ENR 4.3.1 GNSS systems in use for air navigation in BIRD FIR

Nafn gervihnattaleiðsögukerfis/
Name of GNSS element


Nominal SVC area
Coverage area


1575.42 KHZ
640728.80N   0215615.60W
Operated by USA

ENR 4.3.2 WGS-84

ENR Introduction

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.

ENR What is WGS-84?

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




  1. The height of the aircraft above the ground.
  2. The height of the aircraft above mean sea surface / geoid (MSL).
  3. Height of aircraft over ellipsoid (elliptical surface).
  4. Height ground of geoid.
  5. Geoid height over ellipsoid (difference of reference systems).

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.

ENR Precision

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:

  Notkun staðsetninga
Use in phase of flight
Hlutir sem þarfnast nákvæmra útgefina gagna (1)
Facilities requiring aeronautical data quality (1)
NDB100 m
2Að- og fráflug grunnaðflug
Terminal area and departure, non augmented GNSS
ILS Localizer þar sem það er notað fyrir hliðrað aðflug. (1) DME/N, TACAN, VOR, VORTAC, VOR/DME,LOCATOR,
ILS Localizer where used for offset flight
30 m
3Leiðareftirlit Lokaaðflug
Enroute surveillance
Radar10 m
4(Nákvæmnis, allar tegundir)
Precision approach
ILS Localizer, Glide Slope
3 m
5Lendingar og flugtök
Landing and takeoff
Miðlína og þröskuldar flugbrautar
Runway centre line and thresholds
1 m
6Hnitavarpanir innbyrðis
Coordinate system transformations
Landmælinganet flugvallar
Airport reference system
10 cm
Athugasemdir / Comments:
1) Sjá nánar gögn Alþjóða flugmálastofnunarinnar ICAO Doc 9674: WGS-84 manual
    See also International Civil Aviation Organization ICAO Doc 9674: WGS-84 manual


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.

ENR 4.3.3 RAIM prediction services provided by Isavia

ENR Introduction

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 makes RAIM predictions available to a pilot for the region where he intends to do a GNSS NPA. Isavia, in cooperation with Deutsche Flugsicherung (DFS), the German Air Navigation and Air traffic Services, provides pilots with RAIM prediction service. Information on current RAIM predictions will be available on Isavia's home page: http://www.isavia.is/c and also on AFTN lines to such equipped parties.

ENR GPS and RAIM short overview

The GPS system is a satellite constellation composed of minimum 24 satellites (in May 2013 the system was composed of around 32 satellites including backup units) travelling on 6 elliptical planes. The travelling planes reach up to the 55° lateral. For a user to be able to locate himself on the earth's surface a minimum of 4 observable GPS satellites are required while 5 satellites are needed to calculate RAIM (and 6 are required for the so called Fault Detection and Exclusion, FDE feature).

If the GPS receiver is baro-aided then one less satellite is required. These 5 satellites need to be oriented from the user's perspective in such a way that the user can position himself with minimum required precision and for example the value of Dilution of Precision (DOP) are used in the error estimates calculated in certified GPS approach capable equipment. In the event of poor error estimates the certified GPS approach capable equipment will flag a RAIM failure, and the pilot will not have GPS available as an approach option. When this happens the pilot must report it to the next air traffic control unit, abort the RNAV(GNSS) approach procedure and choose between going into holding, proceed to the alternative airport and/or seek other means of approach guidance (conventional systems).

ENR RAIM prediction

RAIM predictions that Isavia is offering is based on locations of 18 airports in Iceland:



14 airports in Greenland:



the airport in Faroe Islands (EKVG) and the airport in Jan Mayen (ENJA). If RAIM outage in Reykjavik FIR and/or Reykjavik CTA is predicted, a NOTAM will be issued.

ENR Construction of RAIM prediction messages

A RAIM prediction is published at least once per 24 hours but if some change on GPS constellation status is expected a new RAIM prediction is calculated immediately and issued, it becomes valid at the same time and invalidates the older prediction. Even though a prediction is published every 24 hours each prediction is actually valid for up to 72 hours if not superseded by a newer one. This in effect means that if no change in constellation status occurs each prediction could be expected to foresee next three days at most.

It is always recommended pilots have the latest RAIM predictions at hand.

The RAIM messages format is demonstrated in the following two examples.

Example 1:


2013-05-21 8:00:00.



Here we have a message that predicts no GPS RAIM outages in Bíldudalur airport (BIBD) during the prediction period:

Example 2:

The second example shows where RAIM outage is predicted. A closer look reveals that RAIM outage is expected in the next 48 hours from the publication time of the message. The times and duration of the outages can be read as 09:11-10:14 on the 23rd of May.


UNTIL 23 May 08:10: 05230911 TIL 05231014 GPS RAIM UNAVBL FOR NPA.



If the circumstances in the second example arise the pilot scheduling to fly an RNAV(GNSS) approach to Keflavik airfield should be aware, before he leaves for the destination, that his GPS approach certified equipment will not allow approach mode during the outage times at this location. So the pilot must make some adjustments for example to change the time of his trip or have some other means of approach navigation instead of GPS.

ENR Contacts for Isavia's RAIM prediction service

RAIM prediction services:Isavia‘s Flight Data Service
 Open H24
Phone:+00 354 424 4141
Technical expertise and
information on Isavia's
RAIM prediction services:
Arnor Bergur Kristinsson,
Projects Manager
(GNSS and R&D), Isavia,
Phone:+ 00 354 424 5171