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GNSS is the most accurate mean of navigation available on the flight deck but it relies on satellite being available and under control. Inertial systems are autonomous, but far less accurate.

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While GNSS and inertial systems relieve authorities from the burden and cost of building and maintaining terrestrial navaids, terrestrial navaids (VOR/DME) are still imposed as a safety net in case instruments fail (GNSS) or drift (inertial).

As a matter of fact it's not possible to deliver an IFR certificate to an aircraft without a VOR-DME navigation equipment.

  • Note: Taking comments into account, this last sentence may be not true for IFR certification in the US. However flying IFR in controlled airspace, or flying some approaches may mandate VOR/DME/ADF, depending on the country. E.g. "ADF, DME: Until about 2013, the UK Air Navigation Order required an ADF for all IFR in controlled airspace. The ADF requirement now remains only for instrument approaches that contain an NDB." (source).

Is there any credible roadmap to remove those constraints and have IFR aircraft relying only on GNSS/Inertial? If so what are the mainlines of this new framework?

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  • $\begingroup$ There is an effort to reduce VOR's to a "minimum operational network", however there are no plans to remove VOR's completely at this time. Until they fully commit to removing VOR/DME stations (or at least letting them rot as a number are), I'm not sure that there will be an available timeline. This may run after NextGen is fully implemented but as far as I can read there is no provision in NextGen to completely remove ground based navigation sources. $\endgroup$
    – Ron Beyer
    Sep 21, 2016 at 13:40
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    $\begingroup$ Are you sure you can't get an IFR certification without VOR/DME? FAR 91.205(d) and (e) states equipment requirements and doesn't specify VOR/DME, only "Two-way radio communication and navigation equipment suitable for the route to be flown", which means you can use just RNAV if the airports you fly into have RNAV approaches. $\endgroup$
    – Ron Beyer
    Sep 21, 2016 at 14:29
  • $\begingroup$ @RonBeyer: Well, maybe it's not required in the US to certify the aircraft. How do fly "DME required" or "ADF required" approaches, like this one? Missed approach fix is still often defined relative to a VOR or sometimes an NDB no? $\endgroup$
    – mins
    Sep 21, 2016 at 15:44
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    $\begingroup$ I read a few years ago that they were working towards "combo" avionics boxes which combined GPS and LORAN (of all technologies). This would still be ground based, but based on an upgraded LORAN chain which requires very few ground stations and work over long distances and higher power output which make it harder to jam, and therefore a good backup for GPS. $\endgroup$
    – Lnafziger
    Sep 22, 2016 at 4:12
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    $\begingroup$ @Lnafziger: Interesting point, thanks. That's actually mentioned in this document about the enhanced Loran project (E-Loran). E-Loran has the advantage of broadcasting an accurate time signal which could backup GPS time too. $\endgroup$
    – mins
    Sep 22, 2016 at 8:35

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I'm not aware of any program at present that does away with ground based infrastructure completely. Present plans are to have the ground based stations as a backup in case of a GNSS outage/interference.

The present FAA plan, for example includes DME while reducing the VOR coverage:

FAA Nextgen

Navigation- En Route Transition; from FAA navigation Programs Update Sep 2013

As far as ground based navigational aids are concerned, the plan is to reduce the VOR coverage with VOR Minimum Operational Network (MON) as a middle step. However, the plan appears to be to retain DME at the expense of other Ground Based Navigation Systems (GBNS).

FAA's Alternative Positioning, Navigation, and Timing (APNT) project is geared up towards maintaining aviation operations in case of a GNSS outage. According to FAA:

The Alternative Positioning, Navigation, and Timing (APNT) project is investigating alternatives for providing higher precision back-up for Global Positioning System (GPS)-based position, navigation, and timing (PNT) services.

However, Ground Based Systems are still a part of the program (though VOR is not) and serve as a backup for the GNSS. One of the options is to modify the existing DME facilities in order to improve coverage/accuracy.

International Civil Aviation Organization (ICAO) 2013-2028 Global Air Navigation Plan also stresses the importance of having some 'conventional' ground based systems as a back-up for GNSS:

Conventional navigation aids (VOR, DME, NDB, ILS) are in widespread use globally, and most aircraft are equipped with the relevant avionics. The vulnerability of GNSS signals to interference has led to the conclusion that here is a need to retain some conventional aids or alternative navigation service solution as a back-up to GNSS.

ICAO also concurs that DME is the most appropriate ground based equipment to be used in the future navigation system:

The implementation of PBN will make area navigation operations the norm. DME is the most appropriate conventional aid to support area navigation operations(i.e. assuming DME multilateration on board capability),since it is currently used in multi-sensor avionics for this purpose.

At present, as far as I'm aware, there is no available roadmap for satellite/inertial only based navigation systems as far as civil air transport is concerned. Though GNSS will remain the primary means of navigation for the foreseeable future, plans still retain ground based systems as a backup.

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  • $\begingroup$ Are any of the RNAV systems out there able to read VOR/DME signals or would you need an older receiver in case of GNSS failure? $\endgroup$
    – TomMcW
    Sep 21, 2016 at 17:38
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    $\begingroup$ FMS is a multi-sensor system that is capable of reverting to IRS, DME/DME, or VOR/DME navigation if GNSS is unavailable. $\endgroup$
    – Gerry
    Sep 21, 2016 at 18:40

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