# A Case Study of an Hybrid RF and Optical Inter-Satellite Link Terminal to Enhance Optical Pointing

Elena Fernandez-Nino, Juan A Fraire, Sergi Figuerola, Adriano Camps, Joan A Ruiz-De-Azua

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Elena Fernandez-Nino, Juan A Fraire, Sergi Figuerola, Adriano Camps, Joan A Ruiz-De-Azua. A Case Study of an Hybrid RF and Optical Inter-Satellite Link Terminal to Enhance Optical Pointing. EuCNC & 6G Summit 2024 - Joint 33rd European Conference on Networks and Communications & 6th edition 6G Summit, Jun 2024, Antwerp, Belgium. pp.640-645, 10.1109/EuCNC/6GSummit60053.2024.10597073. hal-04711316

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# A Case Study of an Hybrid RF and Optical Inter-Satellite Link Terminal to Enhance Optical Pointing

Elena Fernandez-Nino,

Space Communications Research Group,
i2CAT Foundation, Barcelona, Spain

TSC, UPC BarcelonaTech, Barcelona, Spain

elena.fernandez@i2cat.net

Juan A. Fraire

Inria, INSA Lyon, France
UNC-CONICET, Cordoba,

Argentina

juan.fraire@inria.fr

Sergi Figuerola

Space Communications Research Group
i2CAT Foundation

Barcelona, Spain

sergi.f figuerola@i2cat.net

Adriano Camps

TSC, UPC BarcelonaTech, Barcelona, Spain
IEEC/CTE-UPC, Barcelona, Spain

ASPIRE Visiting International Professor, CoE, UAE Universitiy, AbuDhabi, UAE

adriano.jose.camps@upc.edu

Joan A. Ruiz-de-Azua

Space Communications Research Group
i2CAT Foundation

Barcelona, Spain

joan.ruizdeazua@i2cat.net

**Abstract**—The attitude determination and control system of a satellite plays a crucial role, especially in optical communication systems due to the narrow field-of-view of the laser. In the last years, the space ecosystem has shown a large interest in optical technology due to its high speed, reduced size, weight, and power, and reliable communications link. However, its narrow field-of-view puts some challenges in terms of connectivity. For this reason, an optical terminal requires a pointing, acquisition, and tracking system to establish the link. This work proposes the use of a dual-plane communications system in which optics and low throughput radiofrequency technologies are combined in a single module. This hybrid system leverages on the quasi-omnidirectional ability of the radiofrequency antennas to enhance the optical pointing system. The improvement is evaluated by means of a simulator that includes the dynamic pointing between two spacecraft. Results demonstrate that the designed hybrid system can heavily reduce the pointing error.

**Keywords**— Acquisition; pointing; inter-satellite; optical communication

## I. INTRODUCTION

Non-Terrestrial Networks (NTN) have emerged from the need to bring communications connectivity to terrestrial areas that are geographically remote or inaccessible [1]. Thus, global and ubiquitous connectivity is pursued. The 3rd Generation Partnership Project (3GPP) works to standardize such networks. They propose two types of payloads called transparent and regenerative [2]. The transparent payload relays the incoming data as a bent pipe. The regenerative payload provides the satellite with more functionalities, as data can be processed and routed to the next node, whether space or terrestrial. The 3GPP has standardized the use of transparent satellites in release 16 [2], and is currently working on regenerative payload in release 19 [3]. The growing interest on NTN highlights the important role that satellite-to-satellite communications play. The link established between two satellites is known as Inter-Satellite Link (ISL). The number of active nodes can grow and conform a satellite network.

However, there are still challenges in the mobile and transport domain to be addressed in this kind of networks.

The Iridium [4] satellite network features ISLs. The network is composed of 66 satellites equally arranged along 6 equidistant Low Earth Orbits (LEO). The satellites establish radiofrequency ISL (RF-ISL) to relay information between the network nodes. Moreover, this technology has also been onboarded on small satellite missions, such as the GomX-4 [5]. This mission was launched in 2018, and it established an ISL between two 6U CubeSats orbiting in LEO orbits, achieving a data rate of 1.25 Mbps.

