The requirement for up to seven surveillance drones appears to be based on maintaining two patrols. The U.S. Navy expects to sustain surveillance zones 3,700 km (2,000 nm) from Triton bases, each patrol supported by three aircraft, plus one as a spare. For Australia, a single spare could back up two patrol groups. Alternatively, more patrols closer to the country could be maintained—or Australia could just buy fewer Tritons or opt for the shorter-range Predator B.
Although the Predator B is traditionally viewed as a medium-altitude, long-endurance aircraft, that description may be misleading. General Atomics says the aircraft can fly at 50,000 ft., hardly a medium altitude, although not with an array of external pods.
Even if the Triton is chosen, the Predator B may have a second chance at entering Australian service. A senior air force official says that, apart from maritime surveillance, the Poseidons and the chosen drone will be heavily tasked with electronic intelligence—collecting signals emitted by other countries. That work could be so time-consuming as to justify a dedicated electronic-intelligence aircraft, letting the maritime aircraft attend to their primary role. The government has not yet approved any such capability.
Poseidon and Triton are already designed for electronic intelligence. The exact capabilities are classified, but the U.S. Navy has said the Northrop Grumman drone will play a key role in the replacement of the intelligence version of the Orion, the EP-3. Conceivably, Australia could also seek to fit its own electronic-intelligence equipment to the Triton. The aircraft has a little spare space, but not much.