GPS World, March 2018

02 01 0 01 0 6 12 18 24 Time hours 02 01 0 01 02 Position error meters precise but ambiguous phase measurements This filtering is illustrated in FIGURE 1 where measurements are continually added in time in the range domain and errors are modeled and filtered in the position domain resulting in reduced position error in time The result is the characteristic PPP 42 GPS WORLD WWW GPSWORLD COM MARCH 2018 initial convergence period seen in FIGURE 2 where the position solution is initialized as a sub meter dual frequency code point positioning solution quickly converging to the decimeter level in something like 5 to 20 minutes and a few centimeters after 20 minutes when geodetic grade equipment is used at station ALGO Algonquin Park Canada on Jan 2 2017 For static geodetic data daily solutions are typically at the few millimeter level of accuracy in each Cartesian component The primary benefit of conventional PPP is that with the use of state space corrections from a sparse global network there is the appearance of precise positioning from only a single geodetic receiver Therefore baseline or network RTK limitations are removed in geographically challenging areas such as offshore far from population centers in the air in low Earth orbit and so on and without the need for the requisite terrestrial hardware and software infrastructure PPP is now the de facto standard for precise positioning in remote areas or regions of low economic density which limit or prevent the use of relative GNSS RTK or network RTK but allow for continuous satellite tracking These benefits translate into the main commercial applications of offshore positioning precision agriculture geodetic surveys and airborne mapping which also are not operationally bothered PPP Its one of the many acronyms or initialisms if you prefer associated with the uses of global navigation satellite systems It stands for precise point positioning But what is that Isnt all GNSS positioning precise Well its a matter of degree Take GPS for example The most common kind of GPS signal use that implemented in vehicle satnav units mobile phones and hiking golfing and fitness receivers is to employ the L1 C A code pseudorange code measurements along with the broadcast satellite orbit and clock information to produce a point position Officially this is termed use of the GPS Standard Positioning Service SPS It is capable of meter level positioning accuracy under the best conditions There is a second official service based on L1 and L2 P code measurements and broadcast data called the Precise Positioning Service PPS In principle because the P code provides somewhat higher precision code measurements and the use of dualfrequency data removes virtually all of the ionospheric effect PPS is capable of slightly more precise and accurate positioning But because the P code is encrypted PPS is only available to socalled authorized users While meter level positioning accuracy is sufficient for many if not most applications there are many uses of GNSS such as machine control surveying and various scientific tasks where accuracies better than 10 centimeters or even 1 centimeter are needed Positioning accuracies at this level cant be provided by pseudoranges alone and the use of carrier phase measurements is required Phase measurements are much more precise than code measurements although they are ambiguous and this ambiguity must be estimated and possibly resolved to the correct integer value Traditionally phase measurements typically dual frequency made by a potentially moving user receiver have been combined with those from a reference receiver at a well known position to produce very precise and accurate positions If done in real time through use of a radio link of some kind this technique is referred to as real time kinematic or RTK A disadvantage of RTK positioning is that it requires reference station infrastructure including a radio link such as mobile phone communications for real time results Is there another way Yes and thats PPP PPP uses the more precise phase measurements along with code measurements initially on at least two carrier frequencies typically from the users receiver along with precise satellite orbit and clock data derived by a supplier from a global network Precision in this case means a horizontal position accuracy of 10 centimeters or better In this months column we travel along the road of PPP development examine its current status and look at where it might go in the near future INNOVATION INSIGHTS BY RICHARD B LANGLEY 0 10 20 30 40 50 60 Time minutes 02 Position error meters ΔN ΔE ΔU FIGURE 2 Conventional geodetic GPS PPP positioning performance characteristics of initial convergence period and steady state for station ALGO Algonquin Park Canada on Jan 2 2017