Satellites are relay stations in space for the transmission of voice, video and data communications. They are ideally suited to meet the global communications requirements of military, government and commercial organizations because they provide economical, scalable and highly reliable transmission services that easily reach multiple sites over vast geographic areas. Transmissions via satellite communications systems can bypass the existing ground-based infrastructure, which is often limited and unreliable in many parts of the world.
Satellite communications involves four steps: An uplink Earth station or other ground equipment transmits the desired signal to the satellite; The satellite amplifies the incoming signal and changes the frequency; The satellite transmits the signal back to Earth, and The ground equipment receives the signal.
Satellite onboard connectivity defines how the satellite network resources are switched on board in order to meet the service-level connectivity requirements. It, therefore, depends on how the satellite resources (beams, channels, carriers, etc.) are organised on both satellite up- and downlinks and, primarily, on the type of coverage that the satellite system provides. In the case of global coverage, any user within the coverage can, in principle be connected to any other user.
In recent years, the new LEO constellations have received much attention as a means of bringing ubiquitous Internet connectivity, including to rural areas. To this end, more than 1,000 small satellites have been launched into low Earth orbit in 2021. According to Northern Sky Research, approximately 85% of all satellites launched between 2020 and 2030 will be constellations.
High-throughput Satellite or HTS systems represent a new generation of satellite communications systems, that provide a significant increase in capacity than conventional communication satellites. They utilize frequency reuse and multiple spot beams to deliver vast throughput for the same amount of allocated orbital spectrum.
In multi-beam coverage, interconnection of any user within different beams of the coverage requires on-board interconnection of beams and of the resources allocated to those beams. The multibeam satellite systems make it possible to reduce the size of earth stations and hence the cost of the earth segment. Frequency re-use from one beam to another permits an increase in capacity without increasing the bandwidth allocated to the system.
These satellites will not only have consistent contact with ground systems, but many will be communicating with one another directly. Intersatellite links are links between satellites. Intersatellite links (ISL) can be considered as particular beams of multi-beam satellites; the beams, in this case, are directed not towards the earth but towards other satellites. For bidirectional communication between satellites, two beams are necessary—one for transmission and one for the reception.
Equipping each node to send and receive data, reduces the waiting time for a satellite to be in range of a ground station and gives operators more options to task a satellite closest to a downlink location, which enhances time-sensitive data operations, particularly in LEO.
That information which is propagated between the satellites falls into several data types or traffic such as Navigation data, Payload data or Spacecraft health & status while each of these types of data has different levels of data rates and bandwidth requirements. ISLs that represent the network links of these satellite networks which can be either optical or radiofrequency.
Common types of inter-satellite links ISLs
Depending on the location of the satellites, ISLs can be classified into three types: intraorbital plane ISL, which is between two satellites in the same orbital plane; inter-orbital plane ISL, which is between satellites in two different orbital planes; and inter-orbit ISL, which is between satellites in two different orbits.
Satellites in an orbital plane move with the same velocity (i.e., same speed and direction), and intraorbital plane ISLs are relatively easy to establish and maintain. Inter-orbital plane ISLs with satellites in adjacent orbital planes are more difficult to establish. Different orbital planes have the same altitude, and satellites in these orbital planes move at the same speed. However, the direction of satellites in adjacent orbital planes is slightly different, which leads to different relative velocities of satellites.
Also, satellites in crossing orbital planes or satellites in different orbits move at high relative velocities and suffer from challenges like Doppler shift, point-ahead angle (PAA), and acquisition, tracking, and pointing (ATP). Intraorbital plane ISLs and inter-orbital plane ISLs with satellites in adjacent orbital planes are generally stable. Inter-orbital plane ISLs with satellites in crossing orbital planes are make-break or intermittent in nature and cannot last for long durations.
—GEO to LEO links between geostationary (GEO) satellites and low earth orbit (LEO) satellites, also called interorbital links (IOLs);
—GEO to GEO links between geostationary satellites;
—LEO to LEO links between low earth orbit satellites.
