GPS World, January 2016

Real Time Single Frequency Precise Point Positioning for Cars and Trains 66 GPS WORLD WWW GPSWORLD COM JANUARY 2016 WITH RICHARD B LANGLEY GUIDANCE FOR ROAD AND TRACK By Peter de Bakker and Christian Tiberius T he single frequency precise point positioning SFPPP method developed at Delft University of Technology was previously demonstrated to provide lane level position accuracy on a freeway in postprocessing mode Important applications of SF PPP are lanelevel traffic state estimation and lane level specific driver advice for next generation car navigation For a functional system as well as for advanced experiments in this field the computed positions have to be available in real time Therefore a new real time implementation of the SF PPP method was developed as part of the Dutch Dynamic Lane Guidance project In this article we outline aspects of the real time implementation and we present experimental results from this new implementation collected on a busy freeway in the Netherlands and in a parking lot as well as results from a railway experiment In these experiments a test vehicle was equipped with a low end automotive type single frequency receiver with a patch antenna to collect raw GPS observations A 3G mobile communications link was used to obtain data correction streams over the Internet using the Ntrip protocol The SF PPP processing was performed on a laptop computer onboard the vehicle in real time Various forms of ground truth positions were used to assess the real time SF PPP positioning accuracy For some of our tests the vehicle was also equipped with highend GPS antennas and receivers to provide ground truth The position solutions obtained with the SF PPP algorithm have been compared to post processed network RTK solutions using the Netherlands Positioning Service NETPOS Additional validation was performed by means of a 5 centimeteraccuracy road infrastructure map from Rijkswaterstaat the Dutch Ministry of Infrastructure and the Environment and by a centimeter level a priori ground survey The new real time SF PPP software was tested successfully with performance comparable to our previous post processing software and meeting the required accuracy for freeway lane identification Statistics on the performance are provided as well as their dependence on a number of external parameters including the number of available satellites Precise corrections from both the German Aerospace Center Deutsches Zentrum für Luft und Raumfahrt or DLR and the International GNSS Service IGS were used Delays in the correction streams vary between providers and can increase 4 3 2 1 0 1 2 3 4 3 2 1 0 1 2 3 4 700 600 500 400 300 200 3 2 1 0 1 2 600 500 400 300 200 further in the event of a time out of the mobile link The influence of these delays is considered and an optimal approach for dealing with outages is discussed PPP MODEL AND CORRECTIONS The GNSS positioning model is non linear The observations are non linear functions of the unknown parameters plus noise To solve for the unknown parameters including the receiver position coordinates through least squares estimation the model must be linearized around an approximate solution In our SF PPP model the primary observations are from each satellite the pseudorange measurement and the carrierphase measurement The unknown parameters are the receiver position vector and the receiver clock offset both of which are involved in the linearization and also the ambiguity associated with the carrier phase measurement for which the model is already linear In the context of PPP it is important to note that in addition to the linearization around the initial approximate values the computed observations contain a number of a priori model values for parameters which are not estimated including The precise satellite position and clock offset including the relativistic effect The GPS satellite positions and clock offsets are computed from the broadcast products navigation message and corrected with real time data streams via Ntrip The correction streams of DLR and IGS were used at different times as detailed in TABLE 1 In post processing older files the satellite orbits and clocks are taken from sp3 files but to keep the processing as close as possible to the real 4 East meters North meters 100 3 2 1 0 1 2 3 3 Cross track meters Along track meters 100 FIGURE 1 2D histogram of SF PPP position errors with respect to the network RTK GPS solution in horizontal directions for the 2012 test on the A13 freeway expressed in local east and north directions left and in cross track and along track directions right The color indicates the number of samples in each bin