Inter-satellite Laser Communication Research Articles

Linearization of Coupled Pointing and Tracking Dynamical Equations

Cooperative pointing and tracking is one important characteristic of deep space inter-satellite laser communication, so that the dynamic mathematical model is complicated nonlinearity and coupling. The contribution of this paper is to substitute the time-domain error transfer function of state equations and to linearize the state space for the sake of physical realization easily in terms of previous authors’ work. The method applied is state feedback and coordinate transformation in nonlinear field, the calculated result is demonstrated that differential equations modeled can be transferred into canonical controllable form and realized physically.

Design and Beam Quality Analysis of Reflective Telescopes through Diffraction Effects and Third Order Aberrations

In the design of inter-satellite laser communication terminal, the diffraction effects are raised in the optical system, when we operate at the wavelength of 1550nm. This is because of the line of sight stabilization in space domain. In this paper, we designed various cassgrains which operates at aforementioned wavelength to overcome those effects. In addition, we analyzed the design as a linear beam expander to obtain a magnified beam for a long haul free space transmission. Then the beam quality is evaluated through Point Spread Function (PSF), Modulation Transfer Function (MTF), Wave front, Distortion, Energy plots & third order aberrations. Finally, we study the designs of Schmidt and Houghton type cassgrains out of which one is opted for Inter-satellite Laser communication.

Influence of localized distortion in lenses on Strehl ratio in lasercom

Strehl ratio (SR) is used to evaluate the beam quality in inter-satellite laser communications (lasercom) and its variation with localized distortion in lenses is investigated. It is shown that SR varies regularly with the characteristic parameters of localized distortion in lenses. To reduce the influence on beam quality, the conditions that localized distortions should meet are given. PV value, which is a very important parameter to evaluate lens surface error, is determined. This work can contribute to the lasercom system design.

Discreteness of Sum-Capacity-Achieving Distributions for Discrete-Time Poisson Multiple Access Channels with Peak Constraints

This letter introduces the discrete-time Poisson (DTP) multiple access channel (MAC) which is applicable to a number of optical wireless links, most notably intersatellite laser communications. It is shown that the sum-capacity-achieving distributions of the DTP MAC under peak amplitude constraints are discrete with a finite number of mass points.

Strehl ratio versus localized distortion in reflectors in lasercom

In intersatellite laser communication (lasercom), the Strehl ratio (SR) is usually used to characterize the transmitter-beam quality. We study its dependence on localized distortion in reflectors based on the Gaussian model. We show that the transmitter-beam quality depends on three parameters of localized distortion and changes regularly with their increase. Localized distortion in reflectors influences the beam quality near the depth of 0.4 λ, which does not depend on other distortion parameters. To reduce the impact of localized deformation on the beam quality, we use the peak-to-valley (PV) value to evaluate the machining precision of mirrors and show that it should be much better than 0.8 λ. The results obtained can be used in designing intersatellite laser-communication systems.

Doppler effect performance for multiple hops optical connections in LEO satellite constellations

Low earth orbit satellite constellations using laser inter-satellite links are recognized to be capable to satisfy the increasing broadband communication demands. However, Doppler in inter-satellite laser communications can degrade the performance of the optical space network. It is crucial to study the Doppler performance on the high precision system. The expression of Doppler wavelength shift for multiple hops laser inter-satellite links is proposed in the paper. The calculation of Doppler wavelength shift in multiple hops ISL is studied with the consideration of location, motion and onboard characters of the satellites. Theoretical and numerical analysis show different routing way between two certain distant satellites will lead to different Doppler wavelength shift and signal transmitting time, and the variation of Doppler wavelength shift is mostly depend on the number of laser inter-satellite links hops; reasonable onboard time delay can augment signal transmitting time with little influence to the Doppler wavelength shift. It is hoped that the study can help the routing selection of the optical space network and system compensation.

Influence of reflector temperature gradients on bit-error rate in periscope laser communication systems in the presence of detector noise and platform vibrations

We consider the influence of thermal deformations of elliptic reflectors in periscope laser communication terminals on the bit-error rate (BER) taking into account the satellite-platform vibrations and the detector noise. We study the relationship between the average BER and temperature gradients of elliptic reflectors in inter-satellite laser communication systems at different values of the light beam wavelength and truncation rate, and different vibration amplitudes. The average BER increases with increase in the vibration amplitude, and it is noteworthy that the average BER increases with increase in the beam truncation rate, which contradicts the result obtained in [1] without taking noise into account. We find that for some temperature gradients there is an optimum communication wavelength that provides the minimum average BER. We use the back-fixing method for fixing the elliptic reflectors, which has proved to be much less sensitive to temperature gradients compared to the traditional around-fixing method by a press board.

