Angola, the Soviets embarked on a crash program to counter the technology. As a result, the MiG-29 was fielded in 1985 with an HMD and a high off-boresight weapon (R-73), giving them an advantage in close maneuvering engagements. Several nations responded with programs to counter the MiG-29/HMD/R-73 (and later Su-27) combination once its effectiveness was known, principally through access to former East German MiG-29s that were operated by the unified German Air Force. One of the first aircraft with simple HMD devices appeared for experimental purpose in the mid-1970s to aid in targeting heat seeking missiles. These rudimentary devices were better described as Helmet-Mounted Sights. The US Navy's Visual Target Acquisition System (VTAS), made by Honeywell Corporation was a simple mechanical "ring and bead"–style sight fitted to the front of the pilot's helmet that was flown in the 1974–78 ACEVAL/AIMVAL on U.S. F-14 and F-15 fighters. VTAS received praise for its effectiveness in targeting off-boresight missiles, but the U.S. did not pursue fielding it except for integration into late-model Navy F-4 Phantoms equipped with the AIM-9 Sidewinder from 1969. HMDs were also introduced in helicopters during this time - examples include the Boeing AH-64 Apache with the Integrated Helmet and Display Sighting System (IHADSS) demonstrated in 1985. One successful HMD was the Israeli Air Force Elbit DASH series, fielded in conjunction with the Python 4, in the early 1990s. The U.S., UK, and Germany pursued a HMD combined with ASRAAM systems. Technical difficulties led to the U.S. abandoning ASRAAM, instead funding development of the AIM-9X and the Joint Helmet-Mounted Cueing System in 1990. American and European fighter HMDs became widely used in the late 1990s and early 2000s. The first civilian use of HMD on aircraft was the Elbit SkyLens HMD on ATR 72/42 airplane. Technology While conceptually simple, implementation of aircraft HMDs is quite complex. There are many variables: precision – the angular error between the line-of-sight and the derived cue. The position of the helmet is what is used to point the missile; it thus must be calibrated and fit securely on the pilot's head. The line between the pilot's eye and the reticle on the visor is known as the line of sight (LOS) between the aircraft and the intended target. The user's eye must stay aligned with the sight; in other words, current HMDs cannot sense where the eye is looking, but can place a "pipper"