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    Re: Is Celestial Navigation really a backup to GPS Navigation?
    From: Frank Reed
    Date: 2018 Sep 9, 10:19 -0700

    To your headline question, is celestial really a backup to GPS? Clearly, no (with the caveat that it still has some remaining roles), because it does not yield anywhere near the same output product. Manual celestial navigation with a handheld instrument can produce a fix a few times a day when the weather is fair with an accuracy of about one nautical mile under typical conditions (often worse and sometimes a little better). Contrast that with GPS or any GNSS system. You get a "live dot". The position is updated essentially continuously, in all weather, and the accuracy is typical 500 times better than a celestial fix. And in addition, a GPS receiver supplies altitude, exact time, and velocity data.

    Since I started teaching celestial classes on a regular basis eight years ago, I begin every class with a quick promise that I am not going to turn them all into luddites. I begin by saying that we all use GPS today, and I ask them "what is the best backup to a GPS?" I try to do this with a smile and I sometimes wave a sextant and say "I don't mean this!" Usually someone will get the idea and say "another GPS" And everyone laughs politely.

    And this is a good point to talk about the way the expression "GPS" has mutated in the language (at least in US English). People will say "GPS" to refer metaphorically to anything reliable and accurate, but even in common tech jargon when we're not speaking in metaphor, the expression has shifted. It's quite normal for drivers, and boaters, too, to say "I have a GPS system" but what they mean in a more strict sense is that they have a plotting system with built-in mapping capability that uses GPS/GNSS signals to plot the "live dot". When we say "GPS", we mean the whole plotting solution. What is the best backup for that complete plotting solution? It may well be a simpler complete plotting solution with independent access to satellite signals. Or we may skip that level of backup and jump to something like a smartphone or an iPad which can provide some degree of backup of the complete solution, depending on details. And finally the most basic backup is a simple standalone GPS receiver or an earlier generation smartphone, which in either case might only provide the raw GPS position solution: latitude and longitude (and sometimes altitude, time, and velocity depending on the display). A prudent navigator should always have at least two layers of GPS backup available, and "these days" that is not hard to arrange since so many people carry smartphones wherever they go (yes, power management is then the big worry, but that's a separate matter).

    Even ten years ago, I remember a boater telling me a story about how the "GPS failed" and he was thrown "back to the basics" of navigation. Even then, I was aware that he probably meant something quite different from what a NavList reader would mean by "back to the basics". I chatted with him a bit to find out just what he meant. When he said that the GPS had "failed", he meant that the electronic chart display had died. So he had pulled out a Garmin handheld GPS and pulled his boat's (live, exact) position from its display for the rest of the voyage. The "back to basics" element here was plotting that GPS position on a paper chart. He felt like a real navigator because he could track his progress with a pencil on a chart. This was a classic case of backing up the GPS with another GPS, even though from his perspective, the GPS was "out of action".

    So does celestial still have a role to play if it's not much of a backup?

    Yes, it has a number of roles, both cultural and directly navigational. From the cultural side, first consider that the "backup" to GPS is sometimes created by decree. If authorities (law enforcement, licensing agencies, corporate management, insurance agencies) proclaim that celestial is the backup to GPS, then you study and practice it regardless of reality. And that is, in fact, the case today. Celestial is the backup by decree for many users. Other cultural aspects include hobbyist interest, antiquarian gaming, ocean racing gameplay, and so on. We can discuss the lingering "directly navigational" role for celestial navigation another day... Obviously a phrase comes to mind for me: "GPS Anti Spoof".

    Frank Reed

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