Environmental GeodesyLecture 7 (March 8, 2011): Observing Earth's surface displacements: The imaging techniques-Introduction-Satellite altimetry-DORIS-InSAR-LIDAR-GNSS-Reflectometry

Introduction

Space-geodetic tracking methods provide time series of point movements: - high temporal resolution; - high accuracy; - tracking stations normally placed on the land surface.

There are two broad classes of techniques to monitor changes in Earth's geometry:- space-geodetic tracking techniques: - monitor the deformation of a polyhedron (points) defined by ground-based networks of tracking stations; - use either passively signals from satellites (GNSS) or stellar objects (VBLI) or actively send out signals to satellites (SLR and DORIS); - air- or space-born remote sensing techniques: - send signals from airplanes or satellites to the Earth's surface and utilize the reflections to map the surface.

Remote-sensing techniques in general:- have much lower temporal resolution;- provide information with potentially high spatial resolution and much better coverage, including the surface of oceans, lakes, and ice sheets.

IntroductionRemote Sensing in the most generally accepted meaning refers to instrument-based techniques employed in the acquisition and measurement of spatially organized (for the Earth, most commonly geographically distributed) data/information on some property(ies) (spectral; spatial; physical) of an array of target points (pixels) within the sensed scene (anywhere in the Universe) that correspond to classes, features, objects, and materials, doing this by applying one or more recording devices not in physical, intimate contact with the item(s) under surveillance (thus at a finite distance from the observed target, in which the spatial arrangement is preserved); techniques involve amassing knowledge pertinent to the sensed scene (target) by utilizing electromagnetic radiation, force fields, or acoustic energy sensed by recording cameras, radiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, sound detectors, seismographs, magnetometers, gravimeters, scintillometers, and other instruments.

Altimetry

Satellite AltimetryAltimetry: Transition from point measurements to linear measurements.

Satellite Altimetry

Two different techniques: - Radar altimetry - Laser altimetryBoth yield measurements of elevations along paths with narrow footprints. A series of parallel profiles provides a framework for contouring

Altimeters send (active) signals to reflecting surfaces and record the reflected signals.

The signals can be either radio pulses (radar included) or light pulses (laser).

Altimeter measure the total roundtrip times from the satellite to the targets (solid land, tree tops, ice, water, etc.) and back.

Extremely precise determination of transit times provides for accuracies of a few centimeters in determining elevations along the traverse.

Two Links with summaries of Satellite Altimetry:http://rst.gsfc.nasa.gov/Sect11/Sect11_7.htmlhttp://earth.eo.esa.int/brat/

Altimetry: Transition from point measurements to linear measurements.

Satellite AltimetryTwo different techniques: - Radar altimetry - Laser altimetry

Main disadvantage of lasers: cloud interference with the light beam.

Satellite AltimetryTwo different techniques: - Radar altimetry - Laser altimetry

Satellite/Airborne AltimetryLaser altimetry has been used on aircrafts for several decades.

Lasers can be directed in a scanning mode.

More Later on LiDAR

Satellite AltimetrySatellite radar altimetry provides height measurements of the instantaneous surface (sea, ice, or open water on land) with respect to a fixed reference (typically the conventional reference ellipsoid).

Onboard radar altimeter transmits a short pulse of microwave radiation with known power towards the nadir.

Part of the incident radiation reflects back to the altimeter.

Measurement of the round-trip radar signal travel time provides the height of the satellite (altimetric range) above the instantaneous sea/land water/ice surface.

Its difference with the satellite altitude above the reference ellipsoid (computedthrough precise orbit determination) gives sea/land/water/ice surface height measurements with respect to the reference. The range from the satellite to mean sea surface must be corrected for: - various components of the atmospheric refraction; - biases between the mean electromagnetic scattering surface and mean sea surface at the air-sea interface. - a number of geophysical effects.

Satellite AltimetryIceSat in the NASA EOS series was launched in 2001:- uses the Geoscience Laser Altimeter System (GLAS);- designed to measure ice-sheet topography, as well as cloud and atmospheric properties, - also surveys selected land and water surfaces. - 40 pulse/sec beam; - two level energy output: - 1.064 µm (infrared) for surface surveys; - 0.532 µm signal for atmospheric (clouds and aerosols) measurements. - footprint of beam is 70 meters- separation between pulses amounting to 170 m; - orbital altitude is 705 km; - height precision on ice of is 10 cm; - characteristics of the altered return pulse indicate surface roughness.

