Friday, 13 July 2012

SAR End Point

When does Search and Rescue (SAR) start looking for for you? Where do they start looking? How long do they look for? These are three important questions and by understanding the answers, you can greatly increase your chances of survival. SAR Starting Point One answered the first of these and SAR Starting Point Two the second. This article takes the more complex final question. By understanding more about the methods of SAR agencies, you will begin to understand why planning, reporting and signalling are essential in maximising your survival chances.

We already know that SAR starts to look when they know you're missing and begin to look where they think you are. Excluding those times when you have a beacon transmitting your exact location over great distances, searches are generally performed over areas. This area will be chosen based on the number of search resources available, the quality of information about your estimated location and time.

It may be that the casualty (it's just a term), is known to be on foot and was known to be at a specific location 12 hours ago. Arguably then, they could have walked 10 miles in that time and be anywhere in a circle with a 10 mile radius. That's over 300 square miles (mi2), which beyond the capabilities of most search and rescue groups, but some cunning will come in which reduces that area down (see previous articles). Conversely, if SAR had your route plan and you checked in a couple of hours ago, then the search area many only be a few square miles. Alternatively, a sinking ship, which can report its position with great accuracy, being affected by wind and tide over time might be deemed to be in an area of 20 square miles over a particular time period. A plane reporting heading and speed having missed a check in might be deemed to be in some narrow rectangle on the map based on the flight plan, last data given, time and weather.

Probability of Area (POA) or Probability of Containment (POC) is the chances that the casualty is in the area described. This can be 100% if the area is large enough, but if resources are limited, then this area must be reduced in order that sensible coverage can be given to areas of where the casualty is most likely to be. There is little point in skimming over a massive area, since the Probability of Detection (POD) is then very small and the Probability of Success (POS), which is simply POA multiplied by POD, is still very small. The first goal in SAR planning is to maximise the POS by finding a relationship between POD and Search Area. This is quite mathematical and computers get involved.

If we imagine a search area described by a circle of diameter 5 miles (about 20mi2 ) which has been determined as the bounding area with a POC of 100% of where the casualty could be now and over the two hours of search that will operate before dark. SAR in the dark is a different kettle of fish and requires a completely separate plan altogether. If it is then determined that the speed at which the units would have to be travelling would give a POD of 40% then the POS would be 40%. If, however, it were determined that there was a 50% chance that the casualty was, in fact, in an circle half that diameter (about 5mi2), then a slower, closer and more accurate search could be undertaken with a POD of 90%.  This would give a POS of 45%, which is more, but is there a better solution? In this circumstance, a circle of 70% diameter would give POD of around 70% and so a POS of 49%. These numbers are not just picked out of the air, there are some fancy formulae involved, but what it goes to show is that there is an interplay between the quantities and an optimisation process to find the best search area given the resources and time available.

As you can imagine, POD is greatly reduced by fog, darkness, camouflage and affected by the (lack of) size of the search object. If you're going to come down in a plane at all, pray it's a great big orange one that lands gracefully in a lush green meadow, rather than a small white one in the snow.

So how is this answering the question you might ask? Keep with me reader, I'm getting there.

In order to answer the intended question, we have to assume that you (the casualty) was not found. This may be because you were simply not spotted, or because some assumption was made about your position which was wrong. Maybe you deviated from your intended route plan. Equally, if the search area is huge, there is only so much POS that can be achieved, even with helicopters, planes and loads of people. So what happens next?

First, a re-evaluation of the search area is conducted. If you are on the sea, then you have been subjected to more time to drift in the wind and tide. In addition, greater error factors are applied in order to accommodate the failure of the first run. This makes the search area much bigger and so it has to be searched a lot slower. The same process of maximising POS is conducted and everyone sets to work. It may be that if you were overflown but missed on the first day, you might not see anyone else until day three. How annoying would that be of you'd only got one packet of salted nuts between ten of you!

As you can imagine, the more time that passes, and the more search runs that occur, the more time it would take to cover the increasing areas and the lower the POS becomes. Even if you were lost in a Land Rover full of pot noodles and sleeping bags, there becomes a time when POS is so low that reducing it any further by continuing to search is futile.

In addition, there is a consideration that your situation may be so grave that you have expired and that continuing to search is, in fact, putting the search teams at such a risk that the POS is lower the  probability of the SAR team becoming a casualty themselves. Clearly, this is influenced by the weather on both sides of the equation. At this point, SAR teams are operating in Recovery mode, rather than Rescue, which is lot slower paced and is fair weather dependent. Of course, there are limits to this mode also.

As we saw in the previous article on the subject, initial search areas are determined by expected position. If this is large due to lack of quality information, then initial POS is low and ultimately, the lower threshold of POS will come quicker, reducing total search time and giving you less opportunity to be rescued. The more you do to reduce that initial search area, the more opportunity you will have to get yourself found. Oh, is that me talking about route plans and regular communication again? I think it is!

It's quite impossible to expect a hapless survivor to accurately calculate the maximum search time themselves, so here are some rules of thumb. If you're in the freezing Arctic or the burning desert and there is a group of you, then chances are SAR will operate for a week to ten days. If you're on your own in a similar environment or wider area, then chances are, less than a week. If you're in a temperate region, then SAR may operate in one mode or another for anything up to 12 weeks, but as you can imagine is highly dependent on environment, weather, SAR resources and initial conditions.

I imagine the exception to all of this is that which happens when Madonna or some other mega star's plane goes down. There would be sufficient cash to hire enough SAR teams to just keep on going until they are found, dead or alive. So remember, always travel on large, highly equipped, orange planes which transport rich and important people and you'll be fine!

Now we know a little more about SAR, we can think about how to best increase our chances of being seen (increasing POD and hence POS) through timing, placement and types of signal. Look out for further articles on the subject.

As always, I hope you never have to use this type of knowledge in a real situation, but should you find yourself up the proverbial creek and this article helps you get out alive, then please do Like it on the Survival's Cook Facebook Page.

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