Synthetic aperture radar interferometry(7)

时间：2025-07-11

Synthetic aperture radar interferometry is an imaging technique for measuring the topography of a surface, its changes over time, and other changes in the detailed characteristics of the surface. By exploiting the phase of the coherent radar signal, interf

Fig.16.Definitions of interferometric parameters relating to a possible radar interferometer configuration.In this example,the transmitter path is common to both roundtrip signal paths. Therefore the transmitter phase and time delays cancel in the channel difference.The total delay is the sum of the antenna delay and the various receiver delays.

rate knowledge of the baseline length and orientation angle-

millimeter or better knowledge for the baseline length and

10’s of arc second for the baseline orientation angle.These

requirements are typical of most InSAR systems,and gener-

ally necessitate either an extremely rigid and controlled base-

line,a precise baseline metrology system,or both,and rig-

orous calibration procedures.

Phase accuracy requirements for interferometric systems

typically range from0.1.This imposes rather strict

monitoring of phase changes not related to the imaging ge-

ometry in order to produce accurate topographic maps.Both

the TOPSAR and SRTM system use a specially designed cal-

ibration signal to remove radar electronically induced phase

delays between the interferometric channels.

C.Interferometry for Motion Mapping

The theory described above assumed that the imaged sur-

face is stationary over time,or that the surface is imaged by

the interferometer at a single instant.When there is motion of

the surface between radar observations there is an additional

contribution to the interferometric phase variation.Fig.17

shows the geometry when a surface displacement occurs be-

tween the observation

).In this

case,

.The interferometric phase expressed in terms of this new

vector

(34)

Typically,for spaceborne

geometries km,

and

tracked over the time between observations.Intermediate cases include slow and/or variable surface motions,such as volcanic inflation or surging glaciers.

Equations(34)and(35)highlight that the interferometer measures the projection of the displacement vector in the radar line-of-sight direction.To reconstruct the vector dis-placement,observations must be made from different aspect angles.

The topographic phase term is not of interest for displace-ment mapping,and must be removed.Several techniques have been developed to do this.They all essentially derive the topographic phase from another data source,either a dig-ital elevation model(DEM)or another set of interferometric data.The selection of a particular method for topography measurement depends heavily on the nature of the motion (steady or episodic),the imaging geometry(baselines and time separations),and the availability of data.

It is important to appreciate the increased precision of the interferometric displacement measurement relative to topo-graphic mapping precision.Consider a discrete displacement event such as an earthquake where the surface moves by a fixed amount in a short time period.Neither a pair of ob-servations acquired before the event(pair“a”),nor a pair after the event(pair“b”)would measure the displacement directly,but together would measure it through the change in topography.According to(33),and assuming the same imaging geometry for“a”and“b”without loss of generality, the phase difference between these two interferograms(that is the difference of phase differences)

(38)

to first order,because appears in both the expression

for

is the topographic

height above the reference surface.In this formulation,the

phase difference is far more sensitive to changes in topog-

raphy(surface displacement)than to the topography itself.

From

(39),

must change by a substantial amount,

essentially

cm,km,and