Links between geostationary satellites (GEO–GEO)
This kind of link can be used for Increasing the capacity of a system. It is assumed that the traffic demand increases and exceeds the capacity of the satellite. One solution is to launch a replacement satellite of greater capacity and this implies risks, development costs and the availability of a suitable launcher; alternatively, a second satellite identical to the first could be launched with the traffic shared between the two satellites. To avoid interference, the two satellites must be in sufficiently distant orbital positions but not too distant so as to provide sufficiently large common coverage. To ensure interconnectivity among all stations, it is necessary to equip all stations with two antennas, each pointing towards a different satellite.
They can also be used for Extending the coverage of a system. An inter-satellite link permits earth stations of two networks to be interconnected and hence the geographical coverage of the two satellites to be combined.
The principle is to locate several separate satellites in the same orbital position with a separation of tens of kilometres and interconnection by intersatellite links. The satellites are thus all in the main lobe of an earth station antenna and appear equivalent to a single large-capacity satellite which would be too large to be launched by an existing launcher. The cluster is put in place by successive launching of the satellites which form it. As all the satellites are subjected to the same perturbations, orbital control is simplified although station-keeping manoeuvres should be carefully phased. In the case of breakdown of a satellite, it can be replaced in the cluster. Finally, the configuration of the cluster
can be modified in accordance with traffic demands.
Links between geostationary and low earth orbit satellites (GEO–LEO)
This type of link serves to establish a permanent relay via a geostationary satellite between one or more earth stations and a group of satellites proceeding in a low earth orbit at an altitude of the order of 500 to 1000 km. For economic and political reasons, one does not wish to install a network of stations which is so large that at every instant the passing LEO satellites are visible from at least one station.
One or more geostationary satellites are therefore used; they are permanently and simultaneously visible both from stations and low earth orbit satellites and serve to relay communications. This technique also permits overcoming possible limitations of the terrestrial network.
This concept is presently operated in the NASA tracking network by means of the tracking and data relay satellites (TDRS) which, in particular, provide communication with the International Space Station. A European program has successfully launched a data relay payload (ARTEMIS satellite) to provide communications between the ground and low earth orbit spacecrafts.
Links between low earth orbit satellites (LEO–LEO)
There is growing interest in LEO satellite constellations, most notably for science and remote sensing applications as well as for communications with mobile and fixed terminals. Small satellite systems enable a whole new class of missions for navigation, communications, remote sensing, and scientific research for both civilian and military purposes. As individual spacecraft are limited by the size, mass, and power constraints, mass-produced small satellites in large constellations or clusters could be useful in many science missions such as gravity mapping, tracking of forest fires, finding water resources, etc.
Satellites orbiting in low earth orbit present the advantage of significantly minimising the transmission delay, which is of high interest for some services (typically voice). However, a single satellite is visible from earth during a very short period of time thus limiting the duration of communication. This disadvantage can be reduced in a network containing a large number of satellites that are connected by intersatellite links and equipped with switching devices between beams. The IRIDIUM system is an example of a deployed constellation of 66 satellites.
By connecting satellites together, we can cover the globe to establish communication from any place to another on earth. The information is sent by a ground station to the nearest satellite above and then transmitted between the satellites until it reaches the satellite above the destination which then transmits the information down to the destination ground station
It facilitates in eliminating the use of extensive ground-based relay systems and worldwide tracking systems. It also helps to provide attitude control and maintain the relative distance between small satellites. Inter-satellite communications support transmission with high capacity and data rates, real time data delivery, and also can provide absolute interoperability among various spacecraft within the system. The
ISC enables navigation and formation control by exchanging the attitude and position information and also maintains time synchronization between the spacecraft. Consequently, inter-satellite communications enable multiple satellite missions for Earth observations and inter-planetary explorations and observations.
Extending networking to space requires inter-satellite communications which will enable autonomous transfer of data and hence being analogous to terrestrial Internet with autonomous transfer of data with minimum human intervention. Inter-Satellite Communications (ISC) assist in performing advanced functions including, for example, distributed processing, servicing or proximity operations, autonomous
applications, and fractionated operations.
