Yes, but this is an issues with GPS anyway. If you try to use GSP in an urban canyon, you can't get a good fix on location due to multi path propagation.

@whuber and you're right... if every tower can help you estimate your position to within 300 meters of truth, then perhaps if you had 1000 towers around you, then you could get a good fix on your location. So there might be very practical issues about the quality of the information you receive.

@whuber - certainly the time accuracy wouldn't be nearly as good as the GPSs' accuracy, and there is not new information created from the towers simply reestimating the time. However, where the towers would provide more information is in the fact that their location can be very accurately determined because they don't move, and the location only has to be determined once. Also, even if you don't have accurate times, if you can at least quantify the uncertainty statistically, then there may be enough information to still get a good fix on location

@Mapperz Interesting point about price that I had overlooked.(...which is a shame. I thought my next pocket watch was going to be a "Super Cool Caesium-133".) However, let's get around this by having the towers constantly reestimating their time based upon their connection to GPS. They wouldn't be nearly as accurate, but they would be accurate enough to provide more information.