F E AT U R E
Imagine you are given a map with only the Start and Finish marked. You have to find five unmarked controls and get back within a certain time limit. Your only aid is a portable antenna which will (hopefully) point you in the right direction to each control. But each control only transmits for one minute in every 5-minute period. You have to get a good bearing whilst the control is “ON”, then find it. This is RadiO Orienteering.
Tips and Techniques for RadiO Orienteering Bruce Paterson, VK3TJN (VIC) AS I started to write it became increasingly obvious I would have to split this up since there was way too much to cover in one go. Therefore I’ll firstly deal only with something familiar to orienteers - the map.
What is an ARDF Event ? An Amateur Radio Direction Finding event following championship rules has 5 transmitters (control points) which take turns transmitting, so each transmitter only broadcasts for one-minute in each five-minute cycle. Only the Start triangle and Finish circle are shown on the map and there is a fixed time limit in which to find a certain number of controls (like in a Score event). Time limits of two hours or more are common so often there may be a long way between controls.
Why use a map ? I’ve been asked “why not just follow the signal from each radio beacon ?”. Well, some competitors do this; they stuff the map in the back pocket and bee-line for the transmitters in turn. Though this technique, with easy terrain, might even get you to the first transmitter faster, your rate doesn’t improve during the event and you most probably end up doing the transmitters in the wrong order. You will do poorly. The map can be used for navigation around bad terrain (eg. rivers, jungles, ravines), location, remembering bearings, transmitter guesstimation, choosing the control order and getting to the finish quickly.
Location Knowing where you are on the map, at least approximately, is essential to succeed at Radio Orienteering. Obviously you can’t take advantage of good fast routes to where you think the transmitters might be if you don’t know where you are on the map, but you also cannot draw bearings on the map since you don’t know where they are being taken from. A good coarse navigation ability is essential and normal foot Orienteering events are excellent training for this. Re-location is also important since when very close to a transmitter you sometimes have to sprint to “catch” it whilst it is still transmitting in its one-minute period and it’s very easy to lose contact with the map. When you find a control in foot Orienteering you at least know exactly where you are on the map; not so for Radio Orienteering. Finding the transmitter is no help at all for location. 22 THE AUSTRALIAN ORIENTEER MARCH 2006
Bearings One technique nearly all competitors use is to draw bearings on the map. Bearing lines are drawn from your current position in the direction of a transmitter as determined by peak signal from your radio direction finding gear. When a number of bearings to one transmitter taken from various points as you travel around the course start to cross near each other (pray they do!), the intersections are your guesstimate of that transmitter’s location. You have to use your compass to relate a bearing from the gear to a bearing on the map. Originally, I used to sight a plate compass arrow where the antenna pointed, turn the dial to align North, put the plate on the map with the North lines aligned to magnetic North and draw a bearing using the side of the compass from where I figured I was at the time. Luckily there are much faster techniques that are nearly as accurate. I now affix my compass to the antenna and simply read the degrees where the North arrow points. Note this is not the actual bearing, but the Y-axis reflection (eg. a reading of 350º is a bearing of 10º). I then use an A4 compass rose printed on an overhead transparency showing “reflected” degrees to hand draw a bearing on my map. I merely need to be able to draw a line parallel to the appropriate degrees line on the transparency. Other techniques using a rotatable transparency rose can give even more accurate results (fewer inaccuracies in estimating a parallel line) at the expense of a little more manipulation, but still heaps faster than the original method.
Preparation Before the event you need to have your map board well prepared. • You need a reasonably hard but light-weight backing behind the map to be able to draw a straight line quickly whilst on the run. I use 1.5mm clear polycarbonate. • A plastic sheath for the map is a very good idea in case of rain. Bags are not provided in ARDF due to the huge variation in the way people attach their maps. Time is given at pre-start for you to prepare your map board. I have a thick A4 plastic pocket which will contain the board, the map and the compass rose. • Draw exclusion circles around the start and finish once you get the map. The rules stipulate the distance any transmitter must be from the start and from each other. For championship events the start exclusion is usually 750m and from each other 500m. Since the finish chute has a homing beacon transmitter you can also draw a 500m radius circle around the finish. Often templates are provided for this at the pre-start, but it’s safest to assume not and have your own. • Have compass rose transparencies pre-printed for both landscape and portrait formats so you can use the appropriate one for the map.
