Hi. We are developing a system of RF tags for open space storage place. This are mostly containers. I need to make simulation of collisions in simultaneous Tag's transmission. Let's say I need 10 Tags to send update to base stations at exactly the same time. The idea to connect tags with wires is not good as we will have to use kilometers of wire and it can not be placed everywhere. So far I have found some GPS based solution http://www.xdimax.com/interX/interX.html They have GPS and unit can be programmed to output signal at predefined times . Some on---off----on---of sequence. If I take 10 units they will do the same and with good precision. I wounder if there is any other solution? Any chance for instance to program all tags to make synchronous transmits and lay on Tag's crystal synchronization? In other words if I take a number of 4MHz Cristals what can be a drift between them let's say in one hour period?