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How to Select Antenna Front-End Components for Non-GEO Space Applications

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by Jim Ryan, Product Marketing Manager, Space Products Group, Analog Devices, Inc.

The adoption of active electronically scanned antennas (AESAs) for satellite communication has offered greater flexibility for operators and consumers. This article will review the design considerations when choosing antenna front-end components such as low-noise amplifiers and power amplifiers for use in these beamforming arrays.

We have lived with satellite technology for over 60 years, and even though the early satellites were launched to low Earth orbit (LEO) because of launch and size limitations, we are most familiar with satellites in geosynchronous Earth orbit (GEO) where they have provided multiple services such as telecom, satellite TV, Earth observation, and a range of services for governments and their military.

However, a major shift has occurred to now position LEO and medium Earth orbit (MEO) as the most attractive orbits for a range of large constellations offering a range of data-based services (SATCOM, Earth observation and mapping, navigation and positioning, etc.). Figure 1 shows the relative positioning of LEO, MEO, and GEO orbits.

Figure 1: Comparison of LEO, MEO, and GEO orbits

This move to non-GEO has come about due to the confluence of lower launch costs, adoption of mass manufacturing techniques for satellites, technical advancements in communications and antenna technology and sensors, optical technology for inter-satellite links, and the availability of substantial amounts of private capital to fund these large programs.

The increasing use of spacecraft in LEO poses new challenges for designers of on-orbit SATCOM links. The fixed communication links of GEO have been replaced with links that must be adaptable to allow them to communicate with locations on Earth, even as they orbit Earth at speeds in the range of 7.5 km/s.

AESAs used in these SATCOM systems provide the ability to adaptively steer the antenna’s signal in the correct direction of its intended target and support multiple beams, allowing multiple users to be supported simultaneously. On-orbit satellites have unique requirements for component selection, and this is particularly the case for the front-end components that interface the antenna elements with the transmit and receive signal chains.

GEO Satellites Provide Good Service, So Why Change?

Despite the drawbacks of high launch costs, satellites in GEO had a significant advantage: They were at a fixed position in the sky due to the orbit being synchronous with the Earth’s rotation. This allowed the deployment of fixed-position satellite antennas and relatively low-cost VSAT terminals with parabolic dish antennas, key enablers of data services and direct-to-home satellite TV services. Satellites in GEO have the largest Earth coverage (Figure 2), with only three GEO satellites required to provide global coverage.

Figure 2: Earth coverage from GEO, MEO, and LEO

Despite the stated advantages of GEO, the move toward satellites in LEO has several drivers, mostly concentrated on evolving communication networks. We live in a highly connected world, but a considerable portion of the world’s population lives in areas that are either not served or are underserved by Internet connectivity. For example, while GEO satellites are positioned on an equatorial plane, this reduces service availability in polar regions.

Large SATCOM constellations in LEO can bring high-speed connectivity to these areas. For areas currently served by Internet connectivity, LEO constellations promise even higher data rates equivalent to fiber for consumers and private industry. The size of the proposed LEO constellations, including some redundancy, brings the advantage of network resiliency due to the larger number of available satellites. This resiliency interests government and military users and the commercial world. Finally, lower manufacturing and launch costs mean a satellite network can easily be upgraded as new technology becomes available.

Satellite Orbits

A non-GEO constellation is configured using satellites in specific orbits or in a mixture of orbits. The more popular orbits include the equatorial orbit used by the SES O3b mPOWER constellation in MEO where the satellites follow the equator. The inclined orbit is tilted from an equatorial orbit by several degrees and tracks from west to east in the same direction as Earth’s rotation.

The polar orbit is where each satellite will follow a particular line of longitude while orbiting over each pole (OneWeb uses this approach, for example). Several large LEO constellations, such as Telesat Lightspeed and SpaceX Starlink, use a hybrid of inclined and polar orbits to provide optimal coverage in the northern regions because inclined orbits can only operate to a certain latitude.

Figure 3: A combination of orbital configurations for LEO constellations

Polar orbits provide the best global coverage of the three orbit categories, but due to additional fuel for positioning they are primarily used to provide additional coverage of northern latitudes combined with shells of satellites in inclined orbits. Polar orbits are also more exposed to radiation effects. Satellites are arranged in circular planes, each at constant height above Earth. The size of the constellation is given by the number of planes multiplied by the number of satellites per plane (Figure 3).

Enter the LEO Constellation

Some constellations have launched, and there are plans to launch hundreds, in certain cases thousands, of small satellites into LEO. Satellites in LEO offer two distinct advantages for SATCOM over GEO links. First, signal latency is reduced because of the height of the orbit. The signal path from Earth to a LEO satellite is much shorter (around 1/35 of that of a GEO satellite), reducing signal latency by an order of magnitude to about 25 ms. This will allow LEO SATCOM to exploit the expansion of 5G services with its promise of data-intensive real-time services.