Satellite AltimetryState of the art satellite radar altimetry has more than 2 decades of heritage:- GEOS-3 (1975), - SEASAT (1978), - Geosat (1985-1989), - ERS-1 (1991-1996),- TOPEX\-/Poseidon (1992-2006), - ERS-2 (since 1995), - GFO (since 2000), - ENVISAT (since 2002),- Jason-1 (since 2001)- Jason-2 (since 2008).

Main technological improvements: TOPEX/Poseidon, ENVISAT and Jason-1have decreased considerably the instrumental noise for a point-to-point measurement.

With precise modeling of the geophysical and environmental corrections as well as inprecise orbit computation, the total rms measurement accuracy of altimetry-basedsea surface height is currently about 1-2 cm for a single measurement.

Satellite AltimetryJASON-2 satellite: payload is composed of - a Poseidon-3 radar altimeter supplied by CNES, - an Advanced Microwave Radiometer (AMR) supplied by NASA/JPL, and - a triple system for precise orbit determination: - the DORIS instrument (CNES), - GPS receiver and - a Laser Retroflector Array (LRA) (NASA).

Continuation of satellite radar altimetry is very important for measurements of sea level changes.

Satellite Altimetry- Satellite radar altimetry originally developed for oceanographic purposes, - early altimetry missions mainly served to map the marine geoid globally with high precision and resolution,- TOPEX/Poseidon mission (early 1990s) increased the precision of sea surface height measurements by a factor 10 or more: precise determination of the temporal variability of the ocean surface.

Major results:- surface currents and the ocean dynamic topography, - ocean seasons, - El Nino, - ocean heat content, - sea level rise, - ocean tides, - waves, etc.

Satellite radar altimetry has also proved very useful for measuring the change in elevation of ice sheets and water level of lakes, rivers and floodplains on land.

DORIS

DORISStands for:Doppler Orbitography and Radiopositioning Integrated by Satellite-French system, maintained by the French Space Agency (CNES)-Designed to determine satellite orbits-A beacon is installed on the ground and emits a radio signal, which is received by the satellite (some say GPS in reverse)-There are about 50-60 stations equally distributed over the earth and ensure a good coverage for orbit determination.-The best known satellites equipped with DORIS are the two altimetry satellites TOPEX/Poseidon and Jason.-Apart from orbit determination, DORIS can be used for positioning ground stations, and is used in ITRF.

InSAR

InSARInSAR:- is a processing technique for SAR images;- has demonstrated the potential to revolutionize deformation monitoring from spaceborne platforms;- gives deformation information for extended areas (up to a few hundred km across);- truly is a remote sensing technique which can provide spatially smooth three- dimensional maps of surface change, including that from earthquakes, volcanoes, ice sheets, glaciers, fluid extraction, and landslides.

SAR: - radar signals are radiated from a moving platform and are reflected back to the antenna from the surface of the Earth. - The intensity and phase of the reflected signal are measured. - In order to measure topography, two antennas separated in space are used to measure phase differences between the two antennas from a radar signal reflected from one point on the Earth's surface.

SRTM is a radar mission that mapped 80% of the Earth's topography using thistechnique.

InSAR

Topography estimated from SRTMFormatted in MatlabPlotted in GMTWith GPS velocities in Sierra Nevada Reference frame

InSAR

InSARSeveral radar missions have used interferometric techniques for topography andsurface change: - SRTM mapped 80% of the Earth's topography in a 10-day mission in 2000. - The European ERS-1 and ERS-2 missions, the Japanese JERS-1 and ALOS missions, and the Canadian Radarsat missions have provided important data sets for measuring surface change.

The European and Canadian missions are C-band instruments, and the short wavelength signal decorrelates over vegetated regions.

U.S. National Research Council recommends an L-band InSAR mission with 8-day repeat to provide global coverage of Earth's deforming regions.

InSARSuccesses from radar interferometry include: - measurement of ice velocities, - SRTM topographic map, - discovery of actively inflating volcanoes that were thought to be dormant, - measurement of interseismic, coseismic, and postseismic deformation related to earthquakes, - observation of incipient landslides, and - subsidence due to water and oil withdrawal.

InSAR

Earthquakes and Volcanos!

