Editing Inertial frame of reference (section)

===Applications===
[[Inertial navigation system]]s used a cluster of [[gyroscope]]s and accelerometers to determine accelerations relative to inertial space. After a gyroscope is spun up in a particular orientation in inertial space, the law of conservation of angular momentum requires that it retain that orientation as long as no external forces are applied to it.<ref>{{cite book|last=Chatfield|first=Averil B.|title=Fundamentals of High Accuracy Inertial Navigation, Volume 174|date=1997|publisher=AIAA|isbn=9781600864278}}</ref>{{rp|59}} Three orthogonal gyroscopes establish an inertial reference frame, and the accelerators measure acceleration relative to that frame. The accelerations, along with a clock, can then be used to calculate the change in position. Thus, inertial navigation is a form of [[dead reckoning]] that requires no external input, and therefore cannot be jammed by any external or internal signal source.<ref>{{cite book|editor-last1=Kennie|editor-first1=T.J.M.|title=Engineering Surveying Technology|date=1993|publisher=Taylor & Francis|location=Hoboken|isbn=9780203860748|page=95|edition=pbk.|editor-last2=Petrie|editor-first2=G.}}</ref>

A [[gyrocompass]], employed for navigation of seagoing vessels, finds the geometric north. It does so, not by sensing the Earth's magnetic field, but by using inertial space as its reference.<ref>{{Cite book |last=Bowditch |first=Nathaniel |title=The American Practical Navigator |date=25 September 2002 |publisher=[[National Geospatial-Intelligence Agency]] |isbn=9780939837540}}</ref> The outer casing of the gyrocompass device is held in such a way that it remains aligned with the local plumb line. When the gyroscope wheel inside the gyrocompass device is spun up, the way the gyroscope wheel is suspended causes the gyroscope wheel to gradually align its spinning axis with the Earth's axis. Alignment with the Earth's axis is the only direction for which the gyroscope's spinning axis can be stationary with respect to the Earth and not be required to change direction with respect to inertial space. After being spun up, a gyrocompass can reach the direction of alignment with the Earth's axis in as little as a quarter of an hour.<ref name=l>{{cite magazine|title=The gyroscope pilots ships & planes |magazine=Life|date=15 March 1943 |pages=80–83|url=https://books.google.com/books?id=YlEEAAAAMBAJ&pg=PA82}}</ref>{{unreferenced section|date=July 2013}}