In addition to radiofrequency (RF) technology, it has been a recent interest in the use of optical communications due to the high data transmission speed that it offers. In 2008, TerraSar-X and NFIRE LEO satellites established an optical ISL (O-ISL), achieving a data rate of 5.6 Gbps [6]. Moreover, the European Data Relay System (EDRS) [7] onboards a Ka-band and optical communication systems. The EDRS is composed by two Geostationary Earth Orbit (GEO) satellites (EDRS-A and EDRS-C) launched in 2016 and 2019, respectively. Optical terminals have also captured the interest of the small satellite community. One of their advantages is the reduced size, weight, and power (SWaP) of the system, which makes it a very attractive solution to meet the requirements of small platforms, such as CubeSats. In 2020, the FSSCat [8] mission was launched to demonstrate, among others, the feasibility of an optical link between two 6U CubeSats with 1000 km of range. Currently, the integration of optical communication payloads in small satellites continues its course. In 2024, NASA will launch the CubeSat Laser Infrared CrossLink (CLICK) [9] mission. The objective of this mission is to demonstrate the feasibility of a data transmission link greater than 20 Mbps between two 3U CubeSats ranging from 25 to 580 km. There are several companies providing optical payloads as COTS for small satellites, such as Golbriak Space [10], Astrolight [11], TESAT [12], or Stellar Project [13]. Nowadays, advances in communications must overcome the
challenge of providing high speed global connectivity. An example of this is SpaceX's Starlink mega-constellation to provide Internet around the globe. The new satellites carrying optical terminals have established O-ISLs [14]. Despite the technical complexity of optical space communications, its advantages over RF communications compensate.

In general, optical systems offer large communications ranges and high data rates. The narrow field-of-view of the optical terminal increases the communications security, as it is more difficult to intercept and track the laser beam. Moreover, the ability to concentrate power in a narrow beam makes the system more power efficient, while its size and weight are reduced [15]. However, an accurate pointing system is required. On the other hand, depending on the design and configuration of the RF system, the link range can be increased at the expense of reducing the data transmission speed (e.g., LoRa [16]). Omnidirectional RF antennas radiate power almost isotropically allowing good connectivity between the satellites of the network, although the lower Signal to Noise Ratio (SNR) leads to lower data transmission speeds. In short, there is a tradeoff between connectivity and data rate, keeping the same range.

The Pointing, Acquisition and Tracking (PAT) system is used in optical communications system to perform a fine pointing [17]. In the pointing phase, the PAT first orients the platform towards an estimated position. Afterwards, two methods in the acquisition phase can be differentiated: (1) beacon and (2) beaconless. The former one uses a beacon to know the position of the other terminal. However, the link budget is compromised since the transmitted power decreases as the beamwidth increases. Therefore, this technique requires higher transmitted power. In the second technique, both satellites describe a geometric pattern to find the position of each other from a possible intersection produced along the spatial scan. The advantages and disadvantages of the different geometrical patterns (e.g., spiral, rectangular, etc.) are discussed in [18]. The cone that captures the position error margin is known as the Uncertainty Cone (UC) [17]. Each time the beam intercepts the other terminal, the UC is reduced. For example, the time to discover the other terminal in the EDRSS mission is 55 seconds for an UC of 1 milliradian [17]. When the optical pointing system has aligned the transmitter and receiver, the optical link is established, and both satellites track each other to keep the link active during the communications period.

Moreover, in [19] it was stated that the maximum time invested in spatial acquisition was 8 s, and that the minimum duration of a contact was 50 s. Therefore, the percentage of the contact devoted to point two terminals is 16%. This may be due to 1) the initial size of the UC, and 2) the geometric pattern described.

This work focuses on reducing the initial size of the UC by means of a hybrid solution using a low throughput RF technology to leverage the good connectivity of the omnidirectional antennas, and exchange their GPS position to reduce the UC, and an optical technology to leverage the fast

transmission. In short, the main contributions of this work are: 1) the design of the hybrid pointing strategy, 2) the development of a satellite pointing simulation tool, and 3) the evaluation of the pointing improvement achieved by the designed hybrid pointing strategy.

The remain of this work is structured as follows. Section II introduces the hybrid communications system and the pointing strategy. Section III describes the performance of the developed pointing tool and the simulated scenario. Moreover, this section also discusses the pointing results. Finally, Section IV concludes the work.

