Backscatter is the portion of the outgoing radar signal that the target redirects directly back towards the radar antenna. Backscattering is the process by which backscatter is formed. The scattering cross section in the direction toward the radar is called the backscattering cross section. The usual notation is the symbol σ (sigma). It is a measure of the reflective strength of a radar target. The normalised measure of the radar return from a distributed target is called the backscatter coefficient, or sigma nought, and is defined as per unit area on the ground. If the signal formed by backscatter is undesired, it is called clutter. Other portions of the incident radar energy may be reflected and scattered away from the radar or absorbed.
A focused pulse of energy. The antenna beam of a side-looking radar is directed perpendicular to the flight path and illuminates a swath parallel to the platform ground track. Due to the motion of the satellite, each target element is illuminated by the beam for a period of time, known as the integration time
The SAR operating configuration defined by the swath width and resolution.
The area within the total possible swath that is actually illuminated while being governed by the characteristics of a specific beam mode.
β° A radar brightness coefficient. The reflectivity per unit area in slant range which is dimensionless.
A nominal frequency range, from 8 to 4 Ghz (3.75 to 7.5 cm wavelength) within the microwave (radar) portion of the electromagnetic spectrum. Imaging radars equipped with C-band are generally not hindered by atmospheric effects and are capable of 'seeing' through tropical clouds and rain showers. Its penetration capability with regard to vegetation canopies or soils is limited and is restricted to the top layers.
Calibration is the process of quantitatively defining the system response to known controlled signal inputs.
Coherence is the fixed relationship between waves in a beam of electromagnetic (EM) radiation. Two wave trains of EM radiation are coherent when they are in phase, that is, they vibrate in unison. In terms of the application to things like radar, the term coherence is also used to describe systems that preserve the phase of the received signal.
For radar systems, a complex number implies that the representation of a signal, or data file, needs both magnitude and phase measures. In the digital SAR context, a complex number is often represented by an equivalent pair of numbers, the real in-phase component (I) and the imaginary quadrature component (Q).
A data-take is a continuous temporal segment of SAR acquisition without instrument mode change (due to on-board memory handling, a data-take can be downlinked in distinct moments and in different channels).
Processing stage at which the strength of the signal is determined for each pixel value. Detection removes phase information from the data file. The preferred detection scheme uses a magnitude squared method, which is energy conserving, and has units of voltage squared per pixel.
The Doppler frequency depends on the component of satellite velocity in the line-of-sight direction to the target. This direction changes with each satellite position along the flight path, so the Doppler frequency varies with azimuth time. For this reason, azimuth frequency is often referred to as Doppler frequency.
Linear polarisation with the lone electric vector oriented in the horizontal direction in antenna co-ordinates.
Horizontal Transmit - Horizontal Receive Polarisation (HH)
A mode of radar polarisation where the microwaves of the electric field are oriented in the horizontal plane for both signal transmission and reception by means of a radar antenna.
Horizontal Transmit - Vertical Receive Polarisation (HV)
A mode of radar polarisation where the microwaves of the electric field are oriented in the horizontal plane for signal transmission, and where the vertically polarised electric field of the backscattered energy is received by the radar antenna.
Most imaging radars produce two-dimensional images. The two dimensions are range and azimuth.
A technique that uses the measured differences in the phase of the return signal between two satellite passes to detect slight changes on the Earth's surface. The combination of two radar measurements of the same point on the ground, taken at the same time, but from slightly different angles, to produce stereo images. Using the cosine rule from trigonometry to calculate the distance between the radar and the Earth's surface, these measurements can produce very accurate height maps, or maps of height changes. Mapping height changes provides information on earthquake damage, volcanic activity, landslides and glacier movement.
Reconstructed unprocessed data at full space-time resolution with all available supplemental information to be used in subsequent processing (e.g. ephemeris, health and safety) appended. SAR Level-0 products may cover part of an acquisition segment (in case only part of a segment is down-linked at a certain ground station in the same pass) or a full acquisition segment. Level-0 products are generated at the receiving station immediately after acquisition.
Reconstructed data at full resolution, time-referenced and annotated with ancillary information, including radiometric and geometric calibration coefficients, and geo-referencing parameters. Data may be radio-metrically corrected and calibrated in physical units at full instrument resolution, orthorectified and re-sampled to a specified grid.
Derived geophysical parameters (e.g. sea surface temperature, leaf area index) at the same resolution and location as Level-1 source data.
