On this page and the page detailing the sextant I have tried to give some idea of the amount of work that went into obtaining astro fixes in the Vulcan.  And all the time the aircraft is travelling 10 miles a minute with no opportunity to ‘back up and try again’.

Procedure for a 2 star fix Sources of error in an astro shot

I would welcome any input from old Plotter’s on this section.

Because the Vulcan was designed decades before PCs and some of the newer navigation systems, such as DECCA and LORAN, and a generation or two before GPS came on the scene, star shots or Astro was an integral part of Vulcan operations.  On a good day a fix could be obtained which would be in error by only a couple of miles, and I'm sure some nav teams could do better than that.  While a 'couple of miles' do not sound like precision navigation, it has to be kept in context. Astro was used only at high level, there was a training requirement to do a certain number of Astro legs in each 6 month period, and it was standard operating procedure to use it on long water transits where there were no ground stations for Tacan etc, and out of radar range of ground fixes.  Astro was used for star fixes but also during the day when sun shots were used.  Just as in the old sailing days a sun shot taken at 'local noon' would give a latitude line so fixing your north/south position.  Another very useful output from a sun shot was a heading check, using the azimuth ring around the sextant.

The biggest drawback with Astro was the time taken to obtain a fix, it was very time consuming; here's a quick breakdown of what went on. The planning for the 'shots' would normally be done at the flight planning stage, prior to take off. Using star charts and almanacs (no computers then) the Nav Plotter would decide from the profile of the route, where he needed the shots to take place. Based on his expected position on the route, and his heading, he would select stars for the shot, normally three would be selected, failing that, two.  The choice of star would be based on; brightness of the star, would it be expected to be in a good position for it to be seen through the sextant (not on the nose or tail of the aircraft) would the position lines obtained from the shots give a good 'cut'. Ideally if there were three stars then you would want the position lines to result in a 'cocked hat', a triangle with the lines cutting at 60 degrees, if there were two stars then ideally the two lines would cut at right angles. When the Plotter had selected the stars to be used for the astro fixes on his route he would then use the star almanacs to calculate the expected 'height' of each star, which determined the angle above the horizon that would be set on the sextant, he would also calculate the azimuth of the star; effectively the angle between the nose of the aircraft and the star. With the sextant set with the height and azimuth then ideally when the sextant was extended through the pressure hull, it should be pointing at the star.  Because a fix would involve stars on both sides of the aircraft the Radar would need to set up a sextant on each side and move between sextants to cover the different stars.

Procedure for a fix.

A fix using 2 stars requires 4 shots on one star and 3 shots on the other, and each shot takes 1 minute with a minute in-between to reset the sextant, so giving 13 minutes from the start of the first shot to the end of the 7th shot.  It would then take the Plotter time to use the info from the individual shots (actual height and azimuth), convert these into two position lines and plot them on the chart, so a fix would take some 15 -20 minutes of rather hard work by the 2 Navs to get the one fix. The two star fix is the best with the Vulcan as the sextants have a more limited view than on the Victor, because of the position of the pilot canopy.  In the photo below the ‘blister’ through which the periscopic sextant is extended can be seen just below the pilot canopy, this limits the view of each sextant to about 170⁰ rather than a full fore and aft 180⁰.

Photo J Dillon

The following table shows the shooting sequence for a two star Sandwich Fix, the fix time is 20:00 hrs.  In the table you can see the mid time of each shot on each star, that means that each ‘shot’ would start 30 seconds before each mid-time.  You can see that there needs to be a degree of organization within the crew to ensure that each shot happens on time, and there will be a move between sextants, star A on one sextant, star B on the other. Between each shot the sextant  wound need to be wound up for the next shot, and with the various readings of star height and aircraft heading and speed changes all being passed to the Plotter in the interval between one shot and the next.

Fix time 20.00             Stars A & B                             Mid times 19.54 19.56 19.58 20.00 20.02 20.04 20.06 Star A A B B B A A

Sources of error.

