Depends on what accuracy you want to get from hydrophone only methods.
Since hydrophone induces a lot of bearing inaccuracies, you may want to adjust your listen intervals, and/or maneuver your boat to increase target bearing rate in order to reduce the impact of bearing inaccuracies.
I've been doing error analysis on bearing only methods recently with my TMA calculator, which is capable of calculate probability for positional/course error distributions, assuming a random bearing inaccuracy within a range of -0.5 to 0.5 deg. (
http://www.subsim.com/radioroom/showthread.php?t=240508)
If you want a high level of accuracy for firing solutions, then you probably want the target positional error induced by bearing inaccuracies to be within 150 meters.
Consider a typical 4-bearing method scenario (constructed with a software called "Maneubo"): Let's say we have a medium-range medium-speed 6km/5kts target with an average bearing rate of 1 to 2 deg/min. And we choose a 10-minute time interval:
Code:
time t1 = 0 (sec)
ownship heading at t1 (deg): 0
target bearing at t1 (deg): 322.48
ownship straight direction from last position = 0 (deg)
ownship straight distance from last position = 0 (meter)
*******************************
time t2 (sec): 600
ownship heading at t2: 0
target bearing at t2 (deg): 338.23
ownship straight direction from last position (deg): 0
ownship straight distance from last position (meter): 0
*******************************
time t3 (sec): 1200
ownship heading at t3: 0
target bearing at t3 (deg): 355.6
ownship straight direction from last position (deg): 0
ownship straight distance from last position (meter): 0
target course: 78.12deg
probability of target course error within 5deg: 72.2727 %
probability of target course error within 10deg: 98.9091 %
probability of target course error within 20deg: 100 %
3 bearings when stationary can only get course solution.
*******************************
time t4 (sec): 1800
ownship heading at t4: 0
target bearing at t4 (deg): 32
ownship straight direction from last position (deg): 270
ownship straight distance from last position (meter): 2330
target course: 78.12deg, speed: 5.17377knots, distance: 6583.09
probability of target course error within 5deg: 70.1653 %
probability of target course error within 10deg: 98.9256 %
probability of target course error within 20deg: 100 %
probability of target positional error within 75m: 13.4711 %
probability of target positional error within 150m: 29.1736 %
probability of target positional error within 300m: 56.4463 %
Looks like we got a roughly 60% confidence of target positional error to be within 300m, when we consider a -/+ 0.5deg bearing inaccuracies. Looks marginally acceptable if we want a firing solution, but further increasing time intervals will improve the score.
If you only want a rough estimate of course and speed, you can decrease the time interval to 5 or 7 min maybe.
In this case when I'm about to take the 4th bearing, I moved to the opposite direction of
the target course because this will maximise bearing rate. You may want to avoid the following situation since the bearing rate is low and the solution is very error prone to bearing inaccuracies:
