Celestial + Piloting

Sight Planning

Sight Analysis

Sight Reduction

Position Fixes

Cel Nav Utilities

Route Sailings

Current Sailings

Piloting Fixes

Dead Reckoning

Nav Utilities

Calculator Versions

Position Fixes

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 [5]. 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.

Fix by Plotting LOPs

Alternately, to use a graphic method to obtain a fix, use the Fix by Plotting option under the Celestial Fix menu. Fix by Plotting will graphically display the LOPS on the screen allowing the user to actually select the fix from the plot.

When first executed, the plotting function prompts the user for the vessel's speed in knots that was in effect during the sights. Entering 0 for the speed will cause StarPilot to plot the LOPs without advancing them to a common time. Entering a nonzero value for the speed causes StarPilot to prompt for the course (in True or Magnetic) and a WT  in exactly the same manner as the computational fix methods describe above. The program then advances all LOPs to the given time and plots them on the screen. In either case the DR position is plotted in the center of the screen with a circle around it.

The first display shows the DR position (a circle) about 23 miles north of the intersections as we learned from the numerical fix. The automatic scale of the initial plot is about 3 times the distance from the DR position to the farthest LOP intersection. The Lat and Lon of the cursor position are displayed at the bottom right of the screen. Next we use the mouse to move the cursor into the "cocked hat" of intersections, as shown below and click the left mouse button.

The precise location of the fix is displayed in the dialog box.

Pushing [Update DR] will simply take the position you selected and store it in the DR position. [+] or [-] simply zoom into/out of the current plot.

Here we have moved the cursor to the right to coincide more with the choice made by the USNO computation. This latter is more the true centroid of the pattern, i.e. the point that is about 1.0' inside of each of the LOPs. Note the DR to cursor distance is 0.87 nm in direction 057. You can more directly compare USNO to the Plot by updating DR to the USNO fix when you do it, then do the plot, expanding as needed.