Orbiting Satellites (Webcast, 30 minutes), 2015

Polar-orbiting microwave sensor generated rain rate estimates are the most accurate satellite-derived estimates of rainfall because of their direct relationship between liquid and ice hydrometeors and surface precipitation. People may find that surprising because the timeliness, latency and spatial resolution are inferior to geostationary satellite rain estimates. But the primary advantage that microwave sensors aboard operational polar orbiters have over geostationary satellites is the ability to see through the clouds and capture what is below the cloud tops. The current operational polar-orbiting satellite sensors aboard the DMSP (SSMI/S sensor) and NOAA/METOP/S-NPP satellites (AMSU/MHS and ATMS sensors) that generate instantaneous areal average microwave rain rates will be presented and explained, along with the recent development of combining these measurements into a Blended Rain Rate Product and an additional blended product called QMORPH. In the future polar-orbiting microwave rain rates will be used as a calibration of the next generation of geostationary satellite-derived rain estimates. A few recent case studies of the best use of operational microwave rain rates will also be shown, as well as those from more research oriented missions, such as GCOM (AMSR-2 sensor) and GPM (GPI sensors) that are also used in operations.

Instruments onboard Polar and Geostationary Satellites (Webcast, 30 minutes), 2014

This lecture presents an overview on the most commonly used instruments on board geostationary and polar orbiting satellites. The focus is laid on similarities and differences of instrument characteristics between these two types of satellites. Different types of instruments, covering the meteorological relevant spectrum, are shown. Active and passive instruments are explained as well as the presently used scanning principles.

Sounding instruments


Atmospheric sounding instruments provide a measurement of vertical distribution of physical properties of the atmospheric column such as pressure, temperature, wind speed and wind direction (thus deriving wind shear), liquid water content, ozone concentration, pollution, and other properties.

Atmospheric Soundings (Webcast, 35 minutes), 2012

High resolution infrared sounders, such as AIRS and IASI, and microwave sounders, such as AMSU, are a key element of the global satellite observing system and provide a wealth of data important for various operational applications including data assimilation and nowcasting applications. This presentation will revisit the physical basis of infrared and microwave sounding and provide an overview on the state-of-the-art of microwave and infrared soundings.

Altimeter


Altimeter instruments are used to monitor the sea level and sea level changes over longer time periods. The data record from these altimetry missions has given scientists important insights into how global sea level is affected by natural climate variability (e.g. El Nino), as well as by human activities.

Measuring Wind and Waves from Space - Altimeters (Webcast, 60 minutes), 2013

Conventional observations of wind and wave data from ships and buoys are limited by coverage. Due to the increasing size of ships, observers are more removed from the ocean surface and wave estimation is more difficult. Satellite altimeters are a downward pointing radar that scan the oceans and provide very accurate observations of wave height, wind speed, sea surface height anomaly. Our understanding of sea level-rise is due in large part to the record of altimeter-based sea level data.

Forecasters responsible for ocean and coastal waters must rely on any information for situational awareness and to keep forecasts relevant. Satellite altimeters provide very accurate coincident measures of significant wave height and wind speed. Integrated data display and product generation systems offer the opportunity for forecasters to compare a variety of observations, imagery, and numerical model predictions to enhance awareness and communicate hazards to mariners. This session will discuss the use by forecasters of altimeter wave and wind data with other sources in an operational forecast center.

Jason 2 altimetry data (Webcast, 30 minutes), 2012

This presentation reports on the use of significant wave height (SWH) altimetry product, derived from OSTM/Jason-2 data, during a recent wave storm in the Northeast region of the Atlantic Ocean at mid-April 2012. Altimetric measurements provide data for wave model assimilation and also support ocean forecasts. Sérgio Muacho will focus on remote-sensed observations that cover a period of one week and track the wave storm over the Atlantic between Iceland,the Portuguese Continental West coast and the Gulf of Biscay. This study provides a summary of this wave storm and highlights the importance of having altimetry data in ocean areas, where there is a lack of observations, especially in off-shore regions.

The Use of Altimeter Data (Webcast, 40 minutes), 2012

Altimeter estimates of the significant wave height Hs are quite accurate. The assessment of the wave energy resource in deep water locations requires not only the knowledge of Hs but also of the wave period, in particular the energy period Te. Several models have been proposed in order to derive the mean zero-crossing period from the altimeter backscatter coefficient with reasonable accuracy. The attempts to establish a relation between Tz and Te revealed significant constraints. As a consequence, the exclusive use of altimeter data may be suitable only for preliminary wave energy resource assessment purposes. Additionally, given their wide geographic availability, altimeter Hs estimates are typically applied in the validation of wave model data used in wave energy resource assessment studies.

