Geophysical applications of radar interferometry to measure changes in the Earth's surface have exploded in the early 1990s. This new geodetic technique calculates the interference pattern caused by ...the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times. The resulting interferogram is a contour map of the change in distance between the ground and the radar instrument. These maps provide an unsurpassed spatial sampling density (∼100 pixels km−2), a competitive precision (∼1 cm), and a useful observation cadence (1 pass month−1). They record movements in the crust, perturbations in the atmosphere, dielectric modifications in the soil, and relief in the topography. They are also sensitive to technical effects, such as relative variations in the radar's trajectory or variations in its frequency standard. We describe how all these phenomena contribute to an interferogram. Then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments, including the four satellites currently in orbit: ERS‐1, ERS‐2, JERS‐1, and RADARSAT. The next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement: respecting the limits of the technique, assessing its uncertainty, recognizing artifacts, and discriminating different types of signal. We then review the geophysical applications published to date, most of which study deformation related to earthquakes, volcanoes, and glaciers using ERS‐1 data. We also show examples of monitoring natural hazards and environmental alterations related to landslides, subsidence, and agriculture. In addition, we consider subtler geophysical signals such as postseismic relaxation, tidal loading of coastal areas, and interseismic strain accumulation. We conclude with our perspectives on the future of radar interferometry. The objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it.
A study demonstrates that spaceborne radar interferometry can be used to monitor long-term volcano deformation. The results show that it will be possible to use this technique to detect the inflation ...of volcanic edifices that usually precedes eruptions.
Synthetic aperture radar (SAR) inferometry was used to capture seismic displacement caused by the 1992 earthquake in Landers CA. The SAR interferogram provides denser sampling and better precision ...than other imaging techniques.
The ACES/PHARAO space mission Laurent, Philippe; Massonnet, Didier; Cacciapuoti, Luigi ...
Comptes rendus. Physique,
06/2015, Letnik:
16, Številka:
5
Journal Article
Recenzirano
Proposed in 1997, the ACES/PHARAO experiment is a space mission in fundamental physics with two atomic clocks on the International Space Station, a network of ultra-stable clocks on the ground, and ...space-to-ground time transfer systems. The ACES flight instruments are near completion and launch in space is planned for the first half of 2017 for a mission duration of three years. A key element of the satellite payload is a cold-atom clock designed for microgravity environment, PHARAO, operating with laser-cooled cesium atoms. Here we first report on the design and tests of the PHARAO flight model, which is now completed and ready for launch. We then briefly present the status of development of the other instruments of the ACES payload, the Space Hydrogen Maser, the microwave time-transfer system (MWL), and the laser time transfer ELT.
Proposée en 1997, l'expérience ACES/Pharao est une mission spatiale en physique fondamentale avec deux horloges atomiques sur la Station spatiale internationale, un réseau d'horloges ultra-stables à terre et de systèmes de transfert de temps de l'espace jusqu'au sol. Les instruments de vol ACES sont proches de l'aboutissement et leur lancement dans l'espace est planifié pour la première moitié de 2017 pour une durée de mission de trois ans. Un élément-clé de la charge utile du satellite est une horloge atomique à atomes froids conçue pour la microgravité, Pharao, qui fonctionne avec des atomes de césium refroidis par laser. Nous commençons par décrire la conception et les essais du modèle de vol Pharao, qui est maintenant opérationnel et prêt pour le lancement. Nous présentons ensuite brièvement l'état de développement des autres instruments de la plateforme ACES, le maser passif à hydrogène, le système de transfert de temps par liaisons micro-ondes (MWL) et le transfert de temps par lien laser (ELT).
Quasi-simultaneous radar images can be produced by a low cost system using a set of passive receivers onboard a constellation of microsatellites in a special orbital configuration. The combination of ...these images can improve the final resolution in range and azimuth and systematically produce across-track and along-track interferometric data. The author reviews some of the technical particularities of these systems linked to (1) the system, such as the pulse scheduling, the orbital geometry, and the required pointing and positioning, and (2) the image quality, such as the ambiguity level, the required clock stability, the effect of varying baselines at various scales, focusing to higher resolution, and geometric limitations, mainly the behavior of the coherent combinations over terrain with high slopes. The author illustrates three applications of the concept in cooperation with the radar onboard the Japanese ALOS satellite, the ASAR instrument onboard the European ENVISAT, and as a study-case for defense applications of the concept. The author envisions even more innovative applications of the concept, for instance in simultaneous polarimetry and interferometry.