## II. HYBRID COMMUNICATIONS SYSTEM

As previously mentioned, a technologically hybrid inter-satellite communication terminal is proposed that benefits from the advantages associated to the high and low throughput terminals. To the best of the authors' knowledge, it has not been done before in large links (i.e., 1500 km). For example, the CLICK mission proposed the use of an RF link to exchange the satellite position with the counterpart, but the maximum distance expected to reach is 580 km. The challenge posed in this paper is to extend the distance of the link up to 1500 km. In [15], on the contrary, the use of RF in large distance links (i.e., 1500km) is not recommended due to CubeSat performance limitations. This section describes the hybrid ISL concept, detailing its communications architecture, and the designed pointing strategy.

### A. Definition of the Hybrid Concept and Architecture

The hybrid ISL (H-ISL) concept emerges to solve the optical connectivity gap. It incorporates RF and optical transceivers in a single module to take advantage of the benefits provided by these two technologies. That is, this module will allow to establish long distance links, good connectivity between the nodes of the network provided by the RF antennas (i.e., increased beamwidth), and high optical data rates. Table I compares the characteristics of an ideal scenario with those offered by RF and optical technologies, as well as those of the hybrid system already mentioned in this section.

<table><caption>TABLE I. CHARACTERISTICS OF ISL COMMUNICATION LINKS</caption><thead><tr><th rowspan="3">Characteristics</th><th colspan="5">Type of Terminals</th></tr><tr><th>Ideal</th><th>Low throughput</th><th colspan="3">High throughput</th></tr><tr><th>Ideal</th><th>RF-ISL</th><th>RF-ISL</th><th>O-ISL</th><th>H-ISL</th></tr></thead><tbody><tr><td rowspan="3">Comms</td><td>Long range</td><td>Long range</td><td>Long range</td><td>Long range</td><td>Long range</td></tr><tr><td>Omnidir. Connect.</td><td>Omnidir. Connect.</td><td>Direct. Connect.</td><td>Direct. Connect.</td><td>Omnidir. Connect.</td></tr><tr><td>High data rate</td><td>Low data rate</td><td>High data rate</td><td>High data rate</td><td>High data rate</td></tr><tr><td>SWaP</td><td>Not a limitat.</td><td>Limitat.</td><td>Limitat.</td><td>Not a limitat.</td><td>Limitat.</td></tr><tr><td>PAT</td><td>Not a limitat.</td><td>Not a limitat.</td><td>Limitat.</td><td>Limitat.</td><td>Not a limitat.</td></tr><tr><td>Missions</td><td>-</td><td>FSSCat</td><td>Iridium</td><td>TerraSar-X</td><td>-</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>Fig. 1. Block diagram of the H-ISL.</figcaption></figure>

The combination of the strengths offered by each type of terminal brings the performance of the hybrid system closer to the performance of an ideal communications link. Note that communications performance and PAT characteristics are expected to be similar, but not the SWaP because of the limitation imposed by the RF low throughput terminal.

The architecture of the proposed H-ISL consists of two planes called control and data plane. Figure 1 illustrates this concept. On one hand, the control plane uses the RF-ISL to easily reach the nodes of the network and manages the signaling data (i.e., telemetry, telecommands, routing, etc.). The reduced size of this type of data does not require a high data transmission speed, but good connectivity among the nodes belonging to the network in order to have the network status updated. For this purpose, non-directive antennas are needed to transmit data through a quasi-omnidirectional pattern. On the other hand, the data plane handles a large volume of data, which makes high transmission speed essential. On-board memory limitations may result in data loss during the mission as the data generation rate is usually higher than the data transmission rate. Therefore, optical technology is of interest due to its inherent high data transmission speed. Nevertheless, optical pointing is still a challenge to be addressed. This work has designed a pointing strategy that aims to reduce the uncertainty of the position of the other satellite through the hybrid system.