Inter-Satellite Links (ISLs) are considered to be a fundamental component of future Global Navigation Satellite Systems (GNSS) to improve the positioning accuracy and orbit determination. ISLs provide precise pseudorange measurements between satellites in a specific constellation. The combination of ISL and GNSS measurements is one of the key requirements for improving orbit determination. Connections between satellites can be used to transfer information, which might shorten the ephemeris update interval and improve navigation. One of the advantages of the ISL system is the potential to establish links outside of the atmosphere, and thus the ISL measurements are not affected by atmospheric delays and they are also less impacted by multipath and interference than GNSS measurements .
At present, the BeiDou Navigation Satellite System (BDS) is the most advanced system to introduce ISLs. Since March 2015, new generation satellites have been launched to validate, among others, the ISL system. All in-orbit BDS-3 operational satellites are equipped with ISLs . The ISL payload enables observation of other satellites and ground stations with Ka-band single frequency pseudorange measurements . Each satellite operates within a 1.5 s timeslot with another satellite, creating a link pair. However, even with additional ISL measurements, the constellation is still affected by external environmental and technological effects . Observations to anchor stations, described as Ground-Satellite Links (GSLs), use the same communication and measurement system as ISLs. However, unlike the ISL observations, they need to be corrected for tropospheric delays
Microwave Interatellite link technology
Table indicates the frequency bands allocated to intersatellite links by the Radiocommunication Regulations. These frequencies correspond to strong absorption by the atmosphere and have been chosen to provide protection against interference between intersatellite links and terrestrial systems. However, these bands are shared with other space services and the limitation on interference level is likely to impose constraints on the choice of the defining parameters of intersatellite links (CCIR Reports 451, 465, 874, 951). Table also indicates the wavelengths envisaged for optical links. These result from the transmission characteristics of the components.
These new forms of space distributed assets such as satellite swarms and formations, increasingly require that broadband intersatellite links be reconsidered in addition to rapidly reconfigurable interconnectivity. Inter-satellite link (ISL) communications provide a direct link within the space segment without need of an intermediate ground segment to relay the data. As the distributed spacecraft systems (DSS) have become less exotic and more complex, the need and demand for inter-satellite antenna systems has increased and the requirements for the antenna systems more diverse and become more demanding.
Data relay systems, such as EDRS and ARTEMIS, increase the available contact time of the LEO satellites and a number of the upcoming small satellite missions lean to DRSs for communications, either only TT&C or also downlinking the data. Thus the increase of the number and market value of the small missions increases the demand for the data relay services. The major difficulty currently is in forming suitable cross-links with high data-rates. More traditional low data-rate ISLs can be formed with low gain antennas on the LEO S/C, but links of 150 Mbps and 800 Mbps (provided by Russian Luch 5V and US TDRS respectively) require high gain antennas and pointing stability from
With large data relay constellations the payload data can be needed to be linked through several S/C (referred as ISL hops) and this can cause choke points on the network as the capacity cannot be upgraded as easily as with ground network. This is especially important for the high data bandwidth connections, such as upcoming earth observation (EO) missions, where daily data output can be several Tbits. For these optical inter-satellite links are preferred due to the higher data bandwidth and higher directivity, but are impracticable in smaller systems.
Antenna pointing error can be maintained at around a tenth of the beamwidth and this leads to a pointing error loss of the order of 0:5 dB. The antenna temperature in the case of GEO–GEO link, in the absence of solar conjunction, is of the order of 10 K. For practical
applications, antenna dimensions are of the order of 1 to 2 m. Because of the relatively wide beamwidth of the antenna (0.2 degree at 60 GHz for a 2m antenna), establishing the link is not a problem. Each satellite orientates its receiving antenna in the direction of the transmitting satellite with a precision of the order of 0.1 degrees to acquire a beacon signal which is subsequently used for tracking.
