Why space matters: Imaging satellite operations, part 14—making it intelligible

I know—I promised to talk about the different colors on imagery birds (satellites) and in what applications in particular they’re used.

But, someone pointed out to me that perhaps the past few lessons have been a little too space-nerd-tastic and maybe a little too difficult to understand.

SO let me just try to make things a little easier, just in case that someone is right.

Rehash:  the satellite bus is the system supporting the payload.  The satellite bus is made up of several different systems:  Power (solar panels, batteries), data (satellite telemetry and tracking information, antennas), thermal (radiators—active/passive/both), pointing (control moment gyroscopes, thrusters, reaction wheels), and fuel (for the thrusters).

But the big thing to remember is the satellite bus exists to support the satellite’s payload.  There would be no reason for the bus without the payload, because the payload IS THE MISSION, the reason for the satellite being sent into orbit to begin with.  And the payload can be anything, not just cameras (which we are currently focusing on).  It can be radio transmitters and receivers for TV and radio.  It can be some sort of measurement device.

You get the idea.  The payload is what the satellite carries allowing it to get its primary job done, and but that job can vary depending on what the satellite builders are interested in.  And, as we’ve discussed before, there can be more than one payload on a satellite’s bus.

In this lesson, and a few others prior to this, we focus on imagery (or photography) payloads on DigitalGlobe/GeoEye satellites, particularly WorldView-2.  So what does the payload consist of?

Really, the imagery payload consists of things you are already familiar with, especially if you’ve used a digital camera.  There is an image sensor (more than one in this case), some lenses, and mirrors.  There’s also memory for storage of the photos, just like on your cell phone and camera.  There are even batteries and a charging system, maybe a bit more complicated than on your camera.

I’m not going to get into too much detail here, but the image sensor is a simpler and maybe more flexible version of what you’ve used in a digital camera.  In this case, the sensor on WorldView-2 is likely a charged-coupled device (CCD) designed and built for DigitalGlobe by ITT Exelis (they built the sensors for GeoEye, too).  If you wish to learn more about what an image sensor is and how it works, click here and here.  But the satellite’s image sensor works the same way your camera sensor works.  A big difference, though (and I am guessing here), from the technology you may have used is that DigitalGlobe can select a particular color on the image sensor to highlight certain things in the pictures their customers are interested in.

The lenses are bigger versions of what you have in your camera—same for mirrors.  And unlike your camera—maybe more like your cell camera—there’s no real way to focus the lenses.  They have been optimized to take pictures from a particular height in orbit and such optimization takes out another variable DigitalGlobe would’ve had to deal with (plus, the satellite is moving fairly quickly, making focusing very tricky).

A way to compensate for lack of focus is to have a very high resolution image sensor on board, and then accomplish work on the image after the picture is taken (called post-processing).  I am assuming you know a few things about resolution, but, if you want to learn more about sensor resolution, pixels and scale, please click here.

And there you go—you already knew a lot about how a satellite imagery payload works.  You’ve even used similar technology in the form of the digital camera.  Sure, it’s all in space, but that shouldn’t make it harder to understand.  Hopefully this lesson has shown that.

And next lesson, we finally get into the colors of these cameras.

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