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4 Navigations from Ground to Space
drag. By using the Doppler-shifted counts from the ground stations tracking the
satellites, it is also so difficult to precisely determine the satellite’s orbits that
the navigating and positioning accuracies are not high finally. In general, the
positioning accuracy is 200 m by using the Doppler data during a single pass,
and can only reach 10 m by using 15 passes. Thereby, the orbit determination
accuracies are so low that the requirements for high-precision navigation and
precise geodetic positioning cannot be met.
(3) Due to the low frequencies used by the satellites to transmit the signals, it
is difficult to effectively calibrate the ionosphere delay. The Transit satellites
transmit the navigation signals, respectively, at two frequencies 150 and 400
MHz, where the two frequencies are in the VHF band. In the dual-frequency
Doppler navigating and positioning, the low-order term of the ionosphere delay
effect could be weakened by using the two frequencies, but it is difficult to
eliminate the errors from the high-order terms. From the studied results, it
is demonstrated that the high-order terms of the ionosphere delay effect will
cause the geodetic altitude error of more than 1 m near the geomagnetic equator.
As long as high microwave frequencies are used as the carrier frequencies of
navigation satellite’s signals, the ionosphere delay effects will effectively be
eliminated to improve the navigating and positioning accuracies.
For the development of follow-up navigation satellite system GPS, the Transit’s
contributions were mainly to two aspects. One is the dual-frequency technique, which
is used to calibrate the time delay of the radio signal induced by the ionosphere, and
which is incorporated into the GPS to attain the highest positioning accuracy. The
other is the accurate prediction of satellite orbits, another essential GPS technique,
as one of the five fundamental challenges that faced the GPS system designers.
4.6.3 Second-Generation Navigation Satellite System
For the first generation of navigation satellite system based on the Doppler-shifted
navigation principle, it is difficult to meet the requirements for continuous, highly
dynamic and highly accurate navigating and positioning applications. For this reason,
using the passive navigation mechanism with single-way timing and ranging, the
second generation of navigation satellite systems is designed and developed, and
their basic principles are: the user receiver tracks more than four navigation satellites simultaneously, and the pseudorange observables between the satellites and the
receiver are obtained by measuring the arrival-times of the pseudo-random noise
codes; more than four measurement equations are established; finally four unknown
parameters, including three position coordinate components and one clock bias of
the receiver, can be estimated by using the least squares method. The built and
building second navigation satellite systems, including the American Global Positioning System (GPS), Russian Global Navigation Satellite System (GLONASS),
4 Navigations from Ground to Space
drag. By using the Doppler-shifted counts from the ground stations tracking the
satellites, it is also so difficult to precisely determine the satellite’s orbits that
the navigating and positioning accuracies are not high finally. In general, the
positioning accuracy is 200 m by using the Doppler data during a single pass,
and can only reach 10 m by using 15 passes. Thereby, the orbit determination
accuracies are so low that the requirements for high-precision navigation and
precise geodetic positioning cannot be met.
(3) Due to the low frequencies used by the satellites to transmit the signals, it
is difficult to effectively calibrate the ionosphere delay. The Transit satellites
transmit the navigation signals, respectively, at two frequencies 150 and 400
MHz, where the two frequencies are in the VHF band. In the dual-frequency
Doppler navigating and positioning, the low-order term of the ionosphere delay
effect could be weakened by using the two frequencies, but it is difficult to
eliminate the errors from the high-order terms. From the studied results, it
is demonstrated that the high-order terms of the ionosphere delay effect will
cause the geodetic altitude error of more than 1 m near the geomagnetic equator.
As long as high microwave frequencies are used as the carrier frequencies of
navigation satellite’s signals, the ionosphere delay effects will effectively be
eliminated to improve the navigating and positioning accuracies.
For the development of follow-up navigation satellite system GPS, the Transit’s
contributions were mainly to two aspects. One is the dual-frequency technique, which
is used to calibrate the time delay of the radio signal induced by the ionosphere, and
which is incorporated into the GPS to attain the highest positioning accuracy. The
other is the accurate prediction of satellite orbits, another essential GPS technique,
as one of the five fundamental challenges that faced the GPS system designers.
4.6.3 Second-Generation Navigation Satellite System
For the first generation of navigation satellite system based on the Doppler-shifted
navigation principle, it is difficult to meet the requirements for continuous, highly
dynamic and highly accurate navigating and positioning applications. For this reason,
using the passive navigation mechanism with single-way timing and ranging, the
second generation of navigation satellite systems is designed and developed, and
their basic principles are: the user receiver tracks more than four navigation satellites simultaneously, and the pseudorange observables between the satellites and the
receiver are obtained by measuring the arrival-times of the pseudo-random noise
codes; more than four measurement equations are established; finally four unknown
parameters, including three position coordinate components and one clock bias of
the receiver, can be estimated by using the least squares method. The built and
building second navigation satellite systems, including the American Global Positioning System (GPS), Russian Global Navigation Satellite System (GLONASS),