GPS World, November 2017

worst oscillator will take many months to drift off by up to an hour with no GNSS even under horrible vibration conditions so this is an unlikely cause Is it drift or a jump in error Nearby electrical noise could cause GNSS denial jamming but not erroneous data That requires spoofing If you have no reason to believe that it is intentional that makes spoofing unlikely but still possible Is a GNSS repeater or a record playback GNSS tester operating in the area These are spoofers even if they are unintentional If this is a precision agriculture application then an RTK reference station transmitting erroneous data could be the cause What timestamping format is used local time or UTC An unlikely but possible scenario is the unit is changing time zones so local time jumps an hour Is there a processor software app between your output and the actual GNSS receiver This could introduce errors What is the position output indicated when the time drift occurs The best way to diagnose this is to record the time and position output as log files using a laptop PC connected to the serial data Q Do your simulators work as well for testing handheld consumer grade GPS Please discuss the differences in testing techniques or approaches for highprecision vs mass market receivers A From Racelogic We have a range of simulators suitable for all levels of GNSS testing If you dont need the high fidelity and wide bandwidth of the LabSat Wideband then the entry level LabSat 3 will also work with any GNSS device including handheld consumer grade products To fully explore the performance of high precision receivers including multipath effects and P code reception a wider bandwidth and a greater Figure 4 Simulator graphic user interface image racelogic number of bits would be required to capture and replay all of the available signals For these applications we recommend a bandwidth of 56 MHz and at least 4 bits of resolution For testing of consumer grade handheld devices with simpler RF front ends we recommend a much reduced bandwidth of around 9 MHz and only 2 bits of resolution This smaller bandwidth and fidelity will easily reproduce the majority of real world conditions and the resulting data files will be much easier to handle Q How many GNSS signals can a softwaredefined radio produce A From Skydel The theoretical limits of a softwaredefined radio SDR are based on four distinct characteristics of the SDR the digital to analog converters DACs bit resolution the maximum sampling rate the bandwidth and the number of RF outputs With most SDRs available bandwidth is defined by the sampling rate With a 16 bit DAC there is enough dynamic range to generate up to 50 GNSS signals and hundreds of multipath echos with more than 60 dB of range to 40 GPS World www gpsworld com november 2017 accommodate different signal power levels per RF output For example with a sampling rate of 50 MSps a 40 MHz wide signal combining GNSS constellation signals such as GPS L1 C A Galileo E1 GLONASS G1 can be generated Nowadays SDRs can have two or more RF outputs and are able to operate with sample rates of 100 MSps or higher By distributing the GNSS signals across different RF outputs the entire GNSS spectrum can be covered at a relatively low cost in terms of hardware A handful of SDRs can easily be synchronized to form multiple RF output systems In such cases the complete range of GNSS signals for all visible satellites can be generated at the same time Q In a dual frequency receiver would it be possible to still use L1 spoofed jammed with L2 clean to get an accurate position Is it possible to do a combination between the two signals in order to save the spoofed jammed L1 A From IFEN In principal it is still possible to use L1 spoofed jammed with L2 clean in a dual frequency receiver to get an accurate position Such receivers are available as off the shelf products