Acquisition and Tracking Technology for Space Laser Communication

ZHANG Jinlong,CHEN Anhe,ZHANG Wenrui*,ZHANG Jianhua,BIAN Jingying,ZHANG Ruofan,REN Bin

China Academy of Space Technology (Xi’an),Xi’an 710100

Abstract:The divergence angle of laser beam used in space laser communication is usually no more than 100 μrad.Using laser beam with small divergence angle to achieve acquisition and tracking for space laser link has always been a difficult problem.In addition,the random nature of the atmosphere will affect the satellite-ground laser link,which increases the difficulty of the acquisition and stable tracking for the laser link.Thus,taking into account the above challenges for satellite-ground laser communication,an acquisition and tracking scheme of using both beacon beam and signal beam was designed for the Laser Communication Terminal (LCT) of Shijian 20 satellite.In-orbit test results indicated that under the condition of moderate atmospheric turbulence (atmospheric coherence length r0≈3cm),the process of acquisition and tracking for the satellite-ground laser link can be completed within 1s after the initial pointing between the LCT and Optical Ground Station (OGS) is performed,and the tracking error was less than 1 μrad (3σ).In addition,the laser link can be re-established quickly once being interrupted by unsteady atmospheric turbulence,and can be maintained for a long time under moderate twurbulence conditions,which lays a foundation for future application of satellite-ground laser communication.

Key words:space laser communication,Shijian 20 satellite,satellite-ground laser link,acquisition and tracking

1 INTRODUCTION

It is well known that space laser communication is point-topoint transmission.Since the divergence angle of laser beam is very small,accurate alignment of laser beam is needed to achieve communication,hence the pointing,acquisition and tracking (PAT) technology has been a research hotspot for a long time,where the constant pursuit is to shorten the acquisition time,improve the tracking accuracy and maintain the link stable[1,2].The working process of the PAT system for space laser communication includes three stages:initial pointing,rapid acquisition and combined coarse and fine tracking[3].At present,there are mainly two acquisition and tracking methods of using beacon beam and signal beam.The former uses beacon laser with large divergence angle as the scanning and tracking beam[4],while the latter directly uses signal laser with small divergence angle as the scanning and tracking beam[5].

Using beacon beam with large divergence angle for acquisition and tracking can cover the field of uncertainty (FOU) quickly and shorten the acquisition time,but the system is relatively complex.In addition,there is an unavoidable link loss caused by deviation of the optical axis between the beacon beam for tracking and the signal beam for communication.In contrast,the system using signal beam with small divergence angle is relativelys imple,without the above-mentioned coaxial problems,however,it is difficult to establish the laser link initially,especially when applied to the satellite-ground laser link,and the link may be interrupted because the narrow signal beam is easily affected within the unstable atmospheric channel[6].

At present,the above two tracking methods have been successfully applied in space laser communication.The European Data Relay System (EDRS) had successfully established an optical communication link between Alphasat (GEO satellite)and Sentinel 1A (LEO satellite)[7,8],where the link distances is up to 45,000 km and data transfer rate is 1.8 Gbps,and the acquisition and tracking process was performed using only narrow signal beam.In-orbit test results showed that the duration of the spatial acquisition was consistently less than 40 s and single axis tracking error was up to 75 nrad[7].NASA’s Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time duplex laser communications between Lunar Lasercom Space Terminal (LLST) and Lunar Lasercom Ground Terminal (LLGT)[9],uplink beacon beam from LLGT is used for fast acquisition,and signal beams from both LLST and LLGT are used for fine tracking,once the LLCT gimbal completed slewing,the whole acquisition and tracking phase can be finished within about 2.7 s[10].Based on the success of LLCD,NASA developed the Laser Communications Relay Demonstration Project (LCRD) to demonstrate high bandwidth optical communications links between a GEO satellite and two ground stations[11].LCRD uses a similar acquisition and tracking scheme as used in LLCD,but the maximum data transfer rate is raised to 2.88 Gbps,compared with 622 Mbps for LLCD[9].

In the current,the master-slave bidirectional acquisition and tracking scheme of using both beacon beam and signal beam was designed for the LCT of Shijian 20 satellite.The initial acquisition is achieved with the beacon beam,and then switched to signal beam for fine tracking and communication,while keeping Coarse tracking with the beacon beam.Once the signal beam loses lock due to the influence of the unsteady atmospheric channel,the beacon beam will quickly guide the re-establishment of the fine tracking and communication link.

