GPS Precise Point PositioningAn Investigation in Reachable Accuracy
Sammanfattning: Accurate positioning is very important in many various applications. Today one of the mostused methods for this is DGPS, i.e. relative positioning. DGPS can be extended to aWADGPS (Wide Area DGPS) which consists of a network of reference stations which covera whole region, country or continent. This implies that you are dependent on other factors outof control of the user, and that a connection to the reference stations is needed.Precise point positioning, PPP, is an absolute positioning method where no reference stationsare involved. Ordinary single point positioning is based on broadcast ephemeris, and theaccuracy is on the 15m level. Per definition PPP is based on precise ephemeris, with muchhigher accuracy in the orbital parameters and in the satellite clock information. The resultshould therefore be more accurate. Precise ephemeris are available in different levels ofaccuracies (final, rapid and ultra-rapid) and can be downloaded from the internet for free.Precise clock files are not available as ultra-rapid and therefore it is not possible to getaccurate PPP-solutions in real-time.In Sweden there are several networks for relative positioning, e.g. through SWEPOS orEPOS. This might not be the case in other areas. As accurate PPP-solutions are not availablein real-time, PPP could be used to establish a reference station for DGPS/RTK for real-timemeasurements in those areasThe objective of this research is to evaluate precise point positioning regarding accuracy.Observation files from both IGS and SWEPOS-stations have been used in order to find out ifyou can expect the same accuracy wherever you are on earth. A couple of points have alsobeen measured only for this investigation in order to find out if the result will be the sameunder ordinary conditions. Coordinates of each point are determined with different duration ofobservation times and different level of accuracy of the ephemeris (final, rapid and ultrarapid).Bernese 5.0 and Auto-Gipsy have been used to compute the PPP-solutions and thenthe result is compared with the true position. The self-measured points are also determinedwith a WADGPS (Omnistar) in order to easily be able to compare PPP with a traditionalmethod.As expected the result becomes better for longer observation times and with higher accuracyof the ephemeris. The difference in accuracy between using rapid and final ephemeris is sosmall that it can be neglected in most applications. In almost all cases the accuracy is betterthan 10cm after only one hour of observations with rapid ephemeris. The investigation doesnot indicate significantly differences in accuracy depending on latitude and differencesbetween the self-measured points and IGS- and SWEPOS-stations can also be neglected.Using rapid or final ephemeris, the PPP-derived coordinates are much more accurate than theones obtained with Omnistar.
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