Calculation of Azimuth, Elevation and Station Star Distance
Constellation Design: How to Cover
Satellite constellation: A satellite constellation composed of multiple satellites according to certain rules.
Covering zone of satellite ring: The satellite ring consisting of multiple satellites covers the ground along the space orbit.
Types of constellations: Star constellation ("iridium" system) and reticular constellation, the Walker constellation ("global star" system).
The area covered by satellites, and the duration of coverage, depends mainly on the number of satellites in the constellation, their altitude and orbital inclination .
constellation design
Best constellation design principles:
By selecting the best orbital inclination angle and the position of the ascending node, in the orbit with the lowest possible altitude, use as few satellites as possible, make the minimum elevation angle as large as possible, and provide all-weather continuous coverage to the designated area.
Definition of coverage (usually takes the first definition):
Determine the specific range of coverage by calibrating the trajectory of the ground point with the smallest elevation angle;
The area on the ground that is within the half-power angle of the satellite antenna beam is the coverage area.
In the design of satellite constellations, the first method of defining coverage is generally used.
Constellation Type:
According to whether to control the mutual positional relationship of the satellites in the constellation, the satellite constellation can be divided into two types: phased constellation and random constellation.
A phase constellation consists of satellites with relatively fixed positions in time. The advantage is that the required performance can be achieved with fewer satellites; the disadvantage is that the satellites are larger due to the need for fuel and thrusters, and the system also requires a satellite orbit control network.
A random constellation consists of satellites with different orbital altitudes and inclinations. The advantage is that there is no orbit control, and no measures need to be taken after the satellite is launched into orbit, which saves the cost of orbit correction and orbit control within the validity period of the satellite after launch, but requires more satellites to achieve the same performance.
For any satellite constellation to continuously cover the entire earth (including the poles), at least 5 satellites (single-satellite coverage) are required; if 2 satellites are required to be visible at any time, at least 7 satellites are required.
The optimal design of a satellite constellation requires consideration:
The total number of satellites required by the system, the complexity and cost of the satellites, the life of the satellites, the required minimum angle of belief, the influence of the Van Lun belt radiation, the flexibility of satellite launch, the propagation delay and the system reliability, etc.
Communications satellite
Space platform, payload (antenna subsystem and communication repeater)
Selection of working frequency band and characteristics of radio wave propagation
Working frequency:
UHF band: 400/200 MHz;
L-band: 1.6/1.5 GHz;
C-band: 6/4 GHz;
X-band: 8/7 GHz;
Ku-band: 14/12 GHz, 14/11 GHz;
Ka-band: 30/20 GHz
Propagation characteristics of radio waves:
Propagation loss in free space
Atmospheric absorption loss
The Fading Phenomenon of Mobile Satellite Communication Waves
Doppler shift
Doppler shift
Hazard : Coherent Demodulation
Use : positioning
Compensation method:
Terminal-satellite closed-loop frequency control; on-board Doppler frequency shift correction; pre-correction at the receiving end of the link; pre-correction at the transmitting end of the link.