A versatile technology that has existed in its present form for some 40 years only recently has been able to expand into the commercial marketplace in the form of new products. In 2002 the U.S. Federal Communications Commission (FCC) modified its Part 15 rules covering radio frequency radiators to accommodate the unlicensed use of low-power, ultra-wideband (UWB) transmissions. The new ruling, following a contentious debate, protected GPS bands but left stakeholders on both sides of the issue less than satisfied.
Nevertheless, UWB technology shows considerable promise for aviation. A Germantown, Md., company, Multispectral Solutions Inc. (MSSI), is developing two new UWB systems, an obstacle avoidance radar and a radar altimeter, for use in low-flying vehicles, such as helicopters and small unmanned air vehicles (UAVs).
A UWB radar altimeter would perhaps have been a godsend for the 30 Marines and one sailor who lost their lives in the Jan. 26 crash of a CH-53E Super Stallion helicopter in the Iraqi desert. Blowing sand was suspected to be a contributor to the accident during the night-time operation. A key attribute of UWB radar altimetry is its ability to see through blowing sand, a situation the helicopter community commonly calls "brown-out."
Because of its small size, a low-power UWB radar can alert small, low-flying UAVs of nearby obstacles and trigger an evasive maneuver automatically. MSSI demonstrated this capability in September 2002 on a prototype UAV built by Honeywell. The UAV was developed for a Defense Advanced Research Projects Agency (DARPA) program that was once called the Micro Air Vehicle but has transitioned into the Organic Air Vehicle (OAV). (Honeywell is competing with BAE Systems and Aurora Flight Sciences in the OAV program, so-called because it potentially would place UAVs organically within military units.)
Despite the DARPA program's name change, MSSI engineers still refer to their radar as the MAVCAS (micro air vehicle/collision avoidance system). The company has further developed the system for a UAV program and this year plans to deliver a UWB radar that can detect, localize and identify thin wire obstacles at distances exceeding 160 feet (50 m).
UWB technology is unique in that its signal spreads across multiple frequency bands instead of using just one band. One of its primary advantages is that the technology can be employed for numerous applications using the same circuitry architecture. In addition to its potential airborne use, the radar can perform tasks, ranging from detecting buried land mines to tracking animals to alerting drivers of possible conflicts during lane changes on multilane highways.
The U.S. Special Operations Command (SOCOM) would like to employ UWB radar for perimeter intrusion detection around its facilities in the field. Its quest for a detection system follows demonstrations showing that an ultra-wideband radar can detect low radar cross-section (human) targets at ranges exceeding 420 feet (130 m). Other UWB applications include high-speed digital voice, data and video communications on a wireless local area network (LAN) and tracking, with centimeter-level accuracy, the position of inventory or vehicles within a warehouse or factory.
Of course, all technologies have trade-offs, and UWB is no exception. There has been concern, for instance, that this frequency-pervading technology could cause interference and disrupt the transmissions of other frequency users. Of major concern in the aviation and defense communities is UWB's possible interference with the GPS satellite navigation signal, which is vulnerable because it is received at an extremely low power level and operates at a low margin above the thermal noise floor.
Avionics Magazine has covered the pros and cons of UWB several times, starting with Kathy Kocks' report on the technology in the February 2001 issue (page 20). Coverage also has been given to UWB's history, which can be traced back to World War II, when--strange but true--the German actress, Hedy Lamar, and the composer, George Antheil, patented a technology for splitting a broadcast among many radio frequencies to avoid jamming. The technology began in its present form in the late 1950s, however, when high-speed radio signal processing allowed scientists to view electromagnetic wave phenomena from a time-domain rather than from the more common frequency-domain perspective. Carrying that premise to an extreme, the Italian scientist, Enrico Staderini, declared that, with UWB, "frequency is meaningless...and time is our lord." The term, ultra-wideband, didn't originate until the 1990s, when a DARPA study of radar coined it to differentiate UWB radar from conventional radar.
Avionics Magazine also has covered FCC's Part 15 rule, which outlines how UWB is to be regulated for unlicensed use. UWB radar apparently can be produced economically; the challenge is to assure it can work within Part 15 parameters. MSSI believes the challenge can now be better met, knowing that the Part 15 ruling is more or less in its final state. The ruling's last update was in December 2004.
