So from FL285 to FL420, you need MNPS-approved navigation equipment — Page 138, Lesson 211

I want to walk you through the Brest Oceanic Transition Area, or BOTA, and then the communications setup in the North Atlantic region. Let's start with BOTA. Part of the Shanwick Oceanic Control Area is designated as the Brest Oceanic Transition Area, abbreviated BOTA. Even though it's a transition area, the Minimum Navigation Performance Specifications airspace requirements — MNPS airspace requirements — still apply here from flight level 285 up to flight level 420. That's the same vertical extent as the Shanwick OCA itself. So from FL285 to FL420, you need MNPS-approved navigation equipment. The air traffic service in BOTA is provided by the Brest Area Control Centre, and its callsign is BREST CONTROL. Now let's move to communications across the North Atlantic. Because of the huge distances involved — we're talking hundreds of nautical miles from land — the primary method of communication is HF, which stands for high frequency, using single sideband voice, or SSB voice. That's the main tool for talking to ground stations when you're far out over the ocean. Long-range VHF is available, but only when you're within 250 nautical miles of land. VHF is also used specifically for the delivery of oceanic clearances to aircraft before they enter the Oceanic Control Areas. There's also a VHF air-to-air frequency: 123.45 megahertz. That frequency lets an aircraft that's having communications or navigation problems talk directly to another aircraft. And here's a critical requirement: all aircraft flying on North Atlantic routes must monitor 121.5 megahertz — that's the international VHF emergency frequency. It's very important that you, as a pilot, appreciate something about how voice communications work in the NAT region. Routine air-to-ground ATS voice communications are conducted via aeradio stations. These stations are staffed by communicators — not air traffic controllers. They have no executive ATC authority. Their job is to relay messages to and from the air traffic controllers in the relevant Oceanic Area Control centre. So when you speak to a ground station over HF, you're talking to a communicator who then passes your message to the controller. Now, let's talk about HF itself. Long-range HF communications is not user friendly. The correct choice of frequency is crucial, and that choice is time-of-day dependent. Also, HF is susceptible to atmospheric interference, which makes continuous listening on HF painful — the book uses that word deliberately. In common with all aeronautical HF networks around the world, the NAT region uses what are called 'families' of frequencies. This means that all, or selected, ground stations monitor the same frequency. The specific frequencies used are 3, 5, 8, 11, and 13 megahertz. And here's the general rule: to talk to a station 1,000 nautical miles away, you will need a higher frequency during the day than at night. The book gives you a simple memory aid: "the higher the Sun, the higher the frequency." So daytime propagation supports higher HF frequencies; at night, you drop down to lower ones.