## B. Definition of the Hybrid Pointing Strategy

One of the applications of the hybrid system is the reduction of the acquisition time. Specifically, the approach is defined by 1) predicting the position of the satellite propagating the Two Line Elements (TLE) through an orbit propagator and starting a coarse pointing before the two terminals are in line-of-sight (LOS); and 2) exchanging the satellite position through the omnidirectional antennas of the RF-ISL system, and making a second coarse pointing once they can establish a contact. Then, optical pointing system performs the fine tuning. This mechanism prevents the traditional optical pointing from looking for a speck of dust in the universe. Note that the difference between step 1 and step 2 is that the first pointing is based on a position estimation computed by a propagator (i.e., the error on the position is given by the TLE, and the orbit propagator), and it is performed before the satellites are in LOS, whereas the second pointing is performed based on the GPS position data exchanged between the two satellites through the RF-ISL. Therefore, the GPS position error (i.e., in the order of tens of meters) will be considered. Figure 2 diagrammatically shows the designed PAT strategy, where the steps performed by each PAT process are specified according to the distance of the ISL link.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Flow diagram of the hybrid pointing strategy.</figcaption></figure>

## III. SIMULATOR, SCENARIO, AND RESULTS

This section first describes the main characteristics of the simulated scenario. Then, a comparative analysis of the pointing is performed based on 1) the position data obtained by the TLE propagation, and 2) the GPS position data.

### A. Definition of the Simulator Characteristics

The impact of the hybrid system in terms of improving optical pointing is evaluated through a custom, dedicated tool developed in-house. This pointing tool bases the analysis on the satellite quaternions, a four-dimension vector containing angle and axes information. This mathematical tool is widely used in attitude computation because of the fast computation as compared to the rotation matrix, and the avoidance of co-plane singularities as in Euler angles, among others.

First, the tool performs a TLE propagation. Then, the orientation of the satellite is changed according to the distance between the two spacecraft. An ideal actuator is considered to change the orientation of the platforms. Nevertheless, the tool can be extended by incorporating real actuators. Moreover, the cumulative errors produced by the TLE propagation along time are also computed by this tool. The GPS error is also considered.

### B. Definition of the Simulated Scenarios

To illustrate the methodology, the LEO orbits of Okean-1 and Okean-2 have been selected. This simulation comprises 16 orbits where the two satellites communicate at the polar caps. Moreover, a set of fifty TLEs have been obtained from the CelesTrak [20] database. Table II contains the main orbital parameters of both satellites.

<table><caption>TABLE II. ORBITAL PARAMETERS OF THE TWO SATELLITES</caption><thead><tr><td>Satellite</td><td>a<sup>a</sup> [km]</td><td>e<sup>a</sup> [rad]</td><td>i<sup>a</sup> [rad]</td><td>RAAN [rad]</td><td>ω<sup>a</sup> [rad]</td><td>m<sup>a</sup> [rad]</td></tr></thead><tbody><tr><td>Okean-1</td><td>6960.4</td><td>0.0015</td><td>1.4400</td><td>4.1300</td><td>0.6062</td><td>5.6774</td></tr><tr><td>Okean-2</td><td>6975.3</td><td>0.0013</td><td>1.4402</td><td>3.7303</td><td>2.6473</td><td>3.6367</td></tr></tbody></table>

Furthermore, two pointing modes have been implemented: (1) nadir and (2) satellite-to-satellite pointing modes. The nadir pointing mode orients the z axis of the platform towards the

<footnote><sup>a</sup> a: semi-major axis, e: eccentricity, i: inclination, ω: mean anomaly, m: argument of perigee.</footnote>
center of the Earth. The satellite-to-satellite pointing mode orients the z axis of the platform towards the position of the other satellite.

The pointing tool also evaluates the integrity of the position data. In this way, three types of data are evaluated: (1) the ideal one, which does not contain errors; (2) the one containing the TLE error; and (3) the one containing the GPS error. For this purpose, the tool also computes the cumulative TLE error by comparing a set of TLEs. It has been studied that TLE error is cumulative with time, and, in the case of study (i.e. LEO orbits), the along-track component is the one that contains most of the error, as compared to the radial and cross-track components. In order to show the effect of the cumulative error of the TLE propagation on the position data, this work evaluates the pointing by considering the cumulative TLE error after three and eight days. Moreover, the GPS error is assumed to be constant during the contact, and equal to 10 m rms at each axis.

