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Radar has been used in remote sensing for decades as one of the few active remote sensing methods. They have been used extensively onboard spacecraft for a wide range of remote sensing purposes, from meteorological precipitation monitoring to ground-penetrating radar. Radar is also used for other purposes, both civil and military. With improvements in radar technology and computer processing power, the applications and usefulness of radar has increased.
2. Radar Types (Geoscience)
2.1 Pulse-Doppler Radar
Pulse-Doppler is a radar system capable of not only detecting target location (bearing, range, and altitude), but also measuring its radial velocity (range-rate). It uses the Doppler effect to determine the relative velocity of objects; pulses of RF energy returning from the target are processed to measure the frequency shift between carrier cycles in each pulse and the original transmitted frequency. To achieve this, the transmitter frequency source must have very good phase stability and the system is said to be coherent.
The nature of pulsed radar, and the relationship between the carrier frequency and the pulse repetition frequency (PRF) means that the frequency spectrum can be very complex, leading to the possibility of errors and tradeoffs. In general, it is necessary to utilise a very high PRF to avoid aliasing, which can cause side effects such as range ambiguity. To avoid this, multiple PRFs are often used.
Pulse-Doppler radar is based on the fact that targets moving with a nonzero radial velocity will introduce a frequency shift between the transmitter master oscillator and the carrier component in the returned echoes. This is because the signal is subject to Doppler shift, so echoes from closing targets will show an apparent increase in frequency and echoes from opening targets will show an apparent decrease in frequency. Target velocity can be estimated by determining the average frequency shift of carrier cycles within a pulse packet. This is typically done by means of a 1D fast Fourier transform or using the autocorrelation technique. The transform is performed independently for each sample volume, using data received at the same range from all pulses within a packet. In older systems, a bank of analogue filters were used.
The maximum velocity that can be unambiguously measured is inherently limited by the PRF, as discussed above. The PRF-value must therefore be chosen carefully, based on a tradeoff between maximum velocity resolution and the reduction of velocity aliasing and range ambiguity problems, resulting in a highly application dependant trade-off.
2.2 Synthetic Aperture Radar
Synthetic-aperture radar (SAR) is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional beam-scanning means. It originated as an advanced form of side-looking airborne radar (SLAR).
SAR is usually implemented by mounting, on a moving platform such as an aircraft or spacecraft, a single beam-forming antenna from which a target scene is repeatedly illuminated with pulses of radio waves at wavelengths anywhere from a meter down to millimeters. The many echo waveforms received successively at the different antenna positions are coherently detected and stored and then post-processed together to resolve elements in an image of the target region.
SAR images have wide applications in remote sensing and mapping of the surfaces of both the Earth and other planets. SAR can also be implemented as inverse SAR by observing a moving target over a substantial time with a stationary antenna.
Careful design and operation can accomplish resolution of items smaller than a millionth of the range, for example, 30 cm at 300 km.
The process can be thought of as combining the series of spatially distributed observations as if all had been made simultaneously with an antenna as long as the beamwidth and focused on that particular point. The ââ‚¬Å“synthetic apertureââ‚¬Â simulated at maximum system range by this process not only is longer than the real antenna, but, in practical applications, it is much longer than the radar aircraft, and tremendously longer than the radar spacecraft.
Combining the series of observations requires significant computational resources, usually using Fourier transform techniques. The high digital computing speed now available allows such processing to be done in near-real time on board a SAR aircraft. (There is necessarily a minimum time delay until all parts of the signal have been received.) The result is a map of radar reflectivity, including both amplitude and phase. The amplitude information, when shown in a map-like display, gives information about ground cover in much the same way that a black-and-white photo does.
More complex operation
The basic design of a synthetic-aperture radar system can be enhanced to collect more information. Most of these methods use the same basic principle of combining many pulses to form a synthetic aperture, but may involve additional antennae or significant additional processing.
SAR requires that echo captures be taken at multiple antenna positions. The more captures taken (at different antenna locations) the more reliable the target characterization.
