Plotting High Altitude Sights

Using High Altitude Sights to determine position:

The idea behind a high altitude sight is much the same as lower altitude sights, except that rather than using a straight line to represent a segment of the Circle of Equal Altitude, we'll be drawing a curved line, or even possibly an entire circle. Generally these observations need to be above about 87°, otherwise they would become very cumbersome to draw onto standard plotting sheets.

The basics steps are as follows:

1. If you are using 2 (or more) separate bodies, all above 87°, great, but ensure that they are still roughly 60° apart, or your results will become less accurate. Where this type of problem comes into play more often is when taking multiple observations of a body around the time of meridian passage.

2. Run through the proper steps on a sight reduction form to transform your Hs into Ho.

3. Figure out how far away from the GP of the body you were at the time of observation. To do this, convert the arc between your zenith (90° and Ho to a distance, ie. 90° - Ho x 60 = distance (in nm) you are away from the GP of the body.

4. Find the GP of the body/bodies at each time of observation.

5. Set up your plotting sheet, ensuring that the GP of each body and your DR position will all fit nicely on the page.

6. Plot the GPs of each body, making sure to denote each with a unique designator so that you can tell which position corresponds to which body/time of observation.

7. Just like when plotting lower altitude LOPs, you're going to need to advance/retard your observations to a common time. Just like before when you advanced/retarded your AP along your track line, do the same with the GP of the body to find GP' (the advanced GP of the body).

8. Now take a spread on your compass equivalent to the distance you found in step 3 above.

9. Swing this arc from the GP' of the body in the direction of your DR position, and make your arc large enough to ensure that it will intersect the other arcs you will be swinging.

10. Repeat steps 8 and 9 for each observation.

11. If you made 3 observations, you should wind up with 3 arcs, with 4 points of intersection. Three of these intersections will be of only 2 arcs, while the last intersection should include all 3 arcs (or you should at least end up with a small grouping of intersections of the three arcs). Your observed position is where all 3 arcs intersect.

If you only made 2 observations, you'll see that when you swing your arcs, you end up with 2 points of intersection. Your observed position could potentially be either of these points, but choose the one nearest your DR position as your observed position, or take another observation and retard/advance that arc to add accuracy to the original observations.