Page 63 - ITU Journal: Volume 2, No. 1 - Special issue - Propagation modelling for advanced future radio systems - Challenges for a congested radio spectrum

ITU Journal: ICT Discoveries, Vol. 2(1), December 2019

power to extract any induced signal attenuation. bandwidth of 300 Hz and a tracking range of
Due to the satellite’s slightly inclined orbit, its 100 kHz. The output of the PLL receiver is fed to a
apparent position as observed at the ground varies computer-attached Analog to Digital Converter
over time;nevertheless, its position is known at (ADC) for further processing (sampling, digitization
each time using Orbit Ephemeris Messages (OEM) etc).
files.
The available measurement dynamic range is about
2.1.1 Receivers in Greece 20 dB at a 10 Hz data output rate .

NTUA has designed and deployed two identical Ka- 2.2 Ancillary equipment
band beacon receivers in Attica, Greece, one at the Since the beginning of the campaign, collocated
NTUA Campus in Athens and another one at the ancillary equipment has been deployed at all
Lavrion Technological and Cultural Park. Both campaign locations allowing for in-situ
receivers are based on the Software Defined Radio meteorological measurements. All measurements
(SDR) principle, making use of high-grade off-the- are synchronized to the receivers and archived with
shelf parts allowing for lower procurement and the necessary meta-tags for later processing. The
service costs, as well as quicker provisioning.
use of collocated meteorological instrumentation
The Ka-band receivers front-end consist of 1.2 m should allow for further study of the correlation
offset dishes. After undergoing filtering, between the fading statistics and the observed
amplification and down-conversion at the Low meteorological events.
Noise Block (LNB) units, the signals are fed to a 2.3 Data preprocessing
Universal Software Radio Peripheral (USRP) which
further samples them, digitizes them and passes NTUA and RAL Space monitor the data collection
them to a single-board computer for further and visually inspect the obtained time series on a
processing. The signal power estimation is done daily basis to ensure data consistency and integrity.
using a real-time Fast Fourier Transform (FFT) Additionaly, data preprocessing is performed using
algorithm, providing 10 Hz of data measurement the well established RAL method based on Fourier
sampling rate and is built around the popular open- Series [10]. Data preprocessing is critical in any
source GNU Radio framework. It is worth noting propagation experiment campaign and involves the
that apart from the beacon signal measurements, following tasks:
also the noise power in the same bandwidth is • removal of the effect the ground equipment
measured and recorded [9]. has on the received signal;
All oscillators used in the receiver chains are locked • identification of valid data;
to external GPS Disciplined Oscillators (GPSDOs) to
guarantee that extremely high frequency stability is • data merge from beacon receivers and
achieved. Also, both antennas are equipped with an ancillary instruments;
accurate tracking system developed in-house. The • raw and processed measurements archiving.
measured dynamic is in excess of 40 dB. All measurements have timestamps synchronized

2.1.2 Receivers in the UK to GPS time and are stored using a common format
(netCDF) for further processing and evaluation.
RAL Space has deployed receivers targeting the Ka- Apart from the actual propagation measurements,
band ALPHASAT’s beacons at Chilbolton and ancillary data is recorded to support the experiment
Chilton. The measurement setup comprises of with essential metadata (e.g. meteorological
equipment already used successfully in past events).
propagation measurement campaigns; all
equipment is located indoors, in a specially adapted 3. ANALYSIS OF EXPERIMENTTAL
Portakabin with woven PTFE windows. RESULTS AND DISCUSSION
The antennas used are 50 cm in diameter lens horn In the following the results obtained from the first
type. Each antenna has its own (commercially two years of the campaign are analyzed and
procured) tracking system. The beacon reception is presented. The processed beacon data availability
based on conventional techniques, i.e. Phase Locked for the Greek campaign has been 96.11% for Athens
Loop (PLL) envelope detection using a PLL tracking and 95.99% for Lavrion; for the UK campaign the
receiver. The receiver is configured with a noise processed data availability has been 98.33% for

© International Telecommunication Union, 2019 47