344
10 TT&C and Telecommunication Technology …
with bidirectional Doppler or mono/bi-directional Doppler might be a powerful orbit
determination data type and could track multiple spacecraft at the same time [5].
10.4.2 Design Method
1. Architecture Design of Telemetry, Telecommand and Telecommunication
The maximum communication distance of the lunar soft-landing exploration mission
was about 4 × 10
5 km. The S-band, X-band and Ka band could be selected for the
telemetry, telecommand and telecommunication of the lunar probe to the ground. In
the design of telecommunication on lunar surface, it was necessary to consider the
diffraction characteristics of radio waves, the constraints of antenna size and other
aspects. Generally the UHF band was selected.
The common telemetry and telecommand modulation architecture for space
missions included unified carrier modulation and spread spectrum modulation. The
telemetry/telecommand and telecommunication of lunar soft-landing mission had
the characteristics of long distance and weak signal. The unified carrier telemetry
and telecommand architecture could realize high sensitivity reception and integration of interferometric measurement signals, which was more suitable for the
lunar mission than the spread spectrum modulation architecture. It was necessary
to maintain certain orbit determination accuracy in a long distance for lunar softlanding missions. Generally coherent ranging, coherent velocity measurement and
interferometric angle measurement were applied to achieve high accuracy of orbit
determination for the lunar lander.
Generally, the suppressed carrier modulation architecture was applied in space
mission telecommunication. Compared with the unified carrier modulation architecture, there was no residual carrier after the suppressed carrier modulation. In case of
certain transmission power, it was beneficial to improve the transmission energy of
unit code element and increase the transmission code rate.
2. Specification Analysis
There were coverage requirements for antenna pattern of both
telemetry/telecommand and telecommunication. Generally, full space coverage
was necessary for telemetry/telecommand antenna pattern. The final coverage
of telecommunication antenna pattern should be determined after the factors are
considered comprehensively such as manufacture, installation, pointing control
accuracy, and etc. According to the antenna pattern coverage requirements, antenna
efficiency and other factors, the antenna gain could be determined.
The channel budget should be regarded to determine the specifications of the
telemetry/telecommand and telecommunication subsystem. It could be checked
whether the ratio of signal power to noise power (signal to noise ratio) or the ratio
of signal power to noise spectrum density (signal to noise spectral density ratio) at
the receiving end was above the threshold by the channel budget in form of table, as
10 TT&C and Telecommunication Technology …
with bidirectional Doppler or mono/bi-directional Doppler might be a powerful orbit
determination data type and could track multiple spacecraft at the same time [5].
10.4.2 Design Method
1. Architecture Design of Telemetry, Telecommand and Telecommunication
The maximum communication distance of the lunar soft-landing exploration mission
was about 4 × 10
5 km. The S-band, X-band and Ka band could be selected for the
telemetry, telecommand and telecommunication of the lunar probe to the ground. In
the design of telecommunication on lunar surface, it was necessary to consider the
diffraction characteristics of radio waves, the constraints of antenna size and other
aspects. Generally the UHF band was selected.
The common telemetry and telecommand modulation architecture for space
missions included unified carrier modulation and spread spectrum modulation. The
telemetry/telecommand and telecommunication of lunar soft-landing mission had
the characteristics of long distance and weak signal. The unified carrier telemetry
and telecommand architecture could realize high sensitivity reception and integration of interferometric measurement signals, which was more suitable for the
lunar mission than the spread spectrum modulation architecture. It was necessary
to maintain certain orbit determination accuracy in a long distance for lunar softlanding missions. Generally coherent ranging, coherent velocity measurement and
interferometric angle measurement were applied to achieve high accuracy of orbit
determination for the lunar lander.
Generally, the suppressed carrier modulation architecture was applied in space
mission telecommunication. Compared with the unified carrier modulation architecture, there was no residual carrier after the suppressed carrier modulation. In case of
certain transmission power, it was beneficial to improve the transmission energy of
unit code element and increase the transmission code rate.
2. Specification Analysis
There were coverage requirements for antenna pattern of both
telemetry/telecommand and telecommunication. Generally, full space coverage
was necessary for telemetry/telecommand antenna pattern. The final coverage
of telecommunication antenna pattern should be determined after the factors are
considered comprehensively such as manufacture, installation, pointing control
accuracy, and etc. According to the antenna pattern coverage requirements, antenna
efficiency and other factors, the antenna gain could be determined.
The channel budget should be regarded to determine the specifications of the
telemetry/telecommand and telecommunication subsystem. It could be checked
whether the ratio of signal power to noise power (signal to noise ratio) or the ratio
of signal power to noise spectrum density (signal to noise spectral density ratio) at
the receiving end was above the threshold by the channel budget in form of table, as