InSARExamples of Subsidence:

Las Vegas, NV (Bell et al. 2002) Coso, CA Geothermal (Wicks et al., 2001)

Also see USGS page on measuring human induced subsidence from space: http://pubs.usgs.gov/fs/fs06903/images/p01Bbg.jpg

InSARVolcanic Unrest: Three Sisters, Oregon

InSARVolcanic Unrest: Galapagos

InSAR

Slope Stability

InSARRemaining issues: - combination of imaging and point measurements; - connection to a global geocentric reference frame; - reduction of atmospheric effects.

InSARA source of noise:

Signals attributable to the atmosphere

Li et al., 2006

Li et al., 2005

InSAR

Steady “secular” motion measured with InSAR in areas of active crustal deformation.

InSAR

InSAR sensitive to vertical motions as well as horizontal

Signal of uplift of the Sierra Nevada seen in long InSAR/GPS time series.

LiDAR

LIDARLIght Detection And Ranging (LIDAR):- uses same principle as radar;- LIDAR instrument transmits light out to a target;- the transmitted light interacts with and is changed by the target;- some of the light is reflected and/or scattered back to the instrument where it is analyzed; - the change in the properties of the light enables some property of the target to be determined- return time of the light is used to determine the range to the target.

- Space-based, air-based and ground-based.

Fun and informative video content:http://www.opentopography.org/index.php/blog/detail/lidar_illuminating_earthquake_hazards_video_premiered_at_scec_annual_meetin

Check out:http://www.opentopography.org/http://unavco.orgTake a brief tour, explore these incredible resources

LIDARThree basic generic types of LIDAR:- Range finders: These are the simplest LIDARs. They are used to measure the distance from the LIDAR instrument to a solid or hard target.- DIAL: - used to measure chemical concentrations (such as ozone, water vapor, pollutants) in the atmosphere. - uses two different laser wavelengths which are selected so that one of the wavelengths is absorbed by the molecule of interest whilst the other wavelength is not. - The difference in intensity of the two return signals can be used to deduce the concentration of the molecule being investigated.- Doppler LIDARs: - measure the velocity of a target; - moving target changes the wavelength of the light reflected/scattered off the target; - this Doppler shift can be used to determine the relative velocity of the target; - a target can be either a hard target or an atmospheric target (microscopic dust and aerosol particles which are carried by wind); - atmospheric targets are the ones of interest (small and light enough to move at the true wind velocity).

LIDAR

LIDAR

From presentation given by Crosby, available on www.opentopography.org at workshop “Using EarthScope and B4 LiDAR data to analyze Southern California’s active faults”

LIDAR

Baja (Cucayo Major) Earthquake

http://earthquake.usgs.gov/earthquakes/shakemap/sc/shake/14607652/

http://www.unavco.org/community_science/science_highlights/2010/M7.2-Baja.html

And see tour in Google Earth

Reflectometry

Reflectometry

GNSS reflectometry and GNSS scatterometry:

In the past few years the potential of GNSS signal reflections for ocean altimetry and remote sensing of sea surface roughness has generated considerable interest.

The PAssive Reflectometry Interferometric System (PARIS) was the first concept proposed for ocean altimetry using GNSS L-band (~24 cm) signals.

Within the PARIS scheme direct and ocean-reflected signals are detected by spaceborne receivers and altimetric height information is extracted from the delay in arrival times of the reflected in relationto the direct signals.

In the following years altimetric heights with accuracies below 5 cm were determined in a number of airborne and ground-based experiments using special purpose GNSS receiver instrumentation.

Reflectometry

Reflectometry

The shape of the code correlation as a function of time delay and Doppler frequency and its dependency on the reflecting surface's slope characteristics can be used to infer the sea surfaceroughness.

Using parameterizations relating the observed roughness to the surface wind vector GNSS scatterometry allows for the remote detection of wind speed and wind direction as well.

In the future, satellite constellations furnished with GNSS scatterometry and reflectometry instruments could contribute to the long-term observations of ocean topography as well as constitute essential elements of early warning systems for catastrophic tsunami events.

Reflectometry

Space-born receivers:- Ocean altimetry - Can we measure sea ice surface topography (freeboard), to determine sea ice thickness and mass balance? - Can we measure wind for a) improved vertical mixing at the mesoscale; b) monitoring and prediction of severe weather systems; c) high resolution wind forcing and attendant coastal ocean response (e.g., local upwelling)? - Can we measure the sea surface topography with sufficient spatial and more importantly temporal resolutions to monitor the evolution of mesoscale ocean eddies and coastal oceans?- Ice altimetry- Soil moisture

In situ receivers:- tide gauges- soil moisture