In the first lesson, you learned (if you didn’t already know) a spacecraft in geosynchronous orbit is very far away from the Earth—a little over 26,000 miles.
Before going any further, an explanation of Field of Regard (FOR) versus Field of View (FOV) is in order since we will initially be talking about weather imagery satellites. Both terms describe simple concepts that apply not just to imagery satellite payloads, but any kind of image sensor, like the camera in your cellphone. So we will use the camera cellphone to help explain both terms.
Regard the explanation in front of you
Field of Regard (FOR) is the entire area which the camera in your cellphone can possibly photograph. Say you have the cellphone in your hand. Where are you able to aim the camera? Hint: the answer should be almost anywhere you can point the camera. So the possible areas you can point the camera with your arm are within your camera’s FOR. Okay, okay—this definition is more precise and in English, but it sounds like a politician made it up—or someone who didn’t want people to understand what was being defined (same difference). For you visual types, there’s a picture just right below.
View the focus of your attention
Field of View(FOV) is, then, a subset of the camera’s FOR. It is what the camera’s sensor sees immediately in the direction it’s pointed at. Here’s the wiki explanation for you Missouri types. Again, word-impaired get a picture below:
So, using the cellphone example, let’s say you’re tied to a telephone post from the waist down; so, your head and arms are free to move about (or your tongue’s stuck to the pole–but you can still move your arms). The camera cellphone is glued to one of your palms. Immediately in front of you is the beach, the ocean, the horizon and everything that scene entails. That is you and your cellphone camera’s FOR. But you notice a sailboat on the horizon, focus on it, and aim your cellphone camera at it to take a picture. That sailboat is within the cellphone camera’s FOV. Everything else around that FOV is excluded while the cellphone is focusing on the sailboat.
Hopefully these examples help you understand uncommon terms for common experiences in with your vision and cellphone photography applied to satellites: a LEO satellite’s FOR is significantly smaller (satellite is closer to the Earth, not able to see as much) than a geosynchronous (GEO) satellite’s FOR (further away—it can see more). Sometimes a satellite’s FOR will also be its FOV. This just depends on the image sensor used, how the sensor is mounted to the spacecraft bus (more explanations about spacecraft/satellite busses here), the spacecraft’s orbit, and the maneuverability of the spacecraft.
This is why it’s possible for a GEO satellite to take an image of the Earth, like the one in the example used in Lesson 1, the last post. The GEO satellite’s image sensor payload’s FOR is big enough to include the entire disk of the Earth in its FOV. Voila, you have the picture of the whole Earth.
So with this explanation out of the way, the next post will hopefully make more sense to you. But I did include this XKCD post about visual fields as a treat for you. If you click on the link, you might have fun…