They can provide early warning of cholera outbreaks by tracking the plankton that carry the bacteria. They have been used for some time to measure the concentrations of greenhouse gases in the global atmosphere. Most statistics on emissions now come from national governments, and not all governments are equipped to monitor them accurately. Worse, the pressures to falsify data would rise rapidly if the Kyoto Protocol or any similar international agreement were to come into effect. If and when a treaty limits emissions and establishes trading in emissions permits, billions of dollars would change hands annually based on these statistics.
Governments are unlikely to trust each others' data without any independent review.
The United States Senate in particular is unlikely to approve a trading system that does not include independent verification. The Senate's experience with arms control treaties will probably lead it to demand sophisticated and unimpeachable checks on national data. That will be a major hurdle for the Kyoto treaty, if it should survive, or any other agreement imposing legally binding obligations on countries' production or emissions of these gases. It is already clear that satellite sensing can be much cheaper and less intrusive than a similar level of coverage by sensors on aircraft, or by inspectors at ground level. But any kind of surveillance raises political anxieties.