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high-performance standards. However, it is essential that stringent cross-checking procedures are employed,
both to ensure that these systems perform to their full capabilities and to minimise the consequences of
equipment failures and possible human errors.
8.1.2 Navigation systems are continuously evolving and early editions of this Manual concentrated
on offering specific guidance on the use of individual systems. Rather than specifying the types of
equipment required for flying in defined airspace, current philosophy is to specify a Required Navigation
Performance (RNP), effectively a track-keeping capability. As an example, the navigation performance
accuracy of the aircraft population operating in airspace designated RNP X airspace would be expected to be
better than X NM on a 95% containment basis. The NAT Minimum Navigation Performance Specifications
(MNPS) inter alia define a requirement for the standard deviation of lateral track errors to be less than
6.3 NM. Since two standard deviations provide for about 95 % containment, the MNPS statement is
effectively akin to an RNP value of 12.6 in a lateral sense. It is also perhaps interesting to note that actual
measurements of the achieved navigation performance by the entire fleet of NAT aircraft, even before GPS
came into use by a large proportion, indicated an achieved standard deviation of approximately 2 NM.
Note: - Continued development within ICAO of the concept of navigation performance
specifications is leading to the future replacement of the current system of RNP with a
“Performance Based Navigation (PBN)” philosophy. Within this new philosophy, navigation
specifications not requiring automatic monitoring and alerting will be known as RNAV.
Navigation specifications which do require monitoring and alerting will be known as RNP. With
current technology, on-board performance monitoring can only be carried out by aircraft fitted
with GPS. Hence, GPS will be mandatory for RNP airspace. MNPS airspace navigation does not
require an on-board automatic monitoring and alerting function. Instead pilots must remain
vigilant and employ rigorous routine manual monitoring procedures.
8.1.3 MNPS was devised and implemented in the NAT Region long before the RNP concept was
developed. MNPS was established primarily with the NAT OTS environment in mind. The defining
waypoints of OTS tracks are specified by whole degrees of latitude and, using an effective 60 NM lateral
separation standard, most adjacent tracks are separated by only one degree of latitude at each ten-degree
meridian. The traffic density in the OTS is higher than in any other oceanic airspace. In such a densely
populated flexible track system (one that changes twice every day), it is essential that crews avoid (whole
degree) waypoint insertion errors. Such errors in the NAT MNPSA will inevitably result in a conflict with
traffic on an adjacent track. For this reason Minimum Navigation Performance Specifications had to include
not just the technical navigation accuracy of the Long-range Navigation Systems used on the aircraft but also
the crew navigation procedures employed. The MNPS statement thus involves both cockpit/flight deck
procedures and crew training requirements. In the early days of the RNP concept, it was these additional
requirements that separated MNPS from RNP. However, RNP has come a long way since its inception and
the development of the RNP-10 approvals for PAC operations have brought it much closer to the original
MNPS concept. The ICAO Air Navigation Plan for the North Atlantic Region states that the intention in the
future is that navigational performance is expected to be tied to a level of RNP. This will probably require
the carriage of dual GPS to allow the required on-board performance monitoring and alerting which would
be necessary for the closer track spacing then envisaged.
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 8
NAT MNPS 41 Edition 2008
(For more detailed information on RNP and MNPS see the following ICAO Documents: Doc 9613 –
‘Performance Based Navigation Manual’ and NAT Doc 001 – ‘Consolidated Guidance and
Information Material concerning Air Navigation in the North Atlantic Region (available at
http://www.nat-pco.org /.)
8.1.4 Obviously, there are several combinations of airborne sensors, receivers, computers with
navigation data bases and displays which are capable of producing like accuracies, and which with inputs to
automatic flight control systems provide track guidance. However, regardless of how sophisticated or
mature a system is, it is still essential that stringent navigation and cross checking procedures are maintained
if Gross Navigation Errors (GNEs) are to be avoided. A GNE within NAT Airspace is defined as a deviation
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