Signal square

A signal square is an aerodrome equipment internationally defined at the annex 14 of the Convention on International Civil Aviation by the International Civil Aviation Organization.^[1] It contains ground symbols to indicate visually the conditions on the aerodrome to over-flying aircrafts. There are four basic signals:^[2]

The red square
The dumb-bell
The landing T, showing the wind direction on an all-over landing field with the tail of the T pointing downwind. On airfields with runways it indicates the landing and take-off direction.
The right hand circuit indicator

References

^ International Civil Aviation Organization. Council. (July 2009). Aerodromes : international standards and recommended practices. Annex ... to the Convention on International Civil Aviation. Volume I — Aerodrome Design and Operations (5th ed.). Montréal: International Civil Aviation Organization. ISBN 978-92-9231-332-6. http://www.icao.int/icao/en/sales/cat_2009_2010_revised_edition_en.pdf.
^ "The Signal Square & Other Visual Signals to Aircraft". http://www.airwaysmuseum.com/Signal%20Square.htm.