This requirement creates great challenges to the state-of-the-art electronics. On one hand, direct generation of linear frequency modulation LFM signals by means of direct digital synthesizers DDS is limited to a few gigahertz . On the other hand, the precision of analog-to-digital converters ADCs in the receiver drops rapidly as the input bandwidth and sampling rate increase, which severely restricts the radar resolution as well as the processing speed.
Microwave photonic technologies have been proposed as a promising solution to overcome the limitations of pure electronic systems .
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Various schemes for photonic generation of linear frequency modulation LFM signals with a large bandwidth have been demonstrated [5, 6]. However, fast processing of such signals is still a difficult task in either electronic or photonics-based radar receivers.
Figure 1. We have developed a photonics-based radar incorporating optical generation and de-chirp processing of broadband LFM signals. Figure 1 a shows the schematic diagram of the photonics-based radar. After optical-to-electrical conversion, an LFM signal is generated with both the center frequency and the bandwidth quadruped compared with the IF-LFM signal.
With this method, high frequency and broadband LFM signal can be generated applying low-speed electronic devices. In the receiver, the collected radar echo is applied to drive an electro-optical phase modulator PM , which modulates a reference optical signal from the DPMZM. The phase modulated signal contains a pair of optical sidebands, with one sideband temporally delayed compared to the other.
When the two sidebands are selected out by an optical band-pass filter and sent to a photodetector for frequency mixing, de-chirping of the LFM signal can be implemented, as illustrated in Fig. By properly setting the parameters of the LFM signal according to the detection range, the de-chirped signal can be controlled in a low frequency range, and it can be sampled by a low-speed ADC and quickly processed by a digital signal processing DSP unit.
The achievable operation bandwidth of this photonics-based radar is mainly limited by the electro-optical devices, which can reach tens or even hundreds of gigahertz. Therefore, it is possible to realize real-time target detection and imaging with a very high resolution.
Figure 2. Spectrum of de-chirped signal when detecting a a signal target and b two targets separated by 2 cm The established radar prototype works at K-band radar with a bandwidth of 8 GHz GHz. When detecting a small trihedral corner reflector placed 2. The 3-dB bandwidth of the spectrum peak is kHz, indicating an effective range-resolution of 2.
But what will happen in the future, when every car will be equipped with a radar and every radar will demand the entire bandwidth? Our solutions permit drivers to share the available bandwidth without any conflict," Kozlov says. Ginzburg concludes. Note: Content may be edited for style and length.
Science News. Partially coherent radar unties range resolution from bandwidth limitations. Nature Communications , ; 10 1 DOI: ScienceDaily, 2 April American Friends of Tel Aviv University. 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.
A Unique Constellation
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.
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. 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. 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. 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, ,, for a radar using a 0. Then light using, for example, 0. 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    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.
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. 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.
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These curvatures are not evident unless large extents of near-range terrain, including steep slant ranges, are being viewed. When viewed as specified above, fine-resolution radar images of small areas can appear most nearly like familiar optical ones, for two reasons. The first reason is easily understood by imagining a flagpole in the scene.
The slant-range to its upper end is less than that to its base. Therefore, the pole can appear correctly top-end up only when viewed in the above orientation. Secondly, the radar illumination then being downward, shadows are seen in their most-familiar "overhead-lighting" direction.
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Note that the image of the pole's top will overlay that of some terrain point which is on the same slant range arc but at a shorter horizontal range "ground-range". Images of scene surfaces which faced both the illumination and the apparent eyepoint will have geometries that resemble those of an optical scene viewed from that eyepoint. However, slopes facing the radar will be foreshortened and ones facing away from it will be lengthened from their horizontal map dimensions. The former will therefore be brightened and the latter dimmed.
Returns from slopes steeper than perpendicular to slant range will be overlaid on those of lower-elevation terrain at a nearer ground-range, both being visible but intermingled.
This is especially the case for vertical surfaces like the walls of buildings. Another viewing inconvenience that arises when a surface is steeper than perpendicular to the slant range is that it is then illuminated on one face but "viewed" from the reverse face. Then one "sees", for example, the radar-facing wall of a building as if from the inside, while the building's interior and the rear wall that nearest to, hence expected to be optically visible to, the viewer have vanished, since they lack illumination, being in the shadow of the front wall and the roof.
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Some return from the roof may overlay that from the front wall, and both of those may overlay return from terrain in front of the building. The visible building shadow will include those of all illuminated items. Long shadows may exhibit blurred edges due to the illuminating antenna's movement during the "time exposure" needed to create the image. Surfaces that we usually consider rough will, if that roughness consists of relief less than the radar wavelength, behave as smooth mirrors, showing, beyond such a surface, additional images of items in front of it.
Those mirror images will appear within the shadow of the mirroring surface, sometimes filling the entire shadow, thus preventing recognition of the shadow. An important fact that applies to SARs but not to real-aperture radars is that the direction of overlay of any scene point is not directly toward the radar, but toward that point of the SAR's current path direction that is nearest to the target point. If the SAR is "squinting" forward or aft away from the exactly broadside direction, then the illumination direction, and hence the shadow direction, will not be opposite to the overlay direction, but slanted to right or left from it.
An image will appear with the correct projection geometry when viewed so that the overlay direction is vertical, the SAR's flight-path is above the image, and range increases somewhat downward. Objects in motion within a SAR scene alter the Doppler frequencies of the returns. Such objects therefore appear in the image at locations offset in the across-range direction by amounts proportional to the range-direction component of their velocity. Road vehicles may be depicted off the roadway and therefore not recognized as road traffic items.
Trains appearing away from their tracks are more easily properly recognized by their length parallel to known trackage as well as by the absence of an equal length of railbed signature and of some adjacent terrain, both having been shadowed by the train. While images of moving vessels can be offset from the line of the earlier parts of their wakes, the more recent parts of the wake, which still partake of some of the vessel's motion, appear as curves connecting the vessel image to the relatively quiescent far-aft wake.