There are many interpretations of "deeply coupled". SPAN is a deeply coupled system, in that information is both passed from the GPS filter (carrier phase measurements and positions) to the INS filter, and from the INS filter to the GPS filters. Most people call it "tightly coupled" if GPS raw measurements are used to aid the INS filter. It is "loosely coupled" if only GPS positions/velocities are used to aid the INS. Because our GPS and INS filters have two way communication/information transfer, SPAN is considered a "deeply coupled" system. The GPS and INS filters are separate, which is inline with NovAtel's model of SPAN being an add on/upgrade of our standard GNSS receivers. GPS signal tracking is aided with inertial information, as is the RTK filter. Specific details of that are considered proprietary. Because of SPAN's unique architecture, signal reacquisition is vastly superior to any other system, allowing SPAN users to have more GPS measurements in challenging GPS conditions (ie many obstructions) than any other system can provide. Please see the SPAN white paper on our web for details on that. Our tight coupling allows for updates even when less than 4 SVs are available, provided excellent performance in urban canyons, even without a wheel sensor.