A Method for Rapid Deployment of Ground-Based Ultra-Long-Range Terrestrial Optical Communication Links

Abstract

With the growing demand for high-efficiency and secure information transmission, ultra-long-range optical communication has demonstrated significant potential. This paper proposes a method for establishing ground-based fixed-point ultra-long-range atmospheric optical communication links, aiming to overcome challenges such as atmospheric turbulence, transmission loss, and environmental interference. Through theoretical analysis and experimental validation, we developed a high-precision optical communication terminal installation model, determined the terminal’s optical axis direction via stellar calibration, and established a coordinate transformation model from geodetic coordinates to initial pointing angles. By analyzing initial pointing errors, we designed a laser link scanning strategy to compensate for uncertainties in the initial pointing region. The feasibility of this approach was verified through near-field validation and a long-distance link acquisition experiment exceeding 100 km. Experimental results demonstrated successful 100 km/100 Gbps ultra-high-speed optical communication. This breakthrough study is expected to play a critical role in future space-localized optical communication networks.

1. Introduction

The era of optical information demands the establishment of communication network systems with high transmission rates, large data capacities, and extensive coverage. Utilizing optical waves with extremely short wavelengths for space-based satellite communication is the optimal solution to achieve high data-rate communication, particularly in today’s context where the microwave frequency spectrum is limited and space resources are increasingly crowded [1,2]. Companies like Starlink in the United States and OneWeb in the United Kingdom have already planned or launched massive satellite constellations, with inter-satellite communications relying on laser links for information transmission [3].

With the rapid advancement of information technology, the demand for multi-dimensional, integrated information resources in future information services is increasing. Strategic information services in national security, aerospace, environmental monitoring, traffic management, education and healthcare, industrial and agricultural sectors, counter-terrorism, and disaster relief will unfold in a multi-dimensional space encompassing the air, space, and ground. Information utilization within a single dimension cannot meet these comprehensive needs [4,5,6]. Therefore, the development of space-based local area network (LAN) optical communication technology has become an inevitable trend in the evolution of optical communication [7,8,9].

The future trend of space optical communication is to achieve longer distances, higher communication rates, and lower power consumption. To accomplish this, the divergence angle of the laser beam must be compressed to the order of tens of micro-arcseconds [10]. Consequently, rapid capture and stable maintenance of the laser link become crucial [11,12]. Even minor misalignment errors can cause a decline in the communication link performance and result in significant loss of system efficiency [13].

Optical communication, due to its advantages such as easy terminal deployment and high transmission rates, is currently the best solution for achieving high-rate communication in space. However, space optical communication relies on small divergence angle beams to provide high transmit and receive gains. This imposes stricter requirements on the speed and stability of laser beam acquisition and tracking than traditional microwave communication systems. Small divergence angles, low calibration accuracy, platform attitude drift, and micro-vibrations pose significant challenges to the scanning, acquisition, and alignment of laser beams [14,15,16].

In this work, space optical networks specifically refer to inter-satellite or satellite-to-ground laser communication systems operating in the vacuum or near-vacuum conditions of outer space. This is distinguished from terrestrial free-space optical (FSO) communication, which operates within Earth’s atmosphere and must account for turbulence, aerosol scattering, and weather-related attenuation. While both domains utilize optical wireless transmission, their technical challenges diverge fundamentally.

This paper primarily focuses on the research of rapid ground-based laser link establishment technologies. It explores methods to achieve fast deployment and link establishment for optical communication pointing, acquisition, and tracking (PAT) terminals and conducts targeted analytical and verification work to solve the challenges of rapid switching and long-term high-stability maintenance in space-based LAN laser networks. By studying the mechanisms of laser rapid link establishment and the coupling transmission of platform disturbances, this paper aims to provide a basis for the rapid deployment of three-dimensional space LANs and the isolation of beam disturbances using both active and passive methods. This research is significant for improving the overall performance of space optical networks and can be extended to the construction and application of space-based information networks in the future.

2. System Configuration

As shown in Figure 1, two laser terminals were deployed on opposite shores of Qinghai Lake, separated by over 100 km. The link was established using custom-designed optical communication terminals with a stare-scan acquisition strategy. The scanning PAT terminal (Figure 1b) employs an L-shaped folded-axis design with a 100 mm aperture telescope. To achieve high atmospheric transmittance and low solar background noise, the communication wavelength was set to 1550 nm. A Charge Coupled Device (CCD) camera (−70 dBm sensitivity) served as the detector. Two beacon laser (1540 nm/400 μrad and 650 nm/160 μrad) were integrated to enhance acquisition probability and facilitate manual calibration. A dichroic beam splitter separated transmit (1544 nm) and receive (1556 nm) channels.

