Communications in aviation are not as dependent on HF transmission as before, as other modes of communications have overtaken HF
This piece is about radio in general with obvious aviation leanings. The basic thing one has to understand about propagation is that wavelength multiplied by number of times it moves in a second (frequency) gives its velocity. Radio wave’s velocity is derived as 3x108mtr/sec (or 300,000 Km/sec), and this is recognised as “universal constant”. Most people take “SW-FM” as two facets of radio, representing short wave (or HF) and frequency modulation (FM). It may be a bit confusing to talk about “selcal” here, but I could not help it on seeing the similarity. Please bear with it a little longer, as the above term seems to have more in common with “CD-FM” the selcal code of RA’s first B727. It was a mere coincidence that “FM” happened to be common in both but “FM” in selcal has nothing to do with “FM” of frequency modulation. Selcal or “selective call”, as it is called in full, is essentially a tone related signaling method that alerts an aircraft to get in touch with ground control. It would be generally done through the common high-frequency band, shared by many aircrafts spread around the larger area under the ground control.
As such, the radio is hardly ever quiet as both ground and aircraft are updating position, making all sorts of requests/instructions or passing vital flight-related information constantly. And it naturally gets into one’s nerve if one is constantly exposed to all unrelated blabbering in the congested HF environment especially while cruising. With selcal, crews can afford to mute the receiver so that they can have some quiet time. Needless to say, ground station remains open all the time with the aircraft in question merely a selcal away. It consists of a combination of four preselected audio tones specific to each aircraft. The 16 tone/alphabet (A-Q, except I) arranged in all possible four letter format offers nearly eleven thousand different combinations. To human ears it simply sounds as two varying pitch tones with a short pause in between. It is beyond this scribe to describe how it actually sounds. One can always search under the topic and even get to hear it over internet.
It is unbelievable that the terms like SW and FM have gone through a complete reversal in common understanding in a period as short as two decades. It is difficult to explain SW concept to new generation, for whom a radio cannot be anything other than a FM set. There was a time when we were confused by what a FM stood for. Popular three-in-ones of those days came with a radio (sw/mw/fm), a cassette player and a turn table. Cracking the aviation channels in HF was relatively easy while fiddling with the FM band only produced a constant hissing noise. But the diehard radio enthusiasts tried to extract something from it even if it appeared outright bleak to start with. Surprisingly, the first break in eavesdropping into the VHF communications, between an aircraft and a ground station, happened even under such an unfavourable condition but ended being the most cherished eureka moment for a long persistent effort.
How things changed
Fast forward to present, try to do the same thing now, one will be greeted by FM stations that overcrowd the entire band making even tuning into a particular station a truly difficult task, forget listening to aircraft or tower. The other reason for FM crowding is the gross disregard to basic rule of spacing stations for which they need to be at least 0.6 Mhz apart. But sadly, greed seems to be our underlying policy diktat than ensuring delivery of good products. The situation is so rampant that it is as true while commuting, with public packed like sardines, as with market flooded with banks that the government is crying foul now.
Unlike SW a FM reception comes with sharp quality and is not affected by issues related to poor reception, fading, background disturbances etc. But on the plus side, HF transmissions have an unsurpassable range/reach that a FM transmission can never ever dream of. Even with inherent problem HF transmission is still widely used for variety of communicating purpose but the radio listening for entertainment has been rightfully replaced by FM.
But SW behaves strangely as its propagation is not just affected by the time of the day but also by the properties of the layer of ionosphere that helps it reflect back. Ionosphere is generally taken as the region above the earth’s atmosphere located between 100-1000 Km containing atomic particles that become ionised when exposed to sun’s ultra violet radiation. The radio frequency spectrum between 1.8-30Mhz is generally termed as shortwave (SW) and is broken down into different bands for our convenience. During the night, when the radio wave absorbing layer in the ionosphere gets thin, lower band SW signals propagate distances much further off than during daytime. It is the other way around during daytime when higher end of SW band stand better suited.
But radio scientists found a clever way to make radio signal jump even greater distances with some tweaking. Explaining in a simplest way, a speech can be electrically reproduced, with high intelligibility, in a band of frequencies lying between 100-3000Hz and its transmission is possible when these are combined with radio frequency carrier signal. The transmission was improved by removing unnecessary baggage like “carrier” and one of the two sidebands.
Sidebands, whatever they may be, are said to be mirror images of one another and for SSB transmission one was good enough load. The concept can be best explained if we image a basketball pregnant with two tennis balls. Without even throwing any ball we can confidently say that, a basketball, whether pregnant or not, cannot be thrown as far off as a single tennis ball. That is precisely what happens with SSB transmission.
The carrier, on the other hand, is reinserted at the receiving end, making the out-coming audio not just audible but intelligible and clear. This is how Single Side Band (SSB) got its name but it is not clear who was its real proponent. SSB is extensively used in long-distance wireless transmission while its signal output on a normal radio is inherently unreadable to be of any use. With the advent of integrated circuits the sizes of normal radio receiver have noticeably come down and unlike in the past, it is no big problem getting a SSB capable multiband radio either.
Further, communications in aviation are not as dependent on HF transmission as before, as other modes of communications have overtaken HF for those being efficient and more reliable. As we hear high flying jets now have relegated HF radio to a backup status. With increasing dominance of digital and satellite communication the era of voice communication, especially even while flying in far off and remote oceanic sectors, also seem to be progressively diminishing. HF/SSB monitoring of far off traffic was an exciting place for aviation enthusiasts to tune in. It offered a very different sort of satisfaction on being able to monitor flight as far as like Hawaii and US West Coast or even those over mid-Atlantic in the dead of the night, listening alone to the fading voice of the crew passing on position report. It used to be exciting listening to the occasional sound of silence followed by someone flying somewhere reporting spot wind details with outside temperature in minus thirties or forties making us feel the chill here.