Guidance concerning Air Navigation in and above the NAT MNPSA
Inertial Navigation Systems
Insertion of Initial Latitude and Longitude
Two fundamental principles concerning the operation of an IRS are: that it needs to be
accurately aligned before flight; and that the actual position of the aircraft, at alignment, is set into the system. If either of these principles is violated, systematic errors will be introduced. These errors can be corrected whilst the aircraft is on the ground but it is not possible to adequately recover from them whilst the aircraft is in flight, despite any indications to the contrary. Correct insertion of the initial position must therefore be checked before inertial systems are aligned and the position should be recorded in the flight log and/or Master Document. It is recommended that subsequent 'silent' checks of the present position and of the inertial velocity outputs (e.g. ground speed registering zero) be carried out independently by both pilots during (an early stage of) the pre-flight checks and again just before the aircraft is moved. Any discrepancies
should be investigated.
With regard to the insertion of the initial co-ordinates whilst on the ramp, the following
points should be taken into account:
in some inertial systems, insertion errors exceeding about one degree of latitude will illuminate a malfunction light. It should be noted that very few systems provide protection against longitude insertion errors.
at all times, but particularly in the vicinity of the Zero Degree E/W (Greenwich) Meridian or near to the Equator, care should be taken to ensure that the co-ordinates inserted are correct.
e. E/W or N/S).
The alignment of inertial systems must be completed and the equipment put into navigation
mode prior to releasing the parking brake at the ramp. Some systems will align in about 10 minutes, others can take 15 minutes or more; expect alignment to take longer in extreme cold or at higher latitudes or when the aircraft (and hence the inertial platform) is buffeted by winds or rocked during cargo loading. A rapid realignment feature is sometimes provided but should only be used if, during an intermediate stop, it becomes necessary to increase the system accuracy. The aircraft must be stationary during rapid realignment
which typically will take about one minute.
To ensure that there is adequate time for the initial alignment, the first crew member on the
flight deck should normally put the inertial system(s) into the align mode as soon as practicable.
GNSS (GPS) Systems
As with all LRNS operations, GPS LRNS operations must be approved by the State of the
Operator (or the State of Registry for International General Aviation operations) as part of the MNPS operational approval. When both the LRNSs required for unrestricted MNPS operations are GPSs the approval of their operation will include the requirement to carry out Pre-Departure Satellite Navigation Prediction Programmes (as shown below). When only one of the two LRNSs required is a GPS, State
Authorities vary as to whether they require their operators to conduct such pre-departure programmes.
Given suitable geometry:
Four satellites are required to determine 3-D position;
NAT Doc 007