Figure 1. The Earth seen by Es’hail-2

ADI SDR transceivers enable amateur space communication

By Diego Koch, applications engineer, Analog Devices R

adio amateurs recently received another way to provide uninterrupted worldwide radio coverage. By means of a new geostationary satellite, it is now

possible to reliably cover one-third of the Earth in just one hop. In order to contact the satellite, it is necessary to use dedicated equipment because the access frequencies are different from the ones used to bounce radio signals from the ionosphere. The new software-defined radio (SDR) approach to radio transceivers offers multiple advantages, such as flexible reconfiguration and the capability to observe the whole band of interest at a glance, just to name a few.

The satellite

Launched in 2018 from Cape Canaveral, the Es’hail-2 communication satellite of the Qatar satellite company Es’hailSat provides television, voice, internet, corporate, and government communication services across Europe, the Middle East, Africa, and beyond. It has been operational since February 2019 and has been positioned above central Africa in a geostationary orbit. From a height of 36,000 km, it covers an area spanning from Brazil to Malaysia, from the Faroe Islands to Antarctica, as shown in Figure 1.

32 October 2020

Es’hailSat was established in 2010. Based in Doha, Qatar, the company owns and operates satellites to serve broadcasters, businesses, and governments. In order to promote and foster space technology development in Qatar, Es’hailSat initiated the development of new technology for the Qatar Amateur Radio Society (QARS), a national nonprofit organization for amateur radio enthusiasts, together in partnership with Amateur Satellite Corporation (AMSAT), another global nonprofit organization. AMSAT designs, builds, arranges, launches, and operates satellites carrying amateur radio payloads. AMSAT affiliated national organizations exist in various countries, including AMSAT Germany (AMSAT-DL), which became involved on behalf of QARS in December 2012. This collaboration has made it possible to equip the Es’hail-2 satellite with two dedicated transponders, providing the first amateur radio geostationary communication capability that connects users across the visible globe in a single hop and in real time.

Many amateur satellites receive an OSCAR (orbiting satellite carrying amateur radio) designation. These satellites can be used free of charge by licensed amateur radio operators for voice and data communications. So far, they have been launched into low Earth orbits

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(LEOs) and into highly elliptical orbits (HEOs), and what all of them have in common is that it is necessary to track them with antennas when they appear above the horizon for just a few minutes. Once they disappear below the horizon, the communication is no longer possible. Satellites on a geostationary orbit have the advantage that, as observed from Earth, their positions do not shift in the sky. Although the antennas on Earth do not have to move to access them, the big distance of 36,000 km sets new challenges in terms of free space power loss, antenna pointing accuracy, and latency—about 250 ms for a trip from one ground-based transmitter to the satellite and back to another ground-based transmitter. The nickname given to Es’hail-2 is OSCAR100 because it is the one hundredth satellite to carry an amateur radio payload.

Access to Es’hail-2

Radio amateurs have worked with satellites for many years. Traditionally, this has been done using analog downconverters and upconverters that shift the received and transmitted signals to and from the amateur bands where transceivers operate. The uplink (from Earth to satellite) and downlink (from satellite to Earth) frequencies used by satellites are sometimes beyond the capabilities of available transceivers. Es’hail-2 has two transponders: one for narrow-band

(NB) transmissions and one for wideband (WB) transmissions. In this section, we will talk about the narrow-band transponder. Since, on this transponder, the available bandwidth is only 250 kHz, to accommodate multiple channels it is necessary to use appropriate modulation techniques. The types of analog modulation that are most commonly used are telegraphy (Morse code, also called continuous-wave (CW)) or telephony (voice, also called single sideband (SSB)). Uplink is at 2.4 GHz (13 cm band) in right hand circular polarization (RHCP) and downlink is at 10.45 GHz (3 cm band) in either horizontal (H) or vertical (V) polarization. Radio amateurs have the privilege to transmit in the 13 cm band (2300 MHz to 2310 MHz and 2390 MHz to 2450 MHz) as licensed radio operators for satellite communication with sufficient power and high gain antennas. This band overlaps with the civilian radio allocation 2400 MHz to 2500 MHz, which is part of the industrial, scientific, and medical (ISM) bands. One of the most popular unlicensed emissions on the ISM band is wireless LAN. The transponders are detailed in Figure 2.

The SDR approach

The change that came with the introduction of SDR in its many variants also impacted the amateur radio world. Even though most transceivers still have the same controls of the

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