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:
- Sensitivity of receiver in terms of SNR and bit error rate.
- 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.