The development of high-capacity, radio-frequency intersatellite links between geostationary satellite systems implies re-use of frequencies from one beam to another. In view of the small angular separation of the satellites, it is preferable to use narrow beam antennas with reduced side lobes in order to avoid interference between systems. Consequently, and in view of the limited antenna size imposed by the launcher and the technical complexity of the deployable antennas, the use of high frequencies is indicated. The use of optical links may be usefully considered in this context.
Multiple access methods and Antennas
There is a number of challenges and differences to communicating with the ground segment or even with other individual spacecraft when compared to communications within a Distributed Space Systems (DSS). A major challenge is the handling of the multiple members of DSS in a method which does not cause needles interference between the S/C. Despite the differences in the environment, these are the same challenges as encountered with mobile phones and computers and the same technologies can be applied as well. The five basic Multiple Access (MA) division methods are time, frequency, code, spatial separation, and collision detection and combinations of techniques.
The communications solutions for small satellites are applicable for larger satellites and small satellite downlinks can be re-purposed as ISLs. Technical and regulatory solutions for enabling communications are still required and for the antennas following features are desired
• Small and inexpensive antennas for higher frequency ranges (C to Ka – bands)
• Suitable pointing and beam steering systems
• GEO-LEO links for small satellites
• Long distance LEO-LEO links for the small satellites
• Deployable antennas for small satellites
Beam steering is a major challenge on inter-satellite links and despite the progress in the antenna systems, more research is required. The most traditional method of beam steering is to turn the antenna element, but as antenna pointing mechanisms, APM, are problematic for small S/C mechanisms are often heavy and power hungry. Even for larger satellites the APM system can cause a considerable expense and there is interest for lighter and smaller APMs. Higher frequency antennas produced with lighter materials, such as mesh reflector antennas, provide a possibility to use lighter APMs
Use of multiple antennas in a Switched Beam Antenna, SBA, is a common option for S/C to reach sufficient antenna coverage with out moving parts, such as is used in TerraSAR-X – TanDEM-X link. Combination of reflector and antenna array, such as is proposed for H2Sat allows the use of single reflector for large number of antennas, such as PCB patch antennas. Reflectarray can be used as a special type of SBA, but also if combined with phase shifters, can be used as phased array.
Phased array antennas utilise constructive interference of an array of antennas of which individual antennas are phase shifted. Phased arrays can be considered as the most conventional method of electrical beam steering and a wide spectrum of phase shifter techniques have been developed to suit different applications and frequency ranges. Phased arrays for ISLs are not a new concept and for example Iridium satellites utilise phased array ISL antennas.
Metamaterials are synthetic materials in which specific structures, for example sub-wavelenght patterns, allow the material to have positive or negative effective permittivity and permeability. Antennas which utilise this can be made not only to seem far larger electrically than those are, but also to alter the refractive index which can be tuned using active elements such as diodes and transistors. This can be used for beam steering without using mechanical pointing or traditional phased arrays. The metamaterials can be used to fabricate steerable high
gain antennas with modest size and while there are still technical challenges, upcoming applications seem promising.
Optical Intersatellite Links
In the past, the microwave cross links were implemented which have limited data rates capacities, resulting in a limited performance of most
satellite networks when the traffic demand at ISL constellations increases. So, if we want inter-satellite links with capacities larger than the microwave cross links with data rate Gbps require antenna and transmitter powers large enough.
The optical inter-satellite link is a type of Free-Space Optical link that relies on the transmission of invisible, eye-safe light beams. They are transmitted by a laser with light focused onto a highly sensitive photon detector, which serves as a receiver, equipped with a telescopic lens. Free-Space Optical links have quite a simple construction, typically consisting of two identical heads enabling duplex data transmission. These heads are connected via interfaces directly to computers or with a telecommunications network.
Optical links offer these high data rates because optical shorter wavelengths allow modest antenna (telescope) sizes that transmit very narrow beams which achieve high received signal levels. Compared to radio frequency links, optical communications in space offer several advantages such as higher data rates, small size and weight, no interference with other communications systems bands and less consumed power.
Examples of multiple satellite missions with inter-satellite communications are Iridium, Orblink, Teledesic, Proba-3, Edison Demonstration of Smallsat Networks (EDSN) mission,ESPACENET, NASA’s Autonomous Nano-Technology Swarm (ANTS), and QB-50 mission.
