When thinking about navigational electronics, one piece of gear may not, at first, come to mind. The personal digital assistant (PDA) can actually be used as a very effective celestial-navigation tool by performing celestial sight-reduction calculations and a variety of other nav tasks.
For eight years I’ve used my Celesticomp celestial-navigation calculator to plan, reduce and use celestial sights, but the time may have arrived to retire it to the back of the nav table. The software available for PDAs provides the celestial navigator with incredibly powerful and inexpensive tools for not only planning and reducing sights, but also for using them in precision navigation. I’m pretty good with the Celesticomp, but the intuitive interfaces and large display size of PDA screens make them natural companions to a sextant.
This review focuses on PDA software related to those three aspects of a typical day at sea: sight planning, sight reduction and piloting. This is by no means an exhaustive review — instead I’m reviewing the programs I’ve found most useful. All of the programs operate on the Palm OS (version 3.0 or higher), and all work on any machine from the Palm III onward. Most of these programs are mathematically intensive and use a library of math functions called MathLib. Find it (for free) at www.radiks.net/~rhuebner/mathlib.html. Download it first and then add the applications you choose to use.
Planning
The first part of any trip planning involves such simple things as the times of twilight, sunrise and set, LAN, and so on. By far, the handiest application I’ve found for this (and for general star identification) is Andreas Hofer’s Planetarium, now in version 2.2 (www.aho.ch/pilotplanets). The program provides a perpetual (for the Palm, where time runs from 1904 to 2030) almanac of all the navigational planets and stars, as well as a host of other objects of interest to amateur astronomers. The first feature I use is a compass view window, which displays the azimuths and altitudes of major objects at user-chosen times and positions (Figure 1, at left). Tapping on the icon representing a planet (the symbol in the upper right corner of Figure 1) brings up its altitude and azimuth, another tap and you see a sky view of the body, displayed in the surrounding star field. Particularly in the early days of a passage, when one is still learning the sky, these features allow you to quickly identify any object worth shooting.
Planetarium also includes a well-designed rise-transit-set page, which includes full twilight data (civil, nautical and astronomical, Figure 2). Also included is a convenient magnetic declination (or, in nautical parlance, magnetic variation) calculator, valid for any location on earth for the period from 2000 to 2005. This is a huge improvement over the relatively imprecise values one gets from charts. The program does all this using less than 450 kB of memory (only 151 kB with the options I chose) and costs about $15. Nothing else involved in sailing is this inexpensive and this good.
Planning a trip also involves determining courses and distances, and for this I’ve found John Manson’s Pilot Navigator to be rather helpful. This package of sailings, piloting and sight-reduction applets is inexpensive and small ($10 and 75 kB), programs that do one thing, but do it well. Utilities include time to distance, time/speed/distance, and true/apparent and set-and-drift calculations, course determination (great circle, Mercator), and so on. Roughly half of the applications in Pilot Navigator are available in other programs we’ll look at below, but the other half provide clear, fast solutions to the navigator. Find Pilot Navigator at www.pilotgear.com; it includes the additional software you’ll need to run the program.
Sight reduction
Reducing a sight requires two separate steps: determining the apparent position of a body (its declination and LHA) and then determining the intercept from an assumed position and the corrected height of the body. Traditionally, these two steps required two books: a Nautical Almanac and a sight-reduction book, such as HO 229. Not surprisingly, two types of programs are available for reducing sights on the PDA. One can find standalone almanac programs, such as Planetarium or Orrery (www.palmgear.com), and then use the freeware program MG229 (www.mobilegeographics.com), or Manson’s Pilot Navigator to do the actual sight reductions.
But why would one do it this way? I like the freeware MG229 &mdash it’s a tiny, helpful little program that I keep loaded because I’m paranoid. Manson’s program includes a solar almanac, so sun-shooters could get by with just Pilot Navigator. But the power of the PDA comes from its ability to seamlessly integrate a multi-step process. Two integrated programs I’ve used successfully are Astronavigation ($20, 80 kB, www.online-fischer.de) and CelestNav ($50, 210 kB, www.mobilegeographics.com).
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Figure 9: The fun Your Shadow page of Astro-Compass. For a given time and location, you get the relative length and bearing of your shadow. As their different sizes suggest, Astronavigation is a smaller, simpler program that provides an almanac (sun, moon, planets and the 80 navigational stars), sight reduction and fix features. Sights are reduced by choosing a body, then entering the usual information in a slightly confusing screen (Figure 3; the word “down” should be read as “lower”). I like the traditional output of intercept and azimuth, but only in the online documentation can you determine the format for entering angles and times. With at least two LOPs in hand, you can use the fix screen (Figure 4) to determine a fix. Sights can be added or withdrawn from a fix as you see fit. A few utilities allow you to do piloting calculations, including DR and course to make good. Astronavigation is a small, minimalist program that effectively handles the basics of sight reduction. It utterly lacks any bells, whistles or noise-making features whatsoever &mdash and that may well be its greatest strength.
