In 1989, ICAO decided a single global coordinate system for all navigation should be used. This coordinate system is called "World Geodetic System 1984" (WGS-84). Using the WGS-84 was preferred because of the introduction of GPS- basic approaches in 1998 as a necessary precursor to precision approaches and en-route flight based on of satellite navigation. ICAO set the conditions for all coordinates for navigation would be transferred to the WGS-84 coordinate system and published before 1 January 1998. Since then, all coordinates used in aviation are presented in LAT / LON form in WGS-84.

Implementation of the WGS-84 program in Europe was led by EUROCONTROL. The agency provided guidance while responsibility of the change lay with each national state.

Icelandic Civil Aviation Administration and the National Land Survey of Iceland oversaw at the time measurements of coordinates into WGS-84 for aviation in Iceland but Isavia Ltd now carries this responsibility. All navigation systems, airports and en-route fixes in Iceland have been measured in the WGS-84 coordinate system, in most cases by using GPS technology.

WGS-84 (World Geodetic System 84) is a global coordinate system originally designed for use with GPS. Point of origin in WGS-84 is Earth's centre of mass. Z-axis has same direction as the rotational axis of Earth. X and Y- axis are orthogonal to Z-axis and together define the Earth's Equator with the Y-axis orthogonal to the X-axis, according to the mathematical “right hand rule of axis”.
The plane of XZ defines the longitude that draws through the town of Greenwich in the United Kingdom. Ellipsoid is defined to simplify calculations to Earth’s surface. The centre of the ellipsoid is same as Earth's centre of mass, and the surface of the Ellipsoid is optimized as close to the Earth's surface as is best possible. Longitude, latitude and elevation are calculation from the Ellipsoid. Thus coordinates can be calculated at every point on earth from the same reference by the use of WGS-84 coordinate system. This global application is an important benefit for air navigation users, who's position is in the same reference system world wide.

Figure 1

 

Legend: Ellipsoid with coordinates represented in (X, Y, Z).
A common re-representation of these coordinates is to describe a location from a defined ellipsoid surface with latitude, longitude and elevation (j, l, h).

When talking about elevation of a point or object in the WGS-84 coordinate system, this is described as elevation over a mathematical elliptical surface included the definition of the coordinate system. The ellipsoid is the best elliptical mathematical fit to the earth's surface. This implies that in some places the local sea level is either below the surface of the WGS-84 ellipsoid (at the equator) or above the surface (on the northern and southern hemispheres).

To describe in more detail the height of a point or an object above the average sea surface used so called geoid or equipotential surface, which is a mathematical surface which takes into account changes in the Earth's gravity field in different regions of earth. Earth Gravity Model 1996 or EGM96 was measured through satellites in 1996, and the global model used to describe the height above mean sea level in aviation (MSL = mean sea level).

Height of a point or an object can be given in different manner.
Examples can be found in the next picture.

Figure 2

 

 

Legend:

Note: Sea level surface is also called geoid and is equipotential surface extension over landmasses

It is common in aviation related activities that both these height references are provided for points in WGS-84 coordinates, the ellipsis and the MSL (refering to the WGS- 84 Ellipoid and EGM96 geoid), as well as a specify the difference between the geoid and the ellipsoid for individual point. In Iceland, this difference is commonly over 60 m.

Many coordinates issued before 1998 were accurate. Some were drawn from maps, other measured in older national reference system (Hjörsey-55) and others from local coordinate systems of municipalities. Moreover, there was a 40 m error between Hjörsey-55 and WGS-84. While measuring into WGS-84 the updated national coordinate system Isnet-93 was used to connect the earlier observations into the WGS-84 system and new measurements were conducted in both systems. In air navigation, EGM 96 geoid is referred to when describing measurements above mean sea level as previously stated.

The following requirements are set for the accuracy of measurements in WGS-84 for navigation, collected in the following table:

 

Isavia has chosen to exceed accuracy requirements for all air navigation facilities with the best possible accuracy available at any time and as circumstances permit each time. Therefore accuracy air navigation is somewhat higher in Iceland than the table above requires.

RNAV(GNSS) non-precision approach (NPA) have been introduced for several airports in Iceland and more GNSS procedures are expected. Isavia provides pilots with RAIM (Receiver Autonomous Integrity Monitoring) forecasts for GPS according to the guidelines of ICAO.

Isavia uses EUROCONTROL's AUGUR service for RAIM Prediction for Icelandic airports. If Isavia receives a prediction that accuracy is lacking, a NOTAM will be issued. The AUGUR service has been set up for the following airports:

 

 

Note, the AUGUR's RAIM prediction services are publicly available at the   EUROCONTROL website.

Design of RNAV approaches with LPV minimum based on APV-1 procedure design standards has been increasing in Iceland.  Several such approaches have been published for runways on the eastern part of Iceland.
RNAV approaches with LPV minimums are based on space-based augmentation system (SBAS) where EGNOS enhances accuracy and integrity of GPS navigation by adding real-time signals quality monitoring, increasing safety for air navigation.  Additionally, EGNOS can be used for enroute navigation within the service area of the system.
Isavia has plans to increase options with this type of air navigation in the coming years.

The EGNOS approach minimum are based on the SBAS correction signals from the EGNOS system to enhance the accuracy of satellite navigation and integrity monitoring with in-flight satellite navi-gation. Iceland is located on the western boundaries of the service area of EGNOS.
The Icelandic Transport Authority has authorized, following studies on the quality of the EGNOS correction signal and risk assessment, the design of the APV-1 approach and allows the use of EGNOS for the SBAS approach navigation east of 019° west longitude.
Signal-in-space qualifications were conducted for EGNOS APV-1  approach navigation within Iceland and give the foundation for the decision on qualified EGNOS APV-1 service volume in Iceland. 

It should be kept in mind that availability of the EGNOS signal in Iceland is less than in many European countries.  This results in indicators of availability reduce as we move westwards over Iceland.
Thus when  preparing  a  flight  plan  it  is  important  that  pilots  check whether availability of the signal is predicted to be unsatisfactory at the destination, either through SBAS prediction services or through Isavia NOTAM services, available on Isavia webpage:

 

https://www.isavia.is/en/corporate/c-preflight-information/notam

 

Flight  crews  must  be  prepared  in  their  plans  for  LPV  approach, that EGNOS signals may not be available at destination and therefore other approach minimum may apply (RNAV-LNAV, RNAV-LNAV/VNAV or other navigational methods) or diversion to alternate airport.

Pilots  are  encouraged  to  report  any  abnormalities  (poor signal quality, change to approach minimums, go around etc.) or make other comments on the use of EGNOS to:

 

AIS.LPV@isavia.is.