Table 1. Coarse beacon acquisition link budget.

Parameter	Value	Remarks
Transmit Power	30 dBm/1550 nm	1 W
Transmit Antenna Gain	89 dB	200 μrad (full angle)
Receive Antenna Gain	104.2 dB	80 cm aperture
Space Loss	−238.2 dB	100 km line-of-sight
Receive Path Loss	−1.5 dB
Transmit Path Loss	−1.5 dB
Spot Averaging Loss	−9.54 dB	3 × 3 grid
Atmospheric Loss	−15 dB
Pointing Loss	−0.98 dB
Detector Received Power	−43.52 dBm
Detector Sensitivity	−70 dBm
Link Margin	26.5 dB

outlines the coarse beacon acquisition link budget for the 100 km ground-based experiment.

Table 2. Communication link budget.

Parameter	Value	Remarks
Transmit Power	40 dBm/1544 nm	10 W
Transmit Optical Loss	−2.22 dB
Transmit Optical Gain	95.78 dB	65 μrad divergence (60 mm)
Aiming Loss	−1 dB	5 μrad
Space Loss	−238.26 dB	100 km line-of-sight
Receive Optical Gain	104.20 dB
Receive Optical Loss	−2.22 dB
Beam Splitter Loss	−0.45 dB	1:9 ratio
Fiber Coupling Loss	−10 dB	10% (cross-lake, 3500 m altitude)
Atmospheric Absorption	−15 dB	0.15 dB/km (Qinghai Lake)
Received Power	−29.17 dBm
Receiver Sensitivity	−40.00 dBm	100 Gbps (with coding gain)
Link Margin	10.83 dB

presents the communication link budget for the 100 km/100 Gbps experiment. The schematic diagram of the key parameters of the beacon light and communication light of the laser communication link is shown in Figure 2.

3. Ultra-Long-Range Scanning and Acquisition Technology

The success of establishing a ground-based fixed-point ultra-long-range atmospheric optical communication link primarily depends on the high-precision initial pointing accuracy between the two terminals. To achieve a ground-based laser link exceeding 100 km, we have developed a high-precision initial pointing technique for laser terminals. This technique involves integrating dual global positioning system (GPS) receivers into the terminal’s body coordinate system to measure the pointing orientation of the optical communication PAT terminal. Subsequently, the optical axis of the telescope’s exit pupil, including both the azimuth and elevation axes, is calibrated using an arbitrary star. In previous ground-based calibration procedures, the nearly fixed star Polaris was typically employed to calibrate the PAT terminal’s optical axis.

3.1. Initial Pointing

The initial pointing is a crucial factor in determining the scanning strategy of the scanning terminal. The residual error after the initial pointing defines the uncertain range of the system’s scanning area, which needs to be fully covered by the beacon light. This ensures the scanning capture probability [17,18]. To increase the scanning capture probability, the initial pointing accuracy must be maximized. To achieve high-precision initial alignment, the geodetic coordinates of the two terminals obtained from Global Navigation Satellite System (GNSS) must be converted into the azimuth, and elevation angles are required to point at each other. After obtaining the azimuth and elevation angles, the PAT terminal must rotate precisely according to these angles. Otherwise, significant initial pointing errors will be introduced due to the misalignment between the optical axis and the PAT terminal control system [19].

First, the geodetic coordinates of the terminals are converted into Cartesian coordinates, as shown in Equation (1):

$$\left(\begin{array}{c}x\\ y\\ z\end{array}\right)=\left(\begin{array}{c}\left(N+H\right)\cos B\cos L\\ \left(N+H\right)\cos B\sin L\\ \left(N\left(1-e^2\right)+H\right)\sin B\end{array}\right)$$

(1)

where $N={\displaystyle \frac{a}{\sqrt{1-e^2{\sin }^{2}B}}}$, $a=\mathrm{6,378,137}\mathrm{m}$, $e^2=0.006694379995$ and B, L, H denote latitude, longitude, and altitude.

