The Nobel Prize in Physics was awarded this week to three scientists, two of whom invented the charge-coupled device, CCD
This PDF is the public information on the Nobel Prize in physics. Charge-coupled devices have been used for various imaging devices including the Kitt Telescope and the Hubble which are mentioned in the link below. Note that CCD's can be used in ordinary cameras or in telescopes and in various infrared devices. These cameras are used in satellites and in unmanned drones as well. Night-sight equipment allows folks "to see in the dark" by using infrared images. A warm body can be seen in the cold, dark night.
The Nobel link does not mention spy imaging as one of the uses for CCD's but it is. When I Googled my rural address a few months ago, I saw an image of my house with a deer in the front yard. . . damn deer. The image was clearly taken by satellite.
The CCD was invented in 1969, forty years ago, so there is no reason that a virtual fence cannot work. It is only a lack of will. If Google can do it, our government can do it. That is not to say that a virtual fence is the best solution to closing our borders. It is simply to state that it is possible despite open-borders noise to the contrary.