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Several RTCA DO standards require checks for bad navaid tuning including, "In lieu of using ground-based navigation aid designated operational coverage (DOC), the system should provide, checks which preclude use of duplicate frequency navaids within range" (DO-236B for RNP). Are there example locations where you can be just outside the range of one navaid and pick up another on the same frequency? Has anyone seen potential for confusion like this in real-life, possibly with complicating circumstances like IRS drift?

I thought most navaid frequencies were chosen to avoid potential confusion, but an article in Skybrary seems to suggest this is possible when operating on the boundary between two designated operational coverages, although it may only be talking about voice frequencies. There's also an anecdotal account of VOR and DME receiving from different stations that share a frequency by Tom Rogers at AVweb. There's a similar question on here explaining navaid spacing, but it doesn't answer whether and under what cirucmstances two navaids could be confused for each other.

I even spent a few minutes looking informally through a navigation database couldn't find any examples of close navaids using the same frequency (not including non-colocated systems where each part operates on the same frequency). The closest I got was in Michigan where the Selfridge Air National Guard Base localizer and Capital Region International Airport 28L localizer both transmit on 110.10 MHz. They're less than 100 miles apart but both are ILS localizers, so you'd never confuse one for the other without already being seriously lost. Additionally, Mendoza and Vina del Mar's VDM share 114.9 MHz, but there's the Andes Mountains in between so again you'd extremely unlikely to confuse the two.

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Short answer

ICAO recommendations include a protection for same-frequency navaids, restricting the received power of undesired stations to a level preventing interferences. To meet this restriction same-frequency stations must be considerably distant.

The result, in normal conditions, is the receiver cannot switch uninterrupted from the current station to a same-frequency station, there will be a significant period where none of the stations can be received.

However distant stations signals can be significantly increased by propagation perturbations like sporadic E propagation or wave ducting. These instable conditions can break the co-channel protection plan and increase distant stations levels above the reception threshold, possibly above the local station level. When this occurs the receiver can wander between stations unnoticed if the station ID is not checked.

I'll detail the answer for VHF navaids, but it can be extrapolated to other navaids.


ICAO co-channel protection

Are there two navaids on the same frequency close enough to be confused?

The normal range for VHF is the radio horizon, that is 15% further than the geometrical horizon (k-factor = 4/3).

Same-frequency navaids are spaced well further than this horizon and by measuring the signal level rather than the distance. Using the signal level allows to take into account directional transmitters, like ILS components, where energy is concentrated in some solid angular sector.

At any point within the service volume of the desired VOR, the ratio between the reception level of the VOR and the reception level of any same-frequency VOR must be greater than the co-channel protection factor:

enter image description here

Co-channel interference for a VOR, source

This ratio is 20dB (= 100 times) for ILS/VOR, and 15dB (32 times) for NDB. More information in ICAO Doc 9718, chapter 9. Note this ratio is the recommended minimum, but if for some reason this ratio is not effective, then a solution must be found, ICAO/ITU have procedures to work exceptions.

When the desired VOR transmits and is within range, the receiver reduces its amplification to the bare minimum and the undesired signal, given the protection factor, is undetectable. When the receiver is located at the limit of the service volume, the other signal must still be 20dB weaker. It means the two VOR cannot have their service volume next to each other, but must be significantly remote.

But there are circumstances where propagation can be boosted, and distant stations behave like if they were considerably closer they actually are.

Propagation phenomena

Are there example locations where you can be just outside the range of one navaid and pick up another on the same frequency?

Some conditions of propagation allow reception of signals well beyond their usual limits of reception:

  • Night effect, effective under 30 MHz, may affect NDB. This perturbation is common, but its exact result is unpredictable. See What is the night effect?

    enter image description here

  • Tropospheric ducting, due to a temperature inversion in troposphere, mostly applies to the lower VHF band. As refraction index varies with temperature, a tunnel is created between two air layers and the signal is ducted like light in a graded-index optical fiber.

  • Sporadic E, which affects VHF, is a situation where a signal is reflected by ionosphere and can be received at distance larger than 2,500 km.

  • In polar regions auroras are ionized curtains created by solar winds which alter propagation in unpredictable ways.

