A familiar conversation: a customer needs a quarterly site update, orders an optical capture, and then watches the order sit in scheduled while Central European autumn does what it does. Three weeks later the report is late and the imagery still has not arrived.
What optical actually requires
An optical capture needs daylight, a clear line of sight and an acceptable viewing angle, all in the same few seconds. Miss any one and the pass is wasted. In a cloudy region during winter, the odds of a usable capture on a given pass can be low enough that a seven-day window is optimistic.
That is not a failure of the satellite. It is physics, and no amount of paying for rush priority changes it.
What radar gives up, and what it does not
SAR sends its own signal and measures what comes back, so cloud, haze and darkness are irrelevant. A flood at three in the morning under complete cover is a routine capture.
What you give up is interpretability. A radar image is not a photograph: it shows roughness, moisture and structure rather than colour. A grey field and a grey roof are distinguished by how they scatter, not how they look. Analysts who are new to it need a week or two before the images stop looking like noise.
If the question is "is this area flooded", radar answers it tonight. If the question is "what colour is that container", it never will.
A rough rule
- Use optical when you need to recognise things: condition, colour, counting objects a person would identify by sight.
- Use SAR when you need a measurement on a schedule: extent, change, presence or absence, especially under cloud or at night.
- Use both when the schedule matters and the interpretation does: radar to know something changed, optical on the next clear pass to see what.
The cost argument
Per square kilometre, SAR lists higher than optical. Per delivered answer it is frequently cheaper, because the capture happens when it was booked. Three failed optical attempts and a missed deadline cost more than one radar scene, even before anyone counts the meetings.
If you are consistently missing windows, tell us the site and the cadence and we will model the actual success rate for your latitude and season. It is a short conversation and it usually changes the order.