LEO-to-GEO intersatellite links using laser communications bring important benefits to greatly enhance applications such as downloading big amounts of data from LEO satellites by using the GEO satellite as a relay. By using this strategy, the total availability of the LEO satellite increases from less than 1% if the data is downloaded directly to the ground up to about 60% if the data is relayed through GEO. The main drawback of using a GEO relay is that link budget is much more difficult to close due to the much larger distance. However, this can be partially compensated by transmitting at a lower data rate, and still benefiting from the much-higher link availability when compared to LEO-to-ground downlinks, which additionally are more limited by the clouds than the relay option.
Optical technologies are ideally suited to support high data rate intersatellite link, because of the much-reduced dimensions of the equipment required compared to the microwave frequencies. The gain which is feasible with optical frequencies is paid with very narrow beamwidths. The narrowness of the optical beam is typically 5 microradians. This width is several orders of magnitude less than that of a radio beam and this is an advantage for protection against interference between systems. But it is also a disadvantage since the beamwidth is much less than the precision of satellite attitude control (typically 0.1 degrees or 1.75 mrad). Consequently an advanced pointing device is necessary; this is probably the most difficult technical problem.
It has two well-known consequences: a) problems with the initial spatial acquisition, and reacquisition, of the other satellite with which two way communications must be established. b) the necessity of disposing of very fine pointing and tracking systems to co-align the transmitter and receiver optical boresights; The basic structure of the satellite optical communication system: transmitter, receiver, and PAT system. At first, the transmitter converts electrical signals to optical signals using the laser sources
The transmitter telescope collimates the laser radiation in the receiver satellite direction after that the optics of two satellites must be
aligned with the line of sight during the entire time of communication to establish optical communication successfully. To meet this
requirement, the satellites should implement a pointing, acquisition, and tracking (PAT) subsystem, this subsystem has three phases:
– Pointing: laser source in the transmitter points its beams towards the direction of the receiver according to the orbit equation
using Ephemerides data.
– Acquisition: receiver scans the region of space where it can receive the signal if detects the transmitted beam enters the tracking
phase. The duration of this phase is typically 10 seconds. The transmitted beam should be wide to reduce this duration.
The initial or recurring misalignment between the optical beams boresights is unknown and of several tenth or hundredth beamwidths. the beam must be as wide as possible in order to reduce the acquisition time. But this requires a high-power laser transmitter. A laser of lower mean power can be used which emits pulses of high peak power with a low duty cycle. The beam scans the region of space where the receiver is expected to be located. When the receiver receives the signal, it enters a tracking phase and transmits in the direction of the received signal. On receiving the return signal from the receiver, the transmitter also enters the tracking phase. The typical duration of this phase is 10 seconds.
Initially a brute-force, sequential, raster scan approach was proposed leading however to unpractical acquisition times. Variants were conceived, on the spatial scanning approach to decrease the acquisition times, but these remained quite high. Substantial work has been produced worldwide on the ‘all optical’ solutions to pointing acquisition and tracking (PAT) which often resulted in quite sophisticated, complex and costly systems. All this plus the fall off in the demand for commercial space communication has, ‘de facto’, considerably contributed, over the past few years, to discourage further development of this technology
– Tracking: in this phase, the transmission between the two satellites becomes continuous by using beacon signal on one satellite
and a quadrant detector and tracking system on the other.
The beams are reduced to their nominal width. Laser transmission becomes continuous. In this phase, which extends throughout the following, the pointing error control device must allow for movements of the platform and relative movements of the two satellites. In
addition, since the relative velocity of the two satellites is not zero, a lead-ahead angle exists between the receiver line of sight and the transmitter line of sight.
The issue is typically addressed by means of optical systems that apply well-known radar tracking techniques (e.g.: monopulse, conical scanning, step tracking) or other continuous or pulsed beacon tracking systems: but all have in common the fact that, to have a fair chance of aligning the two transceivers in a finite time, the initial angular error between their optical boresights must be in the range of a few beamwidths.
