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Tuesday, August 1, 2006

Radar Transmitting Data

If the radars of modern combat aircraft can gather surveillance data, can they also serve as high-speed data link radios? That's what three military aerospace giants have set out to prove.

by David Jensen

What means of transmitting radar imagery could be faster than having the radar system, itself, make the transmission? Radar antennas are designed to accept and process a large amount of data from a radio frequency (RF) signal; why not use them to transmit data via an RF signal, as well? Either way, it works with digital data.

The active electronically scanned array (AESA) radars developed for today's combat aircraft contain hundreds of small transmit/receive (T/R) modules that, in addition to gathering targeting data from a radar beam, could be modified to perform other tasks, including transmitting that data, employing a wide-bandwidth data link. In a directional environment--i.e., steering the RF signal--radar imagery and other data could be transferred throughout the battle space, to other aircraft, a ground station, even to a satellite. Likewise, with a bidirectional link, this versatile radar array also could be used to receive data critical to the combat mission. Any information put into an RF signal could be transmitted or received, including electronic intelligence data, maps, streaming video, positional or vector data, updated mission plans and raw (unprocessed) data.

Northrop Grumman Electronic Systems, L-3 Communications and Lockheed Martin Aeronautics Co. have joined in the development of such a capability for application in fifth-generation aircraft, such as the F/A-22 Raptor and F-35 Joint Strike Fighter. In a privately funded program, the three companies are developing a radar data link capability by combining an L-3 high-speed, software programmable modem with the AN/APG-77 solid state AESA radar that Northrop Grumman has developed for the U.S. Air Force's F/A-22. As prime contractor for both the F/A-22 and F-35, Lockheed Martin would be responsible for the new capability's integration into the two platforms' final architecture. (Under contract from Lockheed Martin, Northrop also is developing the radar for the F-35, designated the AN/APG-81. It currently is being flight tested.)

The Raptor's X-band radar, with its 120-degree field of view and all-weather, multitarget detection capability, includes an aperture comprising more than 1,000 transmit/receive modules, each about the size of a human toe, which could be used collectively to serve as a wideband communications antenna. This would forego the need for an additional, externally mounted radio antenna, which would be difficult to position on a fighter's limited surface space and adversely impact a stealth aircraft's radar signature.

Fast Working

Indeed, the Air Force is looking into multiple roles beyond target detection for versatile radars, such as the AN/APG-77, including electronic jamming and the monitoring and corruption of enemy communications, among others. The radar would carry out each task, or mode, separately. However, the speed at which the AESA radar can switch modes is so fast (in nanoseconds) that it provides the perception of simultaneous operations.

"The concept of operations by the user community will dictate what uses are provided by the radar in a multifunctional system and how they are switched," says Joseph Ensor, Northrop's vice president of combat avionics. The basic surveillance mission of the radar "will be preserved," he adds.

Equally as fast as mode switching is the speed at which the radar array can be electronically steered to direct the wideband communications signal. At the speed of light, the radar's steering controls will direct "phase shifters"--one per T/R module--to rapidly move the beam. Shifting the aperture angle from one extreme to the other on an electronically scanned array radar is many times faster than with older, mechanically steered, fighter radars.

Adding the high-speed data link capability requires no hardware modification to the radar and no additional power. The new capability calls for a programmable modem that incorporates the software to create waveforms (for satellite and broadcast channels, etc.), plus software installed in the radar processor to accommodate the additional communications mode.

In production the radar will be repackaged, and the modem will become one of the radar line replaceable units (LRUs), says Ensor. This will be fairly straightforward, as modem technology is similar to the frequency and waveform generation that the radar already is capable of doing. "It just forms the transmission for a different function," Ensor explains. "In this sense, we would adapt similar architectures from communications modems to the RF signal generation already residing in the radar."

Retrofits Possible

The radar data link capability could be made available for retrofit, as well. "The process would not be complicated," says Ensor. "However, hardware would have to be added to fielded systems. The length of the process is entirely dependent on the degree to which an elegant retrofit solution can be engineered."

Ensor adds that the retrofit package could be developed for virtually any AESA radar system. Including both the Air Force and Navy as possible customers, therefore, the capability could potentially be acquired to modernize the F/A-18E/F, F-16, F-15C/F and the EA-18G Growler electronic attack aircraft, as well as the Global Hawk and E-10, which would benefit from larger arrays that can receive and transmit massive amounts of graphical information at high data rates and at a much greater range.

"This capability of using radar for a function it previously has not been used for allows next-generation fighters to be a non-traditional ISR [intelligence, surveillance and reconnaissance] asset," says Bruce Carmichael, L-3's vice president of Air Force programs.

"Now we gain the benefit of additional sensors sharing data in the battle space, where it wasn't practical before," Ensor adds.

Northrop's and L-3's decision to develop a data link capability is the offshoot of another radar advancement. "We have this new capability of putting in synthetic aperture radar [SAR] mapping modes, but we didn't have the bandwidth to transfer that data," says Ensor. "That capability was the motivation to develop this new [data link] technology."

Joining with L-3 Communications--which has long produced high data rate communications for aircraft, particularly reconnaissance platforms--was a logical decision for radar maker Northrop Grumman. "We've worked together on other projects," says Carmichael. "For instance, we provide the data links for the [Northrop Grumman] Global Hawk."

"We also provide the satcom [satellite communications] suite for the Predator unmanned air vehicle," he adds. "And we're about to provide line of sight communications for the Predator." The high-flying U-2 reconnaissance aircraft also sports L-3's line of sight communications and satellite communications suites.

Using the CDL

In addition to furnishing the high-speed modem for the AESA radar, L-3 developed a modified version of the common data link (CDL) waveform, which is a government standard for reconnaissance data. The host equipment for this waveform recently has been based on software-programmable terminals and for network centric warfare, says Carmichael.

L-3 was instrumental in establishing the CDL, having developed a set of high-capacity waveforms for near-real time transmission of sensor data from the U-2. These and other waveforms have contributed to the Department of Defense's decision to set up CDL as the standard data link for primary intelligence dissemination. Now, installed in both manned and unmanned aircraft, the data link also is taking on the larger role as part of the joint airborne network.

Further evidence of L-3's expertise in CDL development are current, comparable programs, according to Steve Barham, L-3's director of programs. He points to the company's involvement in the Multi-Platform CDL (MP-CDL) program, in which the Air Force seeks a high-capacity, Internet protocol (IP)-based data link capable of providing point-to-point and point-to-multiple point, wideband data communications. It would include a modem capable of supporting up to three separate links and deliver data rates up to 274 Mbits/s. L-3 also is involved in the Multi-Role Tactical CDL (MR-TCDL) program, in which the company is developing a next-generation, network-enabled CDL system for the U.S. Army.

Tested for High Speed

Testing the radar-CDL capability in a laboratory environment, Northrop and L-3 have demonstrated the transmission of a 72-Mbyte SAR image in 3.5 seconds at a data rate of 274 Mbits/s. Using the current Link 16 standard, such a transfer would take well over an hour, according to Carmichael.

The two companies have achieved a data rate with the radar that reaches as high as 548 Mbits/s and received data rates up to 1 Gbit/s. "We were trying to explore the limits of the technology," says Carmichael.

Is such a data rate the goal for a production radar? "It remains to be seen if 548 [Mbits/s] is required for operational use," says the L-3 vice president. "We don't have a target [and] it remains to be seen what the user requirements will be."

The next step for the three partner companies is to flight test the radar data link capability. An interim plan had the radar installed on Northrop Grumman's BAC 1-11 flying testbed aircraft, to confirm the lab results. However, according to Ensor, "We seemed to have generated so much interest that we don't need to go to the next step as originally planned; the next step may be a government-funded demonstration."

The high-speed data link capability has caught the attention of Air Combat Command, and the three companies are in discussion with the Air Force on flying demonstration. "The demonstration will probably be done on a military aircraft or one of our testbeds that we fly for the F/A-22 program," Ensor adds.

Flight Tests

Flight tests are meant to confirm the static lab results in a dynamic environment, as well as move the program a significant step closer to verification of radar-CDL capabilities. Officials with the three companies say flight testing also could include the following:

➤ Verification of airframe dynamics on data link integrity;

➤ Confirmation of atmospheric effects on the communications channel;

➤ Confirmation of maximum range, and

➤ Tests of dynamic beam steering and antenna pointing to support radar-CDL communications.

Large data packages--for example, a synthetic aperture radar map--will be both transmitted to and received from ground stations and other airborne platforms during flight test. "To fully demonstrate, the aircraft will gather the data package--in other words, take the SAR map--and then transmit the map it just took to the receiving system," says Ensor. "We also will demonstrate long-range performance of the communications."

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