On 5 April 1965 a reconnaissance aircraft brought back the first photographs of a new construction site in North Vietnam.  The characteristic six-pointed star shape around a central hub was well known to USAF intelligence and indicated that the North Vietnamese were building their first surface-to-air missile site.  A second site was found in May and by July several sites formed an irregular ring around Hanoi and Haiphong.  It was assumed that Soviet or Chinese engineers were assisting in the construction of these sites and for this reason Washington decided not to attack the SAM sites during the construction phase.  However, RF-101C reconnaissance aircraft kept a close eye on the construction programme.

The SA-2 surface-to-air missile was revealed by the Soviets in 1957 and was actually designated the S-75 Dvina by the Russians while NATO gave it the code name Guideline.  The US already had first hand experience of the SA-2 when a missile brought down a U-2 flown by Gary Powers over the Soviet Union on 1 May 1960 and another U-2 was shot down over Cuba on 27 October 1962 during the Cuban Missile Crisis.  The SA-2 missile is 35 feet in length and is powered by a solid fuel booster rocket that falls away five seconds after lift off when a liquid fuel rocket motor cuts in for 22 seconds.  The warhead contains 130 kg of high explosives and detonates in a fan-shaped pattern ahead of the line of flight with a kill radius of about 200 feet.  Guidance is by radar homing controlled by operators in a control van situated near the missile battery.  The Fan Song missile guidance radar sends signals to the missile’s receiver guiding it towards its target.  The SA-2 weighs about 5,000lb at launch and accelerates to Mach 3.5 although its speed in the early stages of flight is much slower and allowed US aircraft to avoid it if spotted early enough.  The missile has an effective range of 25 miles and its effective ceiling of 82,000 feet gives it a virtually unlimited high altitude performance.  However, it is relatively ineffective below 1,000 feet as the guidance system does not have enough time to work at such low altitude and the missile has a limited manoeuvring ability.  The North Vietnamese SA-2 batteries were very mobile and could be moved from one location to another in a matter of hours, sometimes being moved in the space between a reconnaissance flight and a strike.  However, the batteries were very distinctive and, unless well camouflaged, were not too difficult to spot from the air being shaped like a six-pointed star with a launcher at each of the points and a control van near the centre with an Fan Song radar close by.

In mid-July an RB-66C ELINT aircraft picked up signals from an SA-2’s Spoon Rest target acquisition radar.  On 23 July another ELINT aircraft detected signals from the missile’s Fan Song target-tracking radar for the first time.  The following day three SAMs were fired minutes after signals were picked up and the North Vietnamese scored their first SAM kill and ushered in a new phase of the air war.  A flight by a Ryan 147E Firebee remotely piloted vehicle over North Vietnam on 13 February 1966 gained much valuable information about the SA-2’s proximity fusing system, its terminal phase guidance signals and its warhead explosive characteristics at detonation.  This and further flights by Firebee drones obtained enough information for effective countermeasures to be built into tactical aircraft and B-52s to give a higher chance of survival against the SAM.