Once you have a set of sights stored in the sight array, you can obtain a fix two ways, either by direct computation or by plotting the LOPs and selecting the fix graphically yourself, discussed later under Fix by Plotting LOPs .
The Fix by Computation option uses the standard US Naval Observatory (USNO) algorithm found in the Nautical Almanac. This is a set of formulas and procedures that take the raw data and compute the corresponding fix. The lines of position are advanced to a common time and a least squares fitting method is applied to compute the fix.
When you select either from within the Celestial Fix menu, StarPilot will prompt you for your current speed in knots, course track (magnetic if the variation is set, otherwise true), and time (WT). The WT of the fix you desire could be a time of one of the sights, or any other time.
All sights will be advanced (or retarded) to the fix time you entered. Entering a value of 0 for the speed causes the application to suppress the course and time prompts bypassing the advance of the LOPs to a common time. Note that the DR position used in the running fix computation need not be the same one used in the original sight reductions since all sights are recomputed for the fix computation.
Note too, that if you are not moving (S=0) then the time of the fix does not matter. It just assumes you are at the same place but took sights at various times.
When doing sight reductions by tables we are used to using Assumed Positions. With calculators, however, we do not have a separate Assumed Position but instead all sights are reduced form the stored or advanced DR position. When using the USNO method, if the DR Position is in error by more than 20 miles StarPilot will recompute your position after temporarily updating your DR position. Once the running fix computation has completed pushing the [Update DR] button will cause StarPilot to update the stored DR position with your newly calculated fix. The following screens depict a running fix computation. Do not update DR yet for purposes of this example.
The R(nm)/Brg(T) on the output screen are the range and bearing from the DR position to the computed fix. This data should be recorded at this stage since it is very valuable in evaluating your overall navigation and it will be replaced in the display in the next step if the R is bigger than 20 miles.
In summary, the 3 star sights gave a fix of 31° 37.2' N, 15° 01.6' W and this fix was a distance of 22.8 miles off where we thought we were in the direction of 183.5 True.
The full effect of "Update DR position" depends on the DR Mode you are in, as explained further in section . When DR Mode = Speed, "yes" will update the stored DR position and the stored DR Time.
When in Log mode, StarPilot will update the position and turn the Log mode to Off, since we lose track of actual log readings during the sights. If you wish, you can switch back to Log mode at this point and reenter the appropriate log reading
Please review the Quick Start Examples and discussion for more details.