4.9 Autonomous Navigation Technology for Spacecrafts
267
The so-called inter-satellite crosslink is essentially a set of radio-frequency transmitting and receiving devices, which used Time Division Multiple Access (TDMA)
to realize the two-way ranging and data exchange between the GPS satellites, so as to
autonomously generate the satellites’ ephemeris and clock parameters by processing
the inter-satellite measurement data with the onboard filters. The original design goal
of autonomous navigation of the GPS constellation was: during 180 days, the URE
of GPS constellation was kept within 6 m, the navigation and positioning accuracy
would not degrade significantly and the integrity of the satellite’s signals could be
monitored, so as to enhance the GPS system’s availability, continuity and reliability.
The autonomous navigation mode and technical features of GPS satellites contain
the inter-satellite crosslink antenna, topological link architecture, communication
mechanism and operational mode of autonomous navigation.
The inter-satellite crosslink is composed of inter-satellite signal transmitter, signal
receiver, feed network, transmitting antenna unit, receiving antenna unit, and signal
and data processing unit, with the functions of inter-satellite two-way ranging, data
communication and instruction release. The transmitting antenna of the inter-satellite
crosslink is an isolated unit antenna with the double-cone helix shape, while the
receiving antenna adopts the direct plane array composed of nine radiating units, of
which one is located at the center of the array, and the other eight are evenly arranged
around the central unit, whose feed phases are opposite to the phase of the central
unit and whose feed amplitudes are configured in proportion. The phase center of
the receiving antenna beam is stable, and the beam edge gain can reach 7 dBi or so.
According to the design of GPS constellation formation and inter-satellite
crosslink antenna beam-forming, the distance of GPS inter-satellite crosslink can
reach 49,465 km. For a constellation with 24 satellites, there are a total of 8−16
links, including the forward and backward links in the same orbit, as well as lateral
links between adjacent orbits.
The TDMA spread spectrum technique is adopted for the inter-satellite communication, with the communication frequency band of UHF (250−290 MHz). Each
satellite is allocated a time interval of 1.5 s for dual-frequency ranging and communication. Thereby, for the constellation with 24 satellites, a sub-frame is 36 s and
master frame is 900 s. The sub-frame zero is the ranging frame; the 1st sub-frame
is used to correct the inter-satellite ranging errors; the 2nd sub-frame is used for the
inter-satellite measurement and data communication; the 3rd sub-frame is reserved
time, which can be used for data processing and communication preparation; the
4th−9th sub-frames are used for the satellites’ ephemeris and clock parameters to
exchange each other; the 10th−24th sub-frames are reserved time for inter-satellite
crosslink signal transceiver, which can be used for the Kalman filtering and navigation message compiling, etc. For the GPS Block IIR and IIR-M satellite series, their
inter-satellite crosslink ranging period can be set, respectively, as 15 min, 1 h, 2 h,
3 h, 4 h and 6 h, among which 1 h is the default value [21].
There are four operational modes for the GPS satellite autonomous navigation: one
is the shut-off mode, only with the inter-satellite ranging and data storage; the other
is the slip mode, in which the inter-satellite ranging, data storage, orbit propagation
and navigation message generation, as well as the updated navigation message that
267
The so-called inter-satellite crosslink is essentially a set of radio-frequency transmitting and receiving devices, which used Time Division Multiple Access (TDMA)
to realize the two-way ranging and data exchange between the GPS satellites, so as to
autonomously generate the satellites’ ephemeris and clock parameters by processing
the inter-satellite measurement data with the onboard filters. The original design goal
of autonomous navigation of the GPS constellation was: during 180 days, the URE
of GPS constellation was kept within 6 m, the navigation and positioning accuracy
would not degrade significantly and the integrity of the satellite’s signals could be
monitored, so as to enhance the GPS system’s availability, continuity and reliability.
The autonomous navigation mode and technical features of GPS satellites contain
the inter-satellite crosslink antenna, topological link architecture, communication
mechanism and operational mode of autonomous navigation.
The inter-satellite crosslink is composed of inter-satellite signal transmitter, signal
receiver, feed network, transmitting antenna unit, receiving antenna unit, and signal
and data processing unit, with the functions of inter-satellite two-way ranging, data
communication and instruction release. The transmitting antenna of the inter-satellite
crosslink is an isolated unit antenna with the double-cone helix shape, while the
receiving antenna adopts the direct plane array composed of nine radiating units, of
which one is located at the center of the array, and the other eight are evenly arranged
around the central unit, whose feed phases are opposite to the phase of the central
unit and whose feed amplitudes are configured in proportion. The phase center of
the receiving antenna beam is stable, and the beam edge gain can reach 7 dBi or so.
According to the design of GPS constellation formation and inter-satellite
crosslink antenna beam-forming, the distance of GPS inter-satellite crosslink can
reach 49,465 km. For a constellation with 24 satellites, there are a total of 8−16
links, including the forward and backward links in the same orbit, as well as lateral
links between adjacent orbits.
The TDMA spread spectrum technique is adopted for the inter-satellite communication, with the communication frequency band of UHF (250−290 MHz). Each
satellite is allocated a time interval of 1.5 s for dual-frequency ranging and communication. Thereby, for the constellation with 24 satellites, a sub-frame is 36 s and
master frame is 900 s. The sub-frame zero is the ranging frame; the 1st sub-frame
is used to correct the inter-satellite ranging errors; the 2nd sub-frame is used for the
inter-satellite measurement and data communication; the 3rd sub-frame is reserved
time, which can be used for data processing and communication preparation; the
4th−9th sub-frames are used for the satellites’ ephemeris and clock parameters to
exchange each other; the 10th−24th sub-frames are reserved time for inter-satellite
crosslink signal transceiver, which can be used for the Kalman filtering and navigation message compiling, etc. For the GPS Block IIR and IIR-M satellite series, their
inter-satellite crosslink ranging period can be set, respectively, as 15 min, 1 h, 2 h,
3 h, 4 h and 6 h, among which 1 h is the default value [21].
There are four operational modes for the GPS satellite autonomous navigation: one
is the shut-off mode, only with the inter-satellite ranging and data storage; the other
is the slip mode, in which the inter-satellite ranging, data storage, orbit propagation
and navigation message generation, as well as the updated navigation message that