S band

• 1. Introduction

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  The S band once linked NASA to the Space Shuttle. It still keeps the SLS launch vehicle and the International Space Station in contact with the ground. This designation comes from the Institute of Electrical and Electronics Engineers. It marks out microwave frequencies from 2-4 gigahertz. The band straddles the boundary between UHF and SHF, which the IEEE fixes at 3.0 GHz. Airport surveillance radar watches for aircraft inside it. Weather radar reads incoming storms inside it. Surface ships and communications satellites use it too. That same stretch of spectrum also carries Wi-Fi signals into homes and offices around the world. How does a single band hold room for both a spacecraft's telemetry and an ordinary household gadget at the same time? And what happens when regulators start auctioning off pieces of it to competing companies? Those questions carry through everything that follows.

• 2. Wi-Fi Everywhere

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  IEEE 802.11b and 802.11g standards use the 2.4 GHz slice of the S band. Together they represent the single largest use of the entire band. Networks built on this frequency have become the most widely used computer networks in the world. Homes and small offices rely on them to link desktop computers, laptops, tablets and smartphones. Smart TVs, printers and smart speakers join that list, all connecting through one wireless router. Coffee shops, hotels, libraries and airports run their own wireless access points on this frequency. That lets travelers reach the internet on tablets or phones without hunting for a cable. That same 2.4 GHz stretch belongs to a wider band reserved for unlicensed devices. Wi-Fi shares it with several other kinds of everyday equipment.

• 3. Everyday Wireless

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  The 2.4-2.483 GHz ISM band sits inside the S band. It is set aside for low power devices that need no license. Cordless phones, wireless headphones, garage door openers and keyless vehicle locks all draw on it. Baby monitors and medical diathermy machines use it as well. Microwave ovens fall under the same designation, listed under the IEEE 802.16a standard. They typically heat food at 2.495 GHz, though some models run at 2.45 GHz instead. Bluetooth, the standard behind those wireless headphones, operates between 2.402-2.480 GHz. Some digital cordless telephones and consumer video senders share that same unlicensed stretch. The 802.16e standard, built under the WiMAX banner, also claims a part of the S band. Most vendors building that equipment work near 3.5 GHz, though the exact range varies by country. Amateur radio and amateur satellite operators hold two allocations of their own. These sit at 13 cm, or 2.4 GHz, and 9 cm, or 3.4 GHz. Amateur television repeaters broadcast in those same bands. None of those amateur or consumer signals need a license. But the megahertz just above and below them are worth real money to telecom companies.

• 4. Auctioning The Airwaves

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  Mobile network operators run services between 2.3 and 2.6 GHz. That spans the 2300-2400 MHz band and the 2500-2690 MHz band specifically. In the United States, the 3.55-3.7 GHz band has been auctioned off for Citizens Broadband Radio Service, or CBRS. Spectrum between 3.45-3.55 GHz, and again between 3.7-3.98 GHz, has gone to 5G under FCC auctions. The FCC itself calls that particular stretch the C Band rather than the S band. Rules adopted by the Federal Communications Commission in April 2015 turned the 3.55-3.7 GHz range into shared spectrum. Those rules came out of the National Broadband Plan. The United States Navy is the single biggest user of that CBRS spectrum. Cable companies have their own designs on it. Charter Communications began testing wireless broadband service for rural areas in January 2018. Regulators were not only carving up spectrum for mobile carriers. Entire satellite companies staked claims to other corners of the S band too.

• 5. Satellite Broadcasting

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  In 1995, the FCC approved Digital Audio Radio Service broadcasting in the S band between 2.31-2.36 GHz. Sirius XM Radio has used that allocation ever since. The FCC has also opened up 2.0-2.2 GHz for Mobile Satellite Service networks paired with Ancillary Terrestrial Components. ICO Satellite Management, now known as Pendrell Corporation, tried to build such a network. So did TerreStar, a company that no longer exists.

  China Multimedia Mobile Broadcasting runs on the 2.6 GHz range, delivering satellite radio and mobile television. Like other proprietary systems built in the United States, it does not work with open standards used elsewhere.

  In May 2009, the European Commission awarded Inmarsat and Solaris Mobile a 2x15 MHz portion of the S band each. Solaris Mobile was a joint venture between Eutelsat and SES, operating as EchoStar Mobile. Both companies had two years to launch pan-European Mobile Satellite Service, with rights to run it for 18 years. Their allocated frequencies ran from 1.98-2.01 GHz for signals traveling from Earth to space. The return trip, from space to Earth, used 2.17-2.2 GHz. The Eutelsat W2A satellite, launched in April 2009, sits at 10 degrees East to carry that service.

  Indonesia takes a different path. Indovision delivers Direct-to-Home satellite television over the S band instead of the Ku band most countries use. Its allocated frequency runs from 2.52-2.67 GHz, with a local oscillator frequency of 1.570 GHz. IndoStar-1 was the world's first commercial communications satellite built to broadcast on S-band frequencies. Those frequencies push through the atmosphere efficiently, delivering high-quality transmissions to small 80 cm antennas even in heavy rainfall. Indonesia is exactly that kind of heavy-rainfall region. Matching that performance on Ku or C band would demand more power or a far larger dish. Every one of these satellites stays fairly close to Earth. Farther out, NASA also leans on the S band to talk with spacecraft headed into deep space.

• 6. Deep Space Signals

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  Many NASA spacecraft, both near Earth and far out in interplanetary space, communicate in the S-band. Most of them reach home through the Deep Space Network. The James Webb Space Telescope, launched in 2021, is one example. It uses 2 GHz S-band signals to send back 40 kilobits per second of real time telemetry. That signal travels from near the Sun-Earth L2 point, the telescope's home in space. Back on Earth, that same 2 GHz signal sits alongside a very different set of technical standards. Those standards were built for machines that have never left the ground.

• 7. Radar And Technical Standards

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  Airport surveillance radar systems typically run between 2700-2900 MHz within the S band. A related classification, the 10 cm radar short band, spans roughly 1.55-5.2 GHz, overlapping much of that range. The National NEXRAD Radar network also operates on S-band frequencies to track weather across the country. Before NEXRAD, weather surveillance commonly relied on C-band frequencies instead.

  Particle accelerators can also run on S-band radio frequency sources standardized at 2.998 GHz in Europe. That frequency corresponds to a 100 mm wavelength, while equipment in the United States instead runs at 2.856 GHz.

  Satellite communication systems can use the S band as a transmit intermediate frequency, replacing the L band with help from a multiplier near 10. This substitution matters most where a single shared coaxial cable, rather than two separate ones, links the modem to the outdoor antenna unit. Splitting transmit and receive signals this way keeps them from interfering with each other on that one shared cable. One modem, for instance, transmits at 2.815 GHz within that S-band intermediate signal. The outdoor unit then up-converts it to 28.15 GHz, squarely inside the Ka band, before the signal ever reaches the satellite.