Multiple captures can be obtained by moving a single antenna to different locations, by placing multiple stationary antennae at different locations, or combinations thereof.
The advantage of a single moving antenna is that it can be easily placed in any number of positions to provide any number of monostatic waveforms. For example, an antenna mounted on an airplane takes many captures per second as the plane travels.
Different materials reflect radar waves with different intensities, but anisotropic materials such as grass often reflect different polarizations with different intensities. Some materials will also convert one polarization into another. By emitting a mixture of polarizations and using receiving antennae with a specific polarization, several different images can be collected from the same series of pulses. Frequently three such RX-TX polarizations (HH-pol, VV-pol, VH-pol) are used as the three color channels in a synthesized image.
New developments in polarimetry also include utilizing the changes in the random polarization returns of some surfaces (such as grass or sand), between two images of the same location at different points in time to determine where changes not visible to optical systems occurred. Examples include subterranean tunneling, or paths of vehicles driving through the area being imaged. Enhanced SAR sea oil slick observation it has been developed by appropriate physical modelling and use of fully-polarimetric and dual-polarimetric measurements.
Rather than discarding the phase data, information can be extracted from it. If two observations of the same terrain from very similar positions are available, aperture synthesis can be performed to provide the resolution performance which would be given by a RADAR system with dimensions equal to the separation of the two measurements. This technique is called Interferometric SAR or InSAR.
Conventional radar systems emit bursts of radio energy with a fairly narrow range of frequencies. A narrow-band channel, by definition, does not allow rapid changes in modulation. A signal with only a slow change in modulation cannot reveal the distance to the target as well as can a signal with a quick change in modulation.
Ultra-wideband (UWB) refers to any radio transmission that uses a very large bandwidth. Although there is no set bandwidth value that qualifies a signal as UWB, systems using bandwidths greater than a sizable portion of the center frequency are most often called "UWB" systems.
The principle advantages of UWB radar are better resolution and more spectral information of target reflectivity.
Doppler Beam Sharpening commonly refers to the method of processing unfocused real-beam phase history to achieve better resolution than could be achieved by processing the real beam without it. Because the real aperture of the RADAR antenna is so small, the RADAR energy spreads over a wide area. Doppler-beam sharpening takes advantage of the motion of the platform in that targets ahead of the platform return a Doppler upshifted signal and targets behind the platform return a Doppler downshifted signal.
This technique dramatically improves angular resolution however, it is far more difficult to take advantage of this technique for range resolution.
Chirped (pulse-compressed) radars
A common technique for many radar systems usually also found in SAR systems, is to chirp the signal. In a chirped radar, the pulse is allowed to be much longer. A longer pulse allows more energy to be emitted, and hence received, but usually hinders range resolution. But in a chirped radar, this longer pulse also has a frequency shift during the pulse (hence the chirp or frequency shift). When the chirped signal is returned, it must be correlated with the sent pulse. Newer systems use digital pulse correlation to find the pulse return in the signal.
Current (2010) airborne systems provide resolutions to about 10 cm, ultra-wideband systems provide resolutions of a few millimeters, and experimental terahertz SAR has provided sub-millimeter resolution in the laboratory.
A radar scatterometer is designed to determine the normalized radar cross section of the surface. Scatterometers operate by transmitting a pulse of microwave energy towards the Earth's surface and measuring the reflected energy. A separate measurement of the noise-only power is made and subtracted from the signal+noise measurement to determine the backscatter signal power which is then processed to produce the normalized radar cross-section. Scatterometer instruments are very precisely calibrated in order to make accurate backscatter measurements.
The primary application of space borne scatterometry has been measurements near-surface winds over the ocean. Such instruments are known as wind scatterometers. By combining sigma-0 measurements from different azimuth angles, the near-surface wind vector over the ocean's surface can be determined using a geophysical model function (GMF) which relates wind and backscatter. Over the ocean, the radar backscatter results from scattering from wind-generated capillary-gravity waves, which are generally in equilibrium with the near-surface wind over the ocean. The scattering mechanism is known as Bragg scattering, which occurs from the waves that are in resonance with the microwaves.
