Typically, if you're trying to measure the earth's surface with an optical system, you would look at the nadir point directly below the system. However, radar systems in the microwave domain are not very good at doing direction. They use time delay from pulses that are transmitted from the antenna. In this regard, if we look obliquely and send out our pulses in the oblique direction, then the time it takes for the echoes to come back from the ground surface is approximately in proportion to the distance along the ground. In this case, this is the earth's surface and our sensor is travelling into the board. We therefore talk about the asimuth direction as the direction of travel of the system. And we talk about the range direction as being the distance away from the sensor and for an imaging system that is obliquely off to one side. The other significant thing that we have to consider is that this means that at the ground surface, we may be looking at different angles. The angle between the normal to the surface and the direction from the sensor to the ground, we refer to as the incidence angle, often labeled as theta subscript i. The incidence angle then can be different at the near side of the swath to the far side of the swath. The swath is the area that we cover on the ground. One of the important consequences of having an incidence angle, which is different across your swath, so across the image, is that the way that the microwaves interact with the surface can be very different. The microwave interaction with the surface is very dependent on the local incidence angle, so the incidence angle where it actually interacts with the surface. At the near swath, the incidence angle might be quite small. At the far swath, it might be quite large. For aircraft systems, there can be quite a range, maybe as much as 20-30 degrees between the near swath and the far swath. For satellite systems, typically it's only a few degrees difference, and so it has less impact on your interpretation of what the image actually means. The other important thing to consider about incidence angle is that when you're interpreting the properties of the image, you have to think of the local incidence angle, not the incidence angle to the reference surface. If the slope is increasing, for example, the local incidence angle might be considerably smaller than you would expect if you're just looking at the incidence angle for the reference surface. If you have a down slope, the local incidence angle might be considerably larger than what you would expect. So because the incidence angle has a big impact on how you interpret the way that the microwaves are interacting with the ground surface, you have to consider not just where it is in the swath, but also the local topography, because that will impact on the local incidence angle.