The propagation of HF across the Atlantic is well researched — Page 138, Lesson 212

I want to walk you through a key part of HF communications over the North Atlantic. You already know that HF—high frequency—radio is the primary way we talk to air traffic control when we're out over the ocean, beyond VHF range. Now, the first thing I want you to understand is a practical rule about frequency selection: to talk to a station 1,000 nautical miles away, you will need a higher frequency during the day than at night. There's a simple memory aid for this: "the higher the Sun, the higher the frequency." So, daytime propagation over that distance demands a higher HF frequency, while at night a lower one will work. The propagation of HF across the Atlantic is well researched. The geophysical anomalies caused by sunspots and magnetic interference are quite predictable. That means the ground station—the aeradio operator on the other side—will generally dictate the choice of frequency. You don't have to guess; they know what's working right now. The first HF frequency you should use when calling the air-ground station will be specified in your oceanic clearance. That clearance is issued before you enter oceanic airspace, and it tells you which frequency to start on. The ground radio operator will be monitoring all the frequencies in what's called the 'family'—a set of related frequencies assigned to that station. So, when you make your first call, you need to tell the operator which frequency you're actually using. You do that with specific phraseology. For example: "Shanwick Speedbird 289 on 88" This tells the ground operator that you are calling him on 8864 MHz. The phraseology is precise: you state the station name, your callsign, and the last two digits of the frequency—in this case, "88" stands for 8864. For the northerly routes across the North Atlantic, both Gander and Shanwick—those are the two Oceanic Area Control Centres, or OACCs, for the NAT Region—will monitor the day/night long range frequencies. If there are propagation problems with Gander, the aircraft should still be able to relay through Shanwick. That gives you a backup path. The two OACCs also have landline data links with each other, just as they do with all the other OACCs in the NAT Region. At the point where control would be transferred from one OACC to the other, there is a standard procedure: you copy the initial position report to the new OACC to the previous one. The phraseology for that is: "Gander copy Shanwick this is Speedbird 289 position ..." So you're asking the new centre—Gander, in this example—to copy your position report to the previous centre, Shanwick. That ensures the handover is clean. Now, let's move to SATCOM voice communications. SATCOM stands for satellite communications. Since oceanic traffic typically communicates with ATC through aeradio facilities—those are the ground radio stations we've been talking about—a SATCOM call made because you are unexpectedly unable to communicate by other means should be made to such a facility, not directly to the ATC Centre. The exception is if the urgency of the communication dictates otherwise. So, if your HF fails and you need to use the satellite phone, you call the aeradio facility first, not the controller directly, unless it's an urgent situation that requires immediate contact with ATC.