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LabAlphaSat OBJECTIVES
The Laboratory of AlphaSat satellite (LabAlphaSat) is a joint initiative of DGTCSI, FUB, and DIET to exploit the Ka-Q band AlphaSat radiopropagation experiment, called "Aldo Paraboni" in honor of prof. Paraboni.
Terabit capacity and very high data rates are required for the near‐future broadband satellite communication systems, mainly for multimedia services. The increased capacity can be obtained by using the larger bandwidth available at higher frequency bands, like Ka and Q/V. However, severe detrimental atmospheric effects impair radio waves at these bands, which require the extensive use of fade mitigation techniques, such as link power control, site diversity, or onboard adaptive power allocation. The Alphasat Aldo Paraboni propagation experiment was designed and supported by the Italian Space Agency (ASI), and implemented by the European Space Agency (ESA), to better characterize the atmospheric propagation channel at Ka band and Q band, to support the design of future satellite systems. In Italy, 3 ground stations have been installed and are acquiring the Alphasat beacon signals: the 2 ASI main ground stations in Tito Scalo (Southern Italy) and Spino d'Adda (Northern Italy) and the Sapienza‐FUB station in Roma (Central Italy). The 3 stations cover quite distant locations in Italy, with different climatic characteristics. This paper describes the main features of the experimental setup in the above stations and presents some examples of measurements and results.
The LabAlphaSat has the following objectives:
to support and foster higher education in antennas and propagation;
to design and develop new microwave remote sensing instrumentation;
to devise new algorithms to couple atmospheric models and electromagnetic waves;
to operate as a ground-based facility for satellite product validation;
to pursue the use of microwave systems for communication applications;
to pursue the use of optical systems for communication applications.
The LabAlphaSat board includes:
F.S. Marzano (DIET, Sapienza UniRome & CETEMPS)
F. Consalvi (FUB, Rome)
S. Barbieri (DIET, Sapienza UniRome & CETEMPS)
M. Biscarini (DIET, Sapienza UniRome)
Rome AlphaSat Ground Station
The Alphasat ground station in Rome has been designed and assembled by refurbishing, after proper laboratory characterization and testing, several microwave and radiofrequency components obtained from previously dismissed satellite missions (eg, Olympus and Italsat). The installation is at the DGTCSI building roof in Rome (viale America 201, EUR)
The Ka-band station in Rome is characterized by an outdoor unit with a paraboloid having a diameter of 1.5 m (providing an antenna gain of 47 dBi), a low noise amplifier (LNA), and double‐stage frequency conversion.10 The 19.701 GHz received signal is filtered and runs through an LNA with a 30.5 dB gain. The noise figure of the LNA, including the upstream filter, is 3.3 dB. After amplification, the signal is processed by a 2‐stage converter. The local signals for frequency conversions are obtained with high stability local oscillator (LO) followed by a multiplier. The first conversion involves a 94.75 MHz LO and a 200× frequency multiplier. The resulting 18.950 GHz is combined with the 19.701 GHz to obtain the first intermediate frequency (IF) of 751 MHz. In the second conversion stage, we obtain the second IF of 69 MHz by mixing the first IF with a generated 820 MHz signal. The first stage is completely realized with microstrip technology, whereas the second one is a printed circuit board (PCB). Both stages provide an overall gain of 55 dB. The output signal, at a frequency of 69 MHz, is sent to the indoor unit where a satellite beacon receiver (SBR) acquires the signal using 1 kHz bandwidth and generates a voltage output proportional to the power of the 69 MHz signal. The received signal power is then collected and monitored by computer software, specifically engineered. The Ka-band receiving station is not equipped with automatic satellite tracking; however, the antenna elevation can be manually oriented, and the Alphasat orbit oscillation is then removed by postprocessing filters.
The Q band station in Rome, as the Ka-band one, consists of an outdoor and an indoor unit.10 The receiving antenna, front‐end, and 2 intermediate frequency conversion chains are part of the main outdoor section, while the indoor one includes an SBR and an acquisition unit. The Alphasat Q band signal at 39.402 GHz, after LNA amplification, is converted at the first IF 3.406 GHz (IF1), using a 35.996 GHz signal obtained by successive multiplications. A second converter provides the final IF at 70 MHz (IF2). Its local oscillator can be locked on an external frequency reference (ie, cesium‐beam frequency standard). The IF2 70 MHz signal is suitably amplified to the level required by the SBR, equipped with a logarithmic detector providing an output voltage signal for data logger acquisition. The selected LNA has a ~50 dB gain and a 3.5 dB noise figure. To maximize the Q band receiver performance, the SBR bandwidth is set at 100 Hz, instead of 1 kHz, to improve the final signal‐to‐noise ratio of about 10 dB. The Q band station mounts a 40‐cm‐diameter paraboloid with an efficiency of 0.6 and an overall gain of about 42.7 dBi. The Q band receiving station has been recently equipped in 2017 with an open‐loop tracking system, operating both in elevation and in azimuth. It automatically ingests from an online server the satellite nominal ephemerids and calculates the antenna pointing based on the receiver position. The tracking system is able to rotate 200° in azimuth and 180° in elevation with continuity; its maximum speed is 20° per minute with a step‐motor position accuracy of ±0.1° and encoders precision of 0.01°.
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