There are a number of possible sources of error when the Radar 'shoots' his astro shots. An obvious one is that he may have selected the wrong star!  The Radar is reliant on the Plotter making the correct pre-shot calculations to set up the sextant, so that the desired star will fall somewhere close to the centre of his field of view through the sextant.  Using his knowledge of what star pattern he would expect to see the Radar should then be able to select the correct star for the shot.  I have to say that I never did find this easy and believe that on occasion I have selected the wrong one. To be fair to both the Plotter and the Radar, the computations for setting up the sextant are made on the assumption that the Plotter knows pretty well where the aircraft will be at the mid point of the star fix.  If wind conditions are very different from those used by the Plotter to decide where he will be at mid-shot, then it follows that this DR position will be incorrect, and so the computations based on this position and set on the sextant will not be correct.  This may well cause the star to be some way off the centre of field of view (or not in it at all!), so making the job difficult for the Radar when he decides which star to shoot. But let us assume the wind is close to that calculated by the Plotter, the DR position is close to the actual position and the Radar is pretty good at recognizing the correct star. There are still other sources of error that can affect the accuracy of the resultant star fix.

Sources of 'system' error are; heading error, acceleration error, sextant errors, and sextant operation errors.  Most of these cause a change of the position of the bubble in the sextant, so causing the sextant operator to incorrectly centre the star in the bubble. The diagram below shows the changes in true and false horizons with speed and heading changes, so resulting in false altitudes measured for the star.

Diagram from my OCU notes.

Heading error.

Any change of heading occurring during a one minute shot will cause the bubble to be displaced in the athwart ships plane. The resultant error will have a maximum effect on shots taken on the beam and nil effect on fore and aft shots. One of the items of data passed to the Plotter at the end of each shot is the heading at the end of the shot. The Plotter can then see if there has been any change from the heading they were on at the start of the shot.  To give an example of the effect; if the heading error was 10 to the right, and the groundspeed was 500 kts then the position line from that shot would need to be moved at right angles to track, 26 nautical miles to the left. It is not insignificant.

Airspeed error.

As with heading, the Plotter needs to know the speed going into a shot, and the speed at the end, he can then see the difference.  For a 1 minute shot the change in Indicated Air Speed (IAS) is multiplied by 6 and the position line for that shot is moved along track by that number of nautical miles for a decrease in speed, and back along track for an increase in speed. So a 1 knot change in IAS would mean moving the position line 6 nautical miles.  The diagram below shows the effect on the bubble of speed changes.

Diagram from my OCU notes.

There are some other errors caused by the shape of the earth and its rotation. Most people are probably aware from looking at the route charts in airline magazines that aircraft tend to fly a curved path between points A & B.  The shortest route between two points on the earth’s surface is a Great Circle. I will quote from my OCU notes for the effect of Coriolis error and Rhumb Line error.

Coriolis.

Since the earth is rotating, and aircraft flying in a straight line path, ie a Great Circle Path, over the earth’s surface is actually following a curved path relative to space. This produces an angular acceleration effect which displaces the bubble/pendulous reference system, the magnitude of displacement varying with the groundspeed of the aircraft and latitude and being a displacement to the left in the Northern Hemisphere.  hence we apply the Coriolis effect to starboard of track in the Northern Hemisphere. Since the acceleration effect is in the athwart ships sense its practical effect will be a maximum on beam sights and nil on fore and aft sights.  The value of the correction is shown in a table on the back cover of the Air Almanac. [This was a reference manual carried by the Plotter]

Rhumb Line error.

As the aircraft is normally steered with reference to a magnetically monitored heading ie flying a Rhumb Line track it is flying along a curved path in relation to the earth’s surface. In the Northern Hemisphere the curved path has the effect of displacing the bubble/pendulous system in the direction of the North pole.  When combined with the Coriolis effect on a track of, for example, 090⁰ (True) the overall effect will be to displace the reference system by the sum total of both the errors; whilst on a track of, for example, 270⁰ the Rhumb Line effect will now be in the opposite sense to the Coriolis correction and the overall effect will be the difference between the two quantities. Rhumb Line effect depends on the track of the aircraft, its groundspeed and its latitude and because of its acceleration effect is in the athwart ships sense, as is Coriolis, the two can be combined in a graphical manner allowing the effects to be summed algebraically for any particular Rhumb Line track.

When you see the number of corrections to be made to each shot, and the amount of work that goes into obtaining astro position lines, you can see that the astro phase of a flight becomes quite hard work, and would not normally consist of more than three astro fixes.