# GPSMAP 66 sr: Assessment of Its GNSS Observations and Centimeter-Accurate Positioning
## 4 Precise Positioning Results
### Dual-Frequency GPS/Galileo Precise Point Positioning  
Precise point positioning (PPP) is a GNSS positioning technique that uses the carrier-phase observations as primary observables and is able to produce cm level coordinates if dual-frequency observations are available and if precise satellite orbits and satellite clock corrections are introduced into the data processing. It requires long observation times of continuous carrier-phase tracking in order to achieve the highest accuracy level. Often, the carrier-phase ambiguities are treated as real numbers (float solution). With some additional effort, especially consisting of applying corrections for fractional cycle biases, ambiguity fixing to their true integer values may be achieved. This would improve positioning accuracy, especially in the east component. Coordinates results are obtained in the geodetic reference system of the satellite orbits, which presently is the ITRF 2014. Thus, the coordinates are time-dependent and must be provided with a time stamp. The requirement of dual-frequency observations reduces the number of satellite signals being available with a GPSMAP 66 sr, as all pure single-frequency observations (of around half of the GPS constellation and of GLONASS) are discarded. At an ideal mid-latitude observation site, eight dual-frequency measurements are available on average (cp. Figure 3). If signal obstructions exist in the vicinity of the receiving equipment, the number of measurements often falls below the required minimum of signals. Our intention was to produce PPP results with one of the free processing services which accept RINEX observation files and return ITRF coordinates and statistical quality measures. We tried to process GPSMAP 66 sr RINEX files with the following sources:  
* CSRS-PPP of National Resources Canada [13];
* Trimble CenterPoint RTX Post-Processing Service [14];
* GNSS Analysis and Positioning Software (GAPS), University of New Brunswick, Canada [15]. However, we failed for various reasons. Often, we received no result at all but only an error message, or the result was based on GPS single-frequency observations only with poor positioning accuracy. The following main reasons were found:  
* The processing service does not accept G5 and/or Galileo observations but only the GPS (+GLONASS) traditional signal frequencies 1 and 2;  
Figure 11: Results of the antenna phase-center calibration of GPSMAP 66 sr for G1/E1 (**left panel**) and G5/E5 (**right panel**). * The GPSMAP 66 sr antenna does not belong to the group of antennas supported by the processing service. Since the Garmin GNSS observations are presently not processable by the free PPP processing services, we used our own processing module WaPPP to obtain results. Antenna phase-center corrections, as determined in Section 3.3, were applied. We utilized precise satellite orbits and clock corrections from the Center for Orbit Determination in Europe (CODE). Their clock corrections, as all clock corrections from analysis centers of the International GNSS Service (IGS), refer to the ionosphere-free linear combinations G1/G2-P(Y) and E1/E5 [16]. Thus, additional corrections are needed for GPS satellite clock corrections to compensate for differences in signal delays between ionosphere-free linear combinations G1/G2 and G1/G5. These corrections consist of two portions:
* Code biases between different signals (frequencies and modulations), which are determined from the observation data of globally distributed GNSS reference station networks and are available either as differential code biases (DCBs) [17] or as pseudo-absolute code biases (observable-specific bias (OSB)) [18]. We applied the appropriate GPS DCBs produced by DLR and obtained from CDDIS [19], similar to their application described in [20]. * In the case of GPS Block IIF satellites, pronounced inter-frequency clock biases exist between G5 and G1/G2, with periods of several hours and amplitudes of up to many centimeters.