Figure 1: RTL-SDR based meteorological balloon ground receiver
In this paper, we first give receiver hardware details in section 2. In section 2.1 design details of quarter-wave monopole and quadrifilar helical antennas are given. Beam pattern of these two antennas complement each other in sky. Both the antennas are fabricated and tested in lab. Description of RTL-SDR hardware architecture is given in section 2.3. In meteorological balloon application, the balloon borne transmitter typically transmit one packet every second. As given in Table 1 transmitted signal is GFSK modulated with data rate between 1200-4800 Kbps. Receiver first tunes to the centre frequency of transmission and then demodulates the signal and recovers the message. Description of RTL-SDR interface with LabVIEW, demodulation method and processes are given in section 3. In section 4, we evaluated the following parameters to prove the case of RTL-SDR based receiver for radiosonde application:
  1. Sensitivity of receiver in terms of SNR and bit error rate.
  2. Expected link coverage of the receiver.
In order to assess the sensitivity of RTL-SDR based receiver, IQ data is fetched from RTL-SDR and processed in real time using LabVIEW. Synthesized modulated signal from a vector signal generator is used to evaluate BER at different SNR levels and modulation parameters
Further to estimate the link coverage of the receiver, routine soundings of radiosonde at NARL are used. SNR of the signal received receiver with respect to the radial distance of the GPS radiosonde are recorded and analysed. Section 5 concludes the paper.