Celestial + Piloting

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Dead Reckoning

Updating DR position

It is often useful to determine a DR position from log book data without actually plotting out the track on a chart or plotting sheet. This functionality is provided in StarPilot by the Update DR function. Considerable effort has been taken to make this a convenient and versatile function since it is so crucial to the day's work in ocean navigation.

The operational behavior of Update DR as well as how this interacts with the Celestial Fix routines is controlled by two settings: DR Mode and Course/Speed .

When DR Mode = OFF, the Update DR function will ask for your course and then the distance run in nautical miles. Then it computes your new DR position. If you did not have a DR position stored in the Settings, then it will first ask for the starting point. At the end of the computation it will ask if you wish to update the stored DR position to this new value.

Example, we are at 47 39' N, 126 50' W. Our log reads 100.0. We wish to find our new position if we sail 123.7 miles on course 245 T, We do this first in the Off mode, then Log and Speed modes.

From the Settings dialog select the DR Parameters tab and type in 47.39 and -126.50 for the DR Lat and Lon, respectively.

DR mode = Off

Now set DR mode = Off from DR parameters tab in the Settings dialog. Then select Update DR from the DR-and-Piloting menu and then input the course and distance run.

Pushing the [Update DR] button will store this new position into the DR, pressing [Done]will just quit with out updating the DR stored in settings. Select [Done] since we want to do other examples.

The output shows the new DR position as well as a summary of the leg we just computed. This is a double check that we entered the right values, but this latter info would be of more interest in the speed mode. Note that these values reported back may differ by a few tenths or so, since this is not just a repeat of what we entered, but an actual independent second computation of the Rhumbline route from departure point to answer point and there may be some rounding off errors.

DR mode = Log

Now set DR mode = Log from the DR Parameters tab in Settings. StarPilot will then prompt for Log (enter 100) and Course (enter 245). In this mode, to figure our position 123.7 miles along from here, we have to realize that our log will then read 223.7 - or if we were not using real log entries, we could just enter 0 here for our current log reading, and then enter 123.7 for the computation, which is like doing it in Off mode.

Now execute Update DR from the learn that all you need to enter is the log reading of interest, i.e. 223.7, and [Compute] to get the new DR position.

You will get the same screen as in the Off mode. If you [Update DR] then this position will replace the one in storage and also the log in storage will go from 100.0 to 223.7. To carry on with these examples, choose [Update DR] followed by [Done].

DR mode = Speed

First do View Settings to confirm what we have so far. Do this from the Edit menu under View Settings. You should have the new DR position stored along with a log reading of 223.7 and a course of 245.

To start doing DR by speed and time, change DR mode to Speed, enter DR time = 12.04, Course = 245, Speed = 8.5.

Now let us ask where we will be at DR time = 1445. Do Update DR, and enter 14.45, and you should get:

Again, the bottom part of the display is the RL (rhumb line distance) from departure to destination. It should be very nearly the same as the timed run of the input, i.e. 14.45 - 12.04 = 2h 41m at 8.5 kts = 22.8 miles.

Updating DR position will now store this new location along with the new DR time of 14.45. Do this and then view settings to check what you have.

Here is a logbook Sample A, to be computed.

Before going on to the next example note the Auto Update DR /w [Compute] check box in the Update DR dialog. Then this feature is selected the DR is automatically updated when the [Compute] button is hit. Auto Update eliminates having to push the [Update DR] button when executing a sequence of log book entries. Use this feature in the following examples to reduce the number of events required to complete the computation.

Log Course Position

102 245 47.39, -126.50

150 260

178 270

190 170

255 170 find position here

Using Update DR in Log mode, you should get 46.098, -128.359

Likewise, this one, Sample B (unrelated to A)....

WT Course Speed Position

1204 245 8.5 47.39, -126.50

1512 260 7.0

1806 270 7.5

1900 170 6.0

1950 170 6.0 find position here

Using Update DR in Speed mode, you should get 47.193, -128.038

Note on DR over midnight

The Update DR function in Speed mode does not read the date stored in the calculator (date is for cel nav functions only). When you must DR across midnight in the Speed and time mode, take one leg up to 24.00, then start another at 00.00 by setting DR Mode again. In the Log mode this is not an issue. Note that you can do running fixes that span midnight in any time zone, since the date is stored with all sight reductions.

DR Mode and Review-Sights Display

When DR Mode = Speed, the Review Sights function uses the course and speed to advance the sights before displaying them. The common time used is the time of the first sight (#1). Hence for most logical presentation of the data, it is best to have the stored DR position consistent with the DR track in effect at the time of the sights. To do that, just set the DR mode to Speed (set it again, even if is set there now) and at the input enter any valid time and point on the DR track. If you care to, you could then do Update DR to the time of the first sight to double check with your printed records that all is correct, although if your input was correct this would not be needed. The program will automatically update the DR before presenting each sight.

Normally this is not a concern, since when the fix is computed it will be done right no matter where the DR is - it might just take a few iterations to converge. But things will work more smoothly and make more sense if you use for a DR position the one that corresponds to the sight time you requested. Needless to say you need to do it this way if you wish to test your GPS with celestial or vice versa.