NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Alternative to running fix
From: Paul Hirose
Date: 2000 Aug 05, 5:31 PM
From: Paul Hirose
Date: 2000 Aug 05, 5:31 PM
Instead of advancing a line of position by several hours for a running fix, why not use it to improve your estimated position immediately? Bowditch shows how in the chapter on celestial LOPs. First plot the LOP and your estimated (or DR) position at the time of the LOP. Then your new EP is the point on the LOP that's closest to the old EP. Plotting is easier this way because only the EP needs to be advanced to the time of the next LOP. As long as you have a good mix of LOP angles, I bet you could navigate forever, never take a fix, yet keep your DR real close. Just take a few sun sights each day and update position after each one. I don't see anything but advantages compared to a running fix. Nobody else has suggested this, though, so I guess there's a flaw in my reasoning somewhere. I do think a computerized nav system (something way more costly than a Celesticomp) would immediately integrate a LOP into the present position solution, and not save it for a running fix later. But I have to admit I've never worked on a system that dealt with discrete LOPs. In my experience, the sensor, such as doppler, puts out continuous data. A closer analogy would be ground mapping radar, which does yield discrete data points, but they are fixes, not LOPs. One airplane I used to maintain had buttons marked QUAL 1 and QUAL 2 so the navigator could tell the computer how much confidence he had in his radar crosshair placement. Interestingly, even when you hit QUAL 1 the system wouldn't simply slave its present position to the crosshair. It would weigh its own dead reckoning ability against your likely accuracy, and accept some of your input. If the correction was unreasonably large, the computer figured you had the crosshairs on the wrong point, and would reject the update. There was a button to override that, but even then the computer got the last word. It would update its position but not its velocities. That way a botched fix would introduce a fixed error but one that wouldn't grow with time.