Remote sensing is a technical term that basically means “looking at something from far above it”. In the early days, it meant taking pictures with a camera from a kite or a balloon. As technology advanced, the kites and balloons were replaced with airplanes and the cameras became more sophisticated. The airplanes allowed the photographs to be taken from much higher up than the balloons or kites could reach, meaning that the field of view of the photographs was wider. As the technology continued to advance, the airplanes were able to reach higher altitudes, travel farther, and carry even more sophisticated cameras. These were used in wartime.
And not too long after the first artificial satellite, Sputnik, went beeping in orbit around the Earth, we started putting cameras in satellites to take pictures of the Earth from space. Some of these cameras were not that different from the cameras used at the time for television while others still used film. The satellites using film cameras were able to take much higher resolution images than the electronic cameras could, leading them to be used for intelligence gathering, so many people referred to them as “spy sats”. Getting the electronic images down from the satellite is a fairly straightforward process: the satellite transmits the image, and a receiving station records it on tape to be processed later.
Getting the film back from the spy sats, however, was a little more complicated. Once the film, which could be thousands of feet long, was exposed, the satellite transferred it into a return capsule that would be ejected from the satellite at the right time. After separation from the satellite, the return capsule would fire a small rocket motor to send it back to earth. Once it was through reentry, the capsule would deploy a parachute and an airplane towing a hook would recover it. This method is how the CORONA satellites would return their film capsules.
The US stopped using film return satellites after the last launch of a HEXAGON satellite in 1986, while the last Russian film return satellite, a Kobalt-M satellite, was in 2015.
Digital camera technology allowed this aerial recovery procedure to be abandoned because it was now possible to obtain high-resolution images in near real-time from virtually anywhere in the world.
Originally, highly classified, commercial satellite imagery started to become available, albeit at lower resolutions than the spy satellites could produce, and the uses of it expanded from monitoring what the enemy was doing to things like land-use monitoring, crop health, and environmental monitoring. Things like oil spills from leaky ships are quite easy to identify using satellite imagery.
But before you get the impression that remote sensing is just about taking photographs, I should mention that it also encompasses radar sensors that use radio waves rather than the light of the sun. Synthetic aperture radar can penetrate clouds, which are the bane of optical remote sensing satellites, and can also be taken at night, which effectively doubles the opportunities to image something.
A few days ago, the European Space Agency (ESA) posted an article on their website talking about the biggest update to Google Earth since 2017, that involves a collaboration between Google Earth, ESA, the European Commission, NASA, and the U.S. Geological Survey that adds 24 million satellite photos spanning the last 37 years. This feature, which is called Timelapse, will allow users to watch the changes that have occurred over almost four decades. The expansion of cities, the changes to coastlines, and the loss of forests can be watched by anyone with a browser or smartphone.
And what does the future hold for remote sensing? Regular viewers will have heard us report on the launches of new remote sensing satellites. New satellites can be smaller and thus less expensive to build and launch compared to older remote sensing satellites, making the ability to operate a remote sensing satellite much more accessible to small companies and researchers. As more and more of these satellites are launched, access to relatively inexpensive, near-real-time imagery will become increasingly affordable. New satellites will also offer higher resolution imagery, which means you’ll be able to make out smaller details in the images. You probably won’t be able to read a license plate from space, but you will be able to make a fairly good guess as to what kind of car it is and what color the car is.
ESA press release
ICEYE X2 info page (Gunter’s Space Page)