Considering 4 dB of margin SNR of 20 dB or more will be required at the
receiver input to achieve BER of 10-4 or better.
Further to establish that developed receiver will be able to receive the
data from long distance, tests are carried out to estimate the maximum
radial distance up to which RTL-SDR based receiver system can be used to
receive and demodulate the signal from the radiosonde flight. For this
launches of radiosonde at NARL are used. To collect vertical profile of
meteorological parameters such as Pressure, Temperature, Relative
humidity, Wind Speed and Wind Direction GPS radiosondes are launched at
NARL every day at 1730 LT. During the tests Meisei RS-11G radiosonde
were launched. This radiosonde transmits modulated signal at 1200 baud
rate to ground receiver in the frequency band of 400-406 MHz. The
transmit signal power this radiosonde is 20dBm.
The data format of this radiosonde is unknown and cannot be used for the
computation of the BER. Since the minimum SNR of 20 dB at the input of
RTL-SDR receiver is required to achieve the BER of
10-4 therefore received signal SNR can be used to
approximate the maximum radial distance for which RTL-SDR receiver
system will be able to decode the receive signal. Maximum radial
distance for which received signal SNR remains lower than 20 dB is
defined as receiving range of developed receiver. Block diagram of the
test setup is given in Figure 10. LabVIEW program is used to log the SNR
of the received signal with corresponding time stamp for whole duration
of the radiosonde flight. Time interval between two readings was set to
one minute.
Figure
10: Block diagram of measurement setup for logging SNR of received
signal from the fabricated antennas.
A separate ground receiver Meisei RD-06G-006 is used simultaneously to
receive and log the data from the RS-11G. Along with other parameters it
also provides geolocation of the radiosonde flight. This geolocation is
used to obtain the radial distance of the radiosonde from the ground
receiver with respect to the time. SNR of the received signal from the
RTL-SDR receiver system are then time synchronized and correlated with
radial distance of radiosonde. Four numbers of profiles of SNR with
respect to radial distance as given in Figure 11, for both the antennas,
are generated by repeating the test on different days. These
measurements confirms that the RTL-SDR can be used to receive the
radiosonde signals up to a distance of about 65-80 kms.