The second advantage is that an individual LEO satellite’s data capacity is concentrated on a much smaller area, potentially giving individual users a much larger data bandwidth, subject to the overall data capacity of the constellation. The satellite would typically generate multiple downlink beams within the coverage area to link to many users. These spatially separated beams allow the reuse of allocated frequencies to avoid interbeam interference and optimize data availability.

High throughput satellites (HTS and vHTS) can also provide this data concentration, but a GEO satellite’s overall data capacity is lower than that of a typical LEO constellation. A limitation of large constellations with high data capacity is that only a fraction (33% to 50%) of that overall data capacity is available to users at one time, as many of the spacecraft will be flying over oceans or uninhabited areas of Earth.

Constellation Size Impact on Cost and Mission Life

Constellation satellites are less expensive to build because they can be mass produced and because lower cost, non-hermetic, often plastic encapsulated components can be used due to the shorter mission life and less harsh radiation environment. The mission lifetime of LEO satellites is typically 5 years to 7 years because of increased fuel usage for maintaining orbit. This is because there is increased atmospheric drag in LEO and LEO satellites with limited fuel capacity due to their smaller size.

Radiation tolerance requirements are typically lower for LEO satellites. For example, the acceptable level for total ionizing dose (TID) performance of a component to be used in an LEO satellite could be in the region of 30 krad, whereas 100 krad is typically required for a GEO mission due to its longer mission life and higher radiation exposure.

Challenges of LEO and Key-Enabling Technologies

There is an increased complexity in managing data flow to the constellation. Data is routed from Earth stations through the constellation via intersatellite links using either radio or optical technology. This is necessary as LEO satellites may not always be in sight of an Earth station.

Non-GEO satellites move across the sky when viewed from Earth as opposed to the fixed position of GEO satellites. This is a factor in the orbital speed required to maintain their orbit. Due to increased atmospheric drag and a lower orbit, LEO satellites must be faster than satellites at higher orbits. One of the satellite shells for the Starlink constellation is positioned 550 km above Earth, and at that altitude flight velocity is 7.5 km/s, which means that an individual satellite in this shell will only be visible to a user for 4.1 minutes. A user of a GEO satellite can use a fixed antenna positioned on the satellite, while a user of an LEO satellite service must use an antenna that can track the LEO satellite as it crosses the sky.

The satellite’s antenna must be able to track the served area on Earth as it moves in its orbit. Satellites in MEO, such as the O3b constellation, have used mechanically-steered antennas because of their slower orbital speed. LEO satellites must use some form of AESA as mechanical steering systems cannot meet the tracking requirements. Concurrent with the need for steerable beams in LEO is the general requirement for multiple beams that allow satellites to optimize service and data throughput to multiple data gateways or served areas. What is needed for LEO applications is an antenna that can support the electronic beam steering of multiple beams independently, and some constellations propose up to 16 steerable user beams per satellite.

The key to the flexibility of these constellations is the adoption of antennas that support beam steering to maintain the communication links, either primary SATCOM/EO uplinks/downlinks or the secondary tracking, telemetry, and control (TT&C) links.

AESA and Beamforming

A conventional parabolic antenna typically has a single feed for the transmitter and the receiver and will either be pointed at a fixed position or mechanically steered. An electronic beam steering array antenna comprises multiple antenna elements whose radiation patterns are designed to combine constructively with those of neighboring elements in the array to form the main lobe (Figure 4).

Figure 4: Beam steering with a one-dimensional array

The main lobe transmits the radiated energy in the desired direction. Ideally, the main lobe would carry all the transmitted energy, but because of nonidealities, there will be some energy directed in side lobes that are not in the desired direction. The antenna design seeks to maximize the energy in the main lobe while minimizing that of the side lobes. The main lobe can be shaped and steered by adjusting the individual amplitudes and phases of the antenna elements.

Modern IC technology can implement adjustable gain and phase that can be updated in microseconds to provide fast steering even on large arrays of elements for satellite and airborne applications. Side-lobe reduction is critical for LEO applications as the side lobes can cause interference due to the satellite’s proximity to Earth.

Front-End Component Selection for AESA

SATCOM systems use frequency division duplex (FDD) in which the transmitter and receiver operate at different frequencies. These systems most often have separate antennas for uplinks and downlinks using the allocated frequency bands.

In common with most applications in the aerospace and defense domain, the size, weight, power, and cost (SWaP-C) are important characteristics that determine the choice of components in systems and subsystems. For on-orbit applications, size and weight are limited by the launch capability, with larger and heavier systems being much more expensive to launch.

In the case of large constellations, for example, each satellite must fit within a predetermined form factor, which allows multiple satellites to be deployed from the launch bay of a rocket. Also, as on-orbit systems rely almost entirely on solar energy and battery back-up systems, power consumption is a critical specification when choosing components.

