This study aimed to measure and validate altitudes from existing sources with direct GNSS measurements and airborne lidar data. For this purpose, 12 mountain peaks located in the south part of Polish territory were selected. Measurements were performed using a GNSS receiver using the Real-Time Kinematic (RTK) or static techniques enabling altitude measurements with accuracy of 10 cm. GNSS was treated as the primary data source, as the direct field measurements can determine the highest point on each peak. The obtained results were confronted with historical, internet sources, and official altitude data. Moreover, each altitude was determined using lidar data from an airborne lidar dataset of Poland from the ISOK program and provided by the national agency. Significant discrepancies in data were already detected during the analysis of internet materials and traditional maps, up to a few meters. The differences between measured and internet sources in altitude of mountain peak range from 27 cm to 504 cm. This study has shown the need to re-measure the altitudes of the mountain peaks and determine the highest point correctly.
The subject of this paper is the analysis of the stability of BeiDou system clocks; currently only signals from two blocks, BSD-2 and BDS-3, are available. For elaboration, 30 s clock corrections from the 2014 to 2020 period for 37 satellites were used (9 IGSO, 28 MEO). Four different Allan variances were used to determine stability, and additionally, the type of noise characteristic for each satellite was also determined. Based on the calculations, it was shown that the BDS-2 segment has a significantly lower stability than BDS-3. Moreover, it was possible to notice a difference in the course of the graphs of the same satellites using different variances. BDS-2 satellites were mostly characterised by the presence of WFM noise, while BDS-3 satellites were characterised by WFM noise for the shortest averaging times and RWFM for the other intervals. Accuracy varies between 10−10 s to 10−6 s for a rubidium clocks in general, in case of the hydrogen masers in is between 10−14 s to 10−10 s.
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