A friend recently wrote me:


Does anyone know? Do PLB's have an internal clock? Is the question irrelevant since they don't have to snych up with a prior known time and point?

How long do PLB's take before they can acquire good GPS coords? I assume it's going to vary widely with terrain, but does anyone have a good link for this?

thanks!

PLBs do not have any internal clock related to the GPS. The GPS is required by the PLB standard to "cold start" every time. This ensures that there is no "confusion" while the PLBs first tries to find itself where it thinks it was, which may be the problem your friend had. Older GPSes were particularly bad about this. Newer GPSes are better, they more quickly recognize they are "lost." When a GPS "warm starts" it assumes it is where it was when turned off and assumes the satellite information is had downloaded (ephemeris data and almanac) is still current. If it is not, then it won't find the GPS satellites where it expects them to be and gets "lost" and "confused." Eventually it has to decide via internal logic to give up and start from scratch and download new ephemeris data and almanac.



By always starting cold, a PLB eliminates this problem, but that means it always has a short delay to download the data. Even doing that, newer units under optimum conditions will generally get a location within a first few 406 bursts, which occur at 50 second +/- intervals. There can be complications and it can take longer when conditions are less than optimum. See the 406 Beacon Reports on ETS for more info.