11_Ilco_V._et_al_Satellite based onSDR.pdf

Radio also allows the creation of new functional

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Radio also allows the creation of new functional blocks that would implement new signal processing operations. 2. SDR TELEMTRY STATION The goal that we set was to design a telemetry station based on SDR and GNU Radio. We used USRP B200 from Ettus Research, which is fully integrated USRP device with continuous RF coverage from 70 MHz –6 GHz with full duplex operational and with up to 56 MHz of real time bandwidth. The proposed architecture for telemetry base station using USRP and GNU Radio is shown in Fig. 2. Figure 2. Software Defined Radio Architecture. In the above architecture is distinguished analog subsystem (feed block) and digital subsystem (GNU Radio, USRP and PC). Analog subsystem acts as an interface for radio frequency environment. USRP, based on the FPGA (Field Programmable Gate Array), in terms of hardware, it has a modular construction, consisting of a base plate (motherboard) to which can be attached daughters boards. USRP is connected to a PC via a Gigabit port or USB. On the PC, GNU Radio operates with further signal processing by digital processing blocks. Conceptually, these blocks performs endless processing of data streams from input ports to output ports. The basic attributes that characterize a digital processing block are: type of data and number on input/output ports. The GNU Radio package includes about 100 digital signal-processing blocks. Conventional diagram in the GNU Radio to receive telemetric information is presented in Fig. 3. USRP block source presents a interactive way to interconnect hardware model. This block also allows interface configuration of SDR’s reception, while ensuring configuration of key parameters USRP Doppler Correction Noise Reduction (FTB) Conditioning (FTJ) Decoding AX.25 PREDICT (UDP) TLE Data GPS Positioning Display Text File Demodulation Signal Figure 3. The reception of telemetry information in GNU Radio. such as reference signal source, the number of acquisition channels, center frequency, sampling rate and others. Doppler correction block is used to adjust the frequency variation on satellite movement, being directly influenced by the satellite orbit data. This correction will be explained in the next section. 3. DOPPLER CORRECTION The Doppler effect (or Doppler shift) is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. [4] This effect appears at achieving communication with a satellite as carrier frequency increases when the source approaches the receiver and decreases when the source moves away from the receiver. Since the frequency deviation is variable it is necessary to make an adjustment for received signal frequency. Frequency adjustment can be achieved so as hardware or through GNU Radio. Hardware method can be excluded because it has a drawback expressed by the additional cost and additional equipment. Using GNU Radio toolkit, Doppler correction may be received with the same precision and speed but with lower budget spending. The software and hardware accuracy correction is the same, but for both methods, time procession is big, because it measures the current frequency then it adjusts the required frequency.
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