# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Advance LOP of High Speed Vessel
From: Bruce J. Pennino
Date: 2012 Nov 8, 21:12 -0500
﻿
Hello:

I've followed the Trans-Atlantic postings closely. Thank you Kevin and all those working on the CN/ sight data. I realize I don't understand the most correct technique to advance the first LOP on a Universal Plotting Sheet based on DR.

For example, for a vessel at 17 knots with the first LOP @ about 49 Deg N, and 3 hours between sights, the first LOP must be advanced in parallel fashion  51 NM . If vessel is going true N or S, LOP moves  N or S   51 minutes of arc.

If going true E or W, we know 51 NM is about 77 minutes of arc (at 49 N there is about 39.5 NM/degree of longitude). For DR we plot the new DR @ 77 mintues E or W and advance first LOP.

For all other courses, change x and y scales to NM and use vectors? Linear or trig approximation?   I just don't seem to find the answer exactly in any of  my three CN books.  The  proper technique is probably there, but I'm not seeing it!

Bruce

----- Original Message -----
Sent: Thursday, November 08, 2012 10:08 AM
Subject: [NavList] Re: TRANS ATLANTIC

REF : [NavList] TRANS ATLANTIC
From: Kevin Coulombe
Date: 25 Oct 2012 14:00
STAR FIX

*******
From Kermit : own results hereunder, assuming that I have correctly understood information given by you Kevin (see my "decoded" values between ### ... ### 's). I am also assuming that TT-UT = 68.0 seconds of time. As for your 22 Oct 2012 former example, Intercepts and Azimuths are given from each DR position individually derived/computed from "Reference DR Elements".
*******
Date : 25.10.2012 Assumed Position : L 42-00 N Lo 034-10 W Course 097 T , SOA 20.8 kt ### Note : I am assuming that this position the one for 17:56:20 LT (or 19:56:20 UT), which is the hour of the ALPHERATZ Observation. All data here-above constitute "Reference DR Elements".###
HEIGHT OF EYE 30 METERS , BAROMETER 1010, AIR TEMP 18.5C ZD: +2 , WATCH ERROR: 5 s fast , INDEX ERROR 0.8 OFF THE ARC

BODY: ALPHERATZ
WATCH TIME: 175625 LT ### UT = 19:56:20.0 ###
SEXTANT ALTITUDE 38-37.3 ### H = 38°38'1 (residual error not taken in account)
### i ALPHERATZ = -5.8 (A) NM , Z ALPHERATZ = 083°0

BODY: ALTAIR
WATCH TIME: 175838 LT ### UT = 19:58:33.0s ###
SEXTANT ALTITUDE 57-58.4 ### H = 57°59'2 (residual error not taken in account)
### i ALTAIR = + 57.9 (T) NM , Z ALTAIR = 184°0

BODY: KOCHAB
WATCH TIME: 180159 ### UT = 20:01:54.0s ###
SEXTANT ALTITUDE 43-22.9 ### H = 43°23'7 (residual error not taken in account) ### i KOCHAB = +0.2 (T) NM, Z KOCHAB = 338°4

BODY: MOON
WATCH TIME: 175430 ### UT = 19:54:25.0s ###
SEXTANT ALTITUDE 29-21.5 UPPER LIMB ### H = UL 29°22'3 (residual error not taken in account) ### i MOON = +1.5 (T) NM, Z MOON = 121.6°

First computation run Fix observed in Azimuth 205° and at a distance of 35.0 NM from DR Position with LOP's Dispersion of 18.6 NM. (not good)

Hence a first computation run "updated" observed position at N 41°28'4 W 034°30'0 for LT = 17:56:20

From this "updated" position, and time get :

ALPHERATZ i = +13.0 (T) NM , Z = 082°4
ALTAIR i = +25.3 (T) NM , Z = 183°5
KOCHAB i = +24.1 (T) NM , Z = 338°6
MOON UL i = - 2.3 (A) NM , Z = 121°1

Second shot Fix observed in Azimuth 070° and at a distance of 0.4 NM from first computation run "updated" observed position, but still with a LOP's Dispersion of 18.7 NM, which is not good.

After a closer look at your data, Kevin, the first observed intercept for ALTAIR is "strange" (57.9 NM) since it is so close from 60 NM. Accordingly it should reasonably be attributable to a classical/typical Typo blunder. Let us then entirely rework your example with ALTAIR sextant height set to 56°59'2 (vs 57°59'2) and see what happens.

From you initial DR position get :
ALPHERATZ i = -5.8 (A) NM , Z = 083°0
ALTAIR i = -2.1 (A) NM , Z = 184°0
KOCHAB i = +0.2 (T) NM , Z = 338°4
MOON UL i = +1.5 (T) NM , Z = 121°6

Observed position is in Azimuth 263° and at distance of 2.5 NM from DR Position, with a LOP's dispersion of 2.7 NM, which is fair.

Hence at 17:56:20 LT, DR position is N 42°00'0 W034°10'0 , and
Observed Position is N 41°59'7 W034°13'4
Final notes :
* (Most) Probably actual ALTAIR Sextant height was 56°59'2 (Kevin, please be so kind as to confirm that it makes sense to you). And
* Any GPS position on file corresponding to your DR ? If so, it will be interesting to rate the quality of your observations. And
*I have most often observed that it is difficult to get good fixes so high from the sea level (from Aircraft Carriers in my case), since - as you must have noticed - the horizon will often be "hazy" and not sharply defined from so high above the water, while it will often be "reasonably good" seen from a lower height.

Best Regards to all

Kermit
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