The vector pointing to the target in the World Geodetic System 1984 (WGS84) coordinate system is denoted as $\left(\left[\begin{array}{c}{x}_t\\ y_t\\ z_t\end{array}\right]-\left[\begin{array}{c}x\\ y\\ z\end{array}\right]\right)$ Based on the attitude information of the laser terminal’s installation on the ground, the observed vector of the target relative to the PAT terminal is transformed into the PAT terminal’s initial coordinate system (East north up (ENU)), resulting in

$$v_0=\left[\begin{array}{c}{v}_x\\ v_y\\ v_z\end{array}\right]={\mathbf{A}}^{-1}\left(\left[\begin{array}{c}{x}_t\\ y_t\\ z_t\end{array}\right]-\left[\begin{array}{c}x\\ y\\ z\end{array}\right]\right)$$

(2)

where

$${\mathbf{A}}^{-1}=\left(\begin{array}{ccc}-\sin L& \cos L& 0\\ -\sin B\cos L& -\sin B\sin L& \cos B\\ \cos B\cos L& \cos B\sin L& \sin B\end{array}\right)$$

(3)

At this point, ${\mathit{v}}_0$ represents the direction of the PAT terminal’s initial coordinate system (ENU).

If the initial azimuth of the PAT terminal is 0° and the elevation is 0°, with the PAT terminal initially pointing true north, it is conventionally agreed that a clockwise rotation of the PAT terminal azimuth is considered positive. To make the PAT terminal point the target, the azimuth and elevation angles the PAT terminal needs to rotate are given by Equations (4) and (5).

$$Az=\arcsin \left({\displaystyle \frac{v_x}{\sqrt{v_x^2+v_y^2}}}\right)$$

(4)

$$El=\arcsin \left({\displaystyle \frac{v_z}{\sqrt{v_x^2+v_y^2+v_z^2}}}\right)$$

(5)

In practice, it is not possible to directly measure the absolute azimuth and elevation angles of the PAT terminal in the Cartesian coordinate system. Therefore, in this study, we designed an adapter structure for the PAT terminal, on which dual GPS receivers are installed. Through manufacturing tolerances, we ensure that the line connecting the two GPS receivers is perfectly parallel to the elevation axis of the PAT terminal. Using this method, the absolute azimuth angle can be measured, which allows the optical axis of the telescope to be calibrated by selecting any star for star calibration.

The schematic of the dual-station GPS installation is shown in Figure 3. The dual GPS measurement devices are mounted on an extension rod parallel to the elevation axis of the optical communication PAT terminal adapter plate. The extension rod and adapter plate are manufactured to ensure their parallelism, with the length of the extension rod being 1900 mm. Through differential calculation, the angle between the line connecting the two GPS receivers and true north can be determined. When the distance between the two receivers is greater than 1 m, the measurement error is less than 0.2°.

3.2. Star Calibration for Optical Axis

By obtaining the geodetic coordinates (latitude, longitude, altitude) of the two terminals from GPS, coordinate transformation can be used to calculate the azimuth and elevation angles required for the terminal to point at the target. To minimize errors, star observations are employed, using the star’s elevation and azimuth angles as references to correct the azimuth and elevation angles of the PAT terminal. This ensures consistent calculations across the terminals and reduces errors between them. Dual GPS antennas are used to measure the angle between the PAT terminal’s initial position and true north, providing the initial azimuth for star observation [20,21].

The theoretical azimuth angle calculation formula is as follows:

$${\theta }_{FW}=-{\theta }_{GNSS}\pm 90°+{\theta }_{FW-STK}+{\sigma }_{FW} , \theta \in \left(-180°,180°\right)$$

(6)

where ${\theta }_{GNSS}$ is the azimuth angle obtained from GNSS, $\pm 90°$ is determined by the relationship between the baseline and the PAT terminal’s zero position, ${\theta }_{FW-STK}$ is the azimuth angle provided by Satellite Tool Kit (STK11.0) software, and ${\sigma }_{FW}$ is the azimuth angle correction value.

The theoretical elevation angle calculation formula is as follows:

$${\theta }_{FY}={\theta }_{FY-STK}+{\sigma }_{FY} , \theta \in \left(-90°,90°\right)$$

(7)

where ${\theta }_{FY-STK}$ is the elevation angle provided by STK, and ${\sigma }_{FY}$ is the azimuth angle correction value.

