A possible synonym for “airmark” could be “aerial landmark”.
An airmark is something that serves as a guide or reference for aircraft. Similarly, an aerial landmark is a feature that can be easily recognized from the air and helps pilots navigate. For example, a tall tower or a distinctive mountain peak can be an aerial landmark. Just as an airmark helps pilots orient themselves in the sky, an aerial landmark provides a visual reference point.
Another synonym could be “aviation beacon”. An aviation beacon is a signal or light that is used to mark a specific location for aircraft. This is similar to an airmark in that it provides a means for pilots to identify and locate a particular area. For instance, a flashing beacon on top of a building or a lighthouse can serve as an aviation beacon. It acts as a guide for pilots, just like an airmark.
“aerial marker” is also a relevant synonym. This term emphasizes the function of marking or indicating a location from the air. For example, a brightly colored balloon or a large sign can be an aerial marker. It serves a similar purpose as an airmark by providing a visible indication for aircraft.