## 4 Result of a Real Flight Experiment  
The European Space Agency (ESA) PolarGap airborne gravimetry campaign was carried out from December 7, 2015 to January 19, 2016 to cover the gravity gap over central Antarctica. A Twin-Otter  
Figure 6: The average RMS values of kinematic PPP and POP solutions after converged for stations CAS1, DAV1, OHI3 and MCM4 with different types of observations in the east, north and up components. For each day, the RMS values are calculated for each type of solutions, i.e., the PPP float with GPS observations in the east component, with reference to the IGS nominal position. Then the average RMS values are obtained over 14 days. Figure 7: The trajectory of this flight with the latitude shown on the radius and the longitude on the arc. FD83 is the reference station. aircraft was used with three antennas named AIR2, 0158 and SPAN installed on the aircraft to collect the GNSS data. Here, we choose the data collected on day 19 December 2015 for analysis, which covers about 10.5 h from 10:30 to 21:00 (UTC). Most of the flight tracks were done from the South Pole along the meridian to the edge of the region and then back. A dedicated reference station SP2X was installed at the South Pole, i.e., in the middle of the region. Such a track pattern is beneficial for traditional relative kinematic positioning. However, the observation data of SP2X on day December 19 is not available, and we use the data from another reference station called FD83 which was installed in a tented field camp (Figure 7). As the observed satellites on these kinematic receivers are at low elevation angles (less than 60\({}^{\circ}\)), a cut-off angle of 7\({}^{\circ}\) is applied to fully use all satellites. Since AIR2 observes GR data, 0158 and SPAN observe GPS data, only GPS data is processed for the three antennas in DD, PPP and POP mode for validation. For DD processing only one reference station FD83 is used, while 42 IGS reference stations around Australia are included for PPP and POP processing. The sampling interval of the receivers in the experiment is 1 s.  
Unfortunately, it is very difficult to evaluate the positional results of the three approaches, since no \"true\" or accurate trajectory is available as reference. As the three onboard antennas are fixed on the aircraft, the inter-antenna distance should remain constant during the flight. It should be pointed out that special attention must be paid to the different signal receiving times for the two receivers of a baseline, although they are programmed to receive the signal at the same epoch. The difference could reach more than 1 milli-second due to online clock steering, which is usually shown in the observations as milli-second jumps, for example, the JAVAD DELTA G3T receiver at AIR2. This is a problem mainly for receivers moving with high speed but not for static or kinematic station with moderate speed. The positions are interpolated to the same nominal epoch time to get rid of such influence. The time series of baseline length of the three processing approaches are calculated and shown in Figure 8. Be aware of that the DD estimates of the first one hour are not shown, because the reference station FD83 was moved during that time period. In general, there are two time periods of baseline results with larger variations, i.e., 15:00-16:30 and 20:00-21:00 for almost all solutions besides the initialization at the beginning. The variations are more obvious in the two baselines with AIR2 than the baseline 0158-SPAN. This indicates that the data quality of AIR2 might be the reason. The major reason is that in airborne kinematic positioning over Antarctica, the receivers occasionally loosing track, and the gain of satellites observed simultaneously by the ground station and the rover is not beneficial for the estimation of the carrier phase ambiguities. It can be seen in the satellite visibility in Figure 9 that the continuous tracking time of one satellite is usually shorter than that in other regions.