The actual use of the frequency, navigation, data, voice, has no significant impact.

Consequences regarding co-channel protection

With this in mind, the coverage of a navaid is not limited by a circle of some radius, you can be at a location where two navaids at very different distances are received with an equal power, or where the remote navaid is stronger than the local one.

Such super-propagation events are more frequent in HF band, and the night effect is present each night. VHF/UHF events are seen during short periods of time, between 10 and 30 minutes a few times per month, more often around summer solstice.

ID check is required before signal use

Communication cannot be protected against these effects. The possibility to receive a remote navaid stronger than a local one can never been eliminated, even with a careful planning of the radio services and ground stations.

That's why when lives are at stake, identifying the actual navaid received is mandatory. Note the protection factors listed at the beginning already include a 6dB (4 times) margin specific to aeronautical communications, not used for common spectrum planning.

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    $\begingroup$ Excellent answer! Do you have source for "Sporadic E propagation [...] is tightly tied to solar activity"? I tried to find some more info on sporadic E and most sources say it is related to season ("Sporadic E activity peaks predictably in the summertime in both hemispheres.", Wikpedia). I found one paper showing an 11-year periodicity though. $\endgroup$
    – Bianfable
    Oct 9 at 17:40
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    $\begingroup$ @Bianfable: Nothing escapes your vigilance! I'll update the answer as it seems incorrect. If you want more details of the overall mechanism, this radioamateur paper looks a very good summary. Layer E propagation is linked to seasons because there are more Sun light, and longer during Summer. $\endgroup$
    – mins
    Oct 9 at 17:57
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#1 I don't know. #2 On December 20, 1995 American Airlines 965 crashed into a mountain top near Cali Columbia when it was flying down the the valley between mountains and the 757 autopilot turned the aircraft left to follow the loaded flightplan... the flightplan was loaded with an NDB called "R". The problem was that the "R" they loaded into the flight plan was the "R" in Bogota, not the "R" NDB on their approach in Cali. When the aircraft turned they were talking about rest rules and duty times and flight attendants and had one of those "What's it doing now moments". The GPWS activated to warn of the mountain they had just turned towards on their approach. In the confusion while executing their evasive climb maneuver they forgot to retract the extended spoilers, which sealed their fate.

After this accident Boeing made it so that spoilers automatically retract with climb power added and Jepessen made sure that there were no longer any single character navaids.

It seems relevant to your question. Maybe?

Relevant to being a pilot: Monitor the autopilot and your progression on the chart/map so you always know where you are and are ready to disc A/P and fly the airplane without having to try to figure out where you are first because you aren't paying attention. Never have a "what's it doing now" moment.

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  • $\begingroup$ Yes, that does provide an answer in that people have died due to trying to navigate to the wrong navaid. A range check could prevent such an issue, and interestingly it's not even that a valid signal was being received from the wrong navaid. $\endgroup$
    – Cody P
    Oct 9 at 8:12
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    $\begingroup$ Your example is a confusion of navaids because of the same name, not the same frequency. Not sure how that answers the question... $\endgroup$
    – Bianfable
    Oct 9 at 9:40
  • $\begingroup$ "#1 I don't know." was my indication to the reader that I was not answering the question. I wasn't sure that my comment was completely relevant either (which is why I said "Maybe"). ;) Consider my comment to be free flight instruction. I usually bill at $75/hour. :) $\endgroup$ Oct 10 at 0:45
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    $\begingroup$ @mins: They are the same frequency to this day, but ROZO was renamed PALMA (PL). This is my point, each is power-limited to its terminal region, but RNAV systems don't care. Also reading the final report, if following the correct procedure the crew could have simply loaded the full approach, instead of manually and hastily entering R when they decided to change the planned runway when offered. $\endgroup$
    – ymb1
    Oct 10 at 18:42
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    $\begingroup$ @ymb1: Ah so the Romeo NDB signal was not received, but the FMS used the known coordinates as a waypoint and changed the route?? That's even more crazy and actually not related to the question, the frequency didn't play any role. $\endgroup$
    – mins
    Oct 10 at 18:46

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