vehicle positioning v1 july08

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Suggestions for the operational

placement of re-broadcast devices

LDSAMRA Comms Sub Committee - Issued Saturday, 29 October 2005


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In order for a re-broadcast device to be effective, its positioning is critical. There are a number of scenarios where such a devicemight be deployed and its virtually impossible to list them all. However, the basic principle remains the same for all - Two indi-vidual people need to be able to speak to each other but are prevented from so doing by the intervening ground.

Intervening ground can itself mean either theres a hill in the way or that the basic distance between to two radios is greaterthan they can manage without help.

Although our current radio frequencies are higher than the old low band ones, this still doesnt mean that they can be used solely

on line-of-sight paths. The transmitted signal wont so much bend around obstacles as be reflected by them. They will bounce offrock faces etc. This is obviously not something you have a great deal of control over as you cant arrange for the right rock to be

just where you need it!!

Its better for efficient deployment of the re-broadcast vehicle, if you assume you will need as clear a view as possible in order forthe transmission to be received at the other end. You will have some leeway but dont bank on it.

Of all the scenarios where a re-broadcast device will be used, two are obvious.

A...Base cant contact a hill-party because the distance is too great between them or there is high ground between them both.

B...Two hill-parties on opposite sides of a ridge or down different valleys, need to be able to speak to each other.


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A...Base cant contact a hill-party because the distance is too great between them or there is

high ground between them both.

For example - A base in Ambleside might find it difficult to contact a hill-party on the far side of Wansfell Pike due to the amountof rock in the way.


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If the obvious direct route( here shown inblack dotted) is ignored and we instead

look at the actually available paths thenits possible to find a point on the mapwhere a re-broadcast vehicle can be lo-cated.

What we are looking for is a positionwhere the driver of the vehicle can effec-tively see both the base and the hill-partyat the same time.

This is exactly what teams have been do-ing for years when theyve used manualrelay. This method of determining theposition of the vehicle will work whetherre-broadcast or manual relay is used.

Go to the source of each transmission. In

the case of the base, this is easy. Find aroute out from the base where no signifi-cant obstacles to a radio signal exist.

Draw a line down the most direct route.Here shown in blue.


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Now go back up the valley to the hill-partyand draw an un-obstructed line back down

the valley. Here shown in red.

Where the two lines cross is where the re-broadcast vehicle should be positioned in or-der to maintain comms with both.

In practice, the vehicle could be positioned onthe road at the point nearest to that indicatedby the crossed lines.

This method is very rough and ready but hasbeen proven over many years, to work relia-bly enough to be worth the effort. Onceyouve done it a few times, it becomes un-necessary to actually draw the lines, you canfollow the contours of the two paths and seewhere they would meet.

If the necessary unobstructed path meansyour vehicle needs to be parked outside yourown teams operating area, as may occasion-ally be the case, then so be it. Make sure theother team is aware of your need to park avehicle in their area and Im sure it wontcause problems.

Keep your lines along as few contour changesas you can manage and go in whatever direc-tion does the job. Sometimes the point ofintersection will be a significant distance awayand then a decision has to be made on theoperational benefits of going this far. In a bigsearch - possible its justified but in small,quick jobs, its your decision.


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The first scenario is relatively easy as the position of the base doesnt change. The driver of the re-broadcast vehicle has a fixed pivot-point on which to op-erate. Providing his transmission path from the base to the vehicle is never compromised, then he or she only has to concentrate on the whereabouts of the

hill-party. Even so, some local judgement on the part of the driver is required and its vital that this person has a good mental image of the ground in bothdirections. Its easier to see radio paths through gaps in the hills when youre looking at a map than when youre looking at the hills!!

The driver doesnt need to actually speak to base or the hill-party, he or she only needs to be able to hear both at the vehicle. Obviously a test transmissionis the best way to do this but subsequently getting a book out to read isnt a good idea and a constant watch needs to be maintained to ensure that commsthrough the vehicle are still good.

The most difficult concept with these devices is remembering that direct comms between the two parties, Base and the hill, are not now taking place. Every-thing is going via the vehicle and if its in the wrong place, the link is broken and you have no comms as a result.

A second and very common error is to think that getting closer to either of the two stations trying to communicate with each other, will solve the problem.Closer in up here in the Lakes usually means nearer to the rock and thats not a good move at all. If anything, getting further back away from all the obsta-cles is the best approach. High frequency VHF signals will bend but the angle is very much more shallow than that seen on the old low band. Quite oftenwhen a hill is in the way, the signal is actually coming down to the ground but some way behind where you are standing. Getting closer has exactly the op-posite effect to what you want.

Base

Hill-PartyPossible point ofreception

The angles in the diagramare obviously much exagger-

ated but the principle isthere. Contact at the end ofthe dotted line is not possiblewhereas it may well be if arelay vehicle is moved backfurther from the hill.

You can see that movingeither the base or the hill-party closer to the hill wouldhave a detrimental effect oncomms by making the angleeven more acute than it was.

Any team with a base facinga rock-wall will already haveexperience of this problem!!


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The method so far described is crude but effective where a fairly obvious straight-line transmission path exists from each end of the link and where the linescan be made to intersect.

Things become more difficult when this isnt the case but programs such as Memory Map which boasts a 3-D viewing aspect, can be used to get a quick ideaof where the hills and valleys are. Images from this program are used throughout the documents so far produced on this subject.

Another way of going about it is to use a professional radio propagation plotting program. These are very expensive and would be seen as outside our reachwere it not for the availability of the ATDI on-line planning tool on their website. This is available entirely free of charge and is there as a service. The mapsare not very good as far as resolution goes but any team who knows its own area would be able to interpret the results accurately enough.

From the previous example A :-

Using the line-of-sight coverage tool on the ATDIwebsite (www.atdi.co.uk)

The necessary information is entered on-screen.

The grid reference for the position chosen and theheight of the aerial above ground level. Thats localground level, you just enter the aerial height, theprogram knows how high above sea level you are.For a handset, just enter 2 metres.

Then enter the frequency you are using, that ofyour team working channel or 158 MHz if you wanta general, centre-of-band frequency.


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Using a separate plot for the base in Ambleside and another for the location of the hill-party, its fairly easy to see where the two plots coincide and sowhere a re-broadcast vehicle would be placed. Anything coloured green is indicating line-of-sight coverage so if an area common to both plots is ingreen, thats the place for your re-bro. Any area in yellow is also usable. Virtually anywhere inside the black circle would provide comms under thesecircumstances. Exactly where that is on a road etc, is down to your local knowledge.

Note that the program is indicating that comms onto the far side of Wansfell can be achieved from well south of Kendal!! From Ambleside base, thedistance is shorter but still includes the position indicated by the valley-line method. Scale of these maps is 1:500,000

Ambleside Base Wansfell Pike

vehicle positioning v1 july08

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