Network-Centric Naval Forces A Transition Strategy for Enhancing Operational Capabilities (2000) / Chapter Skim
Currently Skimming:
Appendix B Current Sensor Capabilities and Future Potential
Appendix B Current Sensor Capabilities and Future Potential
Pages 352-383
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 352... ...
As a result, every Navy platform has several radars: for search, navigation, missile fire control, gun fire control, target illumination, and so on. Although there are still mechanically scanned, microwave tube-powered radars on naval platforms, modern radar implementations, both surface-based and 352
|
From page 353... ...
For an isolated radar, operating in a platform-centric mode, the precision with which the locations of these target reports are defined is of mixed quality for although a radar usually can provide high-precision range and Doppler measurements, the angular precision is generally poor because of the large radar wavelengths and the dimensions of practically sized radar antennas. Beam widths measured in degrees or finite fractions of degrees are not uncommon.
|
From page 354... ...
In the past, computers with this kind of throughput were large and deployable only on the ground. Early air- or space-based SARs were forced to transmit the raw data to dedicated ground stations for rapid, but not necessarily real-time, availability of the images.
|
From page 355... ...
Of course, bearing estimates are not limited to these dimensions, for with a large enough signal-to-noise ratio, beam-splitting interferometric techniques (e.g., monopulse) can estimate directions to small fractions of the beam width.
|
From page 356... ...
transverse to the flight path. B.1.1.4 Geopositioning Accuracy A single traditional radar, whether phased array or not, has poor geolocation capabilities because, although its range measurement uncertainty can be very small if its signal bandwidth is large, e.g., centimeters to a few meters, its angular resolution is always poor in practice because of the limited aperture sizes available; e.g., 0.1° to 1° or 2° beam widths are typical.
|
From page 357... ...
B.1.1.5 Area Coverage Rates Search radars whether mechanically scanned or phased array, ATC, or military scan the full 360° upper hemisphere out to many hundreds of nautical miles in about 5 to 10 s. If a nominal 450 km range and a 6-s sweep interval, similar to that of the SPS-49, are chosen, the corresponding area coverage rate would be about 105 km2/s a very high rate of coverage but the resolution is also quite low.
|
From page 358... ...
But SAR, the true imaging radar sensor that generates data for every pixel, without exception, will require much higher communication bandwidth capability in order to participate in a network-centric sensor grid but not nearly as much as is required by a capable modern electro-optical camera, as discussed in the section on electro-optical sensors (Section B.2~. Practical SAR sensors produce pixel information at rates comparable to what is implied by the Global Hawk performance capability described above under "Area Coverage Rates" (Section B.1.1.5~.
|
From page 359... ...
Similarly, the search rate capability of a radar is proportional to the product of the transmitted power and the area of the antenna. In addition, since low-frequency radars need large physical antenna in order to maintain even modest angular resolution and microwave power is much easier to generate at the lower frequencies e.g., one can obtain T/R modules with hundreds of watts capability at 1 GHz of L-band whereas the current state of the art produces only about 10 W for an equivalent X-band module at 10 GHz and much less than 1 W for frequencies of 35 GHz and beyond search radars are always L-band or lower.
|
From page 360... ...
previously doomed to off-line processing. Other tasks, such as digital beam forming of phased arrays, even at the subarray level, have remained impractical up to the present.
|
From page 361... ...
After some filtering and low-noise amplification, the received signal would be digitized directly at the microwave frequency or after a single stage of down conversion, digitally delayed as appropnate, and sent via fiber optics to the digital signal processor where pulse compression, beam forming, space-time adaptive processing (STAP) , and so on will be earned out at real-time speeds.
|
From page 362... ...
, and others are currently supporting major thrusts in this much-needed ADC technology, and one can expect to find radars with digital receivers, and perhaps digital beam forming and digital true-time delay, deployed within the next 5 years. Many radars today already employ digital waveform generation.
|
From page 363... ...
In assessing its mix of organic versus joint sensors in the battlespace of the future, the Navy should carefully consider the merits of deploying its own SAR-equipped UAVs. B.1.2.4 SiC and GaN High-power Devices Although the modern high-performance phased arrays that are deployed or under development at present are uniformly based on GaAs or InP MMIC T/R technology, it has long been known that semiconductor materials with a larger bandgap than GaAs and InP are possible and would offer enormous benefits.
|
From page 364... ...
The use of low frequencies (i.e., below L-band) would enhance the detectability of the stealth targets, whereas the distributed cooperation would greatly mitigate the poor angular resolution and permit practical-sized antennas to be utilized without compromising the overall performance of the networked radars.
|
From page 365... ...
In addition to this striking size/performance advantage, electro-optical systems are often significantly simpler than radar systems to implement. Optical systems make prolific use of simple mirrors and lenses of common materials that are transparent in the visible and infrared, conveniently supplying the electromagnetic phase shifts needed for precision beam control and focusing; optical imaging detectors are sensitive to the point of being able to detect single photons; and multipixel detector focal plane arrays for imaging can readily be implemented with microelectronic fabrication technology.
|
From page 366... ...
Thus to achieve the high-spatial-resolution performance for which optics is so valued, only a small IFOV can be examined at any instant. As a result of this kind of trade-off, typical fielded optical imaging sensors both visible and infrared (JR)
|
From page 367... ...
