Past and current navigation systems using clocks in orbiting
satellites require earth-based monitor stations which are used to estimate the
space clocks' times and frequencies as well as the satellites' velocity and
positions (orbital elements or ephemerides). The geometries and atmospheric
delays limit current technology such that the best navigation accuracies are
several meters for real-time systems.
Better accuracies, after the tact, can be calculated with large amounts of
averaging. Differential GPS delivers very good accuracies, but is a much more
complicated approach, and it is not conveniently available globally.
This invention combines three technological realities in a
way that provides navigation accuracies more than ten times better (sub 30 cm)
-- than are currently available. These three technologies are as follows.
First, atomic clocks are now being made whose frequencies in
space can be known without calibration or reference to an earth monitor station.
Having an independent frequency standard in space, being measured with an earth
monitor station frequency standard of comparable accuracy, provides frequency
Doppler shift information which can be used for very accurate orbit period
Second, the poorest geometries limiting current navigation
systems can be bypassed if the satellites' basic equations of motion are truly
followed. For example, the period of orbit of a satellite is directly relatable
to its radius vector from the center of mass of the earth. We can utilize the
significant advantage that the satellite moves at right angles to the radius
vector -- giving an optimum geometrical view. Hence, if the orbit period is well
known, the radius is well known. Using the Doppler shift relationships, the
period can be determined very accurately, and its uncertainty is not limited by
atmospheric delay uncertainties. Thus, both the geometric limitations and the
atmospheric delay uncertainties causing most of the inaccuracies in current
navigation systems are circumvented with this invention.
Third, the basic equations of motion will be followed if compensation can be
made for effects perturbing the satellites' orbits. These include but are not
limited to solar winds and pressure, radiation pressure and atmospheric drag.
Drag-free systems have been developed and tested in space which compensate for
these effects. In well designed systems, the residual forces cause accelerations
significantly less than 10-11g, where "g" is the
gravitational acceleration on the surface of the earth. For GPS orbits, 10-11g
would amount to an 8 cm error in one orbit.
As an illustration of combining the three technologies
outlined above, consider Kepler's second law: T2 = (2pi)2r2/GMe, where "T" is the satellite's orbit period,
"r" is the distance from the satellite to the center of mass of the
Earth, "G" is the universal constant of gravitation, and "Me"
is the mass of the Earth.Each time the satellite's clock is observed from a
monitor station's clock, one can conceptualize that point where the Doppler
shift will be zero. The satellite clock's frequency and that of the monitor
station agree with proper relativistic terms included. This can be viewed as a
fiducial point in the satellite's orbit. Hence from pass to pass the Doppler
frequency behavior can be used to determine the orbit period. For example if the
orbit period could be determined with an uncertainty of 175 microseconds, then
the uncertainty of the absolute length of the radius vector to the satellite
from the center of mass of the Earth would be 7 cm at GPS altitudes. This level
of uncertainty would be available using the Doppler effect if the accuracy of
the satellite's clock frequency were 10-11. This approach
effectively bypasses the need of having an accurate measurement of the time-
of-flight of the satellite's signal through the ionosphere and troposphere.
Rather, it is based on the constancy of the gravitational constant and of the
mass of the Earth -- along with the fact that gravitational fields are not
perturbed by the atmosphere.
This invention provides that by having two or more monitor
stations, or by having one with the satellites in non-synchronous sidereal
orbits, the position and the velocity are determinable in real-time for each of
the satellites configured with this invention. Accurate time and frequency
information are natural bi-products.
Utilizing current technology, and experience gained from
previous navigation and timing systems, we have the opportunity, using this
invention, to improve accuracy for navigation and timing by about an order of
magnitude over current GPS numbers. The simulated accuracies are about 30 cm,
and less than a nanosecond for timing.
It may also be possible to simplify system management. For example, as
compared to GPS, only one monitor station is needed, and even if two or more are
used for redundancy and robustness, data do not need to be communicated to one
central location for the calculation of satellite ephemerides. The satellite can
broadcast its position in real-time, and each pass over a monitor station
provides a slight correction update. Neither is communication between the
satellites necessary as is designed into the next generation of GPS Block 2-R
satellites, which will deliver 3 meter accuracy.
- Potential to provide real-time navigation accuracies better than one meter
for avionics and terrestrial vehicles, and to reach accuracies better than
10 centimeters. For example, this invention could provide sub one meter
precision approaches which are so important in blind conditions for
- Potential to provide root mean square (RMS) errors for satellites' orbital
elements of 10 centimeter or better available in real-time. This level of
accuracy can be achieved with state-of-the-art cesium-beam frequency
- Potential to provide an international time reference with sub-one-nanosecond
- Potential to provide normalized
uncertainties between any two timing centers on the surface of the Earth
such that the full accuracy of the best primary frequency standards
involved may be realized.
- Potential to provide an
independent inertial reference from which
fine detail of Earth dynamics may be studied: short-term UTI
variations; Earth and ocean tides; ocean currents; polar motion;
Earth core-mantle slippage; correlations
of Earth dynamics with earthquakes
and as a potential element in earthquake prediction.
- Potential to provide
collision avoidance assist for land, sea
Potential to provide harbor control and avoidance of
sea craft collisions with natural hazards:
reefs, rocks, shallow areas (correlating tides, acean currents and
underwater objects for safety
- Taken to its practical limit with currently available technology, the
invention could reach less
than I cm navigation accuracies. This could be done with hydrogen
maser clocks in the satellites and
in the monitor station(s),
calibrated using timing signals
of both at the 10-14
level. Because of the excellent
short-term stabilities of H-masers,
the frequency during one pass
can be resolved to this accuracy level. By way of example, at GPS
orbits the radius could be resolved to 7 mm.
- This invention has the potential to provide an external-to-the-Earth
inertial-reference frame. Any long-term drift in this frame could be
calibrated using Very Long Baseline Interferometry (VLBI), which can
reference fixed radio stars in space. These two methodologies taken together
could open up some very important opportunities in our understandings of
Earth dynamics. The Earth has day-to-day random variations amounting to
about 7 cm (a point at the equator speeding up or slowing down in an RMS sense,
this amount). VLBI can resolve earth movements,
but this invention pushes the accuracy another
factor often better in the short-term, and could, potentially be much
more convenient and cost effective. There is the very
real possibility that these movements could be correlated with earthquake
phenomena and Earth plate tectonics. Understanding the
Earth's dynamics as a whole system could be extremely
useful. This could also open up increased understanding
of the higher order
moments describing the mass distribution within the Earth as well help observe
Earth core-mantle slippage.
- Potential to provide high-accuracy
orbital elements for satellites
which are semi-independent of the signal delay through the atmosphere. In
other words, most other satellite systems, such as GPS, depend on measured
time delay through the atmosphere. Given
a fixed and unknown delay, the frequency, which this invention
uses does not change.