Synthetic Aperture Radar
Radar instruments emit pulses of electromagnetic radiation in the radio and microwave part of the spectrum and detect the reflection of the pulses from objects in its Line of Sight (LoS). One class of radar instrument is the imaging radar, and includes Synthetic Aperture Radar (SAR).
A SAR signal can be imagined as a sine wave, which contains amplitude and phase information.
- Amplitude is the strength of the radar response.
- Phase is the fraction of one complete sine wave cycle (a single SAR wavelength)
Amplitude and phase of radar waveform
SAR Satellites
A number of satellites host SAR instruments that acquire SAR data of the Earth’s surface. As the satellite orbits, the SAR instrument transmits energy to the Earth’s surface and records the response.
The satellites revolve around the Earth in polar orbits, meaning the satellite passes close to both poles as it orbits the globe. The ascending and descending orbit pass refers to the travel of the satellite from south to north and north to south respectively. SAR satellites systematically re-visit the same area of the Earth’s surface on a 10 to 45 days basis.
The SAR instrument itself ‘looks’ perpendicular to the orbit path of the satellite (easterly on the ascending orbit pass and westerly on the descending orbit pass) and most SAR instruments can be ‘pointed’ to illuminate the Earth’s surface from different angles. SAR instruments can operate at different radar wavelengths (X-band, C-band and L-band).
Ascending and descending orbits for SAR satellites
Many conventional SAR applications make use of the amplitude of the return signal (e.g. Offshore Basin Screening, Geological Interpretation, Flood mapping), but InSAR uses the changes in phase between SAR data acquisitions.
Phase Shift
If SAR data was acquired of the same location on the Earth’s surface, on two separate occasions, from exactly the same position in space, with nothing at the location changing, the phase values of the returning radar signals would be the same. In practice, the position of the satellite between two image acquisitions varies slightly (this difference in position between image acquisitions is known as the baseline), and the corresponding difference in the path (distance between satellite and ground) means there is a difference in phase between the two signals, a phase shift.
SAR wave phase shift
SAR Interferometry - Topographic Mapping
SAR interferometry makes use of the phase shift information by subtracting the phase value from one SAR data acquisition from that of another, for the same point on the ground. The resulting phase difference, represented by interferometric fringes, is directly related to topographic height. The result is an interferogram. Interferometric fringes can be thought of as a collection of height contours, with each fringe corresponding to a phase difference of 0 to 360°. Assuming the interferogram has been flattened (corrected for the curvature of the Earth), each complete interferometric fringe cycle (e.g. from black through to white) represents a specific elevation interval for all fringes across the interferogram. This interval is known as the altitude of ambiguity and is a function of radar wavelength (ranges from 5.6cm to 23.6cm), satellite altitude (in the region of 800km), incidence angle (typically between 23-50°) and the perpendicular baseline (anything from m to km).
If the terrain were flat, then a series of regularly spaced, parallel fringes would result. Any deviation from a parallel fringe pattern can be interpreted as topographic variation. The resulting image is referred to as a wrapped interferogram.
Interferometric fringes
In order to compute terrain heights and generate a Digital Elevation Model (DEM), the correct multiple of 360° must be added to the phase difference at each pixel. This process is known as phase unwrapping and the resulting image is referred to as an unwrapped interferogram.
SAR Interferometry - Deformation Mapping
SAR interferometry essentially maps the topography of the Earth’s surface in the period of time spanned by two SAR data acquisitions. However, as time passes the height of the Earth’s surface can change, so in fact an interferogram not only contains information on topography, but also changes in topography that may have occurred in the period spanned by the data acquisition. Differential interferometry is used to detect these changes in topography.
In its simplest form, a first interferogram is created representing topography (this is typically simulated using an existing DEM) and then a second interferogram created spanning a period of interest in which deformation may have occurred. By subtracting one interferogram from the other (to create a differential interferogram), fringes that relate to common topography cancel each other out, so that remaining fringes only represent a difference in topography i.e. deformation. The phase differences which remain as fringes in the differential interferogram are the result of changes in ground position from one interferogram to the next in the Line of Sight of the satellite. Surface deformation motion away from the satellite causes an increase in the signal path (and therefore phase) difference. Surface deformation towards the satellite causes a decrease in the signal path (and therefore phase) difference.
Wrapped interferogram Uwrapped interferogram
A number of different differential interferometric techniques exist, these include Differential InSAR (DifSAR), Persistent Scatterer InSAR (PSI) and Corner Reflector InSAR (CRInSAR).