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    Re: Experience of using a hand-held bubble sextant in a light aircraft
    From: Brad Morris
    Date: 2019 Sep 28, 13:41 -0400
    Geoffrey, you wrote

    No tigers though and when I complained to the guide, he paused before saying with a twinkle in his eye that if I came again next year, he would do his best to find some tigers for us…. So I may yet have a chance to try some more Cel Nav fixes from a small plane.

    Perhaps your guide was just having some spirited fun.  Tigers are NOT native to Africa.   But he will spend as much time as you want looking for them in Namibia, at a discount too!!


    On Sat, Sep 28, 2019, 1:17 PM Geoffrey Kolbe <NoReply_GeoffreyKolbe@fer3.com> wrote:

    We just returned from a holiday in the Namibian desert, looking at wildlife, and on 'Skeleton Coast' looking at more wildlife but also the rotting hulks of wrecked ships that foundered on this treacherous coastline. The way to get around Namibia in a reasonable timeframe is by light plane and having flown into Windhoek airport, we transferred to the old Eros airport to the South of the city and proceeded from there in a Cessna 220 aircraft.

    I had with me my A12 bubble sextant, which is an aircraft sextant designed perhaps 80 years ago for use in open cockpit aircraft. I have found this to be the most versatile, easy-to-use sextant for celestial navigation on land and have used one in the Sahara desert to keep track of our travellings on various expeditions there. But this was the first time I had the opportunity to use this sextant in the environment for which it was designed - a small aircraft - and I was looking forward to see how I managed. 

    On our first trip out from Eros airport at Windhoek to Mowani, I looked around for the sun and noticed that as I was sitting in the middle seats of the little Cessna, the wing was above me and so I was in the shade. Moving to the back seat in that cramped environment was not practical, so I waited until the next trip, when I would make sure I sat in the back seat and would have a good view of the sun out of the back window. The ‘airport’ at Mowani is basically an earth landing strip, graded flat, with hut on one side serving as the control tower. This was typical of the small landing strips in Namibia.

    A few days later we took off at about nine o’clock in the morning, on a course of about 340 magnetic headed for Orutjundja airstrip, and I managed a sight of the sun out of the side window, which was in the North-East. I was pleasantly surprised to get a sighting within 7 minutes of my estimated position. The Cessna’s ground speed was about 130 knots, or about 2.2 nm per minute.

    A few days later we took off in the morning for a short flight to Mowe Bay, on the coast, on a course of 240 degrees magnetic, and I took a sight out of the back window with the sun behind us. Given I knew the position of the start point, and knew the course, this sight should have been a good estimate of position. However, the sight put us about 42nm away from my estimated position, which was not credible on a flight of only about 63 nautical miles.

    Next day we headed back to Windhoek from Mowe Bay, with the pilot in a hurry to get away so that he could land in daylight. With a take-off time of 15:35 local time, it was going to be a sunset landing as we had to stop at FYDN airstrip to refuel on the way! The course was 139 degrees magnetic and so once again, the sun was behind us and I managed three sights on the way down. 

    The method was to choose a waypoint some point into the flight time. I could calculate the distance travelled from our speed and so choose a point where we would be at a given time. I would spend a minute or so running up to the mark-time trying to average the position of the sun in the bubble to account for the motion of the aircraft. The first fix some 15 minutes into the flight was about 3 minutes off my estimated position and I was very pleased with that! Even well braced on the solid ground and averaging over 5 or 6 shots, I am usually about 2 minutes off my known position. 

    The second sight another half hour later was only about 4 minutes off my estimated position, so I was feeling pretty confident that I had got this cracked. With the sun sinking low in the West, I tried one more sighting before we descended on our approach to Windhoek. There was some turbulence as I was taking the sight and I had difficulty getting an average altitude by the mark-time for the waypoint. I figured it would not be a great sight, but I was chagrined to work the sight and find I was 58 minutes off - pretty much a whole degree - from my estimated position. 

    So, somewhat mixed results, but it was interesting to find that in calm conditions it is possible to get quite good Cel Nav fixes of a small aircraft’s position using what is a very basic bubble sextant. There is an averaging feature on the A12, which consists of a trigger operated by the thumb, which marks the index arm drum with a small pencil mark. Thus a series of sights can be taken over an interval and ‘averaged’ by rotating the drum to the middle of the band of pencil marks and then taking the reading off the arc. I did not attempt to try this on this trip, but if I ever get the chance to fly in a small aircraft again, I will definitely give it a go.

    The rest of the trip was very interesting as we saw lots of wildlife - ostrich, springbok, orix, baboons, jackals, elephants and even a pair lazy lions who had just eaten. No tigers though and when I complained to the guide, he paused before saying with a twinkle in his eye that if I came again next year, he would do his best to find some tigers for us…. So I may yet have a chance to try some more Cel Nav fixes from a small plane.

    Geoffrey Kolbe

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