The U.S. Navy stopped work four months ago on a crucial part of the MQ-4C Triton unmanned air system and is working to find a substitute, service and industry officials confirm. The Exelis Air-to-Air Radar Subsystem (AARSS) sense-and-avoid radar project was halted April 25 due to technical problems and cost overruns, the Navy said last week.

A sense-and-avoid capability is “vital” to the MQ-4C, according to Tom Vice, president of Northrop Grumman's Aerospace Systems segment. Triton is designed to operate in oceanic airspace, where there is no outside radar surveillance. The radar or an equivalent sensor is required so the UAS can avoid colliding with a “non-cooperative” aircraft that is not using any other collision-avoidance system.

This is the second airspace-related challenge to emerge for the Global Hawk this year, following Germany's cancellation of the EuroHawk program. Vice says the radar problem is an “integration issue” and that the company, the Navy and industry partners have “a broad range of options,” beyond a new radar, to allow the MQ-4C to operate safely and autonomously, including the timing of the sensor's cut-in to production. The Navy confirms that “all options” are being reviewed. “This is hard stuff,” Vice adds. “The first UAS with no-kidding sense and avoid is hard stuff.”

Exelis is referring detailed questions about the problem to the U.S. Navy, but notes that “we have been investing to improve our manufacturing capability, and have made significant strides in this area.” The AARSS is a Ku-band, self-contained active, electronically scanned array unit weighing 50 lb., with three sub-arrays covering a 220-deg. field of view.

Other contractors, including Sierra Nevada Corp. and Thales, are also working on sense-and-avoid radars. There are no formal requirements for these radars, but they are generally designed to have a field of view similar to a transport aircraft's pilot, with the ability to detect another aircraft and assess its trajectory in time to perform a successful evasive maneuver. (Design range for the AARSS is around 8 nm in 4-mm/hr. rain.) Some suppliers favor Ku-band because it provides accurate measurement with a smaller antenna than the X band. Radar is one element of an autonomous safe-separation system, which will also use inputs from a traffic alert and collision avoidance system (TCAS), Automatic Dependent Surveillance-Broadcast and possibly electro-optical sensors.

Northrop Grumman is also still trying to save EuroHawk, Vice says. The German government canceled the signals-intelligence Global Hawk derivative in May, saying the cost to certificate the system for national-airspace operation would be more than €600 million ($802 million) and that, even then, there was no guarantee that it could be approved. “The challenge is airworthiness,” Vice says. “It's not the system; it's understanding how to operate it. The need is still there and the solution is the EuroHawk. We don't believe it's time to give up.”