Governments have been using satellites to monitor activity on the Earth’s surface and in its atmosphere, particularly enemy activity since the dawn of space flight. Typically, surveillance satellites are relatively large and expensive, for example US KH-11 satellites weigh between 13,500kg and 17,000kg and cost more than US$1bn each. This means there are insufficient satellites to provide real-time global coverage, so it can take several hours to collect and transmit information for decision makers. This delay is fine for monitoring weather patterns, but not for alerting on potential missile attacks. Latency (the time it takes a signal to travel from the ground to a satellite and back) is also a problem. In addition, now that China and Russia have demonstrated that they both have the capability to destroy satellites, basing a surveillance system on relatively few highly expensive assets presents a big security risk. As a result, governments are planning constellations composed of tens or hundreds of smaller, less expensive (c US$9m) low Earth orbit (LEO) satellites to provide real-time information on threats. Using a network of many smaller satellites connected with optical communications links also means that it is possible to transmit all of the data collected by surveillance satellites, which is not possible at present.