I asked a friend charged with spending money in this area. Here's his reactions, and those of someone he consulted: ================== ..There is a business case flaw in the proposal. The scheme as laid out only provides a high confidence factor on the users location (+- 10M), NOT on the actual identity of the user. Thus it would have to be coupled with some other form of I&A. The business question then arises, is the confidence delta provided by this scheme consistent with the cost delta? My first take is that the answer is no. =================== Re spoofing the system. If you think about the geometry of the problem, the delay to be induced for each satellite is a time varying function of the satellite's position, the reference site, and the target. It will vary from positive to negative values for many satellite passes. It can be precomputed, but the precomputed adjustment will be in error by some amount due to the orbital perturbations mentioned in the original article. The most common prediction error is the in-track postion, which inconveniently for the predictor has the greatest effect on one's ability to calculate the right delays. Anyone with access to a decent orbital prediction code and access to the statistics on orbital perturbations should be able to calculate the expected delay prediction error as a result of the orbital uncertainties. If the delay prediction errors are detectable, then it seems the system is secure after all, allthough other reservations on operational suitability might still apply. Note that the sensitivity is a function of the distance between the reference site and the target. If you could collect your reference signal across the street from the target, it would make spoofing a lot easier. -- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433