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Showing posts from August, 2019

Finding UTC of LAN with the Starpath Custom Sun Almanac

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We published a short book last year that is intended to be the bare minimum celestial navigation training that will serve as a backup to a loss of GPS. In keeping with the backup concept, the book is presented in such a way that it is totally self-contained, which means the book also covers how to use the simple Davis Mark 3 sextant. This is a small device, for under $50, that could be used to safely circumnavigate the world.

Our promise is that this book could be opened up and read for the first time when it was actually needed, and it would be adequate to teach how to take the sights, and then find your position from them using only data in this small book—a large part of which is a custom Sun Almanac designed to make the position solutions especially easy.


GPS Backup with a Mark 3 Sextant
The techniques taught in this book are finding Lat and Lon from "noon sights" (local apparent noon,  LAN) as well as Lat by Polaris in the Northern Hemisphere. What we forgot to include i…

Compact Day of the Year Calendar

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There are several occasions in navigation when we need to know the total number of days between two dates, or a new date after so many days from another date.

One example is when planning a long distance coastal or ocean route which yields the total duration, and we know the starting date and time, and want to know the date of arrival.

Another example is when figuring the watch error in cel nav for a watch set some time ago. We might know the watch gains 0.335 seconds every 10 days and it was set on April 14. It is now July 22 and we want to know the watch error.

We could of course start out with "Thirty days has September, April, June and November..." to find number of days for each month (if we did not know this already), and then count out the days. But we don't want to make mistakes at the Nav Table, so it is much easier to look up that (in say 2019) April 14 is day 104 and July 22 is day 203. This difference is  99 days and (99 /10) x 0.335 = 3.3 seconds.  The watch…

Compare ASCAT and WindSAT Scatterometer Wind Data

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We have two sources of scatterometer data these days, the 3 Metop satellites with ASCAT data and the US Navy Coriolis satellite with WindSAT data. The latter does not get much mention in official NWS forecast discussions, whereas ASCAT is referred to frequently. The reason for this is not clear, so in  a first step toward trying to understand this, we will compare the data whenever we can spot passes at about the same time, over the same region.  It could be we can calculate that conjunction of passes, which if so we will add to our satellite prediction time article, which is underway.

The OSWT site presents the ASCAT data in a 10º x 15º Lat-Lon grid and the WindSAT in a 20 x 30 grid. They both have in principle 25-km resolution, so the reason for this is not completely clear. It could be simply that the single WindSAT data swath is about twice as wide as one of the halves of the two ASCAT swaths, so it would likely take a different file layout to account for it.

If the WindSAT data a…

Tools for Crucial Weather Routing: With an ongoing comparison of GFS and FV3-GFS.

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We will add new data at the end here until we figure out which model wins out!
It can happen that crucial decisions depend on details of a forecast. Usually this will be for a longer term forecast, since one or two days out are generally pretty good.  We have a specific case at hand now, which is a classic, real-world example.  The vessel has two optional routes, on each a crucial turn must be  made roughly 96 hr from now, meaning we are in interested in what happens on Friday, May 31.

The two main primary sources we have are the GFS and the new FV3 GFS, which will replace the former on June 12.  We see below that these two do not agree at all for Friday.




Again, Friday shown here is 4 days out from these  Monday 18z forecasts.  The displays are the meteograms from LuckGrib.  The standard GFS we have used for years shows roughly 20 kts from the east, or just above east.  The in-principle-better model FV3 GFS shows 30 kts from the SE.  For the vessel at hand here, and almost all vessels…

FV3-GFS v. GFS—A comparison of real forecasts

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On June 12, the GFS will be updated to the new FV3-GFS, which promises to be a big improvement in global forecasting.  We have been following the winds off the coast of Queensland, Australia in a real world example of needing the best possible long-range forecast. See Tools for Crucial Weather Routing: With an ongoing comparison of GFS and FV3-GFS. This note here is a summary of those results, which we can do now that we know what the winds turned out to be.

We started with a 90h forecast, using Friday May 31 at 12z as our test case.  

The boat was travelling at about 1.5 kts at the time, so the location did not effectively change relative to ocean wind patterns. But we see that at that location, the wind did indeed change a lot with time, so we stick with our 12z reference. Below are the winds for that day and location.

Observed wind from Emerson on Fri May 31 located 154 nmi from Cairns, in direction 123T.

00z to 08z  12 @ 070

08z to 14z20 @ 130-140

14z  to 20z15 @ 130-140

20z  to 00z20 @ …