Understanding Satellite Links and Their Future Developments

Satellite links are a crucial part of modern telecommunications, connecting remote areas to the global network.

They operate by transmitting data through radio waves between a satellite in orbit and a ground station, allowing for global connectivity.

This process involves a complex system of signal amplification and modulation to ensure reliable data transmission.

A single satellite can serve thousands of users, making them a cost-effective option for rural or hard-to-reach areas.

The first commercial satellite link was launched in 1965, revolutionizing global communication.

This technology has come a long way since then, with modern satellites offering faster data speeds and more reliable connections.

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Satellite links operate within specific frequency bands, each with its own set of characteristics and challenges.

Radio frequency communications for spacecraft are conducted between 30 MHz and 60 GHz.

The lower frequency bands (up to S-band) are typically more mature for SmallSat use, but extensive use of these bands has led to crowding and challenges acquiring licensing.

Higher frequencies offer a better ratio of gain-to-aperture-size, but this is offset by the increased atmospheric attenuation at those frequencies and the higher free space loss that is directly proportional to the square of the frequency.

Here's a breakdown of the frequency bands used for SmallSat communication:

By understanding the characteristics of each frequency band, satellite designers can choose the most suitable band for their mission requirements.

Consider reading: C Band Frequencies

SmallSat communication systems are the backbone of satellite links, enabling data transmission between satellites and ground stations. These systems are designed to be compact, lightweight, and efficient, making them ideal for small satellites.

A typical SmallSat communication system consists of a transceiver, which includes a radio, amplifier, and antenna. The radio produces and modulates an electromagnetic wave to create a signal, while the amplifier boosts the signal's power before transmission. On the receive side, a low noise amplifier minimizes thermal noise and amplifies the weak signal.

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Radios play a crucial role in SmallSat communication systems, as they generate the signal and modulate or demodulate it. They can also add coding to the signal to provide data error detection and correction capabilities. Channel coding, for instance, increases the signal-to-noise ratio (SNR) to ensure reliable communication.

Mixers are used to change the frequency of the signal, while filters reject undesired frequencies. Amplifiers are necessary for both transmit and receive systems, with power amplifiers increasing the RF output power and low noise amplifiers minimizing thermal noise.

Antennas are responsible for increasing and focusing the signal strength in a specific direction. They can be low-gain and omnidirectional or high-gain and directional, depending on the application.

Here's a summary of the major components in SmallSat communication systems:

Optical inter-satellite communications offer a promising alternative to traditional radio frequency (RF) communications. They use high-frequency carrier waves and large bandwidth to achieve high-speed, high-capacity communication. This method is not affected by radio interference and can use smaller and lighter antennas, reducing the weight and launch costs of satellites.

However, optical inter-satellite communications require high accuracy pointing, acquisition, and tracking technology for communication links, making them more secure. They also need to cover very long distances, which can be challenging due to the use of very narrow beams.

To operate a non-Federal US spacecraft with a transmitter, you'll need to get licensed by the Federal Communications Commission (FCC).

The types of RF licenses used by small satellites are Amateur (FCC Part 97) and Experimental (FCC Part 5).

An amateur license is limited to hobbyists and non-profit use, and comes with many FCC restrictions.

Experimental Part 5 licenses are commonly used for university CubeSats and can be granted for a CubeSat operating in the amateur band.

A spacecraft with remote sensing capability must contact the National Oceanic and Atmospheric Administration (NOAA) to see if a NOAA license is required.

A NOAA license is not an RF license and doesn't give you authority to radiate RF energy for communication.

For government missions, the National Telecommunications and Information Administration (NTIA) is the licensing authority.

For amateur licensing, you need an FCC licensed amateur radio control operator.

Downlink telemetry and communications can't be obscured (encrypted).

Use of science gathered via amateur radio downlink for profit is prohibited.

Frequency "assignment" in the amateur-satellite allocations requires coordination, which is administered by the International Amateur Radio Union (IARU).

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Ground stations play a crucial role in satellite communication, and they come in two main types: RF and optical. RF ground stations are common and use a receiving antenna to capture electromagnetic waves, while optical ground stations require a mirrored telescope to focus collected optical energy onto a receiver.

