GPS World, March 2018

60 40 20 From the origins of PPP it may be argued that singlefrequency processing and many decimeter level positioning performance is not precise But we will avoid such semantic arguments here but see Insights and focus on the use of high performance measurement processing algorithms to new low cost hardware We are currently witnessing great changes in the GNSS chip market single frequency chips for tens of dollars or less and boards with multi frequency chips for hundreds of dollars And these chips will continue to undergo downward price pressure with increases in capability and be further enabled for raw measurement use in a wider range of applicable technology solutions There are now a number of low cost dual frequency multiconstellation products on the market with additional such products as well as smartphone chips coming soon To process data from such products with a PPP engine modifications are required to optimally account for singlefrequency measurements in the estimation filter optimize the measurement quality control functions for the much noisier code and phase measurements compared to data from geodetic receivers and optimize the stochastic modeling for the much noisier code and phase measurements The single frequency measurement model can be modified to either make use of the Group and Phase Ionospheric Calibration linear combination commonly referred to as GRAPHIC or ingest data from an ionospheric model Due to the use of low cost antennas as well as the low cost chip signal processing hardware code and phase measurements suffer from significant multipath and noise at lower signal strengths therefore outlier detection functions must be modified Also the relative weighting of code and phase measurements must be customized for more realistic lowcost data processing FIGURE 7 compares the carrier to noise density ratio C N 0 values from 15 hours of static GPS L1 signals collected from a geodetic receiver with a geodetic antenna a low cost receiver chip with a patch antenna and a tablet 46 GPS WORLD WWW GPSWORLD COM MARCH 2018 4 3 2 1 chip and internal antenna as a function of elevation angle Received signal C N 0 values can be used as a proxy for signal precision The three datasets were collected at the same time in mid September 2017 in Toronto Canada with the receivers and antennas within a few meters of each other The shading represents the raw estimates output from each receiver while the solid lines are moving average filtered results Keeping in mind the log nature of C N 0 the high measurement quality of the geodetic antenna and receiver are clear The low cost chip and patch antenna signal strength structure is similar but on average 35 dB Hz lower And the tablet received signal strength is lower still on average a further 40 dB Hz lower with greater degradation at higher signal elevation angles and much greater signal strength variation The PPP horizontal position uncertainty for these datasets is shown in FIGURE 8 Note that reference coordinates have been estimated from the datasets themselves so potential biases in especially the low cost and tablet results can make these results optimistic Given that only single frequency GPS code and phase measurements are being processed initial convergence periods are short and horizontal position error reaches steady state in the decimeter range The geodetic and the low cost results are comparable at the 2 decimeter level whereas the tablet results are worse at the approximately 4 decimeter level Initial convergence of the geodetic solution is superior to the others driven by the higher quality of its code measurements The grade of antenna plays a large role in the quality of these measurements for which there are physical limitations in design and fabrication While geodetic antennas can be used this is not always feasible given the mass limitations of certain platforms or the cost limitations for certain applications COMMENTS REGARDING THE NEAR FUTURE The PPP GNSS measurement processing approach was originally designed to greatly reduce computation burden in 20 40 60 80 Elevation angle degrees 0 C N 0 dB Hz Geodetic mean 462 dB Hz Low cost mean 427 dB Hz Tablet mean 387 dB Hz 0 025 050 075 100 125 150 175 200 Time decimal hours 0 Horizontal position error meters Geodetic r m s 100 cm Low cost r m s 102 cm Tablet r m s 168 cm FIGURE 7 Carrier to noise density ratios of 15 hour of static GPS L1 signals from a geodetic receiver with a geodetic antenna a low cost receiver chip with a patch antenna and a tablet chip and internal antenna as a function of elevation angle FIGURE 8 Horizontal positioning error compared to final epoch solutions for geodetic low cost and tablet data processed with PPP software customized for single frequency and less precise measurements