Control analysis of acquirement and locking in inter-satellite laser communications

Process of acquirement and locking-up of compound axis system in inter-satellite laser communication has been studied. The effect of different condition parameters on the process of acquirement and locking-up have also been researched and simulated. Simulation results show that when the system with appropriate bandwidth has been adopted, both fine pointing system and coarse pointing system can satisfy three requirements of step response, stability criterion and dwell time and then finish the acquirement and locking-up of beacon laser beam. The simulation results provide the suited condition parameters for both fine pointing system and coarse pointing system, which is very helpful to the subsequent point and tracking processes.

Research on Bit Error Rate in the Presence of Local Wavefront Aberration in Intersatellite Laser Communications

Local distortions in lenses of transmitter optical system could further deteriorate the performance of intersatellite laser communications (lasercom). Bit error rate (BER) for on-off keying system is investigated considering the combined influence of local distortion and pointing error, without considering the noise of the detector. It is shown that BER varies regularly with the characteristic parameters of local distortion. Peak-to-valley value that is usually used to evaluate lens machining precision is determined, and the key principles for selecting lenses are given. This paper can contribute to the lasercom system design.

Received power analysis due to antenna deformation based on wavelet in inter-satellite laser communication links

Arbitrary deformation of optical antenna caused by processing technique and complex outer-space environment in inter-satellite laser communications can degrade the performance of the system. It is crucial to study the influence of deformation on the high precision system. Wavelet expression is proposed in the paper for diversified distortions. The influence on received power is investigated with the consideration of location, scale and depth of the deformation. Theoretical and numerical analysis show that the distortion attenuation factor increases with scale factor; with the location of the deformation shift from the center to the edge of the antenna, the distortion attenuation factor will increase till the whole deformation is away from the shelter of the secondary mirror, and then decrease; with the linear increase of depth, when the pattern of deformation can be well approximated by a constant value, the distortion attenuation factor will fluctuate periodically, or there will be no obvious variation regularity between them. It is hoped the study can help the optical devices selection and system compensation.

Received Signal Strength Reduction Analysis Based on the Wavelet Model in Intersatellite Laser Communications

The performance of intersatellite laser communication system can be affected by optical antenna deformations. A deformation model is established based on wavelet analysis for diversified deformations caused by processing technique and complex environment of the outer space. Numerical analyses show that the reconstruction error of a localized irregular deformation by wavelet could achieve 10-12 level, which is far better than Zernike polynomials to some extend. Relationships between diversified antenna deformations and the effect on received signal strength are derived based on the model. According to simulation results, the effect of deformation on received signal strength increases with the dilation factor of the model, decreases with the shift factor of the model, and tends to increase with irregular fluctuations with the linear increase of wavelet coefficient of the model. This study may be of some help in the design of intersatellite laser communication systems.

Mutual alignment errors analysis based on wavelet due to antenna deformations in inter-satellite laser communications

Wavelet analysis is employed in the paper to model diversified optical antenna deformations in inter-satellite laser communications. Mutual alignment errors, which comprise pointing and tracking errors, caused by the deformations are investigated with the model. Theoretical and numerical analysis show that both errors increase with the dilation factor of the model. Tracking error increases monotonously with the shift factor of the model, while pointing error increases first, and then decreases. When the deformation can be well approximated to a constant, both errors fluctuate periodically with the coefficient of the model. Otherwise, there is no obvious regularity for both errors with the increase of the coefficient. A reference for the machining precision of optical antennas is presented, and a method to reduce the effect of deformations is recommended. It is hoped that the study can contribute to improve the performance of inter-satellite laser communication systems.

Influence of wave-front aberrations on bit error rate in inter-satellite laser communications

We derive the bit error rate (BER) of inter-satellite laser communication (lasercom) links with on–off-keying systems in the presence of both wave-front aberrations and pointing error, but without considering the noise of the detector. Wave-front aberrations induced by receiver terminal have no influence on the BER, while wave-front aberrations induced by transmitter terminal will increase the BER. The BER depends on the area S which is truncated out by the threshold intensity of the detector (such as APD) on the intensity function in the receiver plane, and changes with root mean square (RMS) of wave-front aberrations. Numerical results show that the BER rises with the increasing of RMS value. The influences of Astigmatism, Coma, Curvature and Spherical aberration on the BER are compared. This work can benefit the design of lasercom system.