2 ACQUISITION AND TRACKING STRATEGY FOR SATELLITE-GROUND LASER LINK

Beacon light laser is always installed on a LCT independently,due to the error of installation and the influence of space thermal environment,the coaxiality of transmitting signal beam and transmitting beacon beam may deteriorate in orbit,This may result in an interruption of the communication link if the tracking link is kept only by beacon beam.In order to solve this problem and achieve fast acquisition,the Shijian 20 satellite adopted the master-slave bidirectional acquisition and tracking scheme which used both beacon beam and signal beam,hence the satellite-ground laser link can be established based on the following process.

1) The LCT points to the OGS according to the the OGS position and the satellite attitude,meanwhile,the OGS points to the satellite according to the satellite ephemeris.

2) After the initial pointing is established,the LCT (OGS)transmits beacon beam and scan the FOU of the OGS(LCT) as master side,while the OGS (LCT) transmits signal beam and stare at the LCT (OGS) as slave side,once the beacon beam transmitted by the master side is detected,the slave side adjusts the pointing direction quickly and point to the master side more precisely.

3) When the master side capture the signal beam transmitted by the slave side,it immediately stops scanning to track the signal beam,and sends signal beam simultaneously.At this time,the slave side can also quickly capture and track the signal beam from the master side,which marks the laser communication link is successfully established.

The LCT of Shijian 20 satellite has two acquisition modes.It can be used as the master side to transmit signal or beacon beam for spiral scanning,and can also be used as the slave side to stare at and detect the scanning laser beam from the target satellite or OGS.

2.1 “Staring-Scanning” Acquisition Mode

“Staring-Scanning” acquisition mode is commonly used for inter-satellite and satellite-ground laser communication links,which can be divided into laser beam scanning mode and field of view scanning mode.Field of view scanning mode is suitable for air-ground or satellite-ground laser communication systems where the FOU is small and the divergence angle of the laser beam from staring side can cover the entire FOU.Since the OGS is stationary,The LCT on satellite can point to the OGS more precisely and the FOU is small,the laser beam with a small divergence angle from LCT on satellite can cover the entire FOU.

The acquisition time required for this acquisition mode depends on the FOU (θU),the divergence angle of laser beam(θBC),the scanning dwell time (Td),and the scanning times (Nt).The corresponding expression formula is as follows[12]:

Where,krepresents the scanning overlap coefficient,usually 50%.

2.2 “Scanning-Scanning” Acquisition Mode

The “scanning-scanning” acquisition mode,also known as double-ended scanning mode,is a new acquisition mode in the case that neither the divergence angle of the laser beam nor the field of view can cover the FOU.It means that the master side transmits laser beam to scan FOU of the slave side,while the slave side also transmits laser beam to scan FOU of the master side.The bidirectional acquisition is realized when the laser beam on one side appears in the field of view of the other side.The calculation formula for the acquisition time in this acquisition mode is as follows[12]:

Where,the footmarktdenotes the parameters of master side,andrdenotes the parameters of slave side.

3 SCANNING STRATEGY FOR SATELLITE-GROUND LASER LINK

The most commonly used scanning method for acquisition is the Archimedes spiral scanning method,where the LTC scans from the center of FOU where the corresponding target terminal appears most likely,and keep spiral scanning with a constant linear velocity until the entire FOU is covered.This scanning method is efficient and easy to implement,while its actual scanning speed depends on the bandwidth of the scanning actuators.

However,due to the instability in the satellite-ground random atmospheric channel,the failure probability of acquisition increases.Thus,the joint scanning and tracking scheme with both beacon beam and signal beam was designed for the LTC of Shijian 20 satellite.This scheme adopts the spiral scanning strategy of using “Small Circle inside Great Circle”,as shown in Figure 1,that is,the Coarse Pointing Assembly (CPA) scans coarsely the FOU with large step,large pitch and large range,when the CPA completes each step,the Fine Pointing Assembly(FPA) will perform the fine scanning according to its maximum scanning ability,and finally the entire FOU is scanned quickly by using both CPA and FPA.

