We know our ability to navigate by GPS depends on our location's compliance with WGS-84. But when flying in a non-WGS-84 country what do you do? The coordinates in your FMS could be a few hundred feet off from what the country is using so not using GPS on approach makes sense. What about en route?
The G450 Airplane Flight Manual has gone through a lot of changes over the last few years. Where it used to say deselect the GPS whenever in non-WGS-84 area, it now allows you to keep the GPS while flying en route, SIDS, and STARS. You can even keep it selected on approach if you are flying raw data.
Everything here is from the references shown below, with a few comments in an alternate color.
[G450 Airplane Flight Manual §1-34-30 ¶4] When operating in non-WGS-84 airspace or in countries where the airspace is partially compliant with WGS-84, the FMS with GPS position updating meets the required navigation accuracy and may be used for SIDS, STARS and en route navigation. When flying ILS, VOR or ADF approaches and missed approach procedures in these two situations, the GPS updating does not need to be inhibited or deselected provided the appropriate raw data is used throughout the approach and missed approach as the primary navigation reference. For countries that are partially WGS-84 compliant, when RNAV (GNSS) approaches are offered, these approaches may be flown using the FMS with GPS position updating provided the approach chart is annotated with “PANS OPS”.
Figure: GPS Altitude, from G450 Aircraft Operating Manual, §2B-27-00, pg. 71.
[G450 Aircraft Operating Manual §4.B.] GPS altitude displayed is the World Geodetic System 1984 (WGS-84) height above the ellipsoid (geoid height + height above MSL). The GPS altitude is not relative to pressure altitude but is referenced to an earth-centered earth-fixed (ECEF) coordinate system. Pressure altitude is not relative to the same reference frame, but relative to the standard pressure or local pressure settings. Therefore, significant differences are seen between GPS altitude and pressure altitude. This concept is illustrated in [the figure].
[G450 Airplane Flight Manual §1-34-170] Synthetic Vision-Primary Flight Display (SV-PFD) (ASC 037 or ASC 037A (or later approved revision)) SV-PFD synthetic terrain must be deselected when in non-WGS 84 areas. WGS 84 regions of the world are available from the Jeppesen website under Online Publications, and then select IFR Pilot Information: http://www.jeppesen.com/company/publications/wgs-84.jsp
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
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