This would mainly be a question for real-world pilots: if flying an aircraft equipped with a GPS (let’s say e.g a G1000), can your rely on the GPS alone even for those procedures in a SID or a STAR that are defined based on VOR radials, intercept angles to VOR radials, DME distances etc., or do you still use the NAV radios for those parts?
I guess all the required data / guidance etc for those procedure legs are available from the GPS (and I usually use just that in the sim when equipped with a GPS), but I’m curious if in real life you are a) allowed to fly the SID / STAR based on the GPS alone, and b) if so whether you still mainly use the VOR radios in this case (or use them as backup / verification)? What’s the common practice here?
I was on a piloting site and they were discussing VOR as being very dated and never used. Then I was reading another article that mentions most pilots would be rusty in the event of a GPS failure.
Yes you can use the GPS. I typically kept the NAV radios properly tuned to the VOR’s and would display the needles on PFD/MFD as backup. We did practice them as VOR-only also. In the outside chance the GPS RAIM prediction indicates a problem you would be required to use the ground based navigation sources.
Thanks for confirming! Yeah that was kind of my gut feeling, that pilots are primarily using the GPS for those procedures (it is after all a bit more convenient ), but have the ground-based sources still dialed in for reassurance / backup.
Interesting read, thanks! Though it’s not entirely clearing this up for me, as it only seems to discuss the use of GPS vs VOR receivers in the context of VOR approaches,
For ground source-based approaches (be it ILS, VOR, etc) I would think that the primary instruments used will (and for ILS I assume must) still be the LOC/VOR radios. I was more interested in SIDs and STARs that are based on VOR radial/distance/etc.
To answer your question, yes the aircraft would use GPS rather than VOR/DME/ADF to fly SID/STARS & Airways.
The SID/STAR or Airway will be hardcoded into the FMS (or in your case the G1000) courtesy of the AIRAC data. The aircraft will then use GPS to follow that prescribed route.
In the A320 VOR & DME information is still used to update the position of the aircraft if the GPS signal isn’t strong enough but it is not used to fly radials etc. If I were to ask the A320 to fly direct to ‘CPT’ VOR then it would do so using the GPS alone. I believe the G1000 would do the same.
Obviously it would be prudent to back up your GPS with raw data, but in all honesty GPS is so reliable nowadays it’s never really an issue.
If you have a look at this FAA Departure Procedure from Van Nuys, you will see that the fixes are formed primarily of two intersecting VOR radials, but that each fix is also given its lat-long position for use with a GPS…so you can use either method.
Some RNP (Required Navigation Performance) RNAV procedures are only available if the aircraft not only has a GNSS (Global Navigation Satellite System - the generic name for ‘GPS’ as there are other similar systems in use) but also has equipment to validate the accuracy of the received signal as well as certain levels of redundancy. If it’s RNAV then no lat longs are given, only the name, for the fixes as it is assumed these are in the FMS database.
Short answer: conventional SIDs can be flown by FMS but must be crosschecked raw data. Most conventional SIDs are either replaced by RNAV SIDs or have a “RNAV overlay”. In which case you can fly the SID solely using FMS / GPS.
@HeliBrewer most terminal operations are RNAV 1 (P-RNAV) which can be flown without GPS at all, DME-DME or even VOR-DME position updating usually supports RNP 1.0. So RAIM is not necessarily a requirement. For Terminal RNP1 the primary source of positioning is GPS in which case RAIM is required.
To illustrate, this is a conventional departure:
This is a RNAV departure (including non-RNAV instructions):