Within those restrictions companies such as MSSI, Time Domain, McEwan Technologies and others have looked for ways to commercially exploit UWB's huge potential. Many UWB companies have turned to military applications of the technology, which are not subject to FCC regulations.
MSSI, for example, is developing a short-pulse low-power radar that will operate within FCC restrictions. It is a commercial product, says Lester Foster, the company's radar program manager, but "for altimetry it must be installed on government aircraft." (The U.S. federal spectrum watchdog, the National Telecommunications and Information Administration, has ruled that Part 15 devices can be installed on government aircraft but not on civil aircraft.)
MSSI believes that it could make available an FCC-approved, short-pulse radar that has a "sensible" altitude range of 410 feet (125 m) with a range resolution of 3 inches (7.6 cm) and at a cost that could be significantly lower than that of currently available radar altimeters used for small commercial UAVs, according to a company white paper.
Finding the "Sweet Spot"
MSSI has developed a radio architecture that supports UWB signaling in any frequency band, such as UHF, L band and C band. The trick is in MSSI's receiver technology, which collects the radio frequency energy in the transmitter band and senses the arrival of discrete pulses in time. The radio front-end filters the radio band of interest, while the back-end performs the detection. Using this architecture, the front-end can easily be "swapped out" to operate anywhere in the electromagnetic spectrum while maintaining the same back-end circuitry, permitting a flexible radar design.
The company also has developed several UWB transmitters and antennas for operation in the relatively unused spectrum real estate in C band (6 to 7 GHz). This capability will allow a UWB radar to avoid heavily used spectrum, particularly in the L band, which is utilized by GPS, and the S band, which is used by WiFi and other services. According to Foster, "Because the technology is not frequency-based, we can use any frequency, and we found the `sweet spot' to be in C band."
C band is sweet because it is fairly unoccupied between 6 and 7 GHz and because FCC Part 15 rules permit low-power operation there. The 4-8-GHz band accommodates little more than radio altimetry and microwave landing and relay. "Therefore, there's very little interference with our 6.4-GHz system," Foster maintains.
The busy L band--used for cell phones, radars and other applications, including GPS--has become popular due to the recent explosion of wireless technologies. C band provides comparably shorter wavelengths and has a lower ability to penetrate through obstacles. C band radar operation also suffers some propagation loss. But Foster says these limitations are more than made up for by the fact that the devices using C band are not "interference limited" and operate in a much less noisy environment.
Foster affirms that the applications of UWB radar are "infinite," and adds that there is a lot of interest in short-pulse UWB radar right now. "We're providing radar for one company, for non-coherent synthetic aperture radar, to detect land mines and IEDs," he says. (IEDs refer to the improvised explosive devices, or bombs, being employed by insurgents in Iraq.)
UWB radar works quite differently from the continuous-scan radars with which we are more familiar. Instead of continuously emitting energy and always decoding the returned signal, a UWB radar--and, indeed, most UWB devices--emit extremely fast pulsing signals, or bursts, in short durations. While a conventional frequency-modulated, continuous-wave (FMCW) radar, typically found on an automobile cruise-control system, determines range by comparing the modulation characteristics of the received signal to the transmitted signal, UWB radar calculates the returned signal's strength, based on sampling the detector over time. This is called discrete time radio technology.
In a company white paper, MSSI lists the advantages of short-pulse UWB radar over conventional radar. They include:
Higher range measurement accuracy and range resolution;
Enhanced target recognition due to the detection of more information from a target's separate elements;
Immunity to passive interference, such as from rain, fog and clutter;
Increased immunity to co-located radar transmissions;
Increased radar operational security because of the large spectral spreading; and
The ability to detect very slow moving or stationary targets, which would be appropriate for obstacle avoidance.
MSSI's recently developed radar is unique among UWB technologies in that it can transmit a pulse in 2.5 nanoseconds (billionths of a second) and then sample its entire range space at high speed. Other technologies may require multiple pulses to build up processing gain for range calculation. The UWB radar is sensitive enough to measure a single pulse return down to the theoretical limits of the detector.
MSSI has produced a single-circuit card radar, to create a compact radar package for UAVs. It also has built multicard systems that include separate boards for the transmitter, receiver and signal processing functions. The triple-card system, which MSSI developed for the prototype intrusion detection application, allows easier processor upgrades.
