Accurate time measurement was the technology breakthrough that enabled longitude measurement. Much later the Global Positioning Satellite (GPS) was also made possible by a further leap in time measurement – the atomic clock. Position determination accuracy and time measurement are intertwined. Accurate measurement of the length of time for GPS signals to travel from global positioning satellites to your GPS unit is what allows distances from locked satellites to be determined and through the complex number crunching of trilateration your actual position calculated.
Atomic Clock on GPS Chip
Most GPS receivers use inexpensive quartz oscillators as a time reference. The receiver has a clock bias from GPS time but this bias is removed by treating it as an unknown when solving for position. In effect, the receiver clock is continually calibrated to GPS time. If a highly stable reference were used, however, the receiver time could be based on this clock without solving for a bias.
Improvements which can be achieved by having a GPS receiver with an on-board atomic clock include:
A reduction in the number of satellites required for an accurate position determination (3 not 4);
Accelerated GPS satellite re-acquisitions in even challenging environments;
Improved vertical accuracy and
Improved jamming resistance, integrity monitoring, anti-spoofing, fault detection.
Atomic Clock on GPS Chip
Progress to Date
Miniturization of atomic clocks has been under development for a considerable time now. In 2004 (Applied Physics Letters – Aug 30, 2004) scientists from the Commerce Department’s National Institute of Standards and Technology (NIST) reported the development of a minature atomic clock the size of a grain of rice. DARPA’s Chip-Scale Atomic Clock program was initiated to create ultra-miniaturized, low-power, atomic time and frequency reference units. The first GPS receiver in the world to incorporate a chip-scale atomic clock, the Navigation Nugget, remains in military testing/applications enabling navigation in waters and terrains unachievable with current standalone GPS receivers.
The Future of Chip Size Atomic Clocks with GPS Receivers
When one considers the cost of upgrading the infrastructure of 20 plus GPS navigation satellites or handheld receivers the pursuit of terrestrial solutions seems economically sensible. It took time for GPS technology to move from solely military applications to the civilian marketplace and no doubt it will be a similar wait for chip size atomic clocks.
Categories: GPS Technology