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hange in modulation cannot reveal the distance to the target as well as a signal with a quick change in modulation ultra wideband uwb refers to any radio transmission that uses a very large bandwidth which is the same as saying it uses very rapid changes in modulation 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 typically about ten percent or so are most often called uwb systems a typical uwb system might use a bandwidth of one third to one half of its center frequency for example some systems use a bandwidth of about 1 ghz centered around 3 ghz the two most common methods to increase signal bandwidth used in uwb radar including sar are very short pulses and high bandwidth chirping a general description of chirping appears elsewhere in this article the bandwidth of a chirped system can be as narrow or as wide as the designers desire pulse based uwb systems being the more common method associated with the term uwb radar are described here a pulse based radar system transmits very short pulses of electromagnetic energy typically only a few waves or less a very short pulse is of course a very rapidly changing signal and thus occupies a very wide bandwidth this allows far more accurate measurement of distance and thus resolution the main disadvantage of pulse based uwb sar is that the transmitting and receiving front end electronics are difficult to design for high power applications specifically the transmit duty cycle is so exceptionally low and pulse time so exceptionally short that the electronics must be capable of extremely high instantaneous power to rival the average power of conventional radars although it is true that uwb provides a notable gain in channel capacity over a narrow band signal because of the relationship of bandwidth in the shannon hartley theorem and because the low receive duty cycle receives less noise increasing the signal to noise ratio there is still a notable disparity in link budget because conventional radar might be several orders of magnitude more powerful than a typical pulse based radar so pulse based uwb sar is typically used in applications requiring average power levels in the microwatt or milliwatt range and thus is used for scanning smaller nearer target areas several tens of meters or in cases where lengthy integration over a span of minutes of the received signal is possible however this limitation is solved in chirped uwb radar systems the principal advantages of uwb radar are better resolution a few millimeters using commercial off the shelf electronics and more spectral information of target reflectivity doppler beam sharpening edit 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 compared to the wavelength in use the radar energy spreads over a wide area usually many degrees wide in a direction orthogonal at right angles to the direction of the platform aircraft doppler beam sharpening takes advantage of the motion of the platform in that targets ahead of the platform return a doppler upshifted signal slightly higher in frequency and targets behind the platform return a doppler downshifted signal slightly lower in frequency the amount of shift varies with the angle forward or backward from the ortho normal direction by knowing the speed of the platform target signal return is placed in a specific angle bin that changes over time signals are integrated over time and thus the radar beam is synthetically reduced to a much smaller aperture or more accurately and based on the ability to distinguish smaller doppler shifts the system can have hundreds of very tight beams concurrently this technique dramatically improves angular resolution however it is far more difficult to take advantage of this technique for range resolution see pulse doppler radar chirped pulse compressed radars edit further information chirp 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 classically in analog systems it is passed to a dispersive delay line often a surface acoustic wave device that has the property of varying velocity of propagation based on frequency this technique compresses the pulse in time thus having the effect of a much shorter pulse improved range resolution while having the benefit of longer pulse length much more signal energy returned newer systems use digital pulse correlation to find the pulse return in the signal typical operation edit nasa s airsar instrument is attached to the side of a dc 8 data collection edit in a typical sar application a single radar antenna is attached to an aircraft or spacecraft such that a substantial component of the antenna s radiated beam has a wave propagation direction perpendicular to the flight path direction the beam is allowed to be broad in the vertical direction so it will illuminate the terrain from nearly beneath the aircraft out toward the horizon image resolution and bandwidth edit resolution in the range dimension of the image is accomplished by creating pulses which define very short time intervals either by emitting short pulses consisting of a carrier frequency and the necessary sidebands all within a certain bandwidth or by using longer chirp pulses in which frequency varies often linearly with time within that bandwidth the differing times at which echoes return allow points at different distances to be distinguished image resolution of sar in its range coordinate expressed in image pixels per distance unit is mainly proportional to the radio bandwidth of whatever type of pulse is used in the cross range coordinate the similar resolution is mainly proportional to the bandwidth of the doppler shift of the signal returns within the beamwidth since doppler frequency depends on the angle of the scattering point s direction from the broadside direction the doppler bandwidth available within the beamwidth is the same at all ranges hence the theoretical spatial resolution limits in both image dimensions remain constant with variation of range however in practice both the errors that accumulate with data collection time and the particular techniques used in post processing further limit cross range resolution at long ranges image resolution and beamwidth edit sar antenna of the saocom satellites the total signal is that from a beamwidth sized patch of the ground to produce a beam that is narrow in the cross range direction clarification needed diffraction effects require that the antenna be wide in that dimension therefore the distinguishing from each other of co range points simply by strengths of returns that persist for as long as they are within the beam width is difficult with aircraft carryable antennas because their beams can have linear widths only about two orders of magnitude hundreds of times smaller