The Maidenhead grid locator is a coordinate system that encodes latitude and longitude as a short alphanumeric string (e.g. IO91wm for central London). It is the universal standard for exchanging locations in amateur radio, especially for satellite contacts where brevity is essential during short pass windows.
Understanding Maidenhead Grid
How the Grid Encodes Position
The encoding starts at 90°S, 180°W (the anti-meridian/South Pole corner). The first character (A–R) encodes 20° of longitude, the second (A–R) encodes 10° of latitude. The next digit (0–9) subdivides longitude by 2°, the fourth digit subdivides latitude by 1°. The final optional pair (a–x) provides the subsquare at 5' × 2.5' resolution. This gives 18 × 18 = 324 fields, each containing 100 squares, each containing 576 subsquares — totalling over 18.6 million unique 6-character locators worldwide.
Grid Squares in Satellite Work
During satellite contacts, operators exchange 4-character grid squares as part of the standard QSO (contact). This is shorter than exchanging full coordinates and provides enough precision to confirm both stations shared the same satellite pass. In satellite contests (AMSAT Field Day, ARRL VHF+), each unique grid square worked counts as a multiplier, making the propagation heatmap invaluable for planning which passes cover the most valuable grids.
Grid Square Examples
| Location | 4-Char Grid | 6-Char Grid |
|---|---|---|
| London, UK | IO91 | IO91wm |
| New York, USA | FN30 | FN30as |
| Tokyo, Japan | PM95 | PM95ss |
| Sydney, Australia | QF56 | QF56od |
| São Paulo, Brazil | GG66 | GG66qk |