RF ground stations are often mounted on a stable surface, but optical ground stations need to be located at astronomical telescope sites with low cloud cover and calm air. This is because the receiving aperture must maintain an optical-quality surface to focus the collected energy.

There are two primary classifications of antennas: fixed and deployable. Fixed antennas, such as patch antennas and array antennas, remain stationary in their position and don't require power or triggering mechanisms. Deployable antennas, like whip antennas and parabolic reflectors, require power to deploy and use mechanisms to configure into their final position.

Here are some key characteristics of antennas:

The choice of antenna depends on the mission requirements, such as data rate and frequency. Increasing the aperture or diameter of an antenna can increase the link margin, allowing designers to increase the data rate or reduce the necessary transmit power.

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Free space optical communications, also known as lasercom, uses optical wavelengths to transmit messages wirelessly between user terminals.

The higher frequencies of electromagnetic energy used in lasercom result in a much larger amount of bandwidth available for communicating compared to RF systems.

This increase in bandwidth enables much higher data rates, making lasercom a promising technology for small satellites and CubeSats.

The beam width of a lasercom link is typically much narrower than a RF link, and the divergence of a beam is proportional to the wavelength of the electromagnetic wave transmitted divided by the transmitted beam diameter.

Lasercom systems have a low probability of intercept, are difficult to jam, and encounter very little interference due to the narrow beamwidth.

However, lasercom systems require precise pointing of the beam and are susceptible to attenuation due to moisture in clouds, which prohibits communication while there is cloud cover.

The small beam divergence of lasercom transmit beams means that the acceptable pointing error of the narrow beam is much smaller than that of typical RF systems.

Aerospace Corporation successfully demonstrated a 200 Mbps downlink from a 1.5U CubeSat to a 40 cm ground station using lasercom technology.

The Aerospace Corporation transmitter has also flown on follow-on missions that used lasercom systems to downlink science data.

Lasercom systems can be smaller, lighter, and more power-efficient than similar RF systems due to the high frequencies used and the small wavelengths of the transmitted energy.

Optical ground stations are typically mounted inside protected domes or other structures to cover them during bad weather.

These structures typically need to be opened for clear access to the sky.

Optical ground stations are often located at or near astronomical telescope sites, as they are located in favorable "seeing" environments.

Locations with low chance of cloud cover and calm, non-turbulent air are ideal for optical ground stations.

Typical ground-to-space beacons are tens of watts of optical power for low-Earth orbit missions.

Most optical ground stations are experimental facilities used for campaigns with specific research missions.

Future technologies are rapidly advancing the field of satellite links, enabling faster and more reliable communication between satellites and Earth. Quantum key distribution is a protocol that shares a secret cryptographic key through entangled photons, with sources and optical front ends being developed for transmitting these keys from small satellite spaceborne platforms.

Several major programs, such as the European Data Relay System, use intersatellite links to geosynchronous orbit, increasing the communication windows for small satellites in low-Earth orbit. NICT is working on establishing this type of link with a CubeSat through the CubeSOTA program.

The Space Development Agency's Proliferated Warfighter Space Architecture constellation is driving the deployment of larger terminals for larger SmallSats, developed by companies like Tesat, Mynaric, SpaceMicro, and SA Photonics. DARPA has also funded the Space-BACN program to develop a reconfigurable and multi-protocol inter-satellite LCT.

NASA's Small Spacecraft Technology (SST) program, through the University Smallsat Technology Partnerships (USTP) initiative, is funding several projects to advance RF and optical communication systems. The Deployable Optical Receiver Aperture (DORA) project, in partnership with Arizona State University and JPL, successfully launched from the ISS in October 2024 to demonstrate a novel approach to deploying large apertures.

Here are some USTP projects focused on SmallSat communications technology advancement:

There is already strong flight heritage for many UHF/VHF and S-band communication systems for CubeSats, making them a reliable choice for satellite links.

Limited Ka-band systems exist for CubeSats, but high-rate transmitters like the Astro Digital AS-10075 have demonstrated 320 Mbps in a mission.

RF frequencies are considered complementary to optical communications, as they can maintain contact even when clouds block the optical signal.