Since we can obtain the azimuth and elevation angles of the selected star relative to the PAT terminal’s body coordinate system from the ephemeris, we can use these angles to correct the azimuth and elevation angles when the PAT terminal observes the star. Before calibration begins, the PAT terminal must be installed completely horizontally on the ground, and the absolute azimuth of the PAT terminal is measured using dual-station GPS. Using the STK software, we obtain the azimuth and elevation angles of the star to be observed at a specific time and calculate the azimuth and elevation angles the PAT terminal must control to point at the star. These values are then input to the PAT terminal. After the PAT terminal rotates according to the input azimuth and elevation angles, the presence of the target star in the CCD field of view is checked. When the star image appears in the CCD field of view, as shown in Figure 4, if the star is not visible, the PAT terminal’s elevation angle is quickly adjusted in a small range until the target star appears in the field of view. Once the star appears in the field of view, the PAT terminal is fine-tuned so that the star’s trajectory passes through the center of the field of view. After the star passes through the center of the field of view, the PAT terminal telemetry data are checked, and the time at which the star passes the center of the field of view, along with the PAT terminal’s azimuth and elevation angles, are recorded as the “actual elevation angle” and “actual azimuth angle”. The corresponding theoretical elevation angle and azimuth angle are then calculated from the STK software for the star at that moment, which are referred to as the “theoretical elevation angle” and “theoretical azimuth angle”. By comparing the theoretical and actual elevation and azimuth angles, the difference is obtained, providing the correction value for the PAT terminal’s optical axis. To minimize the error, this calibration process can be iterated multiple times. Once the ideal correction value is obtained, the azimuth and elevation angles required for the two optical communication terminals to point at each other can be easily calculated.

Table 3. Star calibration results.

Star	STK Azimuth (°)	STK Elevation (°)	Theoretical Azimuth (°)	Actual Azimuth (°)	Azimuth Error (°)	Theoretical Elevation (°)	Actual Elevation (°)	Elevation Error (°)
113,368	164.766	21.448	−9.2340	−9.9981	0.7641	21.4480	21.5310	−0.0830
113,368	169.560	22.370	−5.2041	−5.0212	−0.1829	22.4530	22.6051	−0.1521
3419	155.569	31.106	−17.6122	−17.1577	−0.4545	31.3411	31.5049	−0.1638
3419	159.636	32.268	−13.0907	−13.1808	0.0901	32.6669	32.8045	−0.1376
3419	159.915	32.339	−12.8117	−12.8981	0.0864	32.7379	32.8545	−0.1166
3419	160.119	32.390	−12.6077	−12.6981	0.0904	32.7889	32.9045	−0.1156
113,368	149.201	15.782	−23.6147	−23.5083	−0.1064	16.3042	16.2164	0.0878
113,368	150.212	16.286	−23.3880	−22.8921	−0.4959	16.2860	16.7039	−0.4179
113,368	153.225	17.669	−19.8791	−19.8362	−0.0429	18.0869	18.0395	0.0474
3419	144.068	26.434	−28.9932	−28.9156	−0.0776	26.8045	26.8649	−0.0604
113,368	147.287	14.769	−25.6966	−25.7980	0.1014	15.1999	15.1618	0.0381

shows the telescope line of sight correction results for observing different stars.

3.3. Scanning and Acquisition

After achieving high-precision initial pointing, considering factors such as GPS measurement errors and PAT terminal execution errors, it is necessary to scan and cover the uncertain range. In this study, a rectangular spiral scanning strategy is employed. Taking into account platform attitude errors, positioning errors, PAT terminal execution errors, and some redundancy, scanning is conducted within an uncertain range of 0.2°. The scanning trajectory is shown in Figure 5.

The 400 μrad coarse beacon is used to scan the uncertain area. The scanning terminal’s beacon light has a transmission power of 1 W, with a scanning range of ±0.1°, a scanning step size of 0.01°, and a dwell time of 0.1 s. The total scanning time is 46.3 s. The dual-terminal collection uses a staring scan approach. When the staring terminal receives the beacon light, it quickly tracks the signal and sends a signal light to the scanning terminal. The scanning terminal stops scanning once it receives the signal light from the staring terminal, completing the laser link acquisition between the two terminals. In this system, the scanning system is mainly used in the coarse alignment stage. It drives the fast reflector (FSM) through a preset program to search a wide range of angles and compensate for initial installation errors and environmental disturbances. In this stage, the system operates in an open loop, with a slow response speed and low link stability. The fine tracking system mainly controls the fast reflector based on the CCD camera’s spot position error feedback to achieve high-precision closed-loop control. In this stage, the dynamic jitter caused by some turbulence can be suppressed to maintain link stability.

