By 1915, air-to-ground radio was in use by reconnaissance pilots during World War I.
Notes on verification
Confirmed by multiple independent historical sources including Marconi Heritage, FirstWorldWar.com, WarHistory.org, and an academic encyclopedia entry, all corroborating wireless transmitters aboard aircraft for artillery spotting by 1915.
Sources
- Aerial reconnaissance in World War I (en.wikipedia.org)
- https://www.firstworldwar.com/airwar/observation.htm (firstworldwar.com)
- https://warhistory.org/article/wwi-spotting-for-the-armys-big-guns-i (warhistory.org)
- https://marconiheritage.org/ww1-air.html (marconiheritage.org)
- https://encyclopedia.1914-1918-online.net/article/wireless-telegraphy/ (encyclopedia.1914-1918-online.net)