So you have an application or use case that lends itself to the use of SAR satellite imagery, for example flood monitoring, disaster response, illegal fishing, land and asset motion etc. You already know that the resolution and/or revisit frequency of open source SAR imagery from the likes of Sentinel-1, PALSAR, RADARSAT-1 etc. won’t meet your needs so it is time to review the offering of the growing number of providers of commercial SAR imagery.
Aspects such as price, imagery resolution and imagery footprint are easy to flush out, but if your use case requires a provider who can provide imagery fast, when and where in the world you need it…..then more caution and questioning will be required to understand what a provider’s true revisit frequency really is.
Here are the main reasons why the true revisit frequency is always lower than the very impressive quoted revisit frequency numbers bandied around by imagery providers…..
If your use case requires, or is best served by the use of coherent Ground Track Repeat imagery then the usable revisit frequency can easily drop from many times per day to once per day or even less. That is because Ground Track Repeat imagery is imagery taken of the same area, with the same look angle and direction. When providers quote revisit frequencies they are talking about how many times per day, on average, one of their satellites can image the same point on the earth unconstrained by look angle or direction. No surprise then that the usable and quoted revisit frequencies are very different numbers. You shouldn’t have too much trouble getting this info out of a provider.
The next killer is a SAR satellite’s payload duty cycle in mins per orbit. Typical commercial SAR satellites take about 90 mins to orbit the earth once and have duty cycles that range from just a couple of minutes up to 10 plus minutes per orbit. Why? Satellites are solar powered so while SAR satellites can capture imagery day and night they can only recharge when they are on the sunny side. Also SAR sensors, which are active sensors sending out and receiving radar waves, require much more energy than optical image sensors, which are passive sensors. These two facts mean that a satellite passing over your target area may actually be recharging and unable to capture your image. Some providers are very open about this, others much more cagey. Rule of thumb here is that the bigger the satellites are, the bigger the onboard solar panels and power storage will be, giving a higher payload duty cycle and more minutes of imagery action.
Imagine two clients have requested different imagery captures that can only be fulfilled by the same satellite sensor, assuming power is not the constraint, there might not be enough time between capturing one image to move the sensor into the correct position to capture the second image. Different providers handle this in different ways, premium tasking payments, customer ranking based on purchase volume, promises not to change an image capture request once confirmed etc. Either way a few questions about the free capacity they have over your geographical area of interest, and how they handle capacity constraints will flush out where you might stand in the customer pecking order.
Satellites are extremely reliable pieces of equipment built and tested to withstand the unfriendly conditions of space but now and again a captured image may not pass the required quality checks. Again some providers will be more transparent about how often this happens than others, so all the more reason to ask the question.
There are a few other subtleties relating to latitude, data storage and downlink duty cycles but covering the 4 points above will give you an accurate overview of what true and usable revisit frequency can be delivered by any SAR imagery provider.
So while the true and usable revisit frequency will actually be well below a providers quoted revisit frequency that isn’t to say that there isn’t an exciting, very real, and exponentially increasing SAR imagery capture capacity being provided by a growing group of innovative providers. There really is. Just that a little care, expertise, and a few extra questions are required to pick the right imagery provider for your use case.
I would love to hear your views and experience on the mysteries of SAR revisit frequencies and how that has impacted your choice of provider.
At Spottitt we provide our clients with the broadest range of automated satellite derived asset monitoring services on the market, all at attractive pricing. Our expertise includes the integration of both open source and commercial data from optical, SAR, GHG and other satellite sensors.
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