On the other end of the features scale is CelestNav, which has every bell, whistle, horn and air-raid siren you could want in a sight-reduction program. On the almanac side, CelestNav includes a screen that gives daily information for the sun, moon, planets and navigational stars. In a sense, it’s like looking at the daily pages of the Nautical Almanac, but customized for your position and time. For the sun, checking the rise/set button provides a rapid way of planning a day’s work, including the LAN sight. User-changeable fields are shown with a dotted outline (See the first line of entries in Figure 5); tapping on them brings you to the data-entry screen, which is a triumph of user-friendly design (Figure 6). It simply couldn’t be easier to enter data in CelestNav. The large buttons and user-selectable formats allow you to quickly and accurately enter your data. This is, in my mind, the greatest difference between CelestNav and Astronavigation.
A nice feature of CelestNav is the ability to record sight times directly on the PDA. (The program includes a time screen, where watch error can be added. Unfortunately, no rate correction is allowed.) Tapping a button records the time; one then enters the sextant angle, chooses a body, and the sight is automatically recorded. This is the greatest weakness of the program &mdash the sight is automatically added to a sight list, without the user having to push a button and without any indication from the program that the sight has been recorded. I’d like to see a button that says “save” or “record,” so both the program and I know what’s going on. The first few times I used the program, I entered sights multiple times. After sights are recorded, one can edit them, calculate an LOP or use them to make a fix.
The fix screen includes a wealth of information crucial to careful celestial. This includes not only the fix position, but also (assuming you get up your DR) the difference between the fix and your DR. Perhaps no part of good celestial is more important than keeping track of the net “current” affecting the boat, and CelestNav does a nice job keeping track of this. The error information (in bold, Figure 7) is also a handy way of evaluating the quality of the fix, and of tracking your celestial skills over time.
In navigational mathematics, one generally knows all but one variable in a problem &mdash thus you might know the vertical angle subtended by a lighthouse, and it’s height, and want to know how far off you stand. CelestNav implements a distance screen and a sailings page by displaying entries for all of the variables in the problem. One fills in the knowns, and the unknown materializes on the screen. The mathematics are generally simple, so the real-time nature of these features is rewarding. The sailings page includes the usual DR and course-to-steer pages, all well laid out and intelligently implemented. At $50, it’s 2 1/2 times the cost of Astronavigation, but ultimately I find it a more useful and user-friendly product.
Utility programs
Time is the navigator’s constant bedfellow. I never take my watch off while passagemaking, and I’m always converting this time to that time. Skip that and get a copy of Sidereal for your PDA. This tiny freeware program (10 kB www.pilotgear.com) displays local time/date, UTC time/date, solar and mean solar time, and even sidereal time and the Julian date. While meant for astronomers, it’s a handy tool to have around, particularly for those parts of a passage where sleep deprivation is catching up to you and you can’t subtract anymore.
I also use AstroCompass, partly because it is so much fun. Enter your location and time, pick a heavenly body, and AstroCompass displays a compass rose, allowing you to use the PDA as a compass (Figure 8). What a great idea! Sure, we all know how to use the Nautical Almanac to do this, but this program does it effortlessly and well. It also has a great Your Shadow feature, which gives the direction and relative height of a shadow at any time (Figure 9). I know, I shouldn’t get this excited about my shadow, but it is undeniably cool that all of this software fits not only into the palm of my hand, but is so quickly and readily available.
There is an undeniable beauty to celestial navigation. You have a watch, a sextant, a couple books and &mdash Shazam! &mdash you know where you are, anywhere in the world. Doing celestial is a personal experience, an experience not lessened by using calculators for some of the work. After all, if you use the traditional method of Nautical Almanac and HO 229, you’re still using the mathematical abilities of others. Using a PDA or a calculator to do the math simply allows the navigator to take and reduce more shots, leading to a better job and a better day’s work. Despite what traditionalists might say, using a PDA actually improves celestial. And a PDA is useful for other tasks on a voyaging boat: keeping track of parts and supplies, and for handling email duties. Best of all, on those long winter nights, you can take out your PDA and plan next summer’s trip. That, too, is an undeniable joy.
Larry McKenna is a sailor, freelance writer, and president of Working Knowledge Inc., an educational services firm that provides support to science teachers.
