The Homeland Security Department has been using pilotless spy planes to patrol the Mexican border for nearly a year. Vigilante groups have been putting unmanned eyes in the sky for even longer. But a new report from the Congressional Research Service is warning that there could be some pretty major drawbacks to using robotic border guards.

The technical capabilities of the UAVs have been tested in a military context, but serious safety and technical issues need to be addressed if the program is to be expanded domestically…

There are concerns regarding UAVs high accident rate. Currently, the UAV accident rate is 100 times higher than that of manned aircraft…If control systems fail in a manned aircraft, a well-trained pilot is better positioned to find the source of the problem because of his/her physical proximity. If a UAV encountered a similar system failure, or if a UAV landing was attempted during difficult weather conditions, the ground control pilot would be at a disadvantage because he or she is removed from the event. Unlike a manned pilot, the remote pilot would not be able to assess important sensory information such as wind speed…

Another consideration is how well the [border patrol] could respond to UAV imagery. Are there enough border patrol resources to investigate all UAV identified targets? Would the lack of human resources render high technology like UAVs less effective?...

A final potential question pertains to civil liberties such as personal privacy. Some are concerned that UAVs deployed over the United States may provide government agencies a new ability to clandestinely monitor citizens…

However, the report suggests, there is an alternative to the drones: aerostats, "the helium-filled blimps that don’t fly horizontally but are instead tethered to the ground with a cable that provides power. Like UAVs, aerostats are unmanned and can loiter for long periods of time. But the blimps crash less, have had extensive testing in civil settings, and may not cost as much as putting robots in the skies.

(thanks to Nick for the tip)