Captured page. http://www.fas.org/irp/program/collect/vpu-001.htm
translated from "Centerline" of March 1993 (a publication of the Valkenburg Aircraftspotters Foundation)
The abbreviation SIGINT - Signals Intelligence - stands for collecting and analyzing of information from radar- and radio-signals. Because these signals come from radar and radio sources that belong to other nations (that do not want the information to become known by others), these activities are falling under the definition espionage. SIGINT activities are carried out from ground stations, ships, satellites and aircraft. Since only a small number of aircraft is involved and the subject is wrapped in mystery, the aircraft are attracting the attention of aircraft spotters.
After aircraft spotters and later on the (aviation) press got wind of it, an espionage-activity is often admitted out of necessity. But this also happens determined. The experience with Americans is that in this case it is meant to divert the attention from another, more secret, espionage-project. For example the SR-71 Blackbird came in the spotlights when they were working on the F-117 stealth project. This same F-117 is now on display at several airshows, while the new supersonic Aurora is only known from very loose stories. Another example are the VQ's that were stripped of their mystery while VPU's were being established. More examples are available.
An espionage-activity is kept secret as long as possible to be able to conduct the mission as optimal and as long as possible. A very good example are the five Atlantics that are flying SIGINT-missions along the Iron Curtain since the seventies, painted in the colours of the German Navy. Still the written history of the German Naval Air Service is only mentioning four lines on the espionage mission of these aircraft. Also the British try to keep their three Nimrod R.1's of 51 Squadron out of publicity. We know better.
SIGINT is a collective noun for a number of activities that have espionage by collecting and analyzing of signals in common. The difference is the kind of signals. Within SIGINT are:
COMINT - listening in to, analyzing and decoding of military radio-traffic, teletype and fax signals (Communications Intelligence)
RINT - detecting and analyzing of electro-magnetic radiation (Radiation Intelligence)
ELINT - collecting and analyzing of radar, IFF, datalink and missile-firing signals (Electronic Intelligence)
TELINT - collecting and analyzing of signals about guided missiles (Telemetry Intelligence)
Well-known aircraft on this matter are the RC-135 variants of the USAF, which are equipped for COMINT, ELINT and TELINT, and the EP-3E Aries of the US Navy. The main mission of these Orions is ELINT in support of the US Fleet. Known variants of aircraft with a COMINT-mission are the five Atlantics of the German Navy, three British Nimrods, two French DC-8s, two Swedish Caravelles, probably two Russian IL-20 Coots in eastern Germany and, until recently, three special Herculesses of the USAF at Frankfurt Rhein Main AB (Germany).
But the USN is involved in COMINT too. Since the end of the sixties they have operated two and later on four P-3B Orions, which have COMINT as their main mission. Aircraft with a ELINT -mission, such as the RC-135 and EP-3E, are easy recognizable because of their large (often black) radomes and large number of extra aerials.
On the other hand, aircraft with a COMINT-mission are less conspicuous. The equipment is hidden within the aircraft. But aircraft spotters are looking for small details, such as the black bulge below the fuselage of the German Atlantics or the missing MAD-boom of the Nimrods. This way the aircraft show up for what they are. It was know for a long time that the three C-130Es of 7045 OS had a COMINT-mission too, but that was because their predecessors, three EC-97s, had an extra black radome, just like the German Atlantics.
Special Orions in the Gulf
The best disguised COMINT-aircraft are the four P-3Bs of VPU. Still these aircraft can hardly be recognized from normal VP Orions and until recently their mission was unknown as contrasted with the other aircraft mentioned. Because these Orions were involved in the Gulf War, some details were released; just enough to give a new impulse to a study of years to the aircraft and their missions. Decisive were four parts of articles about the Gulf War:
"The greatest number of P-3s in the Gulf were ASW types flown by ten different VP squadrons. Another configuration, the "Aries", was flown by VQ-1 and VQ-2, whose primary mission - air reconnaissance - was of accurate importance. A "cousin" of the Aries, at least in terms of its surveillance and reconnaissance role, were the specially configured P-3s of VPU-1 and VPU-2, which employ some "unique" sensors."
"On other fronts, but again within the inclusive boundaries of the electronic "battlefield", VQ and VPU forces continued to provide support to American and allied tactical strike aircraft."
"Mid-September a detachment from Barbers Point based Patrol Squadron Special Unit (VPU)2 arrived in the area with P-3s equipped especially for reconnaissance."
"VPU squadrons provided "real time Battle Damage Assessment" for coastal attacks and many targets ashore."
It was new that the VPUs and the two VQs, of which the ELINT-mission was already common known, were bracketed together. The remaining question: what part of SIGINT? was answered quickly: COMINT, just like the less conspicuous Nimrod R.1s of the British, the Atlantics of the Germans and the three Herculesses of the USAF. The equipment of the VPU Orions is probably partly similar to the equipment of the EP-3Es, like the multi-frequency ALR-52 IFM and ALR-60 COMINT-systems. The main difference with the VQs is that VPUs have mainly a COMINT-mission and that the mission is not conducted in support of the navy itself, but for a national espionage organization such as the National Security Agency (NSA) or the Defense Intelligence Agency (DIA).
The articles about the Gulf War together with other information that was collected earlier and some logical thinking could not led to another conclusion.