GPS World, June 2009

SYSTEM DESIGN TEST Galileo p FIGURE 19 Tracked carrier frequency corrected for the Doppler showing local oscillator drift group delay of about 10 meters is located as expected The measurement based on 20 seconds is inaccurate and only gives a rough estimate as the group delays introduced by the receiver are also included in the error We plan further investigation and measurements analyzing these distortions within the receiver p FIGURE 20 Pseudorange difference between the signals in E5 and L1 indicating the ionospheric delay Phase Coherence For applications involving carrier phase measurements the phase relation between the two front ends is crucial Both front ends are phase locked to the same reference oscillator The phase relation of the two voltage controlled oscillators was analyzed to assess their phase coherence This was complicated by the front ends having two different IF frequencies due to the heterodyne and direct conversion architectures A high power GPS L1 signal from a simulator was fed into both front ends at the same time These were then multiplied to obtain the IF frequency difference only The signal was then mixed with the expected IF frequency difference producing a zero frequency signal proportional to the phase difference of the two front end oscillators The measurements indicated that the phase difference is noisy but remains constant in both front ends with a mean value of 703 degrees The standard deviation is 26 degrees that translates to a jitter of 14 millimeters This is only a first estimation on 100 second data collections Longer tests including thermal stress are needed to fully characterize the phase relation of the oscillators Conclusion This work indicates that direct conversion can be used to implement an affordable dual frequency GNSS receiver The highly integrated nature of such a D C receiver enables wideband GNSS reception at competitive cost The performance of the D C front end is at least comparable to a heritage heterodyne solution A future problem however can arise from the phase noise in the D C receiver which is currently the largest observed difference compared to the heterodyne receiver Phase noise may be an issue for the most demanding carrier phase based techniques and would need attention in such receivers Direct conversion is particularly appropriate to receive High performance antennas for precise operation maximum durability and ease of installation for applications in GPS World June 2009 www gpsworld com 28