VOR ranges incorrect (at least some)

[JRBarrett]

With the release of the PMDG DC-6, there is an increased interest in old-fashioned VOR-to-VOR navigation. Though many r/w VORs have been already taken out of service, and there are more on the chopping block, there will remain a backbone network of (mostly) high-powered VORs going forward to serve as a backup in case of a GPS outage.

There are several HIGH VORs in the current MSFS Navigraph data (2106 Rev 4) which are incorrectly marked as LOW in Navigraph, with a defined range of exactly 125 miles, when in fact they should be set as HIGH with a range of 195 miles. In particular, YYT and YQX in Newfoundland have this error. These are gateway VORs for North Atlantic flights, and are, (and always have been), long-range.

This is confirmed by examining the defining BGLs in /0401/NVX04010.bgl in both the Navigraph and FS-Base-Nav folders using Herve Sor’s “Easy Navs” utility. Navigraph has both YYT and YQX set as “Low” with a 125 mile range, while MSFS default correctly has them set as “High” with 195 mile range.

These are two examples that stand out, but there may be other r/w HIGH VORs in the current MSFS Navigraph dataset that either have incorrect service class and/or incorrect range.

Regards,

Jim Barrett

[NAVData]

@JRBarrett

Navigraph has both YYT and YQX set as “Low” with a 125 mile range, while MSFS default correctly has them set as “High” with 195 mile range.

Hi,
I have checked the both example VHF navaids in the current cycle/revision:

YYT is defined as High with 130NM
YQX is defined as Low with 125NM

For the YQX there is no official range/power available, therefore it´s defined as “unknown” and we set the Range to “low” with 125Nm as default. YYT is correct defined and also correct implemented (at least in our database).

According the FAA standard, the range is depending on the high (feet/FL), and till 45000 feet the distance is defined out to 130Nm. We set the “HIGH” VHF navaids to 130Nm.

I don´t know the max. cruising level of the DC6 but I guess it´s somewhere between 18000 - 45000 feet maximum. So the when the DC6 reachs a “High” VHF navaid, it should be received out to 130Nm, according the FAA standard.

From where are the 195Nm range? Are there any official source?

To the “Unknown” navaid-classifications:
What we can do is, that we set all “UNKNOWN” VHF navaids to default “High” with also 130Nm range but that for all “unknown” than.

Cheers,
Richard

[hervesors]

Since FS9 up to MSFS (and also in all P3D versions) VOR ranges in stock BGL files are set to 195 NM for High, 60 NM for Low and 37.5 NM for Terminal. This is a purely arbitrary choice that is not based on any official source. As Richard said, setting H,L,T ranges to 130 NM, 40 NM and 25 NM respectively appears to be a better choice based on FAA standard service volumes. However, standard service volume limitations do not apply to published IFR routes or procedures (Extended Service Volume approval), but it is somewhat difficult to adjust ranges accordingly since it would be a case-by-case implementation. A still more realistic setting would be to use published Documented Operational Coverage (DOC) when available from country’s AIP ; unfortunately, this information is not available for all countries and I don’t think it is included in the Jeppesen databases. For simulation purposes, I think the best choice for navaids not defined as High, Low or Terminal is to consider them as belonging to the High class. Frequency conflicts are usually not a problem.

Hervé

[NAVData]

Hi Hervé,
thanks for the details. With the newest upcoming AIRAC 2107 all “unknown” VHF navaids are H with a range up to 130 NM as default. I have changed it … I guess also that this makes more sense in MSFS.

Cheers
Richard

[JRBarrett]

Richard and Hervé, thanks for the feedback. I think the 195 nm limit in FSX is more reflective of an H VOR’s actual performance. The official service volume is conservative at 130 miles, though I am not suggesting that you change the limits shown in the official Jepp sources.

But, as a practical matter, an H VOR is often useable well beyond 130 miles for aircraft above FL300. In 2009 I spent several months as a flight mechanic working aboard a US-registered Gulfstream IV based in Africa. I went with the aircraft wherever it flew, riding in the jump seat. We did several transatlantic crossings in both directions, and I know that both YQX and YYT can be received out to about 200 to 220 miles over the Atlantic, when coasting in or out. SHA and CRK in Ireland have a similar range for TA arrivals or departures. Of course, terrain is not a factor for aircraft out over the ocean, though I don’t think MSFS models the effect of terrain on VOR reception in any case.