The radar look direction defines the angle in the horizontal plane in which the radar antenna is pointing when transmitting a pulse and receiving the return signal from the ground or from an object. The look direction is an angular measurement (in degrees) and is usually made with respect to true north. In side-looking imaging radar, the look direction is often orthogonal (normal) to the flight trajectory (azimuth) of the platform carrying the radar and is synonymous with the range direction. The radar look direction is an important parameter when analysing features with a preferred orientation, for example fracture patterns in rock formations, regular street patterns or ocean waves, as these may be enhanced through choice of appropriate radar illumination direction.
Radar terminology refers to individual looks as groups of signal samples in a SAR processor that splits the full synthetic aperture into several sub-apertures, each representing an independent look of the identical scene. The resulting image formed by incoherent summing of these looks is characterised by reduced speckle and degraded spatial resolution. The SAR signal processor can use the full synthetic aperture and the complete signal data history in order to produce the highest possible resolution, albeit very speckled, single-look complex (SLC) SAR image product. Multiple looks may be generated by averaging over range and/or azimuth resolution cells. For an improvement in radiometric resolution using multiple looks there is an associated degradation in spatial resolution. Note that there is a difference between the number of looks physically implemented in a processor and the effective number of looks as determined by the statistics of the image data
Noise Equivalent Sigma Nought
A measure of the sensitivity of a given SAR. It describes the strength of the (additive) system noise in terms of the equivalent (average) power in the image domain that would result from an idealised distributed scatterer of the stated reflectivity. Smaller noise equivalent sigma nought values are better. Within physical limitations, smaller may be achieved by increasing the power of the radar transmitter, or by decreasing the noise figure of the electronics.
The process of confining the vibrations of the magnetic, or electric field, vector of light or other radiation to one plane. Orientation of the plane of the electric field relative to the Earth's surface.
Pulse Repetition Frequency (PRF)
Rate of recurrence of the pulses transmitted by a radar.
In a long synthetic aperture, SAR focusing involves the removal and compensation of path length differences from the antenna to the target on the ground. The main advantage of a focused synthetic aperture is that it increases its array length over those radar signals that can be processed and increases potential SAR resolution at any range. SAR focusing is a necessary process when the length of a synthetic array is a significant fraction of the range to ground being imaged, as the lines-of-sight (range) from a particular point on the ground to each individual element of the array differ in distance. These range differences, or path length differences, of the radar signals can affect image quality. In a focused SAR image these phase errors can be compensated for by applying a phase correction to the return signal at each synthetic aperture element. Focusing errors may be introduced by unknown or uncorrected platform motion. In an unfocused SAR image, the usable synthetic aperture length is limited.
Synthetic Aperture Radar (SAR)
A Synthetic Aperture Radar (SAR) is a coherent radar system that generates high-resolution remote sensing imagery. Signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture to create an image. As the line of sight direction changes along the radar platform trajectory, a synthetic aperture is produced by signal processing that has the effect of lengthening the antenna. The achievable azimuth resolution of a SAR is approximately equal to one-half the length of the actual (real) antenna and does not depend on platform altitude (distance). High range resolution is achieved through pulse compression techniques. In order to map the ground surface the radar beam is directed to the side of the platform trajectory. With a sufficiently wide antenna beam width in the along track direction, an identical target or area may be illuminated a number of times without a change in the antenna look angle
Linear polarisation with the lone electric vector oriented in the vertical direction in antenna co-ordinates.
Vertical Transmit-Horizontal Receive Polarisation (VH)
A mode of radar polarisation where the microwaves of the electric field are oriented in the vertical plane for signal transmission and where the horizontally polarised electric field of the backscattered energy is received by the radar antenna.
Vertical Transmit-Vertical Receive Polarisation (VV )
A mode of radar polarisation where the microwaves of the electric field are oriented in the vertical plane for both signal transmission and reception by means of a radar antenna. In this case, the plane of the electric field of the microwave energy is designated by the letter V (vertical) for both transmit and receive events, i.e. VV; this transmit-receive polarity is also called like-polarised as opposed to cross-polarised (horizontal transmit - vertical receive, HV). The amount of radar backscatter received at a particular linear polarisation state from a particular ground surface or object depends, in part, on the scattering mechanism and depolarisation effects involved.
Zero Doppler Time
Zero Doppler time is the along track (azimuth) time at which a target on the ground would have a Doppler shift of zero with respect to the satellite (i.e. when the target was perpendicular to the flight path). It is also called the closest approach azimuth time. The SAR processor locates targets in the image at the zero-Doppler azimuth time.