非高斯畸变对星间光通信瞄准偏差的影响

非高斯畸变对星间光通信瞄准偏差的影响

Effect of Doppler shift on differential phase-shift keying receivers using interferometric demodulation and balanced detection in intersatellite laser communication links

Effect of Doppler shift on differential phase-shift keying receivers using interferometric demodulation and balanced detection in intersatellite laser communication links

Securing Funds for Research or Product Development

Doppler shift due to relative movement between satellites must be taken into consideration in intersatellite laser links. The degradation of differential phase-shift keying (DPSK) receiver performance caused by Doppler shift is assessed. In order to simplify the complexity of the receiver, we discuss conditions that will bring the new design concept for a robust laser communication system in space, which does not require precise active control. When the precise active control has to be used, the impact of frequency drifts relative to optical filter's center frequency becomes evident. Optimum bandwidths of optical filters yielding optimum performance over the entire range of frequency shifts are determined to further improve the receiver performance. The results obtained here will be helpful in the design of DPSK/self-homodying intersatellite laser communication systems. C 2010 Society of Photo-Optical Instrumentation Engineers.

Pointing error due to temperature distribution of SiC reflectors in intersatellite laser communications

Wave-front aberrations due to thermal deformation of elliptical reflectors in periscopic laser communication terminals are studied, and are fitted by Zernike elliptical polynomials. The relationship between pointing error and temperature distribution of elliptical reflectors in intersatellite laser communication systems is researched. The back-fixing method is introduced for fixing the elliptical reflectors, and is proven to reduce pointing error compared to the traditional around-fixing method using a press board. It is shown that the difference between uniform temperature and reference temperature has a much stronger influence than the temperature gradient. For the back-fixing method, the pointing error changes periodically as the uniform temperature changes with a period value of 1.6 degrees C, and the maximum pointing error value is 16.7murad. The result also tells us that, if the difference between uniform temperature and reference temperature is within 1.0 degrees C, there is no pointing error caused by back-fixed reflectors. We hope this work can contribute to the thermal control of elliptical reflectors in intersatellite laser communication terminals.

Large-aperture double-focus laser collimator for PAT performance testing of inter-satellite laser communication terminal

A laser collimator is necessary for testing and verification of the pointing, acquisition and tracking (PAT) performance of inter-satellite laser communication terminals on the ground. The laser collimator must have a large clear aperture to fit the PAT performance testing system. The PAT subsystem has a large field of view for the acquisition and a high angular accuracy for the fine tracking. To resolve the conflict between large field of view and fine angular resolution, a large-aperture double-focus laser collimator is proposed and its optical design and mechanical structure are described. The collimator mainly consists of a primary lens, a reflector, a beam-splitting plate, a secondary lens, two compensating lenses, two imaging sensors and a laser. The primary lens directly forms the long focal length arm of the collimator. The combination of the primary lens and the secondary lens form the short focal length arm of the collimator. The collimator has an angular resolution <0.75 μrad and a 10 mrad field of view. For the collimator, the incident beam is focused on the two imaging sensors by its two arms, and the beam emitted from the laser is collimated and transmitted. The collimator is combined with an optical scanner and a fine beam deflector to test and verify the PAT performance of the inter-satellite laser communication terminal in a full physical manner.

Receiver power penalty due to localized distortion induced by transmission-type optical antenna in inter-satellite laser communications

Based on a truncated ellipse Gaussian model for localized distortion in transmission-type optical antenna, the receiver power penalty due to localized wavefront deformation is researched. It is shown that the receiver power depends on distortion deepness h, radii a and b, and position d, and changes regularly with them. Localized distortion has the greatest influence on receiver power at a depth of h ≈ 1.45λ, which does not depend on other deformation parameters. The maxima of the receiver power penalty for different distortion radii are given. To reduce the impact of localized distortion on the receiver power, the machining precision of lenses should be better than 1.45λ. We hope the results can be used in the design of intersatellite laser communication systems.

Pointing and tracking errors due to localized distortion induced by a transmission-type antenna in intersatellite laser communications

A truncated ellipse Gaussian model to express localized distortion is developed to a transmission-type optical antenna, based on which the effects of localized deformation on pointing and tracking errors are researched. It is shown that localized distortion has the greatest influence on pointing and tracking errors when distortion deepness h approximately 0.8 lambda, which does not depend on other distortion parameters. To reduce the impact of localized deformation on pointing and tracking errors, the machining precision of the objective lens of the transmission-type antenna should be much better than 0.8 lambda. The requirement of the machining precision for lenses is lower than that for mirrors. The maxima of pointing and tracking errors due to the localized distortion with different radii are given. We hope the results can be used in the design of intersatellite optical communication systems.