We recently came across this story in which a former engineer on the US spy satellites known as Keyhole-9, claims that the imagery they got from those satellites were ‘better than Google Earth’. We thought this was something worth looking in to.
Google Earth has imagery in a wide range of resolutions from a few centimetres per pixel to more than 15m per pixel. The highest resolution photos are taken from very close to the ground, such as some photos of the island of Manihi that Frank captured with a camera attached to a kite. Next highest is aerial imagery that is captured from aircraft and covers the continental US, much of Europe, Japan and a number of other locations around the world. Aerial imagery in general is better resolution than can reasonably be captured by satellite. So, for a fair comparison we need to look at satellite imagery only.
All references we have been able to find say that the best resolution that the KH-9 satellites provided was 2 to 4 feet. That translates to about 70 cm per pixel at best. If you look at this list you will see that many commercial satellites that provide imagery to Google Earth actually do a bit better than that, the best being DigitalGlobe’s WorldView-3 that can manage 31cm per pixel. WorldView-4, due to be launched this month will have similar resolution. So, although the resolutions achieved by the KH-9 satellites are certainly impressive, they were not actually better than Google Earth provides today.
The maximum possible resolution of a camera depends on the size of the collector (typically a mirror) and the distance from the target. The KH-9 satellites were in non-circular orbits with lowest altitude of about 150 km. Most modern commercial imaging satellites are in higher orbits anywhere from 400 – 800 km up. The lower orbit of the KH-9 satellites gave them a resolution advantage, but it may also be the reason why they were only in orbit for around 3 to 9 months each. The best imaging satellites today probably achieve their greater resolution despite the higher orbit through a combination of larger mirrors and better quality optics, although we could not find any actual data on the size of the mirror used in WorldView-3. The KH-9 satellites had 0.91 m diameter mirrors.
In the past it was actually illegal in the US to sell imagery with better than 50cm per pixel resolution, but in June 2014 DigitalGlobe was given permission to sell higher resolution imagery – up to 25cm per pixel.
One of the photos shown in the clip in the CNN story and also shown on Daily Mail’s version of the story, which shows an overhead shot of people having a picknick is clearly higher resolution than 2 feet per pixel and we believe is not a satellite image. Another of the photos they show is of a submarine at the Russian naval yard at Severodvinsk, which we discussed in this post. At the time we noted that it was a remarkably good image for its age, although not quite as good as the Google Earth imagery.
A declassified photo of a submarine (cropped for better resolution) in Severodvinsk, Russia. The image was captured in October 1982 by KH9-17. Full image here.
The same location (and probably the same submarine) as seen in Google Earth.
It is no coincidence that the company Google Earth originally came from was named Keyhole Inc. It was in direct reference to the Keyhole spy satellites. To this day, Google Earth saves files in the KML format which stands for Keyhole Markup Language.
The Corona program (that flew the Keyhole satellites) and most of the imagery from them has been declassified. The imagery can be obtained from the USGS. Some has been digitized and is available to download for free via Earth Explorer (look for the ‘Declassified data’ data sets). For imagery that hasn’t been digitised, for a fee of US$ 30 per scene you can have them scanned. The imagery is not just of Russia and China. In fact the only high resolution imagery we were able to find was of the US and Antarctica.
You can read more about the KH-9 satellites on Wikipedia.
A documentary about the start of the Corona program can be found on YouTube.