Abstract:Beijing is one of the most water-stressed cities in the world. Due to over-exploitation of groundwater, the Beijing region has been suffering from land subsidence since 1935. In this study, the Small Baseline InSAR technique has been employed to process Envisat ASAR images acquired between 2003 and 2010 and TerraSAR-X stripmap images collected from 2010 to 2011 to investigate land subsidence in the Beijing region. The maximum subsidence is seen in the eastern part of Beijing with a rate greater than 100 mm/year. Comparisons between InSAR and GPS derived subsidence rates show an RMS difference of 2.94 mm/year with a mean of 2.41˘1.84 mm/year. In addition, a high correlation was observed between InSAR subsidence rate maps derived from two different datasets (i.e., Envisat and TerraSAR-X). These demonstrate once again that InSAR is a powerful tool for monitoring land subsidence. InSAR derived subsidence rate maps have allowed for a comprehensive spatio-temporal analysis to identify the main triggering factors of land subsidence. Some interesting relationships in terms of land subsidence were found with groundwater level, active faults, accumulated soft soil thickness and different aquifer types. Furthermore, a relationship with the distances to pumping wells was also recognized in this work.

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The main objective of this research was to examine the feasibility of Multi-GNSS precise point positioning (PPP) in precision agriculture (PA) through a series of experiments with different working modes (i.e. stationary and moving) under different observation conditions (e.g. open sky, with buildings or with canopy). For the stationary test carried out in open space in the UK, the positioning accuracy achieved was 13.9 mm in one dimension by a PPP approach, and the repeatability of positioning results was improved from 19.0 to 6.0 mm by using Multi-GNSS with respect to GPS only. For the moving test carried out in similar location in the UK, almost the same performance was achieved by GPS-only and by Multi-GNSS PPP. However, for a moving experiment carried out in China with obstruction conditions, Multi-GNSS improved the accuracy of baseline length from 126.0 to 35.0 mm and the repeatability from 110.0 mm to 49.0 mm, The results suggested that the addition of the BeiDou, Galileo and GLONASS systems to the standard GPS-only processing improved the positioning repeatability, while a positioning accuracy was achieved at about 20 mm level in the horizontal direction with an improvement against the GPS-only PPP results. In space-constrained and harsh environments (e.g. farms surrounded with dense trees), the availability and reliability of precise positioning decreased dramatically for the GPS-only PPP results, but limited impacts were observed for Multi-GNSS PPP. In addition, compared to real time kinematic (RTK) GNSS, which is currently most commonly used for high precision PA applications, similar accuracy has been achieved by PPP. In contrast to RTK GNSS, PPP can provide high accuracy positioning with higher flexibility and potentially lower capital and running costs. Hence, PPP might be a great opportunity for agriculture to meet the high accuracy requirements of PA in the near future.

Abstract. Although the electron density profiles (EDPs) from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) measurement have been validated by ionosonde data at a number of locations during the solar minimum period, the performance of COSMIC measurements at different latitudes has not been well evaluated, particularly during the solar maximum period. In this paper the COSMIC ionospheric peak parameters (peak electron density of the F region -NmF2; peak height of the F region -hmF2) are validated by the ionosonde data from an observation chain in China during the solar maximum period of 2011-2013. The validations show that the COSMIC measurement generally agrees well with the ionosonde observation. The error in NmF2 from COSMIC and ionosonde measurements varies with latitude. At midlatitude stations, the differences between COSMIC NmF2s and those of ionosondes are very slight. However, COSMIC NmF2 overestimates (underestimates) that of the ionosonde at the north (south) of the equatorial ionization anomaly (EIA) crest. The relative errors of hmF2s are much lower than those of NmF2s at all stations, which indicates the EDP retrieval algorithm of the COSMIC measurement has a better performance in determining the ionospheric peak height. The root mean square errors (RMSEs) of NmF2s (hmF2s) are higher (lower) during the daytime than during the nighttime at all stations. Correlation analysis shows that the correlations for both NmF2s and hmF2s are comparably good (correlation coefficients > 0.9) at midlatitude stations, while correlations of NmF2 (correlation coefficients > 0.9) are higher than those of hmF2 (correlation coefficients > 0.8) at low-latitude stations.