Inside GNSS Media & Research

SEP-OCT 2018

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Page 60 of 67 S E P T E M B E R / O C T O B E R 2 0 1 8 Inside GNSS 61 ment. Besides UAV and SDR, our system includes a positioning and ranging unit to obtain the transmitter-receiver ranges in sub-centimeter and millisecond timestamp accuracy. Further- more, two receiving antennas (Rx) with attached front-ends (FEs) are needed, see Figure 1 . With the time synchronized FEs and the knowledge of the true transmission position, it is possible to eliminate the transmission clock error and analyze the code and phase ranging performance of every GNSS signal of interest. Aer a detailed presentation of the concept, and an expla- nation of all relevant components, the performance analysis of the ground system is discussed. ereaer, we discuss the performance analysis of our UAVlite CBOC signal for different power levels. We conclude with a summary and an outlook. Concept e airborne pseudolite (UAVlite), see Figure 6 in a later sec- tion, is composed of a UAV with an SDR and a mini PC as payload. e ground system includes two receiving antennas with a distance of around 40 meters apart from each other. e antennas are connected to clock-synchronized FEs with soware receivers. In this way the two incoming signals are both tracked and processed with the same receiver clock and receiver clock error (dri) (see Figure 2 ). With the two code measurements, it is possible to eliminate the clock error from the SDR (dt sv ) on the UAV and the receiver FE clock error (dt r ). Equations (1) and (2) give the measured code pseudoranges (PR) for antenna 1 and antenna 2 to the UAV antenna. Sub- tracting the observed code pseudoranges P 1 – P 2 leads to the delta-pseudorange code (ΔPRC), expressed in Equation (3), which is independent of the clock errors dt r and dt sv . If the geo- metric range difference ΔGR = ρ 1 – ρ 2 is known, it is possible to investigate the error difference ε 1 – ε 2 . e absolute pseudorange is, in our case, of no importance because we only investigate the pseudorange difference. An identical pseudorange offset in both pseudoranges has no influence on the evaluation and is canceled in the difference. Additionally, it is possible to correct the ΔPRC of the constant clock offset dt Δh induced by hardware delays (dt h1 , dt h2 ) via, e.g., cables or FEs. is correction is done by determining the offset of the functions ΔGR(t) and ΔPRC(t). is is possible because the time dependent clock errors dt sv (t) and dt r (t) are already eliminated. e concept of the ΔPRC also applies for the phase pseudorange measurements. The only difference is that in the phase PR (Equations (4) and (5)) an additional term Nλ occurs, where λ is the RF wave- length and N is an integer number, repre- senting the total number of accumulated waves between Tx and Rx antenna. is ambiguity condition N has to be fixed at the beginning and is thereaer constant during the measurement (as the PLL was always in lock). erefore ΔNλ is like the constant clock offset dt Δh time indepen- dent in the delta pseudorange phase ΔPRP, see Equation (6), and can be determined by determining the offset of the functions ΔGR(t) and ΔPRP(t). FIGURE 1 Testbed with UAVlite, Rx antennas 1 and 2 and the Multista- tion (MS) UAVlite Rx 1 Rx 2 MS FIGURE 2 Concept sketch of the measurement setup

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