Radar maps of the surface deformation field from the 1992 Landers CA earthquake area reveal features that would otherwise have been poorly sampled. The amount of surface slip following the main shock ...is less than a decimeter.
Interferometric combination of pairs of synthetic aperture radar (SAR) images acquired by the ERS‐1 satellite maps the deformation field associated with the activity of the East Mesa geothermal ...plant, located in southern California. SAR interferometry is applied to this flat area without the need of a digital terrain model. Several combinations are used to ascertain the nature of the phenomenon. Short term interferograms reveal surface phase changes on agricultural fields similar to what had been observed previously with SEASAT radar data. Long term (2 years) interferograms allow the study of land subsidence and improve prior knowledge of the displacement field, and agree with existing, sparse levelling data. This example illustrates the power of the interferometric technique for deriving accurate industrial intelligence as well as its potential for legal action, in cases involving environmental damages.
Interferometric analysis of Synthetic Aperture Radar images acquired by the RADARSAT‐1 satellite of Piton de la Fournaise volcano, show that an eruptive fissure which opened on 9th of March 1998 ...caused asymmetric deformation, with displacement and bulging of the volcanic edifice on the seaward side of the fissure. Up to 50 cm ground‐to‐satellite range change occurred in a 3‐km wide area, in response to a shallow (< 1‐km‐deep) inclined dike that opened by up to 60 cm. The erupted magma was transported from more than 7‐km‐depth below sea level, causing no observable volcano‐wide co‐eruptive deflation. A series of pre‐eruption interferograms show also that no significant inflation occurred prior to the eruption. The dike injection slightly reduced the stability of the volcano seaward flank.
At basaltic volcanoes, magma is transported to the surface through dikes (magma‐filled fractures), but the evolution of these dikes as eruptions proceed is rarely documented. In March 1998, after ...five and a half years of quiescence, Piton de la Fournaise volcano (Réunion Island) entered into a new eruptive phase characterized by intense eruptive activity. Coeruptive displacements recorded by interferometric synthetic aperture radar (InSAR) for the first five eruptions of the cycle are analyzed using 3‐D boundary element models combined with a Monte Carlo inversion method. We show that the eruptions are associated with the emplacement of lateral dikes rooted at depths of less than about 1000 m, except for the first March 1998 event where an additional deeper source is required. The dikes are located above preeruptive seismic swarms. This is consistent with nearly isotropic stress caused by repeated dike intrusions and low confining pressure enhanced by the presence of pores in the shallowest 1000 m of the edifice. The volumes of the modeled dikes represent 17% of the volume of emitted lava, showing that exogenous growth plays a major role in building the volcano. By taking into account the preeruptive seismicity and tilt data together with the results of InSAR data modeling, we find that dikes first propagate vertically from a source region below sea level before being injected laterally at shallow depth. This behavior is consistent with the presence of levels of neutral buoyancy at shallow depth in the edifice.
Testing gravity with cold-atom clocks in space Cacciapuoti, Luigi; Armano, Michele; Much, Rudolf ...
The European physical journal. D, Atomic, molecular, and optical physics,
08/2020, Letnik:
74, Številka:
8
Journal Article
Recenzirano
Atomic Clock Ensemble in Space (ACES) is a mission designed to test Einstein’s theory of General Relativity from the International Space Station (ISS). A primary frequency standard based on laser ...cooled caesium atoms (PHARAO) and an active H-maser (SHM) generate a clock signal that is distributed to a network of clocks on the ground to perform space-to-ground comparison. With a fractional frequency stability of 1 × 10
−16
after 10 days of integration time and an accuracy of 1 – 2 × 10
−16
, ACES will provide an absolute measurement of the gravitational redshift, it will search for time variations of fundamental constant, and perform Standard Model Extension (SME) tests. The ACES payload is currently completing its qualification tests before flying. The mission status, the latest test results, and the ACES performance for testing General Relativity are discussed.
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