Growing interest among the NASA science community is using constellations of CubeSats to enhance observations for Earth and space science, with RF communication playing a key role.

Satellite constellations with hundreds to thousands of satellites will provide global data distribution services of unprecedented scale and reach.

These proliferated LEO (pLEO) satellite networks, such as SpaceX's Starlink and Amazon's Kuiper, offer low-latency data transmission and global coverage, even in remote or inaccessible regions.

Satellites can carry only a small number of inter-satellite link (ISL) terminals due to size, weight, and power restrictions, and the range of ISLs is limited.

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Our team is investigating topology designs for several different constellations, each with hundreds of satellites, to examine network performance characteristics such as resilience to failures, time delays from data sources to destinations, and overall system data capacity.

Satellite networks have unique constraints that most terrestrial networks do not, making topology design a challenging task.

Satellite constellations are being developed to provide global data distribution services on an unprecedented scale. These constellations contain hundreds to thousands of satellites orbiting the Earth at altitudes of less than 2,000 kilometers.

Commercial examples of such proliferated LEO (pLEO) satellite networks include SpaceX's Starlink, Amazon's Kuiper, Eutelsat's OneWeb, and Telesat's Lightspeed. Government-owned pLEO systems are also in the works, including the Space Development Agency's Proliferated Warfighter Space Architecture and SpaceX's Starshield for the U.S. Space Force.

Satellites in these constellations join via inter-satellite links (ISLs), typically using laser communications technology. Each satellite can carry only a small number of ISL terminals due to size, weight, and power restrictions.

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Network topology is a crucial aspect of pLEO constellations, and researchers are exploring topology designs for several different constellations, each with hundreds of satellites. They are examining network performance characteristics such as resilience to failures, time delays from data sources to destinations, and overall system data capacity.

To increase the resilience of pLEO network designs, techniques such as adding more ISL connections and dynamically reconfiguring ISL connections after failures are being evaluated. This is especially important for U.S. government systems.

Satellites can theoretically calculate optimum routes, but their processing power is very limited compared to terrestrial nodes. Routes can instead be calculated on the ground and uploaded to each satellite, but a backup scheme is needed in case the ground-based route calculation engine fails or gets disconnected from any satellite.

Here are some key benefits of emerging pLEO networks:

Optical acquisition and tracking technology is essential for establishing a link in optical communications in space, where the laser beam plays two roles: as a means of establishing a link and as a carrier wave for communications.

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Information on the locations of the two communicating satellites can be predicted to a certain degree using orbit calculations, but it's not accurate enough and may be affected by thermal distortions and micro vibration in the spatial environment.

NEC has developed an algorithm for acquiring and tracking the communication partner satellite with certainty in a short period of time, using multiple scan shapes depending on the sequence and narrowing the scan area for each scan to improve accuracy.

A spiral scan, one of the possible scan shapes, is capable of scanning a wide acquisition area at high speeds, as shown in the image of a spiral scan between a satellite in geostationary orbit (GEO) and one in low earth orbit (LEO).

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Future space systems are being developed to take advantage of optical inter-satellite communications. NEC aims to build an optical communication network system that uses its optical inter-satellite communication technology.

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A data relay system that enables faster inter-satellite communications with a higher capacity and increased coverage for data transmission is necessary. This is because satellite observations are generating increasingly large volumes of data.

The European Data Relay System is a major program that uses intersatellite links to geosynchronous orbit. NEC's goal is to improve the ability to transmit data immediately from satellite observations.

Several projects have been funded to advance RF and optical communication systems for small satellites. The Deployable Optical Receiver Aperture (DORA) project successfully launched from the ISS in October 2024.

Here are some SmallSat communication technology advancement projects:

These projects are focused on advancing small satellite communication systems, including optical communication systems.

In optical communications, establishing a link between satellites requires a precise technology to locate and track the laser beam. This technology is called optical acquisition and tracking.

The locations of communicating satellites can be predicted, but the information obtained is not accurate enough due to thermal distortions and micro vibrations in space. As a result, a technology that allows both parties to scan the transmitted laser beam is essential.

The optical capture operation involves scanning the laser beam in the expected direction of the communication partner satellite. The partner satellite detects the laser beam with its own optical acquisition sensor to determine the exact position of the other satellite.