### C. Discussion of the Simulated Results

The pointing tool computes the pointing angle along the 16

orbits. Figure 3 illustrates the pointing angle of the first orbit, where two ISLs are established at the poles. The blue line considers that the position of the other satellite does not contain errors, the orange one considers the TLE error on the position data, and the yellow line considers the GPS error during the contact area. The results show how the TLE update rate can heavily affect the pointing angle. The higher the update rate of the TLE, the lower the cumulative error and, therefore, the lower the pointing error angle. This means that following a TLE-based strategy, the satellite needs to constantly refresh the information to keep a reasonable error. This is an important operation requirement. Figure 3a shows that the maximum pointing error angle obtained is 0.3° when three days of cumulative TLE error are considered. However, Fig. 3b shows that the pointing error angle increases to almost 2.1° when eight days are considered. In addition, the results also show the benefits of using an RF system to exchange GPS data. In this case, the pointing error is significantly lower since the error induced by the GPS data is also lower than the one obtained by a TLE. Specifically, the maximum error caused by the TLE

<figure><img src="image_4.png" /><figcaption>Fig. 3. Pointing angle along an orbit after (a) 3 days, and (b) 8 days.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Histogram of pointing error angle after (a) 3 days, and (b) 8 days.</figcaption></figure>
after three and eight days are 0.29° and 2.12°, respectively, while the maximum error caused by the GPS is 0.001°. In this way, the pointing accuracy is improved with the use of the RF system, and consequently the initial UC of the optical system is also adjusted.

To extend this study, a statistical analysis of 16 orbits has been performed considering the TLE cumulative error of eight days. Table III encompasses the main parameters.

<table><caption>TABLE III. STATISTICAL PARAMETERS OF POINTING ERROR ANGLE</caption><thead><tr><th>Position data source</th><th>Minimum [mdeg]</th><th>Maximum [mdeg]</th><th>Median [mdeg]</th><th>Percentile 0.25 [mdeg]</th><th>Percentile 0.75 [mdeg]</th></tr></thead><tbody><tr><td>TLE</td><td>576.925</td><td>2126.174</td><td>784.513</td><td>0.665</td><td>1.011</td></tr><tr><td>GPS</td><td>0.301</td><td>1.409</td><td>0.514</td><td>0.0004</td><td>0.0007</td></tr></tbody></table>

Figure 4 illustrates the probability to have a certain error in the pointing angles during the contacts produced in the 16 orbits. The pointing error angle is defined as the difference between the ideal pointing angles, which are considered the reference and are colored in blue, and the other two cases. The two histograms differ three or eight days the TLE update rate. Following the analysis of the worst case, Fig. 4b shows that the maximum error angle achieved is between 2.1° and 2.2° if only TLE propagation is assumed. However, the pointing error is significantly reduced to the range between 0° and 0.1° when GPS error is considered.

## IV. CONCLUSIONS

This paper presents the pointing challenge facing optical communications systems given their narrow field-of-view. A hybrid communications architecture is proposed to address this gap. The data plane of the hybrid system is used to send high amount of useful data through the O-ISL leveraging high data rate, while the control plane is used to exchange signaling data through the RF-ISL due to the good connectivity it offers. The hybrid pointing strategy first estimates the position of the other satellite with a TLE propagation, and performs a first coarse pointing. Afterwards, the satellites establish an RF-ISL and exchange their respective GPS positions to reduce the error of their estimated position. Then, a precise pointing is performed. The last process consists of tracking the correct pointing between the satellites so as not to hinder communication between them.

A pointing tool has been implemented with the aim at evaluating the effects caused by the TLE and GPS errors. The simulated scenario is composed of two LEO satellites that establish RF-ISL at the poles. It has been shown that pointing based on GPS position data produces better results, on the order of hundreds, than pointing based on TLE propagation data. Moreover, as it is not always possible to update the TLE, this study has also addressed the effect that an outdated TLE can cause on the pointing. Specifically, the pointing error obtained when the TLE has not been updated for three days is 0.3°, while it increases to 2.1° when the time elapsed since the last updated is eight days.

The team is currently working on evaluating the effects that real actuator will cause in the pointing, and on the design, implementation, and development of a hybrid communications module.

## ACKNOWLEDGMENT

This work was supported and funded by the by the Spanish Ministry of Economic Affairs and Digital Transformation and the European Union – NextGeneration EU, in the framework of the Recovery Plan, Transformation and Resilience (PRTR) (Call UNICO I+D 5G 2021, ref. number TSI-063000-2021-5-6GSatNet-SS), and by the Government of Catalonia in the scope of the NewSpace Strategy for Catalonia. This work has also partially funded by Ministry of Science and Innovation MICIN/Agencia Estatal de Investigación AEI/10.13039/501100811033 and EU Feder through project “GENESIS: GNSS Environmental and Societal missions” grant PID2021-126436OB-C21.

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