Finally, the receiver telescope focuses the received beam onto the optical filter which prevents the background radiation from
entering the next stages of a subsystem. After that the optical filter pass the received beam to the optical amplifier, the output beam
or radiation of amplifier signal convert into an electrical signal by using photodiode
G&H photonic systems were launched on Japan’s Optical Data Relay Satellite in Dec 2020
Japan Aerospace Exploration Agency (JAXA) and NEC recently announced the launch of LUCAS on board the Optical Data Relay Satellite, which was sent into or orbit last month from JAXA’s Tanegashima Space Center in Kagoshima, Japan, using an H-IIA rocket. Photonics technology from diverse optical systems developer G&H is playing an important role in the communication system known as LUCAS (Laser Utilising Communication System), which employs hardware developed by NEC for JAXA.
The LUCAS system pushes the performance envelope in terms of data rates and latency, with the aim of offering near real-time availability of satellite data. G&H says the use of its “pioneering” system will show how laser communications can be a viable solution for future high-speed and scalable space communications.
LUCAS relies on power-efficient fibre optic and semiconductor laser technology operating at 1.55 µm. The laser communication equipment has been designed to withstand the challenging phase of launch, as well as long term radiation exposure whilst in orbit. LUCAS photonics technology deployed on-board the Optical Data Relay Satellite will be also used on JAXA’s future advanced earth observation satellites named DAICHI 3 (ALOS-3) and DAICHI 4 (ALOS-4).
The LUCAS system has two optical amplifier systems, designed and manufactured by G&H, which create an all-optical bidirectional link between geosynchronous orbit and LEO satellites. A high-power amplifier system boosts the optical signal level before it goes out into space. A low-noise amplifier is used at the receiver side to amplify the very weak signals after having propagated in space across tens of thousands of kilometers. Both optical amplifier systems are manufactured in G&H Torquay and include novel fiber-optics, space-grade laser electronics in addition to NEC-manufactured digital electronic boards for telemetry/tele-command.
Fully-hermetic, space-grade semiconductor laser modules manufactured in G&H Boston are also used in both the transmitter and amplifier systems. The amplifier flight models delivered passed system-level acceptance testing, including mechanical shock, vibration and operation in vacuum. Stratos Kehayas, G&H Chief Technology Officer, commented, “The amplifier system launched is packed with G&H technology. A testament to the power of vertical integration, we used our unique component base in Torquay, UK and Boston, USA to develop novel and high-performing space photonic systems for satellite laser communications.
“To the best of our knowledge this is the first time 1.55 µm optical fiber amplifiers have been deployed in the GEO orbit. We also created the necessary space-grade system manufacturing and assembly procedures and applied these in world-leading, purpose-built cleanroom facilities in Torquay, UK to manufacture these amplifiers.”
Defense organizations have been launching communication nanosatellites and microsatellites to provide communication signals to soldiers stationed in remote locations or in dense forests. The military needs more data bandwidth and reliable communications infrastructure for its UAVs, which can be fulfilled using constellations of nano and microsatellites. Multiple advancements across the board of the technology, such as mobile communications, miniaturisation of electronics, new battery types, have made this possible.
A large number of heterogeneous small satellites can be deployed in space as a network using inter-satellite communications to enable command, control, communication and information processing with real time or near real time communication capabilities. The concept of multiple satellite mission is becoming attractive because of their potential to perform coordinated measurements of remote space, which can
also be classified as a sensor network. Multi-satellite solution is highly economical and helps to provide improved spatial and temporal resolutions of the target. A large number of heterogeneous small satellites can be deployed in space as a network with minimum human intervention, and thus demanding a need for Inter-Satellite Communications (ISC).
DARPA’s call for Intersatellite Microsatellite links
Defense Advanced Research Projects Agency is expected to launch a series of risk reduction payloads into orbit to help pave the way for an experimental program known as Project Blackjack. With Blackjack, DARPA wants to demonstrate the military utility of a large constellation of small satellites operating in low earth orbit. These satellites will connect with each other on orbit over optical intersatellite links, forming a mesh network in space. That network will be able to deliver sensor data collected on orbit to terrestrial war fighters in near-real time.
