Hmm. Here's an interesting question. Let's say that there are 3 satellites in view broadcasting signals f1(t), f2(t) and f3(t). The way the system triangulates is to compute the distance from a location to a satellite by timing the arrival of a signal. So if signal f2(t) arrives at t+3 milliseconds, then the receiver is 3 lightmilliseconds away from satellite 2. For sake of simplicity, let the coordinates be expressed in distance from the satellites. (2,3,1) would mean a distance of 2, 3 and 1 light milliseconds from satellites 1,2 and 3 respectively. Okay, so why can't I just tape the signals I get from each of the three satellites. Let these be T1(t), T2(t) and T3(t). Assume we can easily synchronize them so that T1(t-o1)=f1(t). That is, we figure out our coordinates (o1,o2,o3), and subtract the offset from each tape. Then if we want to pretend to be at coordinate (a1,a2,a3), we simply say that we just received values T1(t-o1+a1), T2(t-o2+a2), T3(t-o3+a3). Or course, I could be completely missing some neat feature of DGPS. I really don't know the details of how it works and this could be completely wrong. Any thoughts? -Peter