The road to the signals Galileo has today as a baseline has been tedious and long, but has followed a logic from the start. From the very beginning, one of the main challenges that Galileo set for itself was to offer three wide band signals, satisfying the requirements of mass market and pushing the potential performance to its natural limits. The historical Agreement of 2004 between the US and the EC impacted the initially planned signals but has intensified the cooperation between Galileo and GPS. The final touch to the Galileo signal plan was achieved in 2006 when the Working Group on GPS and Galileo Compatibility and Interoperability finally agreed upon the great interest in a new modulation for the E1/L1 frequency, namely the Multiplex Binary Offset Carrier. Galileo has thus accomplished the original objective of providing three wide band signals for the civilian GNSS community.
Analysis of the open service modulation waveforms proposed for the Galileo signal on L1 shows that the combined binary coded symbol (BCS) and BOC(1,1) waveforms, as proposed at GNSS ION 2005, may induce tracking bias in a receiver designed to receive only the BOC(1,1) waveform. Navigation using a mixed constellation of modernised GPS satellite broadcasting a BOC(1,1) component and Galileo with one of the possible additive mixtures of BCS and BOC(1,1) -would lead to potential location errors if no corrective measures are taken. The bias depends upon the correlator spacing in the code tracking detector. There are several ways to calibrate the bias. One of these uses a stored table of corrections for use with Galileo signals whose entries depend upon the receiver parameters. Another method uses additional satellite signals and establishes a filter state in a Kalman navigation processor. In this case, the presence of multi-path propagation makes such calibration difficult for users with a finite observation time. An alternative solution uses phase alternation of the BCS component at the chip rate to eliminate the measurement bias for correlator spacings smaller than 0.4 chips. The trend is indeed to use smaller and smaller correlator spacings in the near future. The paper illustrates the tracking bias with examples from the current Galileo signal proposals.The paper establishes an effective method of removing or avoiding the bias corresponding to changes in the satellite signal. We show that the tracking bias is sensitive to the choice of the BCS waveform and to its use in a receiver adapted for BOC(1,1) reception. The main contribution of the paper is to show that there are BCS sequences which lead to bias-free pseudo-range estimates through appropriate design rules. These belong to a set known as anti-symmetric sequences. The resulting set of allowed BCS sequences is very limited in number. There are none of sequence length 10, as proposed originally for Galileo. The spectral properties of the best sequence is shown. There are consequential changes to the performance of the composite spreading symbol sequences in the presence of multipath propagation and these effects are also shown and may be compared to previous Galileo signal proposals.
The objective of this paper is to find an optimum correlator configuration that is capable of achieving the best possible multipath mitigation performance for an MBOC modulated signal. A new code multipath mitigation technique will be presented. The followed approach is the optimization of the shape of the code discriminator function, on account of the definition of an optimum S‐curve. A fundamental aspect of the S‐curve optimization will be to search for the right compromise between bandwidth of the pre‐correlation filter and the required resolution of the correlators. The method to calculate the weights of the correlators to obtain an optimum S‐curve will be equally shown. The implementation of the optimum discriminator is illustrated and the results of several simulations are presented. Additionally, the obtained performance will be assessed by means of multipath envelopes. To conclude, a comparison of multipath performance figures using a statistical channel model is given.
Where am I? What time is it? These are intimately related questions that have accompanied humanity for centuries. From past times when sailors were excitingly discovering recondite regions on Earth, to current outer space exploration. From the fascinating development of calendars by ancient civilizations all over the world -typically for agricultural purposes -to current developments of extremely accurate atomic clocks, reliable position and time measures have played an important role in human progress, and will continue to do so.
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