Satellite Radar Interferometry

Tectonic plates creep silentlypast one another; glaciers flowsluggishly down mountains;ground level slowly rises and falls. Thegeologic forces that shape the surface ofthe earth usually act with such stealththat most people remain entirely un-aware of them. But then the suddenbreak of a geologic fault or the explo-sive eruption of a volcano occurs in apopulated area, and the devastation in-stantly makes thousands frighteninglyaware that the solid earth is indeedprone to motion.

To better understand and, perhaps,forecast such catastrophic events, scien-tists have labored to measure the ongo-ing bending and stretching of the earthscrust. For this task, they have employedinstruments of many types, from simplesurveyor levels to sophisticated elec-tronic positioning equipment. With allsuch methods, a person must travel tothe site that is to be evaluated to set upsome sort of apparatus and make ob-servations. Yet this commonsensical re-quirement, as it turns out, is not an ab-solute prerequisite.

In 1985 I carried out a studythenan entirely theoretical exercisethatshowed a way, without putting anyequipment at all on the ground, to mon-itor the deformation caused by tectonicforces. At that time scientists had usedsatellites and aircraft for many years toconstruct radar images of the land be-low, and I envisioned that, with someadditional tricks, these same devicescould detect the subtle shifts that thesurface of the earth undergoes. I imme-diately tried to convince geologists ofthe value of this endeavor, but most ofthe people I approached remained du-bious. Measuring ground motion ofonly a few millimeters from hundredsof kilometers away in space seemed toomiraculous to be feasible. Fortunately, I

was able to persuade my employer, theFrench Space Agency, to allow me topursue this exciting prospect.

It would take years of diligent work,but my research group, along with oth-er investigators around the world, hassucceeded in carrying out what seemedquite fantastic to most scientists just adozen years ago. My colleagues and Ihave used a new technique, called satel-lite radar interferometry, to map geo-logic faults that have ruptured in earth-quakes and to follow the heaving ofvolcanic mountains as molten rock ac-cumulates and ebbs away beneath them.Other researchers have harnessed radarinterferometry to survey remote land-slides and the slow-motion progress ofglacial ice. Former skeptics must nowconcede that, miraculous or not, radarsatellites can indeed sense barely per-ceptible movements of the earths sur-face from far away in space.

Helpful Interference

Although the dramatic successes of satellite radar interferometry arequite recent, the first steps toward ac-complishing such feats took placedecades ago. Soon after radar (short-hand for radio detecting and ranging)became widely used to track airplanesusing large rotating dish antennas, sci-entists devised ways to form radar im-ages of the land surface with small,fixed antennas carried aloft by aircraft[see Side-Looking Airborne Radar,by Homer Jensen, L. C. Graham, Leo-nard J. Porcello and Emmett N. Leith;Scientific American, October 1977].Even thick cloud cover does not obscuresuch images, because water droplets andice crystals do not impede the radio sig-nals. What is more, aircraft and orbitingsatellites fitted with radar antennas cantake these pictures equally well during

the day or night, because the radar pro-vides, in a sense, its own light source.

But the distinction between radar im-aging and conventional aerial photog-raphy is more profound than the abilityof radar to operate in conditions thatwould cause optical instruments to fal-ter. There are fundamental differencesin the physical principles underlying thetwo approaches. Optical sensors recordthe amount of electromagnetic radia-tion beamed from the sun (as countlessindependent light waves, or photons)and reflected from the ground. Thus,each element of the resulting imagecalled a pixelis characterized by thebrightness, or amplitude, of the light de-tected. In contrast, a radar antenna illu-minates its subject with coherent ra-diation: the crests and troughs of theelectromagnetic wave emitted follow aregular sinusoidal pattern. Hence, ra-dar instruments can measure both theamplitude and the exact point in the os-cillationcalled the phaseof the re-turned waves, whereas optical sensorsmerely record the quantity of reflectedphotons.

