Ground-to-air signals

Ground-to-air signals are visible signs used by people on the ground to show an aircraft their location, distress or an agreed message. They include large signs laid out on the ground and other visual signalling equipment. The ground-to-air visual code for survivors conveys specific messages through a few shapes even without a radio. For a message to reach the crew, they must see the complete sign and know its meaning.

The five ground signs for survivors

The international aviation code for survivors uses the following signs. They are made as large shapes lying flat on the ground. Their orientation and complete outline must be recognisable from the air.

The X conveys a need for medical assistance, but neither the type of injury nor the number of people affected. Y and N are replies: without an understood question, the crew do not know what is being affirmed or denied. The arrow is laid in the actual direction of movement; the upward-pointing example in the table does not automatically mean north. It does not mark a landing site either. The separate signs for search and rescue parties report such matters as a completed operation or personnel found. These messages describe the work of search teams and do not replace the survivor's own signs.

Making a readable sign with available materials

Choose the message first, then the site and available materials. Strips of fabric, parachute material, branches, pieces of wood or stones can form the sign. The contrast with the background matters: dark wood stands out against snow, and pale fabric against dark ground. Individual thin twigs or a few widely spaced stones often look like scattered dots from above. The material therefore needs to form sufficiently broad, visibly continuous strokes. The two arms of a V meet at the bottom; a Y has a clearly recognisable stem below its fork.

The international aviation code calls for signs at least 2.5 metres long. In US civilian aviation, a sign height of at least ten feet, approximately three metres, serves as a construction recommendation. In making the sign, also use the available space: a larger, clearly formed shape is easier to read from the air than a small letter with many details. The minimum size does not establish a detection range. Flight altitude, viewing angle, light, background and the crew's attention also affect whether the shape is read.

Keep the strokes straight, the corners distinct and the spaces between them clear. Secure or weight lightweight material so the wind does not change its shape. A sign in snow can also be made by tramping, dragging or shovelling; depressions may appear dark because of their shadows. This contrast changes with the light. When checking the sign, consider its complete outline, rather than merely whether a mark is visible on the ground.

Visibility from the air

The signal area needs an unobstructed view from above. A sign laid beneath branches may be easy to see from the ground while the canopy reveals only parts of it to the crew. Terrain edges can have the same effect. A shallow viewing angle also makes the shape appear compressed in one direction. Broad strokes, clear arms and enough space between the parts help preserve the letter from that angle. Looking at the sign from the ground can reveal such faults, but cannot replace the view from the aircraft.

An available visual signal can draw the crew's attention to the area. A flash of light or a bright marker attracts the eye; the readable sign then conveys the particular need for assistance. An unlit letter made of fabric or stones loses its visible contrast in darkness. The signalling method therefore needs to suit the time of day. Additional conspicuous signals also increase the area from which the location may be noticed.

Which reply shows that the message has been understood?

In an exchange using the international aviation code, an aeroplane rocks its wings in daylight to confirm that it has understood the ground signs. In darkness, its landing lights are switched on and off twice; if it has no landing lights, its navigation lights are switched on and off twice instead. The reply refers to the message transmitted. An ordinary turn, circling over the terrain or a single visible flash of light is not this acknowledgment.

If there is no clear reply, understanding of the message has not been confirmed. The person on the ground can first check faults within their own control: is an arm hidden, is the contrast too weak, is the shape too small, or has the wind changed it? Improving the sign only helps when an aircraft can actually observe the area. Whether laying it out again or making another attention signal is acceptable also depends on who else could see the site besides the intended crew.

Ground signs during military recovery contact

In military isolation, visibility to friendly forces competes with the risk of detection. An enemy aircraft may recognise a large ground sign; making it in open terrain can also expose a person to observers on the ground. Where possible, choose a signal area visible from the air but screened from ground observation by terrain. Portable material can be prepared before laying it out, reducing the time spent arranging it in the open. A mark left continuously visible keeps the site recognisable even after the person has concealed themselves again.

The public survivor code and military recognition serve different purposes. In the US Army system, Isolated Soldier Guidance, or ISG, sets out the signals intended for the operation and the link-up with recovery forces. Allied personnel recovery planning sets near and far recognition and the conditions for international distress signals in the operation's instructions. For an isolated person, this means using a known, agreed sign in the way the intended forces also expect. An invented letter creates no shared meaning. Where no such arrangement exists, a public survivor sign can convey its particular distress message if the crew recognise the code and the person's situation permits open signalling.

A V or X is publicly readable and can be reproduced by anyone. It therefore does not establish personal identity or affiliation. In military contact, agreed mutual recognition and the required authentication follow separately. Prepared identity information from the Isolated Personnel Report, or ISOPREP, may be used for that purpose; the person follows the procedure they actually know or the instructions received. Full personal authentication information does not belong in an openly laid ground sign.

Acknowledgment means that a message has been understood. Pickup additionally requires suitable forces and a method appropriate to the terrain, weather and threat. The observing aircraft may relay a message or record a location while other forces must carry out the pickup. When pickup is imminent, secure loose equipment and clothing; approach the aircraft only when instructed by its crew. Rotors and propellers remain an immediate hazard even after the signalling exchange has succeeded.