At the time it was a very useful technique for many of our users as datum transformation parameters were not readily available and Hypack didn't provide any support for finding even approximated parameters. The only problem is that this signal repeats itself, well every 19 cm, so to find out your position the GPS receiver needs to calculate the number of integer cycles of 19 cm that fit along the path between itself and the satellite. All of a sudden satellites don't follow their predicted path so your GPS receiver is going to be confused. More often than not it will not be able to properly solve the integer ambiguities and it will drop to a standard differential solution. Go out and find the correct datum transformation parameters, set your base station to correct WGS-84 coordinates and Hypack will obligingly calculate correct coordinates on your local datum.
Initial reactions seem to validate our opinion.
If we are at it, you should try to visit sci.geo.satellite-nav and sci.engr.surveying. There you will find a wealth of information about GPS and surveying as well as informed people willing to answer your questions.