Waves from Radar Altimetry Satellites (Webcast, 30 minutes), 2011

Radar altimetry satellites have been measuring the Earth's ocean since 1991. Currently Jason-2, Envisat and Jason-1 are operational. If the main objective of those are the measurement of sea surface height, wind speed (modulus) and significant wave heights are very useful - bonuses - to these techniques. The presentation will explain how these quantities are retrieved from the altimeter measurements, and how they are used in meteorology, climatology and oceanography.

Sea Surface Height Variation (Webcast, 30 minutes), 2011

Radar altimetry satellites have been measuring the whole Earth's ocean since 1991. Currently Jason-2, Envisat and Jason-1 are operational. Their main objective is the measurement of sea surface height variations. With a twenty-years long time series, the possibilities for climate studies are wide. We will describe roughly the data processing and uses, but mostly insist on two different applications:

  • El Niño - Southern Oscillation
  • Global Mean Sea level

Scatterometer


Scatterometer instruments are transmitting a pulse of microwave energy towards the Earth's surface and measure the reflected energy. The primary application of space borne scatterometry is measurements of near-surface winds over the ocean and soil moisture over land.

Sea Surface Wind Vectors (Webcast, 117 minutes), 2017

The lecture deals with modelled winds and winds derived from instruments onboard satellites like MetOp-A and MetOp-B in low orbits around the Earth (polar orbits). Today's models are evolving at a rate that is faster than the increase of density of observations and that presents a problem for forecasts. Here stands the question 'Will meteorology continue to develop and improve?'. The lack of observed data is thus filled with the data from satellites, although this data also has its own constraints due to the way it is derived. In the lecture the characteristics of the satellites carrying instruments for measuring winds and waves will be explained and the logic behind the calculations of winds using satellites will be discussed.

Scatterometer data are used for many different purposes in marine meteorology, e.g. warnings, enhancement of situational awareness for winds, monitoring of storm evolution, low pressure systems, etc., therefore marine forecasters using the products about wind and waves from satellites will be instructed how to use them and when to combine the data with model outputs.

Measuring Winds from Space - Scatterometers (Webcast, 60 minutes), 2013

Winds over sea are essential for marine forecasting and used in nowcasting and numerical weather prediction (NWP) to aid, among others, in off-shore activities (e.g., energy sector), transport and recreation, particularly to secure safety of life and property. Winds over sea are observed by satellites and available from NWP model forecasts. Most satellite winds over sea are provided by scatterometers these days; they provide swath fields of both wind speed and wind direction from polar satellites. More satellite winds are becoming available through a global virtual constellation of scatterometers. Currently, winds from MetOp-A, MetOp-B and the Indian OceanSat-2 are operationally available and provide good day coverage. Moreover, winds from the Chinese HY2A satellite are being tested with good results at KNMI and further scatterometer launches are being approved. The lecture will briefly comment on these developments, but mainly focuses on what scatterometer winds really represent, how good they are for marine forecasting and what aspects need attention when applying these winds in your routine operations.

Scatterometer Winds (Webcast, 30 minutes), 2011

Scatterometers uniquely define the mesoscale wind vector field at the sea surface by measuring the radar backscatter signal from wind-generated cm-sized, so-called gravity-capillary sea waves. Because of the wavelength (5 cm), the signal of ASCAT is not affected by rain, and is therefore an "all-weather" system.

The all-weather capability of the ASCAT scatterometer provides unique wind field products of the most intense and often cloud-covered wind phenomena, such as polar front disturbances and tropical cyclones. As such, it has been demonstrated that scatterometer winds are extremely useful in the prediction of extra-tropical and tropical cyclone). Moreover, high-resolution near-surface winds as provided by scatterometers are very relevant because these winds drive the ocean water circulation, which in turn plays a major role in the climate system and in marine life and its exploitation (e.g., fishery).

ASCAT (Webcast, 30 minutes), 2011

The Advanced Scatterometer (ASCAT) is one of the new-generation European instruments carried on MetOp and will be used to determine information about the wind for use primarily in weather forecasting and climate research. Data from ASCAT will also find applications in a number of other areas such as the monitoring of land- and sea-ice, snow cover and soil moisture.

Lightning imager


The Lightning Imager (LI) instrument is designed to detect lightning discharges from the satellite orbit day and night.

