DIS Scientific & Technical Memorandum 55/1/98 and 55/2/00

This document consists of a series of scientific and technical memoranda from the Defence Intelligence Staff (DIS) regarding the analysis and filtering of Unidentified Aerial Phenomena (UAP) reports. The text provides a detailed breakdown of common visual and auditory illusions that lead to UAP sightings. It explains that observers often misinterpret the velocity and motion of objects due to the lack of reference points in the sky, particularly when viewing small, receding, or rotating lights. The document also addresses the 'rarity of UAP sound' reports, noting that while observers often expect a 'vehicle' to produce noise, many UAP sightings involve objects that are either too distant, are satellites, or are composed of charged air masses (such as aerosols or plasmas) that do not generate traditional mechanical or aerodynamic sound. The memorandum further outlines the 'UK Low Flying System' and provides strict criteria for filtering out reports that are actually conventional military or civil aircraft. It explicitly mentions that certain UAP reports can be attributed to covert aircraft programmes, such as the SR-71 (Senior Crown), and discusses the potential for misidentifying experimental aircraft or foreign developments, such as the Russian 'thermoplane' developed by the Moscow Aviation Institute. The document serves as a technical guide for intelligence analysts to systematically eliminate conventional explanations—including weather, satellites, and known aircraft flight paths—before classifying an event as a genuine anomaly. It emphasizes that many reports are the result of human error, imagination, or the misinterpretation of natural atmospheric phenomena.