Scatterometer wind measurements are used for air-sea interaction, climate studies and are particularly useful for monitoring hurricanes. Scatterometer backscatter data are applied to the study of vegetation, soil moisture, polar ice, and global change. Scatterometer measurements have been used to measure winds over sand and snow dunes from space. Non-terrestrial applications include study of Solar System moons using space probes. This is especially the case with the NASA/ESA Cassini mission to Saturn and its moons.
2.4 Radar Altimeter
Radar altimeters are used extensively in space remote sensing equipment. Radio waves are transmitted towards the ground and the time it takes them to be reflected back and return to the craft is timed.
Alternatively, Frequency Modulated Continuous-wave radar can be used. The greater the frequency shift the further the distance travelled. This method can achieve much better accuracy than the aforementioned for the same outlay and radar altimeters that use frequency modulation are industry standard.
Radar altimeters normally work in the E band, or Ka band or S bands for more advanced sea-level measurement.
2.5 Microwave Radiometer
A Microwave radiometer (MWR) measures electromagnetic radiation emitted at <1mm-30cm wavelengths. They have been primarily used for meteorological and oceanographic remote-sensing onboard spacecraft.
A variety of surface and atmospheric measurements can be calculated with MWR's including air temperature, sea surface temperature, salinity, soil moisture, sea ice, precipitation and atmospheric water vapour and liquid water.
Although microwave radiometers are technically not radars, they are often used in conjunction with them.
HF (High Frequency) 10-100m 3 to 30 MHz
VHF (Very High Frequency) 0.9-6m 30 to 300 MHz
UHF (Ultra High Frequency) 0.3-1m 0.3 to 3 GHz
P (Previous) 1m+ 250 to 500 MHz The P band is now an obsolete radar band that was used for surface studies.
L (Long)15-30cm ~1 to 2 GHz The L band is used in the Global Positioning System carriers and other satellite navigation systems. It is also used in military telemetry systems.
S (Short) 7.5-15cm 2 to 4 GHz The S band is used in weather radar applications, and some communications satellites, especially those used to communicate between space shuttles and satellites. It is also used in military radars.
C (Compromise) 3.75-7.5cm 4 to 8 GHz The C band is used in weather radar systems and extensively in wireless communications which has caused some interference with systems using this band.
X 2.5-3.75cm 8 to 12GHz The X band is used in most radar applications including continuous wave, pulsed, single-polarization, dual-polarization, synthetic aperture radar and phased arrays.
Ku (K-under) 1.67 -2.5cm 12 to 18 GHz The Ku band lies directly below the K band and is used primarily for fixed and broadcast services.
K (Kurz) 1.11-1.67cm 18 to 27 GHz The IEEE K band is used in meteorological studies as it is easily absorbed by water vapour.
Ka (K-above) 0.75-1.11cm 26.5 to 40 GHz The Ka band lies directly above the K-band and is used in communications satellites and high resolution, close range targeting radars.
Q 7.5-5mm 33 to 50 GHz The Q band is used mostly for satellite communications, surface studies, radio astronomy studies and terrestrial microwave communications.
V 6.0-4mm 50 to 75 GHz The V band is not heavily used other than for millimeter wave radar research.
E 6.0-3.33mm 60 to 90 GHz The E band is used for short range, high bandwidth communications and meteorological studies due to the short wavelengths interaction with atmospheric gasses.
W 2.7-4.0mm 75 to 110 GHz The W band is used for satellite communications, millimeter wave radar research, military radar targeting and tracking applications, and some non-military applications.
F 2.1-3.3mm 90 to 140 GHz The F band lies in the S band of the older classification scheme.
D 1.8-2.7mm 110 to 170 GHz The modern D band intersects with the L band of the older IEEE classification system. The D band lies at the approach to upper frequency limit of contemporary electronic oscillator technology, beween 110 and 170 GHz
UWB (Ultra Wideband) 18.75cm-2.8cm