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To: Petz who wrote (1830)	11/12/1997 10:20:00 PM
From: Robert A. Sutherland	Respond to of 3506

I am not qualified to answer this question properly, as I only partially understand the signal processing involved. I will query the expert, and if he is forthcoming, I will respond later. It is covered in a patent, but I haven't read it. I doubt it is quasi- legal. My guess is it was closer to embarrassing. Every armor has it's chink:-) As far as differential issues are concerned, this product was developed as an enhancement to the codeless dual channel receiver (that I worked on) used for survey. The way to dynamically model the ionosphere permitivity (hence, phase velocity) is through carrier phase offset at two-frequencies (they have to be phase coherent at the source.. and since they are both locked to the same cesium, they are). A codeless receiver runs the L2 signal through a squaring loop, which DESTROYS the information on the carrier, but preserverves the phase information at twice the frequency. This phase information is then used as a second frequency collocated to the same differential sites. This give first order survey results. Most first generation dual-frequency survey receivers use this technique. Differential (real-time or post processed) can also be in the form of code. This is used for the lower accuracy applications, and works through a radio link that corrects S/A inaccuracies by telling the unknown location what it's almanac and ephemeris errors are. This is mostly for the benefit of those that don't know, and would like to. Hopefully you don't mind.