A great benefit arises from measuringphase, because radar equipment oper-ates at extremely high frequencies,which correspond to short radio wave-lengths. If a satellite radar functions, for

Satellite Radar InterferometryFrom hundreds of kilometers away in space,

orbiting instruments can detectsubtle buckling of the earths crust

by Didier Massonnet

46 Scientific American February 1997

INTERFERENCE FRINGES (coloredbands at right) obtained from a sequenceof radar scans by the ERS-1 satellite(above, left) show the deformation of theground caused by an earthquake nearLanders, Calif., in 1992. Each cycle of in-terference colors (red through blue) repre-sents an additional 28 millimeters ofground motion in the direction of the sat-ellite. The radar interference caused bythe mountainous relief of the area (black-and-white background) was removed toreveal this pattern of ground deformation. DID

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Copyright 1997 Scientific American, Inc.

Copyright 1997 Scientific American, Inc.

instance, at a frequency of six gigahertz(six billion cycles per second), the radiosignal will travel earthward at the speedof light for only five centimeters duringthe tiny amount of time the wave takesto complete one oscillation. If the dis-tance from the radar antenna to a targeton the ground is, for example, exactly800 kilometers, the 1,600-kilometerround-trip (for the radar signal to reachthe earth and bounce back up) will cor-respond to a very largebut wholenumber of wavelengths. So when thewave returns to the satellite, it will havejust completed its final cycle, and its

phase will be unchanged from its origi-nal condition at the time it left. If, how-ever, the distance to the ground exceeds800 kilometers by only one centimeter,the wave will have to cover an addition-al two centimeters in round-trip dis-tance, which constitutes 40 percent of awavelength. As a result, the phase ofthe reflected wave will be off by 40 per-cent of a cycle when it reaches the satel-lite, an amount the receiving equipmentcan readily register. Thus, the measure-ment of phase provides a way to gaugethe distance to a target with centimeter,or even millimeter, precision.

Yet for decades, most practitioners ofradar imaging completely overlookedthe value of phase measurements. Thatoversight is easy to understand. A sin-gle pixel in a radar image represents anappreciable area on the ground, perhaps100 square meters. Such a patch willgenerate multiple radar reflections fromthe countless small targets containedwithin itscattered pebbles, rocks,leaves, branches and other objectsorfrom rough spots on the surface. Be-cause these many radar reflections willcombine in unpredictable ways whenthey reach the antenna, the phase mea-

Satellite Radar Interferometry48 Scientific American February 1997

RADAR REFLECTIONS (red lines) from a pair of nearby ob-jects can interfere constructively (right) or destructively (left).Minor differences in geometry can therefore give rise to largechanges in the amplitude of the pixels in a radar image.

OPTICAL REFLECTIONS from a pair of objects always pro-duce a similar number of reflected photons (orange), regardless ofthe exact position of the objects. Thus, the brightness of a pixeldoes not vary with slight shifts in the configuration of reflectors.

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SOAP FILM of tapering thickness can separate light into its component colors (above),each of which corresponds to a particular wavelength of electromagnetic radiation. Afringe of one color shows where the light rays of that wavelength reflect from the topand bottom surfaces of the thin film and combine constructively (right).

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sured for a given pixel seems random.That is, it appears to have no relationto the phase measured for adjacent pix-els in the radar image.

The amplitude associated with a giv-en pixel in such an image will, however,generally indicate whether many or fewelementary reflectors were present at thecorresponding place on the ground. Butthe amplitude measurements will alsohave a noisy aspect, because the indi-vidual reflections contributing to onepixel can add together and make theoverall reflection stronger (constructiveinterference), or they can cancel one an-other out (destructive interference). Thisphenomenon in the reflection of coher-ent radiationcalled specklealso ac-counts for the strange, grainy appear-ance of a spot of laser light.

For many years, scientists routinelyovercame the troubling effects of speck-le by averaging the amplitudes of neigh-boring pixels in their radar images. Theyfollowed this strategy in an attempt to

Satellite Radar Interferometry Scientific American February 1997 49

TAPERED FILM

INCOMINGLIGHT

FIRST CYCLE SECOND CYCLE

GLACIAL ICE at the margins of Antarc-tica flows toward the sea relatively rapidlyalong confined channels, or ice streams,such as the one mapped here using a pairof satellite radar images. Two parallelbands of highly sheared ice (speckled ar-