A unit of angular measure. One radian is about 57.3 0. One microradian (0.000001 radian) is the angle subtended by an object 1 meter across at a distance of 1,000 kilometers
Federation of American Scientists, Glossary of Acronyms and Terms, Washington, DC,USA 2004. https://fas.org/pubs/_docs/10072004163905.pdf, (DPrep - Amer) {1003}