5.6 Methods of Large-Scale Navigation
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pulsars generate the signal radiations in the electromagnetic wave bands from
the radio to the infrared, visible, ultraviolet, X-ray and gamma-ray. The radio
signals can be observed on the ground and in space, but require the larger
aperture telescopes. Because the X-ray signals are unable to pass through the
Earth’s dense atmosphere, they can only be detected on the spacecrafts in the
outer atmosphere of the Earth, or the rovers on the surface of the planets without
dense atmosphere.
(5) There is the discrepancy in ranging way. For the GPS, the distances between the
GPS satellites and the receiver can be determined by using the pseudo-random
noise codes with the time-stamped, so as to achieve the receiver positioning.
The pulsars are very far away from the solar system and emit the pulse signals
without the time-stamped message, so it is difficult to accurately measure their
distances. Obviously, the distances between the pulsars and spacecrafts cannot
be used as the observables to carry out the positioning.
(6) There is the discrepancy in phase-measured way. For the GPS, the integer
ambiguity can be determined by the combination of carrier phase and ranging
code signals. For the XPNAV, the pulsars emit the X-ray signals, and the integer
ambiguity is solved by measuring the pulse phase signals rather than the carrier
signals.
(7) There is the discrepancy in phase change rate. The GPS receivers can accurately
track the carrier signals transmitted by the satellites and measure the carrier
phase change rates, i.e., the Doppler shifts. However, the carrier signals emitted
from the pulsars cannot be directly tracked and measured, so it is difficult to
measure the carrier phase change rates. Even though, the motions of spacecrafts
relative to the pulsars will produce the Doppler shifts of pulse phases, and thus
the change rates of pulse phases can be measured.
For the GPS carrier phase measurements, the commonly used fast ambiguity resolution methods include exchanging antenna, P-code dual-frequency technique, ambiguity function method, least squares searching and ambiguity covariance, among
which the latter three methods are referred to as the space-searching method of the
ambiguity, which are also the basic method to solve the integer ambiguity for the
XPNAV. The so-called ambiguity space-searching method is a way to search and
solve the ambiguity in a three-dimensional coordinate space symmetrically about a
central point. The central point of space-searching is selected according to the actual
application, usually taking the location of defining the pulsar timing model. For the
spacecrafts flying around the Earth or the Earth–Moon system, the center of mass
of the Earth or the center of the Earth–Moon system can be selected as the central
point of space-searching. For the space missions to investigate the planets in the
solar system, the centers of mass of the planets can be selected as the central point of
space-searching. For the spacecrafts lost in space, the last known orbit point can be
selected as the central point of space-searching. In theory, any known position can be
used as the central point of space-searching. The closer is the known position away
from the real position of the spacecraft, the smaller the range of space-searching will
be, and the faster the ambiguity resolution is.
355
pulsars generate the signal radiations in the electromagnetic wave bands from
the radio to the infrared, visible, ultraviolet, X-ray and gamma-ray. The radio
signals can be observed on the ground and in space, but require the larger
aperture telescopes. Because the X-ray signals are unable to pass through the
Earth’s dense atmosphere, they can only be detected on the spacecrafts in the
outer atmosphere of the Earth, or the rovers on the surface of the planets without
dense atmosphere.
(5) There is the discrepancy in ranging way. For the GPS, the distances between the
GPS satellites and the receiver can be determined by using the pseudo-random
noise codes with the time-stamped, so as to achieve the receiver positioning.
The pulsars are very far away from the solar system and emit the pulse signals
without the time-stamped message, so it is difficult to accurately measure their
distances. Obviously, the distances between the pulsars and spacecrafts cannot
be used as the observables to carry out the positioning.
(6) There is the discrepancy in phase-measured way. For the GPS, the integer
ambiguity can be determined by the combination of carrier phase and ranging
code signals. For the XPNAV, the pulsars emit the X-ray signals, and the integer
ambiguity is solved by measuring the pulse phase signals rather than the carrier
signals.
(7) There is the discrepancy in phase change rate. The GPS receivers can accurately
track the carrier signals transmitted by the satellites and measure the carrier
phase change rates, i.e., the Doppler shifts. However, the carrier signals emitted
from the pulsars cannot be directly tracked and measured, so it is difficult to
measure the carrier phase change rates. Even though, the motions of spacecrafts
relative to the pulsars will produce the Doppler shifts of pulse phases, and thus
the change rates of pulse phases can be measured.
For the GPS carrier phase measurements, the commonly used fast ambiguity resolution methods include exchanging antenna, P-code dual-frequency technique, ambiguity function method, least squares searching and ambiguity covariance, among
which the latter three methods are referred to as the space-searching method of the
ambiguity, which are also the basic method to solve the integer ambiguity for the
XPNAV. The so-called ambiguity space-searching method is a way to search and
solve the ambiguity in a three-dimensional coordinate space symmetrically about a
central point. The central point of space-searching is selected according to the actual
application, usually taking the location of defining the pulsar timing model. For the
spacecrafts flying around the Earth or the Earth–Moon system, the center of mass
of the Earth or the center of the Earth–Moon system can be selected as the central
point of space-searching. For the space missions to investigate the planets in the
solar system, the centers of mass of the planets can be selected as the central point of
space-searching. For the spacecrafts lost in space, the last known orbit point can be
selected as the central point of space-searching. In theory, any known position can be
used as the central point of space-searching. The closer is the known position away
from the real position of the spacecraft, the smaller the range of space-searching will
be, and the faster the ambiguity resolution is.