London Info

[jezbowman]

I've never called up London Information since I've always planned routes with airfields en-route to talk to. When you speak to London Info, you must be in LOS with a reciever tower. For aircraft flying at low altitudes, maintaing contact with London Info across large areas of the UK with one frequency would require many towers. So if I talk to London, then other aircraft near me would hear my call, but those over the horizon would not. However, all aircraft would hear the reply from the controller. Is that right?

Does the system use the same network of transmitter towers as D&D 121.5?

[Kolibear]

From experience London Info coverage is coast to coast. I've been over Kent, listening to a pilot looking for Haverfordwest in Wales.

[david viewing]

Does the system use the same network of transmitter towers as D&D 121.5?

Hopefully not, because London Info has dead spots over the Irish Sea where you might need 121.5!

There was a discussion of London Info a while back where it was revealed that one outlet is at Clee Hill, directly shadowed by Snowdon when flying near Holyhead. Possibly 121.5 is there as well, but perhaps someone can advise if 121.5 has other outlets, like on top of Snowdon for instance?

It's a strong argument for always talking to someone when in remote areas. You can't know for sure that 121.5 will work. In the Irish Sea Dublin or Shannon are always there, long after Valley etc. have gone home. (Maybe they'd respond to 121.5?). Otherwise, it's relay by airliner if you're lucky and have time.

[SwanFIS]

London Info has three frequencies each with three transmitter sites so coverage is pretty good. As David says there is a known problem around the TOLKA, WAL, Valley area where both 124.75 and 125.47 coverage is poor.

All three frequencies are "cross coupled" which means that a call from an a/c on one frequency is almost instantaneously transmitted on the other two. This means that in theory you should be able to hear all the traffic on on all three frequencies wherever you are. This feature stops a/c stepping on other transmitions (sometimes!) causing even more chaos than normal on a busy day!!!!

I am not sure if D&D have other sites, I am sure someone else will tell you before I get back to work and enquire.

[alphaalpha]

Hi Swanfis:

You saidAll three frequencies are "cross coupled" which means that a call from an a/c on one frequency is almost instantaneously transmitted on the other two.

You must have found that it's quite common for an aircraft not to hear the transmissions of another aircraft on the FIR frequencies, but to clearly hear the ground reply.

Given what you have said about cross-coupling, this seems to mean that the aircraft which a pilot doesn't hear must be transmitting on the frequency to which he is listening. The two aircraft are then further apart than line-of-sight, but each is within 'sight' of a transmitter. Am I right?

I'm amazed that there are only nine transmitter sites to cover the whole of the London FIR. When I'm really bored I'll do the maths and prove to myself that it works.:eek:

AA.

[High Wing Drifter]

I'm very bored at the moment:

At 2000' you would get a max theoretical receive/xmit distance with VHF of 56nm, 69nm at 3000' and 97nm at 6000' (1.25*SQRT(height')). Explains why there will probably be holes in the coverage with only 9 transmitters in the UK. If you loose contact climbing might work.

Assuming the UK is square: If the UK is 115,000 sq nm then SQRT(115,000/9) = 130nm. Knowing that the UK isn't square then some transmitters will be closer and some further so assuming that max distance between transmitters is 130 * 1.5 = 195 then with this ludicrous guestimation/approximation you would have to fly at 24,000' to ensure constant coverage (195 = 1.25*SQRT(x) = 24000)) throughout the UK.

OK, I know it is a worthless calculation but that's 10 minutes less time to fill :8

[david viewing]

Swanfis

Does 'cross coupling' mean that the aircraft calls are actually re-broadcast by ground stations on the other channels? That's quite a novelty to my mind and might explain how I can hear planes at Lydd when I am at STU.

My experience is that 125.47 does work in the 'Tolka triangle' although it can't be found on the S. England 1/2 mil chart. I have previously suggested that the frequency be put on the chart in place of or as well as the 124.75 which doesn't work up to 5000' right where it is printed in great big letters on the sea W of Holyhead!

David

[jezbowman]

The UK isn't really 115,000 sq. nm's is it? According to my map, it's 94,270 square miles, which is about 82,000 sq nautical miles. Now that's for the UK - i.e. including Scotland and Northern Island. London FIR covers less than than that, so your calculation was based on 9 towers to cover the London and Scottish FIR's. My guess would be somewhere more in the (conservitave) region of the London FIR being 60,000 sq nm.