than the range spacecraft carryable ones can do 10 or more times better however if both the amplitude and the phase of returns are recorded then the portion of that multi target return that was scattered radially from any smaller scene element can be extracted by phase vector correlation of the total return with the form of the return expected from each such element 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 synthetic aperture 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 although some references to sars have characterized them as radar telescopes their actual optical analogy is the microscope the detail in their images being smaller than the length of the synthetic aperture in radar engineering terms while the target area is in the far field of the illuminating antenna it is in the near field of the simulated one careful design and operation can accomplish resolution of items smaller than a millionth of the range for example 30 centimetres 12 in at 300 kilometres 190 mi pulse transmission and reception edit the conversion of return delay time to geometric range can be very accurate because of the natural constancy of the speed and direction of propagation of electromagnetic waves however for an aircraft flying through the never uniform and never quiescent atmosphere the relating of pulse transmission and reception times to successive geometric positions of the antenna must be accompanied by constant adjusting of the return phases to account for sensed irregularities in the flight path sar s in spacecraft avoid that atmosphere problem but still must make corrections for known antenna movements due to rotations of the spacecraft even those that are reactions to movements of onboard machinery locating a sar in a crewed space vehicle may require that the humans carefully remain motionless relative to the vehicle during data collection periods returns from scatterers within the range extent of any image are spread over a matching time interval the inter pulse period must be long enough to allow farthest range returns from any pulse to finish arriving before the nearest range ones from the next pulse begin to appear so that those do not overlap each other in time on the other hand the interpulse rate must be fast enough to provide sufficient samples for the desired across range or across beam resolution when the radar is to be carried by a high speed vehicle and is to image a large area at fine resolution those conditions may clash leading to what has been called sar s ambiguity problem the same considerations apply to conventional radars also but this problem occurs significantly only when resolution is so fine as to be available only through sar processes since the basis of the problem is the information carrying capacity of the single signal input channel provided by one antenna the only solution is to use additional channels fed by additional antennas the system then becomes a hybrid of a sar and a phased array sometimes being called a vernier array data processing edit 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 amplitude data edit 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 variations in processing may also be done in either vehicle borne stations or ground stations for various purposes so as to accentuate certain image features for detailed target area analysis phase data edit although the phase information in an image is generally not made available to a human observer of an image display device it can be preserved numerically and sometimes allows certain additional features of targets to be recognized coherence speckle edit unfortunately the phase differences between adjacent image picture elements pixels also produce random interference effects called coherence speckle which is a sort of graininess with dimensions on the order of the resolution causing the concept of resolution to take on a subtly different meaning this effect is the same as is apparent both visually and photographically in laser illuminated optical scenes the scale of that random speckle structure is governed by the size of the synthetic aperture in wavelengths and cannot be finer than the system s resolution speckle structure can be subdued at the expense of resolution optical holography edit before rapid digital computers were available the data processing was done using an optical holography technique the analog radar data were recorded as a holographic interference pattern on photographic film at a scale permitting the film to preserve the signal bandwidths for example 1 1 000 000 for a radar using a 0 6 meter wavelength then light using for example 0 6 micrometer waves as from a helium neon laser passing through the hologram could project a terrain image at a scale recordable on another film at reasonable processor focal distances of around a meter this worked because both sar and phased arrays are fundamentally similar to optical holography but using microwaves instead of light waves the optical data processors developed for this radar purpose 51 52 53 were the first effective analog optical computer systems and were in fact devised before the holographic technique was fully adapted to optical imaging because of the different sources of range and across range signal structures in the radar signals optical data processors for sar included not only both spherical and cylindrical lenses but sometimes conical ones image appearance edit the following considerations apply also to real aperture terrain imaging radars but are more consequential when resolution in range is matched to a cross beam resolution that is available only from a sar 25cm resolution sar image of downtown cleveland ohio by umbra range cross range and angles edit the two dimensions of a radar image are range and cross range radar images of limited patches of terrain can resemble oblique photographs but not ones taken from the location of the radar this is because the range coordinate in a radar image is perpendicular to the vertical angle coordinate of an oblique photo the apparent entrance pupil position or camera center for viewing such an image is therefore not as if at the radar but as if at a point from which the viewer s line of sight is perpendicular to the slant range direction connecting radar and target with slant range increasing from top to bottom of the image because slant ranges to level terrain vary in vertical angle each elevation of such terrain appears as a curved surface specifically a hyperbolic cosine one verticals at various ranges are perpendiculars to those curves the viewer s apparent looking directions are parallel to the curve s hypcos axis items directly beneath the radar appear as if optically viewed horizontally i e from the side and those at far ra...