I'm not sure whether that's really fine or not. I have used a sirf-based GPS (GT11) that can record sirf binary data which can then be converted to RINEX and have post-processed these files using PPP. All of that went ok, and standard deviations reported by PPP (static mode) are usually between 0.5 an 1.5 metres, sometimes even better. But if I compare two PPP positions from the same point, they can sometimes be several metres apart. In kinematic mode, I get very many outliers and large scatter. When I asked the PPP people at NR Canada, they told me that PPP was developed for dual frequency receivers and was never intended for single frequency receivers. I think they were actually surprised that it worked at all and told me that dual frequency data are required for the correction of stratospheric effects.
For the differential post-processing with base station and rover (as, for example, with the good old Magellan Pro Mark X), single frequency should be ok, because both receivers see the same stratospheric effects. For some unknown reason, I never got the RINEX data (that were converted from sirf binary) to really work with the old Magellan differential post-processing software.
So... I'm not quite convinced how well this would work. Do you know what kind of receiver is actually used in Hagen's devices?