Meteosat Third Generation Lightning Imager (MTG-LI): Flash and Accumulated Products and Test Data for User Readiness Activities (Webcast, 30 minutes), 2016

EUMETSAT has developed with ESA the geostationary Meteosat Third Generation (MTG) satellite system to continue and enhance the service currently provided by Meteosat Second Generation (MSG), from 2020 onwards. One of the new missions of MTG is the Lightning Imager (LI) mission, which is intended to provide a real time lightning detection and location capability in support to nowcasting of high impact weather. The LI measures the total lightning, i.e. the combined cloud-to-ground and intracloud/cloud-to-cloud lightning. One of the major tasks in the MTG program is to provide the Level 2 product processor. The initial L2 products are based on the detection of the optical pulses on top of clouds following a false event filtering and clustering of the detected triggered lightning events in time and space. These are composed to the initial L2 products, i.e. groups and flashes representing geophysical flashes and strokes. A further L2 product category is the Accumulated Products, which integrate the Flash products into a fixed 2 km grid (same as for the MTG Flexible Combined Imager IR channels) with a 30 sec accumulation period. These 30-sec periods can be further stacked by users. The products are disseminated as short duration chunks in order to meet the timeliness requirements.

Preparing the operational community for satellite-based total lightning Observations (Webcast, 30 minutes), 2016

Short-term Prediction Research and Transition (SPoRT) Center, part of the United States' NASA program, has been a leader in transitioning total lightning observations to operational forecasters. This effort began in 2003 with a single, NASA-owned ground-based lightning mapping array. The effort has since expanded to include multiple networks supporting a variety of end users for severe weather, safety, and aviation. This effort serves as the foundation for preparing the United States forecaster community for the Geostationary Lightning Mapper (GLM) set to launch aboard GOES-R. This presentation will discuss the GLM and its observations, what total lightning is, and how these data can be used operationally based on ongoing activities to prepare the United States for GLM. The GLM instrument is very similar to the Meteosat Third Generation's Lightning Imager and presents an opportunity for collaboration between U.S. and EUMETSAT forecasters.

An Introduction to the GOES-R Geostationary Lightning Mapper (Webcast, 30 minutes), 2016

The NOAA Geostationary Operational Environmental Satellite (GOES-R) series provides the continuity for the existing GOES system currently operating over the Western Hemisphere. The Geostationary Lightning Mapper (GLM) represents an advancement over current GOES by providing an entirely new capability for total lightning detection (cloud and cloud-to-ground flashes). The GLM will map total lightning continuously day and night with near-uniform spatial resolution of 8 km with a product latency of less than 20 seconds over the Americas and adjacent oceanic regions. The total lightning is very useful for identifying hazardous and severe thunderstorms, monitoring storm intensification and tracking evolution. Used in tandem with radar, visible and infrared satellite imagery, and surface observations, total lightning data has great potential to increase lead time for severe storm warnings, improve aviation safety and efficiency, and increase public safety.

MTG: Lightning Imager (Webcast, 30 minutes), 2015

The Meteosat Third Generation Imaging satellite will include in addition to the Flexible Combined Imager (FCI), which is a continuation of the imaging mission, a Lightning Imager (LI) which is a completely new mission. The MTG LI will complement existing ground based capabilities for the detection and location of lightning with information on cloud-to-ground (CG) and cloud-to-cloud or intracloud (CC/IC) discharges, i.e. total lightning. The presentation will cover the background for lightning detection from space, including the detection and processing principles, and will concentrate on the meteorological products from the lightning imager. These include lightning flashes as well as accumulated lightning products.

Use of Total Lightning Data in Convective Events (Webcast, 30 minutes), 2015

NASA's Short-term Prediction Research and Transition (SPoRT) center has been a leader in transitioning total lightning observations from ground-based lightning mapping arrays (LMAs) to operational forecasters. This effort began in 2003 with a single, NASA-owned LMA and has since expanded to include multiple LMAs focusing on warning decision support, lightning safety, and aviation forecast needs. This presentation will cover SPoRT's total lightning activities, focusing on operational applications both with the LMAs directly and in preparation for the GOES-R Geostationary Lightning Mapper.

Radar


Space borne Radar onboard meteorological satellites are primarily used to map the ocean's surface (waves, oil spills,...) or to monitor precipitation from space.

