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





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