Why does ACARS default to VHF if HF has a better range?
For example, when requesting ATIS from an airport, it will first try VHF, and if that fails, it will try HF. And if all hope seems lost, it will try Satcom. Why is this?
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4$\begingroup$ The audio with HF is generally awful even under good conditions. It's always the last resort if other means are available. $\endgroup$– John KJun 3, 2022 at 12:37
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4$\begingroup$ If you are outside of VHF range you don’t need ATIS. $\endgroup$– Michael HallJun 3, 2022 at 14:15
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3$\begingroup$ @MichaelHall Not necessarily. The altitudes & (close in) waypoints of a STAR often depend on which flow an airport is using, so it's very helpful to know that "ATL is landing east" pretty far out so that you can set up, verify, and brief everything in the FMS as far out as possible. $\endgroup$– Ralph J ♦Jun 3, 2022 at 18:06
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4$\begingroup$ @JohnK That's true, but the question is about ACARS data rather than voice communication. Of course, the things that make audio quality so awful on HF also reduce its bandwidth, as the accepted answer explains. $\endgroup$– Ralph J ♦Jun 3, 2022 at 18:08
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4$\begingroup$ @RalphJ, I agree getting ATIS as far out as possible is helpful, and I have tried and been unable until being closer, my comment was an admittedly slightly snarky jab at the overkill of using a transoceanic capable radio to get terminal information. $\endgroup$– Michael HallJun 4, 2022 at 14:49
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1 Answer
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The main concern is that radio spectrum is a shared resource, meaning that while you’re using it, nobody else within the range of your transmission can use it too. That means you always want to use a shorter-range band whenever possible.
Another is that HF channels are significantly narrower (lower data rate) than VHF ones, so not only are you consuming a more scarce resource, you’re also consuming it for longer periods.
