Inside GNSS Media & Research

SEP-OCT 2018

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62 Inside GNSS S E P T E M B E R / O C T O B E R 2 0 1 8 Pseudolite (Transmitter) A soware defined radio reconfigurable device is used as the pseudolite (see Manufacturers and Additional Resources). e most critical part of the SDR is the clock. erefore the clock characteristics and stability are tested and evaluated for the usage as a pseudolite. ese results were presented by D. S. Maier et alia (2017) and show that the OCXO clock of the so- ware defined radio reconfigurable device is sufficient and suit- able for our system. In this measurement campaign, the device is used for: digital-analog conversion, the up-conversion of the IF-samples to the target RF, and the transmission of the RF. e IF-samples are generated with nominal signal parameters in advance, either with an in-house MATLAB toolbox or with the soware transceiver MuSNAT (D. S. Maier et alia (2018)). ese IF-samples are stored on a mini PC on the UAV. On the mini PC a LabView soware runs to configure the USRP (file, power, RF, and sampling rate), reads in the IF-samples, and sends them to the soware defined radio reconfigurable device. e mini PC and the soware are controlled via remote control over WiFi by the PC3 on Ground (compare to Figure 2). e computational power of the mini PC allows us an I/Q sampling rate of 40 MS/s with a bit depth of 8 bits per sample. In an earlier study (D. S. Maier et alia (2017)), the USRP FPGA was also used for the IF sample generation, but this task is now done beforehand and sent to the soware defined radio reconfigurable device by the additional mini PC. e mini PC increases the payload weight and decreases the maximum sam- pling rate, however, it allows us greater flexibility, e.g., power control under operation, and an easier and broader usage of signal generation tools. A frequency offset of +750 kilohertz was applied for trans- mission, so the used carrier frequency was 1.57617 GHz (1.57542 GHz + 750 kHz). With this offset we can guarantee the operation of the system without influencing the GNSS ser- vices in the surrounding area or the GNSS system on the UAV. Furthermore, the maximum signal power of the transmission is adjusted to a level such that an increase of the noise floor on the ground will never occur (satellite-like signal). Receiver System e UAVlite signals as well as the signals in space (SIS) are captured by two geodetic GNSS antennas. The antennas are sepa- rated by a distance of approximately 40 meters. On the receiver side we are using multi-GNSS soware receiver front-ends (FE) (see Manufacturers and Additional Resources). e setup for signal recording is sketched in Figure 3 . The Rx antenna 1 is connected via an RF-splitter to the Single-FE (S-FE) and the first RF input of the Dual-FE (D-FE1), with cable length of approximately 10 meters. e Rx antenna 2 is connected to the second RF input of By correcting the delta pseudorange ΔPR(t) for the constant offset cdt Δh (cdt Δh + ΔNλ) and subtracting the geometrical range difference ΔGR, we yield the remaining residual error ε 1 – ε 2 . is error comes mainly from receiver noise, tracking delay, and noise from the electronics, but also from multipath, jam- ming, and other interference. erefore, the influence of these effects on the signal can be studied. One simple idea of our measurement setup is to influence the line of sight from one antenna by foliage and let the other line of sight be unobscured. erefore ε 1 changes differently than ε 2 and ε 1 – ε 2 is directly related to the influence of foliation on the signal. In this way, we can test GNSS signals on the robustness against foliation. Components ere are five relevant components for using UAVs as pseudo GNSS satellites and performing signal analysis: 1. the transmitter system (pseudolite); 2. the receiver systems (capturing, sampling, and recording); 3. the front-end clock synchronization, 4. the positioning and ranging systems, which are used for precise position measurements of the phase centers of the receiving (Rx) and transmitting (Tx) antennas, and 5. the UAV as payload carrier. e following section gives a detailed description and per- formance details of the used components. WORKING PAPERS FIGURE 3 Signal recording, synchronization, and interface connections

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