Terminal NDB reception range at high FL's

[Lawrence of Quebec]

I know NDB reception range is a function of many, many variables, but generally speaking I'd be interested to know how far back of a terminal NDB (ie., one used on an ILS approach course), can you acquire a usable signal when you're high-up in the cruising flight levels? 100NM's, 50 NM's?
Is there a general formula for this?

Maybe a turbine/jet driver would the answer to this one?

Thank you from a instructor.

[Manflex55]

Well it would depend on the NDB's pwr output, which varies between 25W & 10kW. Obviously the low-powered NDBs related to IAPs, aka locators, won't give anything reliable beyond 20 to 25nm. If you're flying over the sea, U might pick up some high-pwrd NDB signals up to 500nm away, though I wouldn't use them.
Actually, I don't think I've EVER used the ADF in commercial ops... or maybe once in some obscure carribean airfield when the GPS system was having a moment ! Thing is U can't couple the autopilot to the ADF, so it's too much hassle. Enter the coordinates of your NDB in the FMS & forget about the beacon itself.

MF

[whoop-whoop]

Terminal NBD's have a published promulgated range eg. EGKK GY (365)15 nm.

Whereas airways NDB's are considerably higher powered, hence range increases eg. LPPT CP (389) 250 nm.

Although not regularly flown, we find ourselves flying of couple of such approaches a year. (Usually when the ILS is off the air)

Whilst agreeing with the previous comments about GPS equipped A/C (flying an NDB monitored GPS approach) Only a few of our A/C have GPS updating to the FMC

The remainder of the A/C have only IRS with traditional radio updating. Problems occur with the fact that if you are required to fly an NDB approach then by implication there are no other more favourable NAV aids to use. With the lack of more accurate NAV aids the quality of radio updating decreases, and hence errors in the FMC map display can occur (map shift).

My opinion is that on a CAVOK day when you are using the NBD as a airfield approach aid, then monitoring of the LNAV is acceptable. But on any other occasion then the only safe method is to ignor the LNAV and go back to raw data.

And back to the original question, the FMC will preferencially update using DME/DME fixes therefore even when flying towards an airways NDB the FMC will not actually tune the NDB, it will fly to the co-ordinates whilst using accurate radio updating from other surrounding aids.

[OzExpat]

Lawrence... I've seen many formulae for this but none of them are ever consistently anything but wrong. As has been said already, the published range is based on lots of variables. One of the biggies here is signal field strength. In essence, the greater the field strength, the better the range.

The trouble with this is that field strength has many variables, so you can end up with vastly different strengths in different approach sectors. The published figures can take this into account by either averaging the strength throughout the whole azimuth, or by defining sectors with different ranges.

Now, the actual range at which you'll pick up an identifiable signal may vary considerably from the published range. But, if the navaid is sited in an area of poor conductivity - and especially if it has a weak earth mat - the actual range may be only a mile or two more than the published range.

But the point to remember is that your enroute LSALT, MEA or whatever you want to call it, could be based on that navaid. In that event, it's unwise to rely on the signal beyond the published range. And, as you may not know whether or not that is the case on any given track, it's generally not a good idea to trust the signal while you are outside its published range.

This is most especially true for Locators, because of their much lower power output which, in turn, results in a generally much lower signal field strength. I know this hasn't actually answered your question, but it IS the way things are with this type of navaid.

[RatherBeFlying]

Terminal NDBs are usually low powered so that they do not interfere with other NDBs at the same frequency. Analogous considerations are given to frequency allocations for VORs. High Altitude VORs with the same frequency will be farther apart than terminal VORs.

So, while you might receive a terminal NDB from a long distance, you will want to monitor the identification as it may change until you are within the published range.

For more amusement you can monitor marine NDBs which share a common frequency, but transmit at seperate intervals.

[fireflybob]

Protection ranges for NDBs are only guaranteed by day (and therefore not at night).

I have been trying to locate the reference in the UK AIP but I recall that if you are within the protection range (by day) this guarantees that the strength of the desired signal will exceed the undesired signal (i.e. other NDBs on the same or nearly adjacent frequency) by 2 to 1.

I can't see that aicraft height is a significant factor since it is normally the "ground wave" which is being used, although "sky waves" become a major factor at night (which is why PRs are not guarenteed at these times).

[Tinstaafl]

Australia publishes both Day & Night ranges for its NDBs.

[pigboat]

Oz, here's one for ya. The airways beacon here used to transmit on freq 251 kc - they just kept the old radio range freq when the range was de-comissioned. Along about the mid 70's, we began to notice interference on the frequency. Some nights, you could actually hear weird music for short periods, and the RMI needles would head off to the east. After a few complaints by us and a few other operators, one night a radio tech from Transport Canada did a jump seat trip with us to see for himself. The reception was really great, and you could even hear the announcer, as well as the music. They NOTAMed the beacon as unreliable, subject to unknown interference. About a month later, they discovered the culprit. Bangaladesh had gained its independance a short time earlier, and not being a member of ICAO, they were using frequencies below the AM band for broadcast radio purposes. Turns out what we were hearing on 251 kc was the Bangaladeshi hit parade. The output of that station was something like 200,000 watts. They changed our freq to 273 kc and got rid of the problem.

[Red Snake]

As FFB says, and unlike VHF, the aircraft height has very little to do with the useful range of an NDB. During daylight the range is limited mainly by power as ground wave is the predominant propagation mode. At night, the audible range of the beacon is greatly increased & is limited mainly by noise (interference or otherwise) as skywave propagation become dominant. But note that any bearing information received via skywave is highly inaccurate.

[OzExpat]

Just to support the point made by Tinstaafl (g'day mate! are ya gettin' cold up there yet? :eek: ), day and night ranges are also quoted in PNG. We do it a bit differently tho ... if the night range isn't much different to the day range, we publish the night range as the range by day and night.

BTW Tinny, when are ya gunna change yer location from London, UK? :p

pigboat ... 200,000 watts? Sheeeiitt! Get too close to the transmitter and ya get fried! :eek: A good example of why it's useful to identify a signal properly before using it, eh!

A few years back, we had a NDB that migrated from place to place but used the same frequency the whole time. We had it published in one location, but when it started to migrate, nobody told us! Had to get a bit nasty about that when I found out.

We have another problem tho and it's getting worse each year. There's a heap of foreign fishing vessels blathering away in (langage withheld to protect the guilty) on the HF comm frequencies used for ATS. It seems to go on all day and there's nothing we can do about it without creating diplomatic incidents with at least two other countries.
:(

[ 25 October 2001: Message edited by: OzExpat ]

[Steamhead]

Long Range NDB's also suffer from a night effect (if my memory is correct)
i.e. The reciever will pick up two signals,
one the normal ground signal
and two the signal which skips of the ironised layer in the atmosphere.(The layer is lower in the atmosphere at night.)
The skipped signal is polerised 90 deg.and the two signals combine to give false indication (I Think)
Just to add confusion
regards
Steamhead