NEC has developed an algorithm that allows for the acquisition and tracking of the communication partner satellite with certainty in a short period of time. This algorithm uses multiple scan shapes to narrow the scan area for each scan, improving accuracy.

One of the possible scan shapes is the spiral scan, which is capable of scanning a wide acquisition area at high speeds. This scan shape is particularly useful for satellites in geostationary orbit (GEO) and low earth orbit (LEO).

The distance between a geostationary satellite and an earth observation satellite is 40,000 km, which poses a significant challenge for signal transmission.

High-power optical amplifiers, such as erbium doped fiber amplifiers (EDFAs), are used in the transmitter section to compensate for signal attenuation.

The EDFA is implemented in a complicated manner to ensure high power characteristic and long-term reliability in the high vacuum environment.

A digital signal processor with accurate demodulation is used in the responder section to handle noisy signals.

Error correction codes are also used to ensure reliable data transmission.

The optical transponder achieves high-speed signal transmission of 1.8 Gbps.

The transponder's configuration includes a high-power optical amplifier, a digital signal processor, and error correction codes.

The equipment is designed to withstand vibration, shocking, vacuum, and high-temperature environments, as well as radiation.

A unique perspective: Transponder (satellite Communications)

NEC has developed three sets of laser communication terminal, including one for a geostationary satellite and two for earth observation satellites.

The system for geostationary satellite was installed in the optical data relay satellite launched in November 2020 and successfully established an optical communications link between the satellite and an optical ground station 40,000 km away.

NEC's laser communication terminal employ the 1.5 µm band wavelength, widely used in underwater cables and LANs, and also has excellent parts availability.

The inter-satellite transfer rate of 1.8 Gbps is at the world's top level and more than seven times faster than conventional radio wave communications.

In the near future, NEC plans to demonstrate and practically use optical inter-satellite communications between the optical communications equipment onboard the advanced land observing satellites ALOS-3 and ALOS-4, which will be launched in FY2021 or later.

Access to Space is a game-changer for satellite links. SpaceX is the only satellite operator that can launch its own satellites as needed, giving them a huge advantage in terms of flexibility and reliability.

With frequent, low-cost launches, Starlink satellites can be constantly updated with the newest technology, ensuring they stay ahead of the curve. This means that users can expect the best possible performance from their satellite links.

SpaceX's ability to launch its own satellites has revolutionized the satellite industry, making it possible for more people to access space and enjoy the benefits of satellite links.

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Starlink is a constellation of thousands of satellites that orbit the Earth at about 550km, significantly reducing latency compared to traditional satellite internet services.

This constellation covers the entire globe, making high-speed, low-latency internet accessible to millions of active customers worldwide.

Starlink operates with the most conservative maneuver thresholds in the industry, ensuring space safety and publicly sharing its high-precision ephemerides to facilitate coordination with other satellite operators and launch service providers.

The satellites are produced and operated in Redmond, Washington, and customer kits are manufactured in Bastrop, Texas, leveraging SpaceX's expertise in spacecraft and on-orbit operations.

Worth a look: Satellite Internet Constellation

Starlink is a game-changer when it comes to satellite internet. Most satellite internet services come from single geostationary satellites that orbit the planet at 35,786 km, which results in high latency.

This means that the round trip data time between the user and satellite is high, making it nearly impossible to support activities like streaming, online gaming, or video calls.

Starlink, on the other hand, is a constellation of thousands of satellites that orbit the planet much closer to Earth, at about 550km. This low orbit significantly reduces latency to around 25 ms.

Starlink's satellites are produced and operated by SpaceX, leveraging their deep experience with both spacecraft and on-orbit operations.

Starlink's satellite constellation is the largest in the world, with over 6,750 satellites currently in orbit.

This massive network serves millions of active customers worldwide, providing them with high-speed and low-latency internet.

Starlink operates with the most conservative maneuver thresholds in the industry, prioritizing space safety above all else.

By sharing its high-precision ephemerides publicly, Starlink makes it easier for other satellite operators and launch service providers to coordinate with them.

A space safety service has also been introduced by Starlink, further streamlining coordination and ensuring a safer shared space environment.

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