Reply To: Air Navigation

[kollsmanUser]

No, the Air Almanac and Nautical Alamanac are optimized for different things but they contain very much the same information. The Air Almanac is optimized to speed up the calculations required to reduce a sight – in particular, determining the GHA of the object that was sighted.

The NA lists GHA for objects on the hour. But what if the shot was taken at 14 minutes and 37 seconds after the hour? To compute the correct GHA, the navigator has to read an extra table to account for the minutes and seconds after the hour to then calculate the GHA.

What the AA does is list GHA for objects at 10 minute intervals. This means that the navigator has fewer calculations to do. On the other hand, to use the AA in this way, the navigator has to time the sight so that it falls on the selected time. (The AA does contain the same tables to convert time to GHA for use in a pinch, but why do it if you don’t have to?)

If you look at the NA and the AA on the same day and the top of each hour, you will see that the entries are the same.

So using the AA means that the air navigator does the precalculations ahead of time for a shot at a specific time rather than the nautical practice of taking the shot whenever and then reducing using the recorded time of the sight – notice that the order between shooting and reducing is reversed between the two methods.

As an example, in practice, I might plan to shoot fixes at 1 hour intervals, and let’s say it’s dark and I choose to do 3-star fixes. I would look at my plan and perhaps say that the time is 1430Z and I want to plan a shot for 1450Z. I would then look at the AA for the day and find 1450 and record GHA-Aries. At this point, I can then work out my assumed longitude at time 1450 and use that to compute LHA-Aries. LHA-Aries then becomes an input to Pub249V1 where I can pull out 3 candidate stars. After a bit of simple math, I can then set up my sextant. Say I am doing 2 minute shots (averager), I would start the first star at 1441Z, the second at 1445Z, and the last at 1449Z which result in shots centered at 1442Z (+8 minutes), 1446 (+4 minutes), and 1450 (+0 minutes) – these nice integer numbers make MOO and MOB calculations a snap.

The whole air navigation process is aimed at generating a fix very quickly so that many fixes can be done in the course of the flight. Aircraft move fast and running out of gas is catastrophic.