NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sights perpendicular to course
From: Gary LaPook
Date: 2014 Nov 11, 19:02 -0800
From: David Pike <NoReply_DavidPike@fer3.com>
To: garylapook@pacbell.net
Sent: Friday, November 7, 2014 9:31 AM
Subject: [NavList] Re: Sights perpendicular to course
From: Gary LaPook
Date: 2014 Nov 11, 19:02 -0800
I've never heard of "sandwich fixes" before, how are they done?
Since you do flight navigation you might find my website interesting:
gl
From: David Pike <NoReply_DavidPike@fer3.com>
To: garylapook@pacbell.net
Sent: Friday, November 7, 2014 9:31 AM
Subject: [NavList] Re: Sights perpendicular to course
In the Vulcan we called them ‘fore and aft’ and ‘beam’ shots. At 480kts,
acceleration errors were a problem. One aspect was that for a fore and aft shot, the pilot had to concentrate most upon keeping a constant speed, whereas for beam shots the pilot had to concentrate most on keeping a steady heading. We also had an extra slide for our Dalton computers called the Mears slide, which allowed the navigator to correct the intercepts for acceleration errors.
In
the Vulcan, shooting astro was an exercise in crew cooperation. The aircraft was fortunate in needing two sextant mountings, because the mountings had to be either side of the fuselage. (The life raft was behind the pilots in their cockpit blister.) This meant you could draw an extra sextant before flight and have one in each mounting. This allowed a reduced time between one minute shots. You could set up on both stars and simply move across the fuselage from one sextant to the other. In general, we used five shot or seven shot sandwich fixes using two stars. The navigator-radar would pre-calculate the shots (over the Atlantic there wasn’t much else for him to do apart from munching rations and serving coffee), raise the sextants, and get the stars in the eyepieces. The flying-pilot would fly the aircraft as accurately as possible. The non-flying-pilot would note any changes in airspeed. The navigator-plotter would monitor his very accurate heading read-out at the start and end of every shot and write down any changes; write down height sextants as they were called out by the nav-radar plus the speed changes from the non-flying pilot; calculate the averages, use his Mears slide to correct the intercepts and put lines on the chart.
The air electronics officer would call out the timings for the shots. For a heading check, he would also call out mid-time. The nav-radar would take the shot, call sextant working, call shot complete, and call the height sextant to the plotter. After the shots, the nav-radar would lower the sextants; retrieve the record from the plotter and check he hadn’t made any mistakes with the averaging. Then the two navigators would put their heads together and decide how far to tweak the system towards the fix. The nav-plotter had a feeling for how the
system was working, and the nav-radar had a feeling for how the astro had gone The one problem with sandwich fixes was that, although you didn’t have to bother with MOO and MOB, the fix didn’t appear on the chart until well after fix time, but as we were just using these to monitor the system that wasn’t particularly important. Steady For Astro. Dave
