HF band and propagation
HF radiotelephony provides the main air-to-ground link where VHF is not practical: long over-water sectors and remote continental areas. Long ranges are possible because the HF sky wave refracts off the ionosphere and returns to earth well beyond the horizon.
The aviation HF frequency bands in use run from about 2650 to 28000 kHz. HF transmitters use higher power output than VHF sets, and the aerials are shaped to the airframe, with high-speed aircraft carrying specially designed aerials built into the vertical stabiliser. The mode of operation throughout the aeronautical HF network is SSB, single sideband.
Ionospheric refraction changes between day and night, so each route is allotted a day frequency and a night frequency. Crews change between them around dawn and dusk on the advice of the HF station. VOLMET weather broadcasts are also carried on HF.
Oceanic control usually takes place on HF. The direct pilot-to-controller relationship of VHF is often replaced by relay through a communications officer or air-ground operator, so messages from the aircraft are passed on to the controller and replies are routed back the same way. Pilots and controllers must allow for the delay factor that comes from working through a third party.
The HF network: RDARA and MWARA
The high frequency network is divided into two families of route areas. Both operate on SSB, and the published frequencies are subject to change by ICAO.
RDARA stands for Regional and Domestic Air Route Areas. In India these operate on SSB and are grouped into five overlapping contours, each served by a radio station with its own set of frequencies.
MWARA stands for Major World Air Route Areas. Large areas of the world, much of the airspace over the Atlantic and Pacific, lack the VHF coverage needed for reliable air-ground contact, so a group of HF frequencies is allocated instead. The world is divided into 31 overlapping contours, all on SSB. International flights operate on MWARA frequencies as their route requires, and India uses three international MWARA contours.
| Network | Full name | Coverage | Mode |
|---|---|---|---|
| RDARA | Regional and Domestic Air Route Areas | India grouped into five overlapping contours | SSB |
| MWARA | Major World Air Route Areas | 31 overlapping world contours; India operates on three | SSB |
Indian RDARA stations include Chennai Radio, Delhi Radio, Guwahati Radio, Kolkata Radio and Mumbai Radio. The same stations also hold MWARA frequency allocations for the international contours. Because the assignments change from time to time, the current frequencies are always taken from the flight documentation, not from memory.
SELCAL
SELCAL, selective calling, replaces the constant listening watch on an HF channel. The ground station transmits a code that is unique to the aircraft, and the cockpit call system answers with a light and a chime.
A single selective call is a combination of four preselected tones and takes about two seconds to send. A decoder connected to the audio output recognises the aircraft's assigned SELCAL code and triggers the cockpit alert, so the crew does not have to monitor the frequency continuously. In India the SELCAL provision is only on HF frequencies; air-to-ground calling still uses standard voice calling in every case, and the crew keeps a conventional listening watch if one is required.
The code uses letters A to S excluding I, N and O. The two letters of each pair are written in ascending order, so CDAM is a correct code and CAIX is not. The operating agency and the aircraft are responsible for making sure every station the flight will work knows the SELCAL code: it is entered in the flight plan and is included in the initial call to a station if there is any doubt that the station holds it.
| Situation | Phrase |
|---|---|
| Pre-flight check, done on the secondary frequency first, then the primary HF frequency | REQUEST PRE-FLIGHT SELCAL CHECK |
| Check received correctly | SELCAL OK |
| Coded signal weak or the call system does not activate | NEGATIVE SELCAL, TRY AGAIN |
| SELCAL equipment is or becomes unserviceable | INOPERATIVE SELCAL |
HF reports and receiver failure
HF reporting follows a fixed shape. Each report has a stated addressee, a set of mandatory items, and a requirement to monitor the read-back and correct the station if the message was taken down wrongly.
A departure message goes to the destination or to any other agency named in the question paper, on the primary HF frequency. It gives one estimate before and one estimate after the FIR, estimates for the mandatory reporting points marked as triangles on the chart, and the aircraft's endurance and SELCAL code.
A position report goes to control when the aircraft is inside a control area. A FIR boundary report goes to both flight information centres on the primary HF frequency, using the callsign suffix RADIO, and gives one estimate after the FIR together with endurance and SELCAL code. An air report gives the elements of weather in the sequence temperature, wind, turbulence, icing and humidity where available.
Receiver failure on HF is when the aircraft cannot receive any transmission although the transmitter checks serviceable on the built-in test equipment, dials and lights. The call is made twice, each transmission prefixed to show the receiver has failed, and it carries the aircraft position and intentions and the time the aircraft will call again. If the failure happens at an FIR boundary, both stations are addressed.