It appears that the default nav data is using the same 195/60/37 mile ranges for H, L and T class VORs as FSX and P3D do.

[crowmium]

I just wanted to post in response to a thread that was locked: VOR ranges incorrect (at least some) - #2 by NAVData

It is incorrect for any NAVAID’s actual reception distance to equal its service volume. The ICAO service volume of a NAVAID is only the GUARANTEED volume of the NAVAID that is actually checked by the civil aviation authority. In reality the radio range and strength of a NAVAID is much larger than the guaranteed range. For instance, in real life H VORs can be detected at ranges exceeding 200 nautical miles at high altitudes. In fact, some procedures actually REQUIRE this increased distance. In those instances the civil aviation authority will specifically check the route to verify the NAVAID is acceptable to use outside of the normal service volume along that route. Similarly, most ILS signals can be detected well beyond the supposed ICAO defined 18 nautical miles. At places like KATL the localizer is used at ranges of upwards of 30-40 nautical miles on a daily basis in real life.

I have had this cause issues in a real human ATC/pilot environment like VATSIM where there are actually enough aircraft to necessitate operating at the edges of NAVAID reception ranges like real life. The issue is MSFS does not simulate a weak or tapering signal strength, so the service volume is a BOOL-type statement of whether the client detects the NAVAID or not (not accurate at all).

Since it is impossible to know if any given NAVAID is needed to be used in a way greater than its standard service volume and there is no simulation of a weak or tapering signal strength, I would suggest increasing all service volumes of all NAVAIDs by approximately 50-100%. This would solve the problem in 99% of cases and also be more realistic than the current implementation.

[NAVData]

… exactly, you wrote it and that’s what the data must offer, a minimum/guarantee range. All above is a feature but can’t be guaranteed due several factors, in the different areas of the world. Not more and not less.

Cheers
Richard

[JRBarrett]

I just returned a few minutes ago from Lincoln, Nebraska to our home base in upstate NY. The aircraft was returning from maintenance, and as aircraft crew chief, I rode along in the jump seat. The flight was GPS direct, so no VORs were actually required. I asked the flight crew to pre-tune a couple of “H” VORs along the flight route, so that I could check the reception ranges with this very forum topic in mind.

The cruise altitude was FL410. The H VORs were: IOW (Fort Dodge, Iowa), and JHW (Jamestown, NY). In both cases, we achieved DME lock at 230 miles, and VOR lock at 203 miles (for IOW) and 211 miles (for JHW). The VOR receiver will not blank the “NAV” flag nor permit the RMI needle to appear unless the received signal strength is adequate for reliable navigation.

Although perhaps “arbitrary”, the long-standing VOR reception ranges in FS9, FSX, P3D and MSFS of 195 miles for H, 60 miles for L and 37.5 miles for T VORs is (IMO) a more accurate representation of how far VORs can be received in actual real-world operations.

As an example, many FAA non-RNAV arrival procedures define specific waypoints as DME distances from “L” VORs that technically exceed the FAA’s own service volume specification. For instance: the SHAFF SEVEN arrival to KEWR defines waypoint EXTOL as 68 miles DME from the ROC VOR, even though ROC is “L” facility.

Although most modern GPS/FMS-equipped aircraft would fly this procedure using RNAV representations of the procedural waypoints - it is designed to be flyable by an aircraft using only VOR and DME.

The problem with using strict FAA service volumes for VOR ranges is that it makes Navigraph an outlier compared to all previous simulators in the MS franchise, (including P3D). The main issue is that the sim treats the embedded range in the nav data as an “absolute”. If the “L” range is defined as 40 miles, then the moment an aircraft reaches 40.1 miles from the facility, the VOR will immediately disappear.

For the sake of consistency/compatibility with the default I would prefer to see Navigraph use the same 195/60/37.5 ranges for VORs that has been the de facto standard in MS sim’s for 20+ years, and which is closer to actual r/w reception performance as opposed to using strict service volumes.