ALOS-2 has three observation modes: Spotlight, Stripmap and ScanSAR mode. By selecting the most suitable mode depending on a purpose, an optimal observation will be made.
When observing with the spotlight mode, which has the highest resolution of the ALOS-2 observation modes, it is necessary to specify the altitude. SAR satellites basically irradiate microwaves toward a point of 0m. Therefore, it is possible to observe the target by specifying the altitude when observing an area with a high altitude. As shown in the figure on the right, if the altitude is not specified when observing a high altitude area, ALOS-2 receives the reflected wave (backscattering) of the microwave irradiated toward the point of 0m. Thus, a point different from the observation target will be observed. When ordering a new data acquisition with spotlight mode, you need to specify the appropriate altitude (unit: m) according to the observation target. You can see how to order a new data acquisition from here.