B.2.1.3 Detection Modern imaging optical sensors use highly integrated, monolithic arrays of semiconductor detectors known as focal plane arrays (FPAs)
|
From page 368... ...
In many forms of optical imaging cameras, the update rate greatly exceeds the gimbal slew capability, and so redundant images are collected and the area coverage rate is determined solely by the telescope stewing characteristics and the platform motion. The IFOV projected on the ground is determined by the number of pixels in the detector array and the pixel resolution.
|
From page 369... ...
, it takes only a simple computation to discover that such imaging sensors generate raw data at rates between 90 and 720 Mbps. Attempting to transfer this data throughout the network-centric sensor grid by means of general purpose communication links would be disastrous.
|
From page 370... ...
In many scenarios, image update intervals of seconds, that is, frame sampling rates measured in fractions of hertz rather than tens of hertz, may be adequate, thereby reducing the raw data communication requirements by factors of 100 or more, even without sophisticated local processing. Active imaging optical sensors, such as a rapid pulsed radar that generates three-dimensional, range-to-pixel imagery, differ from the passive FPA imaging sensors just described in optical detector focal plane arrays, which are capable of simultaneous, independent, multiple time-of-flight measurements (i.e., need to define range-to-target at each pixel)
|
From page 371... ...
B.2.1.8 Spectral Issues The optical portion of the electromagnetic spectrum, stretching as it does from the ultraviolet, through the visible to the near, mid, and far infrared, encompasses a broad range of physical phenomena that can help alleviate some environmental obstacles in certain circumstances and that can be exploited to enhanced target detection and classification (e.g., ATR)
|
From page 372... ...
Even though the progress of computational resources will eventually accommodate hyperspectral imaging, today the data from such sensors cannot be processed in real time. B.2.1.9 Environmental Limitations As indicated in the discussion above, environmental factors seriously limit the usability of electro-optical sensors.
|
From page 373... ...
Moreover, both the flare and the laser weapon beam, even if actual destruction does not result, can, by diffraction, cause large numbers of the detector elements around its image in the focal plane to saturate, thereby temporarily blinding large portions of the IFOV. B.2.2 Technology Trends and Future Growth in Electro-optics B.2.2.1 Uncooled IR Focal Plane Arrays One of the most exciting advances in electro-optics in recent years has been the migration of microelectromechanical systems (MEMS)
|
From page 374... ...
B.2.2.3 Special-Purpose Focal Plane Arrays Over the past decade, a number of interesting special-purpose multispectral FPAs have been developed. Both hybrid and monolithic techniques have been
|
From page 375... ...
In all other respects, it acts like any other traditional FPA with respect to sensitivity, update rate, and so on. Since manmade objects tend to retain polarization and natural background objects generally depolarize, this special sensor offers interesting potential for target discrimination.
|
From page 376... ...
Application to high-dynamic sensor/shooter/weapons scenarios seem unlikely, but longer-latency situational awareness might be considerably enhanced if the needed algorithms can be developed. B.2.2.6 Optical Phased Arrays Considering the performance advantages phased-array electronic beam steering has given to radar such that it completely dominates modern high-performance radar today, it is not surprising that considerable effort has been expended seeking ways to extend electronic beam agility to electro-optics.
|
From page 377... ...
are needed for long range but cannot achieve high angular resolution because of the very large antenna sizes required and the unpredictable spatial variations in sound propagation. And although high angular resolution is possible at high frequencies (e.g., 35 to 350 kHz)
|
From page 378... ...
But the beam width alone is not the limit as fractional beam width accuracy is certainly possible through interferometric techniques or what is called in radar "monopulse." Because of the relatively large wavelengths associated with acoustic radiation, the phased arrays used for sonar typically do not have more than a few tens of elements (e.g., 10 to 40) along any direction, whatever the frequency range employed.
|
From page 379... ...
to have the same beam width. B.3.1.2 Field of View and Field of Regard As phased arrays, all sonar antennas can be readily steered over large fields of regard.
|
From page 380... ...
B.3.1.6 Area Coverage Rate Active sonar for medium or long range is limited by the long round-trip return time of the transmitted energy for a range of 20 nautical miles, the roundtrip time is about 23 sec. If the sonar explores the 20 nautical miles radius + 60° FOR permitted by the phased array by 30 different 4° beams, the area coverage rate is only about 2 km2/s.
|
From page 381... ...
On the other hand, if we wished to employ an acoustic communication link, say, between cooperating UUV platforms, the necessary data rates could stress the system, and further local processing with ATR-like algorithms and perhaps the application of data compression techniques would be called for. B.3.1.X Environmental Issues The effects of the low acoustic propagation velocity, the media variability and inhomogeneity, and the frequency-dependent absorption that seriously limit sonar performance in the open ocean have been discussed.
|
From page 382... ...
It is in this context that interest in synthetic aperture sonar (SAS) has been reawakened in the past several years, with the hope that the imaging resolution and range (and hence area coverage rate)
|
From page 383... ...
Deployed like sonobuoys and drifting or moored in place, each node is envisioned to possess considerable on-board signal and data-processing resources and to be capable of passive automatic detection and classification as well as active signal processing. These individual capabilities, complemented by the networked acoustic/RF communications, would permit them to operate as a single large and very capable coherent distributed sonar.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.