Observing Precipitation with Radars in Space (Webcast, 30 minutes), 2015

Radars provide the most direct means of remotely measuring precipitation over large areas. The value of ground-based radars for indicating the location and intensity of rainfall was recognized shortly after the first surveillance radars were developed during World War II. Since then radar networks have been widely used to estimate precipitation accumulations, track storms, and provide valuable information to the public. However, vast areas of the Earth including oceanic, mountainous, and densely forested regions are largely inaccessible to ground-based instruments. As a result, much of our knowledge of precipitation in such areas derives from passive satellite measurements that can suffer from uncertainties owing to the indirect relationship between radiation measured at the satellite and rainfall at the surface. Satellite precipitation radars offer some of the benefits of their ground-based counterparts with the enhanced coverage afforded by orbiting the Earth. To date three such radars have been launched providing a new perspective of global precipitation from tropical rainfall to polar snows that complements the longer-term record provided by passive instruments. This lecture will describe the physical principles used to relate the reflectivities measured by these radars to precipitation intensity. We will examine the main features of the associated precipitation retrieval algorithms and some of the important factors that need to be considered will be highlighted through illustrative examples. We will conclude by contrasting the strengths and limitations of satellite radars against passive satellite sensors and ground-based radar systems.

SAR Image Analysis (Webcast, 30 minutes), 2011

Synthetic-aperture radar (SAR) is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional beam-scanning means. It originated as an advanced form of side-looking airborne radar (SLAR).

SAR is usually implemented by mounting, on a moving platform such as an aircraft or spacecraft, a single beam-forming antenna from which a target scene is repeatedly illuminated with pulses of radio waves at wavelengths anywhere from a meter down to millimeters. The many echo waveforms received successively at the different antenna positions are coherently detected and stored and then post-processed together to resolve elements in an image of the target region.

Current airborne systems provide resolutions to about 10 cm, ultra-wideband systems provide resolutions of a few millimeters, and experimental terahertz SAR has provided sub-millimeter resolution in the laboratory.

SAR images have wide applications in remote sensing and mapping of the surfaces. In this presentation the applications for monitoring the ocean surfaces are discussed.

Ocean colour instrument


The prime objective of this instrument is to observe the colour of the ocean, both in the open ocean and in coastal zones. These data are used to derive estimates of the concentration of chlorophyll and sediments in suspension in the water.

Ocean Applications with MERIS (Webcast, 30 minutes), 2011

Webcast on the study of ocean color which helps scientists to gain a better understanding of phytoplankton and their impact on the Earth system. These small organisms can affect a system on a very large scale such as climate change. Phytoplankton use carbon dioxide for photosynthesis and in turn provide almost half the oxygen we breathe. The larger the world's phytoplankton population, the more carbon dioxide gets pulled from the atmosphere, hence, the lower the average temperature due to lower volumes of this greenhouse gas. Scientists have found that a given population of phytoplankton can double its numbers about once per day. In other words, phytoplankton respond very rapidly to changes in their environment. Large populations of these organisms, sustained over long periods of time, could significantly lower atmospheric carbon dioxide levels and, in turn, lower average temperatures. Carbon can be "stored" in oceanic sediments when organic matter sinks and is buried in the ocean floor.

Understanding and monitoring phytoplankton can help scientists' study and predict environmental change. Since phytoplankton depend upon sunlight, water, and nutrients to survive, physical or chemical variance in any of these ingredients over time for a given region will affect the phytoplankton concentrations. Phytoplankton populations grow or diminish rapidly in response to changes in its environment. Changes in the trends for a given phytoplankton population, such as its density, distribution, and rate of population growth or diminishment, will alert Earth scientists that environmental conditions are changing there. Then, by comparing these phytoplankton trends to other measurements - such as temperature - scientists can learn more about how phytoplankton may be contributing to, and affected by, climatic and environmental change.

Satellite Chlorophyl Patterns (Webcast, 30 minutes), 2011

Presentation about satellite images of phytoplankton chlorophyll in the North Atlantic. André Valente introduces how phytoplankton chlorophyll is measured from space, describing the spatial and temporal patterns observed in the satellite images and identify the physical processes responsible for the observed variability.

The most important light-absorbing substance in the oceans is chlorophyll, which phytoplankton use to produce carbon by photosynthesis. Due to this green pigment - chlorophyll - phytoplankton preferentially absorb the red and blue portions of the light spectrum (for photosynthesis) and reflect green light. So, the ocean over regions with high concentrations of phytoplankton will appear as certain shades, from blue-green to green, depending upon the type and density of the phytoplankton population there. The basic principle behind the remote sensing of ocean color from space is this: the more phytoplankton in the water, the greener it is....the less phytoplankton, the bluer it is.