Lockheed P-3C Orion
The US Navy, Australia and Norway are known to operate Orion P-3Cís with a SIGINT capability, in addition to employing the aircraft for maritime patrol and anti-submarine warfare.
The US Navy have operated two Special Projects Squadrons, VPU1 based at NAS Brunswick and VPU2 based at MCAS Kaneohe Bay, since the late 1960's. The current aircraft are P-3C Orions and their activities are usually disguised by 'hiding' them within regular P-3C squadrons.
The VPU Orions are specially modified under a programme for a specific reconnaissance mission which undergoes frequent name changes. In the past the names for this programmes have included 'Storm Jib' and 'Reef Point' and it is believed that the current programme is known as 'Iron Clad'. The equipment carried by the 'Iron Clad' aircraft is believed to include highly classified communications systems, long-range cameras, a satellite uplink to allow the near real-time transmission of optical images together with a variety of sensors and intelligence gathering systems. The lack of obvious external antenna make the VPU aircraft difficult to identify, but the frequently visit, RAF Mildenhall and Lakenheath in the UK and NAS Sigonella.
The upgraded Australian aircraft are known as AP-3Cís. Given the endurance and capacity of the P-3C, it would be surprising if other nations equipped with this aircraft do not also exploit its presence over international waters for SIGINT or COMINT gathering.