You hear people refer to the “Google satellites” from time to time when discussing Google Earth and Google Maps. In truth, Google doesn’t have any satellites up in space to capture imagery. While they capture imagery via planes and cars (and even boats, trains and snowmobiles, the satellite imagery is purchased by Google from large providers such as DigitalGlobe and GeoEye.
These satellites operate just a few hundred kilometers above the Earth. This means they only see a small part of the Earth with their camera as they orbit over. They typically go around the Earth every 90 minutes, but only cover about 1% of the Earth on each pass – but, most of the area covered in a pass is water. Not only that, but imagery for Google Earth is only going to be good if the sun is at a high angle when the satellite goes over (fewer shadows), when there are no clouds, and as little haze/pollution as possible. Believe it or not, the times when these factors all come together are pretty rare.
Once the imagery is taken, it takes time to process the data before it is available to customers. Google is one of these customers (a really big one). Google has to evaluate the new imagery against the current imagery to determine whether the new is better than the current. I assume they are trying to automate as much of this as possible. But, for important areas with large populations the process most likely involves people. This process takes time – especially when you think about the quantities of land mass of the Earth. Once an image is selected, it has to be processed into the format and coordinate system of Google Earth’s databases. Then it has to go through a quality control process and fed into a processing system before it gets distributed to the live Google Earth database servers. This is one reason why you usually do not find any imagery younger than about 3 months in Google Earth and why updates only happen about once every 30 days.
For more about how Google Earth imagery is captured and processed, you can read this excellent post that Frank wrote a few years ago.