The Defense Advanced Research Projects Agency has awarded a contract to speed development of technologies that could improve communications amongst its growing fleet of very small satellites. Under the two-year, $5 million contract, LGS Innovations of Herndon, Virginia, will prototype a lightweight, low-power optical communication terminal system that will enable light-based communications between micro-satellites in low-earth orbit.
The desire for rapid revisit rates or persistence from low-earth-orbit (LEO) satellites is also driving the development of large constellations of small satellites, potentially with ~100-400 satellites. In addition to performing their core sensing mission, such satellites also have the potential to provide inter-satellite data relay, providing a highly survivable mesh of nodes capable of relaying data before downlink to ground stations.
Many applications of these satellites will benefit from jam-resistant, high-data-rate, low-latency communications between satellites, whether for cooperative sensing applications or for data relay back to ground stations. Since ground stations may be unavailable in many locations, relaying data between satellites to reach one with a connection to a ground station is an attractive option when low data latency is needed.
DARPA had solicited proposals to develop and demonstrate lightweight, low-power, and low-cost inter-satellite communications links (ISCLs) suitable for use on a wide range of small LEO satellites. Specifically, this program seeks to develop ISCLs with high communication data rates (>1 Mbps) while having a per-link average weight of less than 2 pounds and an orbit-average power dissipation of less than 3 watts. Both optical and radio frequency (RF) links will be considered.
Space Development Agency considers optical intersatellite links key enabler of National Defense Space Architecture.
The SDA was established in March 2019 to design the Department of Defense’s future threat-driven space architecture, a setup it has since defined as a multi-layered constellation of hundreds of small satellites providing several capabilities from LEO. Key to the entire enterprise is the Tracking Layer, a family of satellites in low earth orbit that will facilitate the flow of data between satellites in orbit and between satellites and the ground. The Transport Layer will be essential in connecting the various sensors and capabilities on orbit with weapons systems on the ground or in the air.
In order to build that capability, the SDA plans to use Optical Intersatellite Links. The optical links will also need to provide range estimates of the distance between satellites in orbit and between satellites and the ground to within a meter in order to provide highly precise timing and positional data for the constellation. The SDA also envisions each satellite utilizing a chip-scale atomic clock as well as GPS signals.
Derek Tournear, SDA director, has stressed that OISLs are a key enabler for the Transport Layer of the agency’s seven-layered National Defense Space Architecture.
The problem is that there are currently no industry standards for those links. Several companies make advanced optical terminals for NASA’s deep-space communications and follow common standards. But there is no accepted industry standard for optical inter-satellite links in lower orbits below geosynchronous range, according to SDA.
According to the request, the SDA plans to issue a solicitation for Tranche 0 of the Transportation Layer in Spring 2020, with additional solicitations for the other capability layers to follow in the summer. That first tranche, known as the war fighter immersion tranche, will consist “of tens of satellites providing periodic, regional sensing and data transport capabilities, including the capability to detect hypersonic glide vehicles and to disseminate time sensitive targeting solutions over tactical data links.” According to the agency, that initial tranche could be delivered as early as fiscal year 2022
DoD has a very different requirements than NASA’s deep-space programs that seek to establish communications with the moon and beyond. “NASA has proved the technology. But their terminals are not affordable for LEO programs.” A key advantage of laser links is that they are extremely secure, said Strobel. “They’re virtually impossible to intercept.” Before DARPA started the Blackjack program it awarded LGS Innovations in 2016 a contract to develop two small laser communication terminals to be used as inter-satellite links. LGS Innovations, now owned by CACI International, delivered the terminals and DARPA plans to launch them in a future space experiment, Chris Simi, program manager at DARPA’s Strategic Technology Office, told SpaceNews.
