2.1.3 Introduction to GPS
TECHNOLOGY ADVANCES DRIVING GPS
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Rarely have so many seemingly unrelated technological advances been required
to make possible such a complex system as GPS. Briefly they are (PARKINSON, 1983):
- Space System Reliability: The U.S. space
program had by 1973 demonstrated the reliability of space hardware. In
particular, the TRANSIT system had important lessons for although the TRANSIT
satellites were originally designed to last 2-3 years in orbit, some of
the satellites have operated well beyond their design life. TRANSIT has
continued to perform reliably for over 25 years.
- Atomic Clock Technology: With the development
of atomic "clocks" a new era of precise time-keeping had dawned.
Before the GPS program however, these precise clocks had never been tested
in space. The development of reliable, and stable, space-qualified atomic
frequency oscillators (rubidium, and then cesium) was a significant technological
breakthrough. The advanced clocks being used on the GPS satellites routinely
achieve long-term frequency stability of the order of a few parts in 1014
per day (about 1 sec in 3,000,000 years!). This long-term stability is
one of the keys to GPS, as it allows for the autonomous, synchronised generation
and transmission of accurate timing signals by each of the GPS satellites
without continuous monitoring from the ground.
- Quartz Crystal Oscillator Technology:
In order to keep the cost of user equipment down, quartz crystal oscillators
were proposed (similar to those used in modern digital watches), rather
than using atomic clocks as in the GPS satellites. Besides their cost,
quartz oscillators have excellent short-term stability (Table
-- section 1.3.2), and their drift can be corrected as part of the
GPS position determination process.
- Precise Satellite Tracking and Orbit Determination:
Successful operation of GPS depends on the precise knowledge and prediction
of each satellite's position with respect to the earth. This is the task
of the Control Segment. Tracking data collected by the monitor stations
is analysed to determine the ephemeris over the period of tracking (typically
one week). This reference ephemeris is extrapolated into the future and
the data is then up-loaded to the satellites. Prediction accuracies for
one day, of the satellite coordinates, in the sub-dekametre range have
been demonstrated.
- Spread-Spectrum Technology: The ability
to track and obtain any selected GPS satellite signal (a receiver will
be required to track a number of satellites at the same time), in the presence
of a lot of ambient noise is a critical technology. This is now possible
using spread-spectrum and pseudo-random-noise coding techniques.
- Large-Scale Integrated Circuit Technology:
To realise the aim of low cost, low power and small size for much of the
user equipment, the GPS program relies heavily on the successful application
of VLSI circuits, and the powerful computing capabilities built onto them.
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© Chris Rizos, SNAP-UNSW, 1999