DARPA eyes tiny radar system for future miniature aircraft
ARLINGTON, Va. Throughout the course of military history, perhaps the single most consistent and important question to any battlefield commander has been, what is over the next hill?
By J.R. Wilson
ARLINGTON, Va. — Throughout the course of military history, perhaps the single most consistent and important question to any battlefield commander has been, what is over the next hill?
The answer typically comes from scouts. In the past, scouts traveled on horseback and used their eyes and ears to ascertain vital information about the enemy. More recently, they use aircraft and a variety of electronic and optical sensors.
A major problem, however, involves reconnaissance aircraft, such as the future Army Boeing-Sikorsky RAH-66 Comanche scout/ attack helicopter and the Air Force Northrop Grumman Joint Surveillance Target Attack Radar System aircraft. They are expensive, in short supply, and often are not able to provide the kind of detail that field commanders need — especially when they fight in the close confines of cities.
Defense officials are seeking a partial solution with the use of Unmanned Aerial Vehicles (UAVs), but these also can be expensive and are not widely available. They also are vulnerable to ground fire if they fly too close to the target.
One solution under consideration at the Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., is a tiny unmanned aircraft. This so-called miniature air vehicle (MAV) would be able to operate close to the enemy without detection, cheap enough to be lost without great concern, and yet capable of providing detailed data on precisely what is over the next hill.
To be viable in the field, MAVs will need a comprehensive set of infrared, acoustic, bio-chemical, and radar sensors. It is the last of those that poses the greatest problem, which DARPA officials are hoping to resolve with the development of a miniature synthetic aperture radar — a microSAR.
SARs are common aboard UAVs, but generally weigh from 60 to 100 pounds. The development of a 30-pound SAR is considered cutting edge. But systems designers would measure a microSAR in ounces and inches.
"The microSAR itself is an enabling technology to put reconnaissance capability into the platoons," says Richard DeFrancisco, vice president-sensing tech- nologies for GORCA Inc. of Moorestown, N.J.
"The enabling technology for the microSAR is how the radar is done, and the principal enabling technology for that is special motion compensation," DeFrancisco says. "The hard challenge we have is that SARs generally require stable motion to generate the synthetic aperture, which has been a challenge for all SAR platforms. But when you take a tiny aircraft such as an MAV, it is going to be bouncing all over the sky. You can`t burden the platform with excessive GPS or INS or whatever."
GORCA officials, operating under a Small Business Innovative Research (SBIR) contract with DARPA — the only public microSAR effort in the world — are keeping their work secret, but some details are known.
They are using an iterative spatial correlation algorithm to measure the clutter, determine what the platform is doing in real time, and take the motion out. Experts have demonstrated the process on synthetic data; part of the SBIR is to put it into an air vehicle to make certain it works in actual use. GORCA officials say they hope to do that this fall with a proof-of-concept demonstrator — weighing about two pounds.
GORCA`s first step is to build a prototype "that will actually fly but is not geared to go onto a MAV," DeFrancisco says. The second step is to reduce the size of the prototype. "The radar is only 2-by-3-by-1/4 inches, about the size of a credit card size calculator," he explains. "Even that is too big to put on an MAV, even though it is only a few ounces in weight."
The concept-development unit, which DARPA leaders want to weigh less than two pounds, can go on any UAV or helicopter; the first true microSAR would be the 2-by-3-by-1/4-inch version.
"One of the enabling tech-nologies will be the algorithms for the motion compensation," notes Bruce Johnson, DARPA`s microSAR program manager. Another enabling technology involves what GORCA engineers designed with the signal link, the radar onboard the UAV, and with the use of microwave/millimeter wave integrated circuit (MMIC) technology, which greatly reduces the weight and power.
A partitioning of the radar function between the UAV and the ground station makes the datalink key to the concept, Johnson says. To shrink the SAR down to fit on a MAV will require designers to offload data processing to the ground station — which itself must be extremely small and portable to move easily with mobile platoons.
DeFranciso says the signal link solution GORCA is pursuing does not involve compression, which he says falls victim to phase coherency problems. It also does not use the "store and forward" technique popular in other applications because of bandwidth limitations.
