Stand by The stand-by status is attached to the different facilities which can take over the chain having the status PRINCIPAL in case of failure. Two levels need to be considered: Cold Standby is attached to a non-operational back-up processing chain, needing first to be loaded; Hot Standby is attached to a chain which processes the same inputs that the Principal chain, in parallel, and which is ready to be switched in case of outage. Stand-Alone GNSS System An airborne GNSS configuration which may use altimeter aiding and augmented GNSS signals without reliance on any other navigation system or sensor. Standard Arrival Route Are standard ATS routes identified in an approach procedure by which aircraft should proceed from the en-route phase to an initial approach fix. Standard Deviation A measure of the dispersion of random errors about the mean value. If a large number of measurements or observations of the same quantity are made, the standard deviation is the square root of the sum of the squares of deviations from the mean value divided by the number of observations less 1. Standard Error of Measurement Standard deviation of errors of measurement, that is, differences between true and obtained scores (SIOP, 1987). Standard Geodetic Reference System The co-ordinate system built into the GPS constellation for its navigational output, as the universal co-ordinate system for International Civil Aviation applications. ICAO Annex 15 specifies that with effect from 01 January 1998, the co-ordinates of all points mentioned in AIPS and aeronautical charts be expressed m the WGS-84 System. Standard Geodetic System Approved ICAO geodetic system (earth centred) upon which all latitude and longitude co-ordinates will be based for air navigation. It will ensure total uniformity of latitude and longitude grids between aeronautical charts and aeronautical information (published by world aviation authorities) and airborne inertial and satellite navigation systems for international civil aviation. Standard Instrument Departure (Route) standard ATS routes identified in an instrument departure procedure by which aircraft should proceed from take-off phase to the en-route phase. Standard Length Message An exchange of digital data using selectively addressed Comm-A interrogations and/or Comm-B replies. Standard Manoeuvre The aircraft followed an optimum path to avoid the other aircraft, terrain or airspace and did not need to undergo rapid change in velocity. Standard Operating Procedures An ATC Unit specific set of procedures that specify how the unit’s controllers’ ATC responsibilities are to be co-ordinated. Standard Positioning Service The standard level of GPS service, as specified in the Federal Navigation Plan (FRP) which is available, without qualifications, to any user on a continuous worldwide basis.