The Ins and Outs of Satellite Communications

Getting a refresh on the fundamental aspects impacting satellite communications (SATCOM) is more critical than ever, as new market opportunities fuel the commercialization of space. The potential to monetize space is spawning a multitude of engineering breakthroughs and advancements. Much of this activity focuses on the low Earth orbit (LEO) because of decreasing launch costs and the increased functionality of smaller, lighter satellites. These satellites support various use cases, including fifth-generation (5G) and eventually sixth-generation (6G) cellular non-terrestrial networks (NTN). Examples include communications coverage over vast distances and enhanced situational awareness on the ground for military and critical communications.

Rapid development creates unique challenges when designing and building these complex systems, including the orbits involved, link budgets, antennas, modulation standards, and the need to thoroughly test these systems. By understanding the various factors and their impact on design, satellite developers can optimize performance throughout the satellite development life cycle, regardless of orbit and application.

To ensure optimized performance throughout the satellite life cycle, manufacturers and network operators must account for unique variables such as Doppler shift, ionosphere distortion, and atmospheric losses. They also need to ensure that satellite systems can navigate the crowded electromagnetic spectrum — a landscape that will only grow more congested as commercialization continues.

Testing for situational awareness will be a significant part of effectively managing space traffic as new systems go into orbit. Satellites play a vital role in transmitting data to government, military, and commercial entities. They help provide connectivity for remote locations without terrestrial infrastructure while supporting real-time data sharing. Satellite use cases continue to expand beyond imaging and broadcast to include disaster response, navigation, weather research, and high-speed communications.

As NTNs expand across multiple orbital heights, and LEO in particular, SATCOM capabilities will increase. Expect LEO satellites to transform communication, targeting, navigation, situational awareness, and early-warning systems in the aerospace and defense sector. As satellite technology evolves, it will be pivotal in advancing technologies like autonomous military aircraft, image intelligence, and future battle networks from the lower Earth through geosynchronous equatorial orbits.

Satellite Orbits

Thousands of satellites in orbit constantly transmit and relay data from one location to another above the Earth. Yet they occupy distinctly different orbits. A satellite’s design must account for the unique characteristics of its orbit, which would likely be one of the following:

• geosynchronous equatorial orbit or geostationary earth orbit (GEO)
• medium earth orbit (MEO)
• low Earth orbit (LEO)

Satellite Standards

Numerous standards guide the satellite industry. The industry also has a few well-defined and mature modulation standards. The most broadly used are Digital Video Broadcasting — Satellite (DVB-S), Digital Video Broadcasting — Satellite Second Generation (DVB-S2), and Digital Video Broadcasting — Satellite Second Generation Extensions (DVB-S2X).

The Digital Intermediate Frequency Interoperability (DIFI) Consortium provides an open, interoperable digital intermediate frequency / radio frequency (IF / RF) standard that serves as a modern alternative to traditional analog IF signals. DIFI interoperability and standards for space ground systems aim to prevent vendor lock-in.

And of course, the 3rd Generation Partnership Project (3GPP) focuses on cellular standards, including NTN communication. Releases 17 through 19 include new features to bolster connectivity, including sidelink enhancements, reduced-capability New Radio (NR) devices, NR operation to 71 GHz, and improvements on multiple-input / multiple-output for NR. The scope for Release 20 is underway, with initial studies scheduled for completion by June 2025.

One of the primary reasons to include NTN in 3GPP standards is the ability to access satellite networks with existing unmodified fifth-generation (5G) and Long Term Evolution (LTE) devices. The 3GPP considers LTE NTN synonymous with IoT NTN. Both narrowband IoT (NB-IoT) NTN and enhanced machine-type communication (eMTC) are subsets of IoT NTN.

Atmospheric Effects

The main challenge to address with satellite communication is the effects of the atmosphere on the communications channel. All satellite signals pass through the atmosphere, including the ionosphere. The ionosphere can disrupt satellite communications in several ways, such as through added noise, signal depolarization, refraction and multipath, propagation delays, and intersystem interference. Most of these disruptions are attributable to the changing reflectivity of the ionosphere and the amount of water and ice between the satellite and Earth station.

Intersystem interference results from three sources: diffraction, ducting, and scattering. Diffraction happens when a wave encounters an obstacle and bends or diffracts around the point of contact. The shape of the obstacle determines the resulting diffraction pattern of the wave. Ducting is a type of propagation of electromagnetic radiation, usually in LEO, where atmospheric refraction bends the signal wave. Finally, scattering occurs when particles or gas molecules in the atmosphere interact with and redirect the signal wave from its original path.

Satellite Links

Satellite communications rely on links among satellites, ground stations, and user equipment. When working with user or mobile equipment, two more links are defined: forward link and reverse link. The forward link sends data from the Earth base station to the satellite and then to the user equipment. To do so, it uses the uplink from the Earth station to the satellite and the downlink from the satellite to the user equipment. The reverse link is in the opposite direction. The uplink goes from the user equipment to the satellite, and the downlink goes from the satellite to the Earth base station. The white paper dives into many link budget details to explain contributing factors and how much margin needs to be counted.

Among other topics covered are software-defined satellites and SATCOM bands and antennas. Typical satellite communications happen from the L-band to the Ka-band. Satellite operators are leveraging lowering orbits, developing better technology, and increasingly opting for higher-frequency communications to meet the growing demand for data throughput.

Satellite Antennas

The network channel is continuously dynamic, with fading, Doppler, and angles of arrival. For example, dynamic Doppler changes with the angle of elevation and exhibits more complicated fading characteristics. Maintaining continuous links requires beam steering and adaptive techniques. Commercial services require more ground terminals, interference troubleshooting, and spectrum monitoring. Here, electronically scanned arrays (ESAs) become a vital technology. ESAs can be steered to point in different directions, offering the ultimate flexibility. They help maintain continuous links with ground stations, other satellites, and mobile transport like cruise ships, high-speed trains, and aircraft.

Support Your Satellite Workflow

With the rapidly rising complexity of SATCOM systems and networks, you must optimize your workflow. Keysight provides solutions for every stage of development, from design and conceptualization through prototyping, manufacturing, operations, and optimization.