Manual targeting - The Ekelund Range

This is an interesting read. http://www.news.navy.mil/navydata/cn...5/ekelund.html

I got it from searching the words Ekelund and Range in google. The whole idea about doing that was from reading through old ancient posts on the Pacific Thunder Yahoo group http://games.groups.yahoo.com/group/PacificThunder

I came upon a post of Good old Don Reed - (heh you were back there then as well DonOne) citing an old submariner from the SlideRule yahoo group, as the old Sub'ber gave a description of how to use the Bearing Rate Computer (Kim Rønhof's Course Calculator Mark 4B)

The whole thing is about how to get solid range data in a passive way at large range. Quite imperative for calculating AoB fx.

Anyway - Go have a read - i have quoted the whole old post from DonOne here - Hayle to Don for dragging that stuff out of Mr. Dirickson

Manual of the Bearing Rate computer

Kim’s Course Calculator Mark 4B

The BRC is used for two computations:

• Calculating the target's speed across the line of sight (xDMht) as a function of target speed (DMt) and angle on the bow (Cqt??)

• Calculating target range (Rh) as a function of true bearing rate (DBy) and speed across the line of sight with the usual variations on what is known and what is not.

The sine and speed scales are used for the first operation:

xDMht = DMt sin(Cqt).

Determining the own-ship component is done the same way: xDMho = DMo * sin(Br). Both values have a magnitude (the calculated number) and a direction ("left" or "right"). Mechanically, you point the "arrow" that goes through 90 on the sine scale at the speed, move the index to the target's angle on the bow (for xDMht) or relative bearing (for xDMho), and read the xDMh(whatever) speed component.

Range calculations come from the basic relationship: at one mile (nautical mile, 2000 yards), 1 knot across the LOS (2000 yards/60 minutes) = 1 degree/minute of bearing rate

(sin(1 degree) = 0.01745; 1/60 = 0.01667; 4.5% is close enough for government work)

That "1 knot across LOS" is the *net* speed, i.e., you have to adjust for the own-ship contribution to the speed.

There are several ways to apply this basic equation:
1) If xDMht and DBy are known, you can calculate the range of the target
2) If the target is stationary (a landmark, anchored ship, etc.), you can calculate the range (since xDMho is always known)
3) If Rh and DBy are known, you can calculate xDMht; with the sine relationship and an estimated angle on the bow, you can calculate the target's speed



The range equation is also used in its differential form as the basis of the "EkelundRange" technique:
1) Determine DBy on one "leg" of the analysis
2) Change course across the LOS and determine the DBy on the second "leg"
3) Since the target's contribution to the bearing rate is (nominally) the same on both "legs", the change in bearing rate is simply a function of the change in own-ship speed across the LOS (which is known) and the range; this gives you a completely passive way to estimate range when you don't have reliable target-speed information.

The reason you find these BRCs with loops of string through the center hole is because, during fire-control, approach, entrance/egress operations, and pretty much any other "where is it?" situation, you find BRCs hanging around the neck of the CO, the OOD, the FCC, the T-B analyzer, the Navigator, and various other members of the Fire Control or Maneuvering party.


I've found this on groups/yahoo.com/group/sliderule/

A kind gentleman by the name of Steve Dirickson provided the information.
Don Reed