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.