Figure 1 Spiral scanning strategy of using “Small Circle inside Great Circle”

4 THE GROUND TEST FOR THE LCT OF SHIJIAN 20 SATELLITE

The LCT of Shijian 20 satellite,as shown in Figure 2,uses a periscopic CPA with two-dimensional rotation function.The rotation range of the CPA can cover 0－180° in azimuth and-90°－90° in elevation,so the LCT can establish laser communication links with targets such as optical ground stations,low-orbit satellites,and high-orbit satellites.The laser link is established by the strategy of master-slave bidirectional acquisition and tracking,and the spiral scanning process is completed through coordinated action of CPA and FPA.

Figure 2 The LCT of Shijian 20 satellite

Before the launch of the Shijian 20 satellite,the laboratory test and outfield test are carried out separately to validate the acquisition and tracking performance of the LCT.The laboratory test is based on an optical platform,which can simulate the farfield space laser beam,and the outfield test is carried out under real atmospheric channel conditions.

The optical platform can obtain the far-field pattern of the signal beam by performing Fourier transform on the beam emitted by the optical antenna,thereby realizing the transformation of the signal beam from the near field to the far field.Simultaneously,the optical platform can also simulate environmental conditions such as micro-vibration of satellite platform,orbital motion,and space background light.During the laboratory test,the frequency range of the micro-vibration spectrum was set to 0－200 Hz,the orbital speed was set to 0.5°/s,and the illuminance of space background light was set to 0.3 times the solar constant.The test of acquisition and tracking performance is completed by establishing a laser link between the LCT and the analog terminal of the optical platform.Figure 3 shows the variation of average acquisition time with FOU in different scanning modes.

Figure 3 The variation of average acquisition time with FOU in different scanning modes

Figure 4 The tracking error (right) after applying sine vibration (left) with amplitude of 40 μrad and frequency of 1.0 Hz

Figure 5 The tracking error (right) after applying sine vibration (left) with amplitude of 25μrad and frequency of 5.0 Hz

Figure 6 The tracking error (right) after applying sine vibration (left) with amplitude of 10μrad and frequency of 30 Hz

Figure 7 The tracking error (right) after applying random vibration (left) with frequency range of 0 -200 Hz

The LCT’s capability of suppressing the micro-vibration of the satellite platform is tested based on the optical platform,as shown in Figure 4－Figure 7,the LCT can well suppress the micro-vibration of the satellite platform,and the tracking error is less than 2.8μrad (3σ) under various vibration conditions.

The outfield test,as shown in Figure 8,was carried out between two buildings 1.0 km apart,under real atmosphere was used as the space laser link channel.The docking target of the LCT is a ground analog LCT with beacon beam and signal beam transceiver functions.Figure 9 shows the variation of average acquisition time with FOU in laboratory test and outfield test.And Figure 10 shows the change of laser spot center on the tracking detector before and after fine tracking.

Figure 8 The outfield test for the LCT of Shijian 20

Figure 9 The variation of average acquisition time with FOU in laboratory test and outfield test

Figure 10 The change of laser spot center on the tracking detector before and after fine tracking

5 SATELLITE-GROUND LASER LINK DEMONSTRATION FOR SHIJIAN 20 SATELLITE IN ORBIT

The Shijian 20 satellite was launched in December 2019,and is operating in geosynchronous orbit.The optical axis of the LCT was firstly calibrated in orbit,it is proved that the self-calibrating function of the LCT performed well in orbit.After self-calibrating in orbit,the deviation between the acquisition optical axis and communication optical axis is less than 2μrad,as shown in Figure 11.The deviation satisfies the performance index requirement (≤5μrad) and is consistent with the ground test results.

Under the condition of moderate atmospheric turbulence(atmospheric coherence length r0－3 cm),the laser link has been successfully established between Shijian 20 satellite and Lijiang optical ground station by adopting “Staring-Scanning”acquisition mode,the link distances is about 37000 km and data transfer rate is 9.6 Gbps,as shown in Figure 12.Figure 13 shows the process for establishment of the satellite-ground laser link.Firstly,the optical ground station stares at the satellite according to the satellite ephemeris and transmits uplink beacon beam,while the LCT points to the optical ground station according to the OGS position and satellite attitude where the,and transmits downlink beacon beam.After the above step initial pointing is finished,the LCT captures the uplink beacon beam quickly and switches to the tracking state within 1 s.Figure 14 shows the telemetry data from the tracking detector of the LCT,and the statistical results shows that the fine tracking error is less than 0.75μrad (3σ) in both azimuth and elevation.

