How Far is Far Enough? Understanding Diver Detection Range Requirements
When evaluating a diver detection sonar system, one of the most basic metrics to consider is threat detection range. To be effective, the system must provide underwater surveillance with good coverage out to a tactically significant range. Many diver detection systems advertise their maximum detection ranges. However, most of these ranges are under optimal acoustic propagation conditions. In reality, the local underwater environment can greatly affect the maximum performance of all types of sonar systems. To evaluate if a particular underwater intrusion system meets your needs, it is not enough to only understand what your minimum detection range requirements are. You must also determine if the system is capable of detecting an intruder at that range in real-world conditions. In this blog posting, we will explore how to determine such detection range requirements.
Determine Effective Security Zones to Ensure Shipboard Safety
One can easily calculate the range of an effective security zone by considering threat characteristics (i.e. realistic diver swim speed, etc.), detection/alarm capabilities, and countermeasure response time. This is a simple chain of events in which each link in the chain has an effect on the detection range requirements of the system.
In the case of a sonar based diver detection system, it takes a finite amount time for a system to confidently detect and track a diver target through a series of pings. Once detected and tracked, the system will require some amount of time to classify the target as a threat/non-threat. Possible threat targets within the user defined security zone should trigger a clear alarm allowing the operator and security team to verify the threat and make an informed decision whether to start their response.
The "math" is quite simple:
[Minimum Detection Range] =
[time to detect, track & classify diver] x [diver speed] +
[time to verify and make a decision] x [diver speed] +
[safe response distance]
Let's consider a worst case scenario:
- a diver with closed-circuit rebreather (CCR)
- a challenging summer propagation condition
- shallow water (10m)
- rough bottom
- with a sea state 3
With this scenario, it might take up to 100 sequential pings to reliably detect, track and classify a diver threat. A typical ping rate for a modern diver detection system is less than 2 seconds. So:
[time to detect, track & classify diver] = 100 pings * 1.8 seconds/ping
= 180 seconds
= 3 minutes
Typical maximum speed for a CCR diver over any significant distance is 1 knot. So:
[diver speed] = 0.5 meters/second
Let's guess that a reasonable summary time to make a decision is 2 minutes. First the bridge officer sees the alarm and then reports it to the captain. Then he/she decides on a response and makes the commands. So:
[time to verify and make a decision] = 120 seconds
Finally, assume the vessel wants to be able to respond while the threat is at least 100 meters from the vessel. So:
[safe response distance] = 100 meters
[Minimum Detection Range] = 0.5 m/s *180s + 0.5 m/s *120s + 100m = 250m
This means that the minimum range requirement for a diver detection sonar using this scenario is 250 meters.
What's the Best Performance to Expect?
One should note that some diver detection sonars only provide specifications for the best conditions, not for the worst ones. It is obvious that in this example one requires good detection out to 250 meters range, even under poor conditions. Detections out to 500m would be additionally beneficial for the security team's response.
Promises of detection out past the 500 meters for this example should be received with a healthy bit of skepticism. Electronic surveillance is not a magic wand and cannot see beyond physical limits.
The above example shows that the sonar must provide a good enough detection range and reliable tracking/classification. It also clearly shows that the security staff must be well organized. An additional 3 minute loss in decision making and response may be fatal.
Download our white paper SPS for the Ship Operator: Diver Detection Sonar Explained to read more about what to consider when selecting an underwater security system. Our engineers are always happy to evaluate your diver detection needs and recommend possible solutions. Don't hesitate to contact us with your project's requirements. Our Ship Protection System is designed to provide real time navigation capabilities while underway and underwater threat detection while on anchor.