Posted on: 6 July 2012   By alex.yakubovskiy
obstacle avoidance events
Figure 1: Obstacle avoidance events define minimum sonar detection range.
tracking obstacle avoidance sonar targets
Figure 2: Two bridge pilings detected and tracked with FarSounder sonar. Each dot represents historic target location relative to vessel.
bridge with sonar, radar, and ECDIS
Figure 3: Sonar, radar, and ECDIS at the helm.

As more and more yachts choose expedition style itineraries, the risk of collision with poorly charted obstacles or wrongly placed obstacles (due to GPS malfunctions) increases significantly. Groundings with large underwater structures such as rocks, reefs, sandbanks and shoals are unfortunately significant risks for the adventurer. Having a navigation sonar installed which is capable of detecting such hazards at long range is important in these scenarios. More importantly, the detection range of the installed sonar should be suitable for the vessel. In this blog posting, we discuss how to calculate a suitable detection range for your ship's obstacle avoidance sonar.

Obstacle Avoidance as a Chain of Events

Regardless of the sensor (sonar or radar), the obstacle avoidance is a chain of events, in which the sensor performance, the officer on duty's actions and decisions, and the ship guidance are linked tightly. Figure 1 shows this chain of events in terms of time to collision and in terms of distance to the obstacle.

Distance to Alarm

Let's analyze these timing and distance components starting with Dalarm. It takes at least a few sequential detections of the same target to make a reliable (manual or automatic) decision of "that is the real target". With a single observation there is always a risk of false alarm.

Some radars have "true motion" mode to assist with decision making. If we take a sneak peek at FarSounder's next major software release, automatic target tracking will be one of our new major features. In fact, figure 2 is a screenshot from one of our software developers with the sonar automatically tracking two bridge pilings while the boat is approaching the bridge.

For the strong targets (obstacles), it usually takes about 10 sequential observations to make a reliable decision. The time interval between the observations is the sensor update rate. Mechanically scanned sonars (a.k.a. searchlight sonars) must rotate the antenna slowly, waiting for the echo at every look angle. With update rates of a minute or more at navigationally significant ranges, searchlight sonars are not really suitable for obstacle avoidance.

The only acceptable solution is a multibeam sonar. For these sonars, tracking the target during 10 pings before raising the alarm will take Talarm = 10 * ping_rate, and Dalarm = V * Talarm where V is ship speed. Depending upon model and range, navigation sonars generally have a ping rate of 1-2 seconds (similar to the update rate of typical marine radars). As an example, with ping_rate = 1 second, Dalarm = 60 meters for a vessel traveling at 12 knots.

The next component of minimum detection range requirement is Ddecision. It is driven by the time it takes the human operator to evaluate the risk of collision and make a navigation decision, Tdecision. Three factors contribute to that parameter:

  • IMO and internal ship regulations

  • computer aid for operator decision

  • human behavior (discipline, reaction speed, etc.)

We cannot change the last factor directly. However, over the last several years, factors influencing human component have been studied. The studies show that improvements in both the first and second factors have a strong impact on the operator behavior. With a well trained crew, good bridge procedures, and effective navigation decision aids (i.e. radar, charts, ais, etc), Tdecision could be as low as 30 seconds for a smaller ship where the bridge officer is also the helmsman. At 12 knots, Ddecision becomes 180 meters. For a larger ship where the helmsman may be operating under the direction of another bridge officer, the decision time may be longer, perhaps 60 seconds. At 14 knots, Ddecision becomes 420 meters.

The next component is Dmaneuver. Let's consider the case of a sharp turn with minimal (tactical) diameter, or even a crash stop. General IMO regulations require crash stop track reach to be <15*L and tactical diameter to be <5*L where L is the length of the vessel. In reality, modern expedition ships have much better maneuverability: crash stop reach is about 3*L, and tactical diameter is about 2*L. For example, crash stop distance of cruise liner Costa Victoria is 3.3*L from full speed 24 knots. So let's guess Dmaneuver = 3*L. This is obviously dependent on the size of the ship. For a 40 meter yacht this is only 120 meters. Whereas for a 100 meter yacht this is 300 meters.

The final component is the distance for Closest Point of Approach, DCPA. This distance is not determined by IMO in numbers, let's guess L/2 as a minimum safety requirement. Again, this is dependent on vessel size and could be as small as 20 meters for a 40 meter yacht.

Case Study – 40m Yacht

As an example of the minimum detection range requirement for a 40 meter yacht traveling at 12 knots is calculated below:

V = 12 knots = 6 m/s

ping_rate = 1 sec

Dalarm = 10 * ping_rate * V = 10 * 1 s * 6 m/s = 60 meters

Tdecision = 30 sec

Ddecision = Tdecision * V = 30 s * 6 m/s = 180 meters

L = 40 meters

Dmaneuver = 3 * L = 120 meters

DCPA = 0.5 * L = 20 meters

Minimum Detection Range Requirement = Dalarm + Ddecision + Dmaneuver + DCPA = 380 Meters

Case Study – 100m Yacht

The minimum detection range requirement for a 100 meter yacht traveling at 14 knots is calculated below.

V = 13 knots = 6.5 m/s

ping_rate = 2 sec

Dalarm = 10 * ping_rate * V = 10 * 2 s * 6.5 m/s = 130 meters

Tdecision = 60 sec

Ddecision = Tdecision * V = 60 s * 6.5 m/s = 390 meters

L = 100 meters

Dmaneuver = 3 * L = 300 meters

DCPA = 0.5 * L = 50 meters

Minimum Detection Range Requirement = Dalarm + Ddecision + Dmaneuver + DCPA = 870 Meters

Products to Match the Need

FarSounder's products are uniquely suited for exploration cruising. Navigation obstacles such as reefs, rocks, corals, and shoals are generally good sonar targets and can be detected out to the full range of our sonars. Our FarSounder-500 and FarSounder-1000 obstacle avoidance sonars operate out to 500 meters and 1000 meters respectively, matching the needs of today's expedition vessels.