LEO Satellite Optical Downlink with PPM Uplink Tracking
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Solution Overview
Problem
Low Earth orbit to Earth optical communication systems face challenges in achieving high-speed, reliable downlink data transfer while minimizing complexity and energy consumption, especially for space-based terminals.
Innovation Solution
The system employs a low Earth orbit satellite with a communication subsystem and an optical ground terminal connected via an optical downlink channel and a pulse position modulated (PPM) uplink channel, using a wide angle beam for acquisition and a guidance beam for tracking, with a selective repeat ARQ protocol and negative acknowledge mechanism to ensure efficient data transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional modulation schemes are used for uplink communication, then the system can maintain simpler demodulation processes, but energy consumption increases and transmission efficiency decreases
Solution Approach 1:
The patent replaces conventional continuous-wave modulation schemes with pulse position modulation (PPM) for the uplink channel. PPM encodes information in the temporal position of light pulses rather than continuous amplitude or frequency variations, which reduces the energy required for transmission while maintaining demodulation feasibility through correlation-based detection methods.
Solution Approach 2:
The invention changes the fundamental modulation parameter from continuous amplitude/frequency encoding to discrete temporal position encoding. By varying the time position of pulses within a modulation period rather than continuously adjusting amplitude or frequency, the system achieves lower energy consumption while the receiver can still reliably detect signals through timing correlation techniques.
2Productivity
If high data rates are transmitted during satellite pass, then productivity increases, but the risk of transmission errors increases due to limited contact time
Solution Approach 1:
The patent implements a selective repeat automatic repeat request (ARQ) protocol where the ground station sends negative acknowledgments (NACKs) for erroneously received frames. This preliminary error detection and request mechanism allows the satellite to retransmit specific failed frames during the same pass or subsequent passes, ensuring reliable delivery without sacrificing overall productivity.
Solution Approach 2:
The invention establishes a feedback loop through the uplink channel where the ground terminal monitors downlink transmissions, detects errors, and sends NACK signals back to the satellite. This feedback mechanism enables the system to identify and correct transmission errors systematically, maintaining high reliability even at high data rates during limited satellite passes.
3Reliability
If complex error correction protocols are implemented, then transmission reliability improves, but device complexity and energy consumption increase
Solution Approach 1:
The patent extracts the error correction function from the physical layer modulation and places it at the protocol layer through selective repeat ARQ. Instead of implementing complex forward error correction codes that would increase device complexity and energy consumption, the system uses simpler channel coding combined with NACK-based retransmission, separating error detection and correction into distinct, manageable layers.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides reliable and high-speed data transfer with reduced energy consumption and complexity, ensuring efficient data transmission and minimizing errors through PPM modulation and selective repeat ARQ protocol.
Implementation Method 1
The uplink channel is an acquisition and tracking beacon channel controlled by said point-acquisition-track subsystem, the uplink channel comprising a wide angle beam for acquisition and a guidance beam for tracking
Implementation Method 2
The satellite transceiver is adjustable so that it can be adjusted with respect to the spatial direction of the uplink channel
Implementation Method 3
transmission of said frames of payload data (PD) by the communication subsystem (CS) when a downlink channel (DL) is available connecting the low earth orbit satellite (20) with said optical ground terminal (30)
Data Source
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AI summary
Optical communications system (10) and method for transmission of payload data (PD) from a low earth orbit satellite (20) to an optical ground terminal (30), the low earth orbit satellite (20) being connectable with the optical ground terminal (30) via an optical downlink channel (DL), and the optical ground terminal (30) being connectable with the low earth orbit satellite (20) via an uplink channel (UC); wherein said uplink channel (UC) is an acquisition and tracking beacon channel by means of a ground beacon (GB) controlled by a point-acquisition-track subsystem (PAT), the ground beacon (GB) comprising a wide angle beam (W) for acquisition and a guidance beam (G) for tracking; and wherein the ground beacon (GB) for the uplink channel (UC) is a pulse position modulated PPM channel.