Interferometer Beam Interruption for Satellite Distance Measurement
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Heterodyne laser interferometers face challenges in measuring relative displacement between satellites at long distances (>1 km) due to the significant reduction in power of the measurement beam reflection, leading to increased noise and decreased signal-to-noise ratio, as a result of beam splitter inefficiencies and the presence of a spurious beat note.
Innovation Solution
The interferometer employs a method to interrupt the measurement beam periodically, synchronizing the interruption cycle with the round-trip time of the measurement beam, ensuring that the response beam and spurious measurement beam do not overlap at the detector, thereby reducing noise and enhancing signal clarity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the measurement beam is continuously transmitted to the satellite, then the distance measurement can be performed continuously, but the spurious beat note from the beam splitter leakage overlaps with the measurement signal, degrading the signal-to-noise ratio
Solution Approach 1:
The patent applies periodic interruption of the measurement beam using an acousto-optic modulator. The beam is modulated at a specific frequency to create periodic on/off cycles, allowing the measurement signal to be separated from the spurious beat note in the frequency domain. This resolves the contradiction by enabling continuous measurement through periodic modulation while maintaining high signal-to-noise ratio through frequency separation.
Solution Approach 2:
The patent uses preliminary action by pre-modulating the measurement beam before it travels to the satellite. The acousto-optic modulator imparts a known periodic modulation pattern to the beam in advance, so that when the beam returns, the modulation frequency serves as a reference to distinguish the measurement signal from noise. This allows continuous measurement while maintaining signal integrity.
2Power
If a polarising beam splitter is used to direct the measurement beam, then the beam can be efficiently directed towards the satellite, but a fraction of the beam leaks through to the detector, creating a spurious beat note that interferes with the measurement
Solution Approach 1:
The patent extracts the harmful spurious beat note from the measurement signal by using frequency modulation. The acousto-optic modulator shifts the frequency of the measurement beam, and the periodic interruption separates the spurious signal (which occurs during beam presence) from the useful measurement signal (detected during beam absence), effectively extracting and eliminating the interference.
Solution Approach 2:
By periodically interrupting the measurement beam, the patent creates temporal separation between the spurious beat note generation and the measurement signal detection. The spurious note occurs when the beam is present, while the measurement is made when the beam is absent, allowing clear distinction and elimination of the harmful interference.
3Length of stationary object
If the satellite distance is increased beyond 1 km, then the formation flying arrangement can be maintained, but the reflected measurement beam power becomes much smaller than the spurious leakage, making accurate measurement difficult
Solution Approach 1:
The patent uses periodic beam interruption to create a time-gated measurement system. By knowing when the beam is absent, the system can selectively process only the relevant measurement signals and reject the continuously present spurious beat note. This allows accurate displacement measurement even when the reflected signal is much weaker than the leakage, enabling operation at distances beyond 1 km.
Solution Approach 2:
The acousto-optic modulator serves as an intermediary that introduces a known periodic modulation to the measurement beam. This intermediary element creates a frequency signature that allows the detection system to distinguish the weak reflected signal from the strong spurious leakage, enabling accurate measurement at long distances where the reflected power is minimal.
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 effectively reduces the impact of noise and spurious signals, improving the signal-to-noise ratio and enabling accurate measurement of relative displacement between satellites, even at long distances, by ensuring that only the response and reference beams contribute to the interference signal during designated intervals.
Implementation Method 1
a laser source (30) aligned with a frequency shifter (32)
Implementation Method 2
frequency shifter (32) which is operable to frequency shift the laser beam
Implementation Method 3
a polarising beam splitter (42) which is aligned with the frequency shifter (32)
Implementation Method 4
The phase of the interference signal at the detector (72), measured against the phase of the interference signal at the reference detector (56)
Data Source
AI summary
An interferometer for determining a measure of the distance to an object, comprises means (30) for providing a measurement beam, means (40, 42, 54) for directing at least a portion of the measurement beam towards the object, a receiver (54) for receiving a response beam from the object in response to the at least a portion of the measurement beam and for passing the response beam to a detector (72) to form part of an interference signal at the detector (72), and processing means for determining a measure of the distance to the object in dependence upon the interference signal, and is characterised in that it further comprises means (34) for interrupting the measurement beam.