A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2017 Feb 11, 02:58 -0800
Mike you wrote: I had concluded as a result of needing a whole degree for LHA for entry into AP3270 and each selected HB having a different GHA I would need a different assumed position for each LOP. HOWEVER... Either in a reply to one of my previous posts or in a PM I received someone said "use only one assumed position". Really confused as how to use the same assumed position for all the LOP's
Mike. This post disappeared into space for a couple of days, but in case you’re still confused, I'm re-posting it, It all depends how you use AP3270/HO249. The marine navigator tends to take an observation when he can with a marine sextant and work everything out afterwards, so it’s logical (and there’s time) to use a separate assumed position for each observation. However, the tables were originally designed for air navigation using a bubble sextant using one or two minute shots, and the thing about air navigation is you have to be slick and quick, or you won’t keep up with the aircraft, so a lot of the calculations have to be done in advance. Also, if using a periscopic sextant with a two degree field of view, you have to do this, or you wouldn’t know where to point the sextant.
For air navigation, if sandwich fixing, i.e. shots spread about a mid-time e.g. AABBBAA or ABCCCBA look up GHA Aries for the mid-time of the middle shot. Select an assumed position nearest your DR position which gives you a whole number of degrees for LHA Aries. Enter Vol 1 on the page for your assumed latitude. Run along the line indicated by LHA Aries for your assumed position. You will find Hc and Zn for seven stars. Pick two that give you a nice cut around 90 or three with a cut near 120 (or ones that you know you’re bound to see!). Draw the two or three azimuths from the assumed position and label them as you draw them (or you’re bound to get them mixed up) with an arrowhead towards the star. Average the intercepts for each star and plot each average from the assumed position. Label each position line and put an arrowhead on each end of the lines, so that you don’t get your PLs and your Zn lines mixed up.
If not sandwich fixing e.g. ABC, you can still start with one assumed position at the mid-time of one of the stars, A say. Work as before, but this time you have to adjust the position of B and C position lines for the time difference. If stationary, you just allow for Motion of the Body (MOB) using Table 2 in AP3270/HO249. If moving you also have to allow for Motion of the Observer (MOO) using Table 1. You can correct in any of three different ways. You can move the position line or adjust the intercepts as part the instructions under Tables 1 and 2, or you can move your assumed position before plotting the PL for that star.
Some air navigators preferred to move the assumed position, because they had to do that anyway to allow for Coriolis acceleration. The Brits had a cricketing initialism for this. We said LBW, not leg before wicket but late back west. I.e. if the time of your observation was four minutes after your pre-computation time you would move your assumed position four minutes of groundspeed back along track and one degree of longitude west. To get the intercept you use the difference between Ho actually measured and the pre-computed Hc. The time you write against the PL is the pre-computation time.
I used four minutes in the paragraph above for simplicity, but you can use whatever time interval you end up with e.g. two minutes 30 seconds late. The sub-tables under Tables 1 & 2 inside the cover of AP3270/HO249 allow you to do this. Alternatively, if moving the assumed position, move it 2.5 minutes of groundspeed back along track and 60x2.5/4.0 minutes of longitude west.
However, if you’re only interested in marine navigation and all the above is confusing, ignore it. Use a separate assumed position for each observation. DaveP