Competitive issues, however, may end up placing a strike against GORCA. The company`s approach "is a nice concept that greatly reduces the size and weight, but they are holding it proprietary," DARPA`s Johnson says.
The real push, Johnson says, will be whether GORCA engineers can build it affordably and will it work in the field. "One of our interests, if this concept proves out, is something like a relay station, where the MAV is line-of-sight to another UAV, which would extend its datalink," he says. "A radar of this size also could open up a host of other applications — putting one on a pole as part of a security system, for example. That`s not a SAR, but the technology opportunities extend beyond SARs and MAVs."
For field reconnaissance, DARPA leaders told GORCA engineers to consider the need for Special Forces and front-line units to move quickly without carrying large new pieces of equipment. To that end, they suggested GORCA to incorporate the ground station into a ruggedized laptop computer — not what is in the current inventory, but what will likely be available in five years.
"We look at all stages of weight and power, even on the ground unit," DeFrancisco says. "If there is an integrated Army processing station in somebody`s backpack, we won`t keep adding boxes but try to dovetail with the backpack of the future, as it were. The idea is not to have redundancies but to highly integrate it with other systems."
Another consideration is how fighting forces will employ MAVs and microSARs in the field. "What we`re looking at is about one foot square at a range of 2 kilometers," DeFrancisco says, explaining that this resolution "is great for finding out what`s coming down a pass toward you or what`s over the hill. But for urban warfare, where you want to peer through a window, the resolution will shrink down and should be able to tell you what`s going on inside a room without great difficulty."
DARPA officials, in fact, are interested in using MAVs at ranges measured in yards or even feet from their targets, thus making the microSAR resolution more than adequate.
"The device we`re building has a rather sophisticated microstrip antenna with a very complex antenna pattern," DeFrancisco says. "For a single-element system, it gives a good beam shape. It`s totally flat and integrated into the body of the package." Since the microSAR will be part of the MAV, the final design cannot be fixed until they have an actual MAV design to work with as well, he points out.
He says the ground processor will come largely from commercial off-the-shelf (COTS) components, but the flight element must be a custom design because it must fit in the body of the MAV. The initial proof-of-concept demonstrators, however, will be COTS to the extent they will rely heavily on MMIC.
"It probably has more than a dozen MMICs in it," DeFrancisco says. "Of course, the architecture and design are still proprietary and not yet ready for public release. But we are talking about such small geometries on a true MAV; we would have to know the vehicle intimately, because it would have to be integrated into the fuselage. So there will be some real challenges."
With so little space and weight to deal with, power also is a consideration in designing a viable microSAR. "We`re basically looking at 12 volts at 10 watts on power," he says. "The limiting factor on power is the amount of RF we have to pump out. This is a short-range device, not intended for much more than two kilometers from the MAV to the target. It`s true standoff from the base station could be five to 10 kilometers."
In the field, true MAV-based reconnaissance would not necessarily be limited to a single SAR or other sensor. "If you could have a sparrow-size UAV that could see some limited area — and it`s cheap — you no longer think in terms of one UAV," Johnson says. "You would send in multiple UAVs. And that would give you multiple aspects. You could look at something from more than one angle simultaneously, which makes camouflage pretty tough. You can`t do that with a big platform-the risk is too great. But if you can build a very small, cheap platform, you open up a lot of new possibilities."
Before any of that happens, however, the microSAR SBIR must evolve into a sensor program for a real platform. For that to happen, leaders of the military services must believe the MAV is real, the microSAR is real, and both will sufficiently enhance military capabilities in the field. The first step is the flight demonstration scheduled to conclude the second phase of the current SBIR.
"The idea is to demonstrate the key enabling technologies-the microstrip antenna, the key algorithms and the partitioning of what you do aboard the UAV versus what goes over the datalink to the ground to reduce what you need to put on the UAV," Johnson says. "Success hinges on those three things working together."
The strongest interest in using MAVs comes from the U.S. Army, and its missions involving close-in surveillance. "I just don`t think you can move forward on a paper design at this time," Johnson says. "If it pans out — and I have no reason to think it won`t — it`s just a matter of manufacturing to get it form-fit into a MAV."
This artist`s concept illustrates how systems designers might use miniature radar systems in reconnaisance platforms of the future.