Different radar applications require varying levels of processing. A radar altimeter needs to process only the first return signal from the reflective field to precisely determine an aircraft's height above the ground. Conversely, a noncoherent synthetic aperture radar using UWB would require substantial return signal processing to create imagery. Obstacle avoidance radar--which extracts more information from the reflective field than a radar altimeter, such as obstacle localization and size--would require processing power that is between these two extremes, says Foster. He says obstacle avoidance radar would require "about Pentium II or III capability."
Different radar applications also are created by varying the signal pulse's transmission and range resolution, or how fast you sample the range space. If, for example, you use a single-nanosecond pulse, as with a UWB radar altimeter, you will "see the world" within a 6-inch (15.2-cm) measurement, or range of propagation. In other words the precision of the measured range is no less than 6 inches.
Used to prevent mishaps caused by brown-outs, a UWB radar altimeter on a helicopter would not have to be activated until the crew enters into an approach. This would save power and keep UWB emissions to a minimum. "The radar altimeter becomes very accurate at less than 50 feet, when the radar area of reflection on the ground becomes more uniform in height, " says Foster. By attaching UWB sensors as close to the helicopter's wheels as possible, the UWB radar altimeter can accurately detect the aircraft's pitch and roll in relation to the ground. Inputting this information into a cockpit panel display would assist the pilot in making a safe, soft landing.
Likewise, UWB radar altimetry could be used to guide UAVs to a landing spot, "instead of flying the last 70 feet [21 m] blind," Foster adds. It could guide the UAV operator but would probably interface with an autoland system to ensure accurate landings. In fact, with its precision and ability to define edges, UWB radar altimetry could allow the most advantageous positioning, for example, perching a UAV on the corner of a building's rooftop, where the vehicle could loiter to monitor a street scene.
Selecting an Antenna
Key to a UWB radar's performance and function is its antenna. "We have many antennas to choose from," says Foster. "The choice determines the radar's field of view and gain." Generally, UWB radar is a volumetric sensor, resolving reflective surfaces over the range surface of the antenna field of view. By sweeping a field of view, the radar can give increased information about reflected target location. MSSI radars also incorporate dual antenna designs, with an antenna devoted to each function of transmit and receive for maximum sensitivity. These antennas can be comparably simple in design, as they do not have the conventional radar's geometric sensitivities of relative location between the transmit and receive.
A 14-dBi (decibels gain over isotropic) antenna "would be good for a radar altimeter," Foster adds. For the latest version of its developmental MAVCAS obstacle avoidance radar, MSSI has selected an 18-dBi antenna with a 5-watt transmitter. The C band radar, with its smaller frequency wavelength, allows the use of relatively compact antennas, a plus when the system is being fitted to micro UAVs.
To operate within Part 15 limits, the UWB radar's transmitted energy must meet low-level field strength requirements. With a small antenna, this factor would make the transmitted field of view large, less concentrated, and therefore less sensitive than a high-powered, concentrated field of view. To enhance radar sensitivity, Foster says, you can use a high-gain receiver antenna that concentrates the returned signal's field of view and thus enhances the radar's sensitivity, to better detect small targets or obstacles. "FCC only regulates radiated energy," he explains. "It doesn't care about the amount of energy a system receives."
Because the peak gain of transmitted energy is in the center of the burst, Foster foresees the use of an electronically steered antenna that can localize elements within the radar's various fields of view. In other words, by turning the antenna's field of view, an object that might be at the edge of one transmission would show up in the center of another transmission, allowing improved obstacle location.
What does MSSI plan to do with its UWB radar technology? It recently received a phase 1, Small Business Innovation Research (SBIR) contract to build a proof-of-concept, UAV obstacle avoidance radar for the U.S. Army Space and Missile Defense Command in Huntsville, Ala. (Previously, MSSI had received SBIR radar contracts from U.S. Naval Air Systems Command, DARPA and SOCOM.)
The company also wants to provide a radar developer evaluation kit, which would show corporate, government and academic institutions how to incorporate low-power, FCC-approved UWB radar for whatever applications they may choose. MSSI plans to have this product available later this year.