Tom Wood, CACI’s senior director for optical communications and networking, said optical communications is attracting more interest because of the challenges and limitations of RF systems, said Wood. “The way engineers have addressed it in the last 15 years is by going to higher frequencies. But you’re still using electromagnetic waves to transmit information.” The electromagnetic spectrum is in tight supply whereas the available spectrum is much greater in the optical band. “It’s about 8,000 gigahertz,” said Wood. “You can get essentially unlimited spectrum for transmissions. And you can get much higher throughput through optical systems than through radio systems,” he added. “This spectrum is unlicensed by the Federal Communications Commission or other authorities. You can build your equipment and go.”
General Atomics is one of two firms — the other is SA Photonics — prototyping space-based laser communications systems for the Space Development Agency (SDA). The company is building its own Cubesats to carry its Laser Interconnect and Networking Communication System (LINCS) payload, Bucci said.
SDA plans to launch four experimental satellites in June — the two LINCS sats, and two satellites (called Able and Baker) built by Astro Digital, and carrying the SA Photonics payloads. The SA Photonics payloads were designed as part DARPA’s Blackjack program, and are known collectively as the Mandrake 2 experimental satellites.
“These OISL [Optical Inter-Satellite Links] terminals are early versions of our CrossBeam™ system that is being developed to support DARPA Blackjack and SDA Tranche 0 applications,” Dave Pechner, chief technical officer at SA Photonics said in an email. “The goal of the Mandrake-2 mission is to demonstrate OISL crosslink capabilities as well as space-to-ground links,” he added.
General Atomics Partners with Space Development Agency to Demonstrate Optical Intersatellite Link
General Atomics Electromagnetic Systems (GA-EMS) announced that it has partnered with the U.S. Space Development Agency (SDA) to demonstrate and conduct a series of experiments for an Optical Intersatellite Link (OISL) utilizing GA-EMS’s internally developed 1550nm (nanometer) wavelength laser communication terminals (LCTs). This will be one of the first Department of Defense contracted efforts to develop and deploy a state-of-the-art 1550nm LCT to test capabilities to increase the speed, distance, and variability of communications in space.
“This is an exciting opportunity for GA-EMS to leverage work currently underway to advance OISL technologies,” stated Scott Forney, president of GA-EMS. “For several years we have been developing a series of optical laser communication terminals to improve and increase satellite crosslink data transfer rates and downlink data rates. These experiments will demonstrate robust communication capabilities through multiple mediums, from Earth, to and between satellites in multiple orbits, and on in to deep space. Our LCT technology will modernize and enhance space communications permitting faster communication transmission across longer distances and with greater fidelity.”
The OISL demonstration will consist of two GA-EMS internally designed and built 12U CubeSat spacecraft, each of which will host an Infrared payload (IRPL) and LCT payload, with an anticipated launch date in March 2021. Satellite development, integration and testing is being conducted at GA-EMS facilities in San Diego, CA and Huntsville, AL. GA-EMS will also provide mission control capabilities from its mission control centers in Centennial, CO and Huntsville.
“GA-EMS’ small satellite designs offer unique solutions to many of the challenges that arise when developing demonstration assets such as LCTs including addressing specific size, weight, and power requirements,” said Nick Bucci, vice president of Missile Defense and Space Systems at GA-EMS. “With our proven spacecraft and their capabilities coupled with our significant investment in LCT research and development, we anticipate this demonstration to show data rates up to 5GB a second at ranges up to 2500 km, and this LCT can support out to greater than 4500 km. This increased speed in communications is necessary to advance a variety of space applications in intelligence, surveillance, telecommunications, reconnaissance, and more.”
Experts have also propsed combining RF and optical technologies, instead of insisting on an ‘all optical’ approach, may lead to solutions which are more acceptable from a technology-risk and cost viewpoints. Indeed, the acquisition problems are bound to become even worse with the upcoming distributed space assets characterized by time-evolving spatial geometries and the requirement for very rapid handover, in the data transfer from any satellite of the group to any other in visibility, to maximize the data volume transfer per unit time. In this scenario, system techniques are sought enabling an effective and quasiinstantaneous narrowing of the angular acquisition cell of the partner satellite with which to establish two-way wideband communications via optical ISL.
References and Resources also include: