Software Defined Radio: From Revolution to Evolution

Despite the problems encountered in developing a cross-service family of interoperable software defined radios (SDRs), the U.S. military has determined that it’s still well worth the effort. Without these Internet-enabled radios the vision of total connectivity would stop at the tactical command center, well short of the soldiers and airmen in the thick of the fight. Aircraft are key nodes in this network–critical sources of intelligence for commanders and users of information from them.

Extending connectivity to the field–what the military calls "the first tactical mile"–is the goal of the Joint Tactical Radio System (JTRS) program. The networked devices have a lot in common with desktop computers. While SDRs need hardware to transmit and receive data, they use software to control the modulation, frequency and all the unique aspects of the waveforms, the physical layer characteristics, such as pulse widths, channels and signals. The radios would share a common operating environment, roughly analogous to Windows on a PC. Waveforms, stored in a central library, would be available to different radios as applications like PowerPoint or Excel. Using the networked waveforms, then, users could layer higher-level applications such as streaming video or voice over Internet protocol (VOIP).

Also key to the vision are requirements such as waveform portability–the ability to port a new or upgraded waveform to an existing radio or to reuse an existing waveform in new radios–and cross-banding, the ability to extract data from one type of signal and embed the data in another type for transmission. The radios also need to be able to run multiple channels simultaneously.

But the military has found the program a trying experience. The original plan for JTRS turned out to be too ambitious, its requirements too changeable, schedules too optimistic, budgets too unrealistic, and many technologies too immature. These problems came to a head in the ground radio segment, which already had produced engineering development models when security requirements significantly changed.

Security issues, in particular, were underestimated. Information assurance is a huge concern with Internet-capable radios that link fighters back to strategic networks in the home country. The current definition of the underlying software communications architecture (SCA) supports dynamic behavior such as downloading and running waveforms and over-the-air updates. The size and complexity of the code will make it challenging to troubleshoot, let alone prevent compromise. And the data transmitted or received may span multiple security levels. How do you prove that the radio not only meets all the operational requirements, but that it does nothing else, i.e., it never fails into an insecure state?

The JTRS Joint Program Executive Office (JPEO), formed in February 2005, narrowed the program’s scope and stabilized its requirements in order to contain costs and reduce risks. JPEO has instituted an incremental approach, which defers many tasks to future phases.

"What we’ve seen is a bounding of requirements," sums up Bruce King, vice president and general manager of communications systems for Rockwell Collins. All of the waveforms may eventually be done, but over a longer period of time. "We’re going to field things smartly and we’re going to do a better job of managing the risk."

JPEO has reduced the number of waveforms from 32 to nine and the number of form factors from 26 to 13. Channel counts in certain form factors also will be cut. Helicopter radios have been moved from the ground to the airborne segment, which aims at smaller footprints. There are still numerous hurdles, including delivering waveforms, designing small form factor hardware, integrating radios with existing avionics, devising cooling technologies for high-density electronics and resolving intellectual property issues. On the security front, "Each one of the [JTRS] programs has updated and solidified its Increment 1 security requirements," says Allen Boyd, Collins’ senior director of JTRS systems. Schedules have been remapped out and coordinated with the National Security Agency, he adds.

The reorganization’s impact on aviation radios has been relatively small so far. The Airborne, Maritime and Fixed site (AMF) program has expanded to include not only fixed-wing but rotary-wing aircraft. And the previously standalone element developing a multichannel software radio within the same hardware footprint as today’s Multifunctional Information Distribution System (MIDS) data link radios has been folded into the Airborne and Maritime domain.

The original AMF program was in the pre-system development and demonstration (Pre-SDD) phase when the JPEO was formed. The Pentagon is expected to select one of the Pre-SDD contractors, Boeing or Lockheed Martin, to go on to the SDD phase in early 2007.

AMF Extension

One result of the JTRS reassessment was to extend Pre-SDD from the end of 2005 to October 2006. This helped the teams to accommodate such changes as the addition of helicopter radios, says Leo Conboy, Boeing’s AMF JTRS program manager. The Boeing team has used the additional time to reconcile additional helicopter-related requirements with AMF’s prior requirements and to incorporate rotary wing needs into its specifications.

AMF developers still face challenges in getting radio frequency (RF) hardware small enough to fit into airborne radios. The small form factor airborne radio would provide two "universal" 2-MHz-to-2-GHz channels within a physical footprint similar to today’s ARC210 radios–about 276 cubic inches in volume and 12 pounds (5.4 kg) in weight. That’s much smaller than MIDS JTRS equipment built to the existing MIDS low-volume terminal (LVT) footprint. AMF radios will employ the wideband networking waveform (WNW), the soldier radio waveform, Link 16 and the mobile user objective system (MUOS) satcom waveform, according to Boeing.

The small AMF radios would be appropriate for some of the smaller, "disadvantaged" platforms that don’t have a lot of real estate, according to Collins. These include unmanned air vehicles such as the Predator and Global Hawk, as well as the Osprey tiltrotor and Apache, Chinook and Blackhawk helicopters. But they are also planned for large aircraft such as C-130s (multiple variants), C-5s, C-17s, KC-135s and KC-10As.

The Boeing team has demonstrated AMF hardware. This "was lighter than the [ground radios] right from the get-go," Conboy says. The team demonstrated an Internet-enabled WNW signal running simultaneously with a JTRS version of a legacy signal. They ran common IP services such as streaming video and voice over WNW. Boeing even demonstrated NetMeeting–conferencing software that provides whiteboard, text chat, file transfer, audio and video–over WNW.

WNW will be key to AMF, Conboy says. The initial backbone capability will include two signals in space–orthogonal frequency division multiplexing (OFDM) and anti-jam. OFDM will be the primary signal. The version of OFDM currently under test in Boeing’s system integration lab supports a data rate of 1 Mbit/s in the point-to-point mode, the company says. The goal is to reach a data rate of 2 Mbits/s by 2008. Range is hardware-dependent, but recent field testing at Ft. Huachuca, Ariz., demonstrated the ability to close a link at about 14 miles line of sight.

Applications such as streaming video could enable intelligence, surveillance and reconnaissance (ISR) without the use of specialized ISR aircraft. Voice over IP also could allow multiple aircraft nodes to act as relays, overcoming line of sight limitations. Lockheed also has demonstrated prototype AMF radios over a dynamic, ad hoc airborne network, according to company reports.

Boeing’s demos were intended to increase the military’s comfort level with fundamental JTRS concepts such as waveform portability. "Our prototyping demonstrated that the concept of being able to move [SCA waveforms] over rapidly to an entirely different radio does work," Conboy states. Boeing ported software versions of the HaveQuick and VHF/UHF line of sight (V/ULOS) waveforms to its prototype.

Both AMF teams passed their preliminary design reviews last year. Critical design reviews are expected to take place in the fourth quarter of FY07, as part of the full SDD program. A low-rate initial production decision is expected in FY10, according to Steven Davis, the JPEO spokesperson.

MIDS JTRS

The MIDS JTRS segment, meanwhile, is developing a larger, four-channel radio. Today MIDS LVT terminals provide traditional Link 16 and Tacan. The MIDS JTRS radio will feature one dedicated channel supporting Link 16 and Tacan plus three programmable, 2-MHz-to-2-GHz channels that could host a variety of SCA waveforms. The radios are designed to operate WNW, Link 16 and the new Tactical Targeting Networking Technology (TTNT) waveforms simultaneously, Davis says. TTNT, a new waveform transitioning into MIDS JTRS, "can bring a broadband wireless Internet protocol capability to the airborne community," he adds. TTNT "increases bandwidth capacity at an order of magnitude over what today’s tactical data links can provide [and] allows very low-latency data exchanges to support time-sensitive missions."

The MIDS JTRS program took an incremental approach from the outset. The contractors, Data Link Solutions (DLS)–a joint venture of Collins and BAE Systems–and ViaSat, produce the LVT terminals today. The approach leverages the same LVT form factor, specification, host I/O and many of the same connectors to reduce integration costs, says Davis. MIDS JTRS completed preliminary design review in August 2005 and critical design review in May 2006. Low-rate initial production will begin in FY07, he adds. The government will continue the LVT program’s two-vendor, competitive acquisition strategy to drive prices down.

According to the JPEO, aircraft slated to receive the new MIDS radios include F/A-18E/F, E-2D, MH-60R/S, B-1, F-15E, A-10, E-8C and the RC-135S/U/V/W reconnaissance platform. The initial rollout will focus on fielding a JTRS version of Link 16, as well as Tacan and J-voice capability.

TTNT

TTNT took on higher visibility during tests at the Navy’s China Lake, Calif., facility in September 2005 and again at the Joint Expeditionary Force Experiment (JEFX) exercise in 2006. On both occasions there were a maximum of 12 TTNT nodes in play at one time, all running 10 to 15 applications, says Mike Heater, Collins’ principal program manager for TTNT. Collins has simulated up to 1,000 users.

TTNT is often described as a next-generation Link 16 because of its targeting applications. But there are significant differences. TTNT, for example, provides much greater bandwidth. A Link 16 network probably would have 250- to 270-Kbit/s total capacity, compared with greater than 10 Mbits/s of total capacity for TTNT, Heater points out. That’s how much data you can get into the air at any instant.

An individual terminal can transmit at up to 2 Mbits/s. That doesn’t mean that only five can transmit simultaneously, however. Data rates, determined by range and user application selection, include 2 Mbits/s, 1 Mbit/s, 500 Kbits/s and 250 Kbits/s. TTNT also manages traffic by enforcing eight priority levels. It is designed to operate at up to Mach 8.

And unlike Link 16, TTNT is not a time-slotted network. "If you have a TTNT terminal and the daily network entry key, you can join the network without any prior planning," Heater says. The plan, which has not yet been approved, is to have a longer-term key (perhaps a month long) that would be physically loaded into the radio, he says. On a daily basis, as aircraft sit on the carrier deck, for example, the daily key could be downloaded over the air.

Another IP-capable waveform, TTNT supports the concept of nontraditional ISR. "Instead of sending out a U-2 [spy plane], you can take sensor information already available from a fighter aircraft or ground guy and bring it back to the CAOC [Combined Air Operations Center] for evaluation," Heater says. TTNT can pass a still image about 450 Kbytes in size in "seconds." The equivalent transaction with Link 16 terminals would take 10 to 15 minutes. During the China Lake tests, Collins also demonstrated voice over IP: the backseater of a Boeing F-15 test aircraft at China Lake actually talked to a general in the Pentagon, Heater says. (The call was downlinked to the CAOC in China Lake and then patched into the commercial phone system.)

Collins also has demonstrated text chat, which could be useful in coordinating communications between command and control aircraft and ground forces. While it’s hard to envision pilots of single-seat aircraft surfing the military Internet during combat missions, "you could give him push-to-acknowledge," capability, Heater says. A text chat message with geographical coordinates could cut down on verbal acknowledgments. The Boeing test aircraft had a high-level PC interface between TTNT and the existing mission systems, so that TTNT information was presented on existing displays.

TTNT also can help with extremely high-precision operations such as automatic refueling and landing of UAVs. "You’re trying to run a pretty tight control loop through an automated piloting system," Heater says. The machine-to-machine links between the GPS systems in the tanker and the UAV need to be very low latency. "You don’t want to lag by 5 seconds," he stresses. (TTNT claims a latency of no more than 2 milliseconds.) These communications are basically telling the UAV how to drive. The idea is to keep the drone within a 12-inch box, he says.

Weapons Network

Rockwell Collins is developing an advanced networking technology that may be of future interest to the Joint Tactical Radio System program. Among other things, the technology would let weapons communicate imagery to air and ground nodes, potentially improving the time-critical targeting of mobile systems. Collins’ team recently won the sole Phase 2 contract for the Quint Networking Technology (QNT) program led by the Defense Advanced Research Projects Agency (DARPA). The concept envisions networking smart weapons, small and large unmanned air vehicles (UAVs), dismounted soldiers and fighter aircraft.

If a fighter launches a compatible, camera-equipped weapon, the munition could send still images or streaming video back to the manned platform, explains Mike Heater, Collins’ principal program manager for Tactical Targeting Networking Technology (TTNT), a closely related DARPA program. The data stream can be viewed–and the weapon controlled–by a QNT ground node if the weapon goes beyond the fighter’s line of sight. The ground node could direct the weapon away from a target that’s already been hit, for example, or cause it to explode in midair.

Among QNT’s challenges are how to make sure the right people join and stay on the network and how to obtain adequate power within a very small form factor. Although humans would initiate the link and receive the data, QNT’s operation would be automated. Under Phase 2, the team is developing a networking approach, as well as software and hardware to implement the concept. QNT is not expected to be deployed on manned platforms, but will be compatible with TTNT, which is aimed at those aircraft. Phase 2 will run through December 2007, leading to a flight demonstration in Phase 3. QNT is intended to operate at better than Mach 1.6.

Increment 1 Waveforms

➤ Wideband Networking Waveform

➤ Soldier Radio Waveform

➤ Joint Airborne Networking-Tactical Edge

➤ Mobile User Objective System

➤ SINCGARS

➤ Link 16

➤ EPLRS

➤ HF

➤ UHF Satcom