So working on the basis of a circular transmission footpint, the area of each circle will be 60,000sq nm / 6 = 10,000sq nm = A. 6 comes from a fudge that you'll need 4+2 transmitters to cover the area in which four would fit without overlapping, so we'll assume that 6 trasmitters fit in the London FIR without over lapping, leaving 3 to cover overlap. This just simulated for the numbers of course.

The radius of said circle will be r [nm] =SQRT(A[nm^2]/PI).
r= SQRT(3183) = 56nm = 64 miles.

d [miles] = 1.23 * SQRT(h [feet]) so
h = (d / 1.23)^2
h = (64 / 1.23)^2
h = 52 ^ 2 = 2704'

Which seems more realistic.

Now, on a seperate issue. If the signals from a aircraft are re-broadcast on the other transmitters, assume the following:

A/C [A] is at 2000ft, LOS ONLY with Tx [X].
A/C [B] is at 2000ft, LOS ONLY with Tx [Y].
A/C [C] is at 12000ft, LOS with Tx [X], Tx [Y].

Now, [A] Transmits, so [B] gets to hear what was said since [Y] is \'coupled\' to [X]. That\'s cool. Now what does [C] hear? [C] recieved the same transmission from two sources [X] and [Y]. Now, it\'s not impossible that the carrier frequency of these two stations are perfectly syncronised, therefore this could be possible. But not easy.

However - let\'s be more realistic...
A/C [C] is at 12000ft, LOS with Tx [X], Tx [Y] and A/C [A] & [B].

No way will that work! - [Y] (and I guess [X]) will be broadcasting at the same time as [A] as they attempt to re-broadcast what he\'s saying. :confused:

SCRREECCHH!

[High Wing Drifter]

Jez,

Hmm, I got the figures of of Google! Anyway, looking at MY map (The Sunday Times atlas of the world no less) England + Wales is 50,000 sq nms. So using my assumptions that would be 8000' altitude to guarentee constant coverage everywhere in the FIR - a little lower that 24,000!

What fun :p

You'll have to help me out here as I'm not sure about this but:-

Over these short distances, are you sure the phase shift would have that screech effect? With a wavelength of about 2.5 meters and assuming something like a 10nm overlap that would be 0.05 degrees (as I make it) out of shift. Would that have any kind of noticable affect?

Then again, maybe that's why East and West of A1 are different frequencies...prehaps?

[SwanFIS]

David

Yes, we usually work with all three frequencies bandboxed unless it is busy when we usually split 124.6 off. If we did not have cross coupling the band boxing of frequencies would be a nightmare. The airways sectors use the same system.

A change in the promulgated freq in that area might be an idea to highlight the problem.

[jezbowman]

Over these short distances, are you sure the phase shift would have that screech effect? With a wavelength of about 2.5 meters and assuming something like a 10nm overlap that would be 0.05 degrees (as I make it) out of shift. Would that have any kind of noticable affect?

I wasn't as much thinking about propogation delay (too many years since uni and forgot most of it :uhoh: ).

I was thinking more about the affect you get when two aircraft transmit at once on the same frequency, that screeching noise which I always assumed was due to inaccuraces between the two carrier frequencies causing a sub-harmonic which is in the auidable.

Perhaps I'm wrong (am only guessing at all this), but just can't see how it's any different... :confused:

SWANFIS - sorry to be a pain, but can you just clarify this - does the broadcast of an aircraft on one of the three frequencies get 're-broadcast' onto ALL the towers on ALL the frequencies, or just the towers on the OTHER frequencies (i.e. not the frequency the aircraft is tuned to)?

[BEagle]

Aren't the ground transmitters slightly off-set in frequency to avoid the audible heterodynes which would otherwise be present with multiple transmissions?

And why 8.33 won't work with such a system?

[jezbowman]

SWANFIS - I just read your earlier posting where you said a call from an a/c on one frequency is almost instantaneously transmitted on the other two
which makes sense now.

So two aircraft which are low altitude in their own frequency area may not hear each other, while they would both hear transmissions in other frequency areas, probably further apart.

But I guess, technically, if your so far away from the other aircraft that you can't get LOS for VHF with him, then you probably don't care about what he has to say anyhow. And you may be near the frequency border and traffic information from across that border may be very relevent. But can't help but think it must be a little annoying having to listen to traffic at the other end of the country!?!

Anyway, I'm going to have a listen this afternoon. I wont be able to call as there are to many other stations I need to work on this afternoons flight, but just have a listen and see what crazy locations I can pick up!