For designers of array antennas for on-orbit applications, the array size and element spacing require that front-end components (LNAs for receive antennas and driver and final amplifiers for transmit antennas) be as small as possible as each element of the array will have its own front-end. They often require multiple components that must be placed as near the antenna element as possible to reduce trace losses that can add to noise figure.

A typical implementation has a beamforming core chip dedicated to several antenna elements, with each element having its own front-end devices (LNAs for the receiver and driver and the power amplifier for the transceiver). High-gain receive antennas may implement front-ends by placing several high-gain LNAs in series to achieve the required input gain. Component size is important in this context as interelement spacing will reduce as frequency increases.

In the case of a Ka-band receiver (26 GHz to 28 GHz), the element spacing is about 5 mm for a half-wavelength lattice pitch, while maintaining the wide scan angles for LEO applications dictates that the array elements be placed at the half-wavelength pitch. For antenna arrays used on GEO platforms, the scan requirements are not as critical (± 9), allowing more flexibility in minimal element spacing. The latest LNA form factors in 2x 2 mm packages make it easier to manage component placement and many also include DC blocks and RF chokes within the package to further simplify the layout task.

Device performance is critical when selecting amplifiers for on-orbit applications. For LEO satellite receiver antennas, noise figure is the most important as it contributes to the system noise figure, which directly affects the number of elements required in the array and hence the antenna size. As LEO satellites are smaller than those in GEO, the space to house an antenna may be constrained. For a typical array, a system noise figure of less than 2 dB is required to keep the array size manageable.

A reduction of system noise figure by 1 dB allows a halving of the number of antenna elements, so LNA noise figure as a contribution to system NF is critical. LNA gain is also important as high gain will be required to recover and amplify the receive signal. Several stages of front-end LNA are typically deployed to provide sufficient gain. Communication links must be maintained despite variable atmospheric conditions, so front-end device linearity measured by output IP3 is a key specification.

While receiver signal strength is largely determined by the transmitting ground station, receiver linearity is important to maintain the maximum possible data rates using complex modulation schemes. Devices such as the ADL8142 low power Ka-band LNA can scale their linearity by adjusting power consumption (IDQ) to compensate for variations in the receive path. For transmitting antennas, the front-end will be a driver or final amplifier, and linearity is critical to ensure the highest possible transmission rates, but in this case the output power (OP1dB) will determine the amount of power that each antenna element can contribute.

For on-orbit applications, the power-added efficiency (PAE) of the output amplifier is important because there is limited power available from solar panels or battery backup, and inefficient amplifiers need more cooling to deal with the heat generated by nonconverted power.

ADI ICs for SATCOM

Analog Devices has developed a range of devices that meet the requirements of various applications that employ beamforming, including SATCOM, civil and military radar, and 5G communications. For SATCOM applications, the ADAR3000 and ADAR3001 provide on-satellite Ka-band transmit and receive beamforming, respectively. Each has 4-beam/16-channel beamforming capability using programmable time delay and attenuation. They are housed in BGA packages.

Complementing the beamforming ICs are the ADAR5000 4:1 Wilkinson splitter/combiner for beam distribution while antenna front-end options include the ADL8142 LNA designed for on-orbit applications in Ka-band (23 GHz to 31 GHz). Packaged in a 2 x 2 mm LFCSP/QFN package, the ADL8142 is optimized for low noise figure (1.6 dB), high linearity (20 dBm OIP3), and high gain (27 dB) with power dissipation of only 50 mW from a voltage rail of 1.5 VDC. Figure 5 shows the gain and noise figure of the ADL8142.

Figure 5: ADL8142 gain (a) and noise figure (b) vs. frequency
Figure 6: ADL8107 gain (S21) (a) and P1dB (b)

The ADL8142 is available in both COTS and commercial space versions. On the transmit side, devices such as the ADL8107 (8 GHz to 15 GHz, 28 dB gain, 19 dBm P1dB) or HMC498 (17 GHz to 24 GHz, 22 dB gain, 26 dBm P1dB) with their high gain and linearity can be used as element drivers. Figure 6 shows the gain and output P1dB for the ADL8107.

Conclusion

Beamforming antennas enable the latest non-GEO satellite constellations to deliver on their promise of ubiquitous, flexible, and high-bandwidth data communications. Designers of beamforming antennas can take advantage of ADI’s flexible offering of signal chain components from data converters through frequency converters and beamformers to front-end components. The antenna front ends are critical in the overall signal chain as not only will they determine the system noise performance, but they also must conform to mechanical and power consumption constraints.

For Further Reading

1. Keith Benson. “Phased Array Beamforming ICs Simplify Antenna Design.” Analog Dialogue, Vol. 53, No. 01, January 2019.

2. ADL8142 data sheet. Analog Devices, Inc., 2022.

3. ADL8107 data sheet. Analog Devices, Inc., 2022.

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