Time on watch (TOW) is the metric we use to compare aerial and robotic security architectures. For a single drone-in-a-box with on-station time per cycle of :
A 35-minute sortie with 27 minutes on station and a 55-minute recharge gives 30%. Fleets raise TOW until they hit the site's flyable fraction. A perching sentry's TOW is reduced only by relocation time, typically above 95% even with hourly moves (see perch vs. hover).
TOW is always for a specific point; a system can have high availability and low TOW.
How to measure it
Pick a point on the site plan, then log, minute by minute over a representative month, whether the system had a sensor with line of sight and adequate resolution on it. Divide minutes covered by minutes elapsed. Include weather holds, maintenance and relocations. Vendors who report fleet uptime or launch readiness instead are answering a different question.
Typical values
Single dock: ≤30%. Four-dock fleet with relief scheduling: the site's flyable fraction (91% in our synthetic year). Fixed tower: ~100% from one viewpoint. Perching sentry: ~99% in our weather model.