Figure 11 The deviation of acquisition optical axis and communication optical axis after self-calibrating,left is inorbit test results and right is ground test results

The satellite-ground laser link had lasted for about 72 minutes.Due to the influence of random atmospheric channel,the link was interrupted twice in continuous tracking phase,However,once the link was interrupted,the LCT can automatically scan the OGS according to the pointing direction recorded before the lost of uplink signal beam,and the laser link was re-established within 1 s.

Figure 12 Shijian 20 satellite to Lijiang optical ground station laser link

Figure 13 The process for establishment of the satellite-ground laser link

Figure 14 The fine tracking error in azimuth and elevation

The LCT of Shijian 20 satellite has completed the satellite-ground laser link demonstration of the first stage in orbit,and the key technologies such as self-calibrating,acquisition and tracking mode for satellite-ground laser link have been validated.In the future,in-orbit test will continue to be carried out to accumulate sufficient test data,which lays a foundation for the application of satellite-ground laser communication.

6 FUTURE TECHNOLOGICAL TREND

6.1 The Combined Acquisition and Tracking Mode of Using both Beacon Beam and Signal Beam Will Be the First Choice for Satellite-Ground Laser Link

With the technical validation of LCTs in orbit at home and abroad,the acquisition and tracking mode used to establish the inter-satellite laser communication link has gradually developed from using wide beacon beam to narrow signal beam.While avoiding the deviation of the optical axis between the beacon beam and signal beam,the complexity of transceiver system has been significantly reduced.However,due to the influence of random atmospheric channel,scanning failure and tracking loss are easy to occur when narrow signal beam is used alone for acquisition and tracking.Through in-orbit test of Shijian 20 satellite,the method of using wide beacon beam for initial acquisition and then transferring to fine tracking by narrow signal beam has been proved to be feasible.In addition,the function of coarse tracking by beacon beam is retained,thus,once the signal beam loses lock,the fine tracking link can be guided to be re-established quickly by the coarse tracking.

6.2 The Tracking Mode Will Develop from Incoherent Tracking to Coherent Tracking

Traditional incoherent tracking methods usually use an energy detector,which are susceptible to stray light,this will cause the signal-to-noise ratio to decrease or even the energy detector to be saturated,making the tracking link out of lock or even unable to be established.Coherent tracking adopts the signal intensity generated after mixing the signal beam with the laser beam emitted by local oscillator as the tracking error,and the control loop compensates for the error in real time.Due to the introduction of coherent laser beam emitted by local oscillator,the ability to suppress background noise is greatly improved.The coherent tracking method has great advantages in solving the above problems of satellite-ground laser communication links during daytime and is the future development direction.In the second stage of in-orbit test for the Shijian 20 satellite,the characteristics of coherent tracking and communication will be further validated during the daytime.

6.3 The Optical Carrier Used in the Satellite-Ground Laser Link Will Develop from Near-infrared Waveband to Infrared Waveband

A large number of in-orbit tests have been carried out based on the LCT of Shijian 20 satellite,and the test data (e.g.the successful communication probabilities,acquisition time and tracking error) of satellite-ground laser links have been accumulated under meteorological conditions such as clouds,fog and wind.The interruptions of communication link shows that the near-infrared laser beam currently used in the LCT of Shijian 20 satellite cannot transmit through atmosphere well,to solve the problem,the laser carrier used in satellite-to-ground laser link will be developed towards a more compatible band for the atmospheric window,such as wavelength of 4.0μm or 10.6μm.It is worth mentioning that the acquisition,tracking and communication technologies validated by the Shijian 20 satellite can be fully applied to the LCTs,which use a middle and long wave infrared laser beam as carrier.

7 CONCLUSIONS

This paper introduces the scanning,acquisition and tracking strategy adopted by the LCT of Shijian 20 satellite,and gives a detailed description of laboratory test,outfield test and in-orbit test.The master-slave bidirectional acquisition and tracking technology which using both beacon beam and signal beam has been fully validated in orbit,this lays a technical foundation for future application of satellite-ground laser communication.