4. Experimental Results

We conducted an in-depth study using the system developed in this research and performed the first-ever ground-based ultra-long-distance atmospheric optical communication experiment over 100 km/100 Gbps in the Qinghai Lake region, China. Prior to this, we also conducted numerous near-field outdoor experiments to verify the effectiveness of our system. Although the Qinghai Lake area is at a high altitude, with atmospheric particle concentrations much lower than at lower altitudes, it is important to note that ultra-long-distance atmospheric optical communication experiments still face severe atmospheric turbulence, which significantly impacts the laser energy coupling efficiency. Our system does not include any turbulence compensation systems, such as adaptive optics. However, to validate our laser scanning capture, we can perform this verification at any time of the day. To demonstrate the 100 Gbps high-speed optical communication, we chose the time window with the lowest atmospheric turbulence intensity during the day for the communication experiment.

When the signal light spot emitted by the staring terminal is detected on the scanning terminal’s CCD camera, an image is recorded, and the position of the spot is tracked in real-time. The position of the spot on the detector is adjusted by manually adjusting the PAT terminal and fine-tracking fast mirrors, so that the spot is positioned at the location with the highest fiber coupling efficiency. Figure 6 shows the images of the light spot received under weak, moderate, and strong turbulence conditions. Under weak turbulence, the laser power coupling efficiency is approximately 10%, whereas under strong turbulence, the laser power coupling efficiency is less than 1%.

We used a high-speed optical power meter with a sampling frequency of 1 kHz to statistically measure the received laser energy under weak turbulence, and the results are shown in Figure 7. From the figure, it is clear that, under favorable atmospheric link conditions, the maximum optical power coupled to the scanning terminal through the optical system is −34.24 dBm, the minimum optical power is −50.23 dBm, and the average optical power, calculated from the statistics, is −40.31 dBm, which achieved the expected ideal result. Under these power coupling conditions, 112 Gbps atmospheric optical communication was achieved in the Qinghai Lake area, with the optimal bit error rate (BER) of 3E-3.

Although the current experiment utilized a 10 W transmitter in a controlled, remote environment, practical deployment mandates adherence to international laser safety standards. The beam’s narrow divergence (65 μrad) ensures rapid power density attenuation: at 1 km, the spot diameter expands to ~6.5 cm, reducing power density to <1.5 mW/cm² (far below the IEC 60825-1 Class 1 limit of 10 mW/cm² for 1550 nm lasers). This inherently safe design eliminates public exposure risks under normal operation. For additional safeguards, we propose integrating real-time LiDAR-based intrusion detection (triggering beam shutdown within 50 ms) and deploying transceivers in restricted-access zones (e.g., rooftop towers) with public hazard signage. Compliance with IEC 60825-1 certification will further ensure operational safety across all scenarios.

5. Discussion and Conclusions

Although there have been numerous reports on free-space optical communication, demonstration systems for ground-based local area information transmission networks are still worth exploring. Unlike space-based optical communication, ground atmospheric environments require faster network setup capabilities and the ability to cope with more complex atmospheric conditions. Therefore, to effectively address the rapid acquisition of ultra-long-distance atmospheric laser links, we designed a high-precision initial pointing technique for the laser terminal. By observing any star and correcting the optical axis pointing errors of the optical communication terminal, the initial pointing can be completed rapidly, and scanning acquisition can be performed in a short amount of time. This technique enables the fast deployment of ground-based local optical communication networks and demonstrates ultra-100 Gbps high-speed optical communication in weak turbulence windows.

In conclusion, our research experimentally demonstrated the rapid acquisition and ultra-high-speed optical communication of ground-based ultra-long-distance atmospheric optical communication links. The high-precision laser terminal initial pointing method we proposed effectively reduces the scanning uncertainty region, and the coarse beacon scan enables rapid scanning acquisition. Multiple experimental results show that dual-end link scanning acquisition can be achieved within an uncertainty range of ±0.1°, which represents a breakthrough for ground-based atmospheric optical communication systems. In the future, this research will continue, and we will further investigate high-precision light spot position extraction techniques under turbulence effects, to enable long-term stable tracking of moving platform atmospheric optical communication links.