T9, oceanic clearances, VHF relays and the cost of it all

[BOAC]

Taking my poor co-pilot on his first foray into Shanwick etc on the way to FNC yesterday out of CWL, and as we studied the map and wrestled with ionospheric distortion, Santa Maria, and all the other lovely facets of Oceanic HF, he said " why not use VHF". Ah! I said, I asked the same question a while back of a Shanwick supervisor and he said "cost".

So, why not a repeater at Santiago and the tip of the Brest peninsula? How much, exactly?

[Spitoon]

Part of the 'cost' is interference - you know what happens when two transmitters open on the same frequency at the same time. There were some neat technical tricks played up in northern Scotland at one point - don't know if it still happens - to get good low level coverage for helis. 'Repeater' transmitters were offset from the nominal carrier by 5KHz but last I heard aircraft kit designed to work with 8.33KHz-spaced channels would probably not work with such arrangements.

Just a coiple of thoughts from a non-technical person - no doubt engineering colleagues can give a fuller answer and correct mine!

[HEATHROW DIRECTOR]

Spitoon - Multicarrier systems have been in use throughout the whole of the UK for 50 years plus. Several transmitters carry the same "basic" frequency but each transmitter is off-set by a few khz to prevent the QRM you mentioned. For example, some of the distant sectors from LACC have transmitters in Cornwall and Gloucestershire on the "same" to provide adequate coverage for the traffic they handle.

[threemiles]

The 3 or 5 kHz offset system is called climax and all new 8.33 receivers can handle this when set to a 25 kHz spaced frequency. This is why it is so important for pilots to select 134.700 and not 134.705. Though the carrier freq is the same the bandwidth inside the receiver is adjusted from narrow to wide.

So, why not a repeater at Santiago and the tip of the Brest peninsula? How much, exactly?
I bet Brest and Madrid have receivers in these corners already, so it is just a good wire or in modern thinking a bit sat up and down to feed them. Investment less than 50k, running costs near nil. I believe ICAO NAT Paris is the responsible unit to address this to.

http://www.paris.icao.int/natspg/main_natspg.htm

[BOAC]

Thank you 3m - eventually found the tiny email link there. Email away but I'm not holding my breath..........................

[javelin]

Didn't think 737's could go that far away from land :E

[BOAC]

We've got some extra-long (JAA approved) string here, but we like sunny days so we know where France is.:mad:

[rab-k]

Comes down to € at the end of the day.

Any VHF tx site on the Galician coast in order to extend VHF cover way north of BEGAS would have to be paid for and maintained by AENA. As they wouldn't receive any additional revenue from such a project what would be their incentive? A/c would continue to transit through the Santiago sector whether they routed into Shanwick or Brest, so a big outlay by AENA for no reward = non starter.

Likewise the same applies to the IAA in order to reach further south of LASNO, but I guess they would see some increase in traffic routing to Shanwick via Shannon from London's west-end sectors

However, France's DGAC, in placing a VHF tx on the Brest coast able to reach up to and including 009W, would lose revenue by having non-HF equipped a/c route around their airspace and take the cheaper route charges via Shanwick, so definately no incentive for them.

If Shanwick, i.e NATS/IAA, were to pay for and maintain the additional tx sites it would have to be proved that the additional traffic routing via T9, at $55 per trip, would be sufficient to justify the added expenditure in establishing and maintaining such facilities. Dunno if that case could be proved to be honest.

CPDLC, ADS and ORCA (via ACARS) may be the way forward for non-HF equipped aircraft to route via T9, thereby negating the need for requesting clearances and making waypoint position reports via HF, but such a move may be a few years down the line and for the time being; no HF = no T9!

[clicker]

Bear with me as not aircrew (now ex ops) but I though Shanwick have a couple of VHF freqs, 127.90 and 124.175 I believe. That said not sure if they only cover the 15W areas.

clicker

[BOAC]

+ 123.95 but they will not reach down to Begas.

[rab-k]

"Shanwick Control" has two 'Clearance Delivery' frequencies, 123.95 and 127.65, staffed by ATSAs (Assistants) at the Prestwick OACC, for the issue of Oceanic Clearances to westbound flights about to enter Shanwick. As BOAC states, they cover only as far south as LASNO and ETIKI. (On a good day!)

"Shanwick Radio" has two 'HF Assignment' frequencies, 127.9 and 124.175, staffed by Shannon Aeradio Officers at the Ballygirreen Radio Station, for the allocation of HF primary and secondary frequencies to westbound flights about to enter Shanwick , but they also extend only as far south as LASNO.