Long term stability of medium price commercial GPS receivers

Long-term phase stability of GPS receivers is important in time-keeping applications. MIKES and Nokia Mobile Phones have studied in co-operation the performance of medium price commercial GPS receivers both in the short term and long-term sense. MIKES has run five GPS receivers of two manufacturers for over five years. An automatic measurement system has collected data from those receivers and from environmental conditions every 10 minutes, having a Cs- clock as a master one. By comparing time differences of the receivers, both short term and long-term stability can be evaluated assuming that all receivers do not drift into the same direction. Long-term stability seems to be about 1...5 ns per year but unexpected transient phase jumps occurred in some receivers when they were rebooted. The magnitude of these jumps varies from 10 to 50 ns. The older receivers show a diurnal phase variation of 10...20 ns due to ionospheric effects. According to Modified Allan Variance (MDEV) the long-term stability has been 10^-13 diurnally and 10^-15 annually. Thermal effects seem to be rather small though the ambient temperature can vary from -30degC to +30degC.