Volume 12, Issue 4

The Pseudo Random Noise (PRN) codes is an important part of the GNSS system based on Codes Division Multiple Access (CDMA), and it is the key to channel separation and satellite distinction. The GNSS system excellent performance characteristics of multiple access and spread spectrum are realized by the PRN codes. According to the officially released Interface Control Documents (ICDs), the Beidou global navigation satellite system (BDS-3) B1C and GPS L1C have chosen the Weil codes as PRN codes whose generation is based on the number theoretic method. The Weil codes has the advantages of flexible codes length, good performance and large candidate set, therefore, this paper mainly studies 63 pairs of data/pilot channel PRN codes for BDS-3 B1C and GPS L1C. On the basis of introducing BDS-3 B1C and GPS L1C PRN codes, mainly for the Weil codes generation principle, codes evaluation criteria and intra-system and inter-system odd/even auto-correlation and cross-correlation characteristics are simulated in detail. The experimental results show that the upper limit value, lower limit value, the mean and standard deviation of the odd auto-correlation and cross-correlation of the PRN codes of the BDS-3 B1C signal were better than those of the GPS L1C PRN codes. The performance of odd and even cross-correlation are almost similar between BDS-3 B1C and GPS L1C signal inter-systems, which are slightly worse than the intra-system, and the upper limit value of the inter-system is lower than 2.94 dB in the GPS L1C intra-system.

The Hartebeesthoek Radio Astronomy Observatory is currently building a Lunar Laser Ranging station. This geodetic technique requires a good timing system to measure a round trip of laser photons from the telescope to the Moon and back to the telescope. We test the newly acquired timing system using examples of the Global Positioning System applications. Data in Receiver Independent Exchange Format was processed using GAMIT/GLOBK software. The results were compared against those derived from the Global Positioning System receivers that were integrated with a frequency standard from a hydrogen maser and a standard internal quartz. The results indicate that (i) the rubidium clock operates optimally and the clock drifted to within error margins of sub-centimetre level during the period of 2.5 seconds, (ii) the selected site for the permanent installation of the timing antenna has minimal multipath effect and (iii) we observed no improvement in Global Positioning System products derived from receivers that were integrated with different frequency standards.

The accuracy of georeferenced TLS point clouds is directly influenced by site-dependent GNSS effects, deteriorating the accuracy of the ground control point coordinate estimation. Especially under challenging GNSS conditions, this is a crucial problem. One common approach is to minimize these effects by longer observation durations, which in turn increases the effort in time and cost. In this paper, an algorithm is proposed that provides accurate georeferencing results, even under challenging measurement conditions and short observation durations. It iteratively improves the georeferencing accuracy by determining and applying obstruction adaptive elevation masks to the GNSS observations. The algorithm is tested and assessed using the data of a field test. It is demonstrated that after only 5 minutes observation duration, the ground control point coordinates can be estimated with an accuracy of 1 to 2 cm, independent from the GNSS measurement conditions. Initial states of the elevation masks are determined from a point cloud that is georeferenced using coordinates from a single point positioning solution, enhanced by a RAIM-FDE approach. Afterwards, the coordinates are estimated in a weighted least-squares baseline solution and both, the elevation masks and the coordinate estimation, are iteratively improved. Besides the significant reduction of measurement time, the proposed algorithm allows for increasing the amount of ground control points and can be applied to other direct or indirect GNSS-based georeferencing approaches.

Laser scanner has become widely used nowadays for several applications in civil engineering. An advantage of laser scanner as compared to other geodetic instruments is its capability of collecting hundreds or even thousands of point per second. Terrestrial laser scanner allows acquiring easy and fast complex geometric data from building, machines, objects, etc. Several experimental and field tests are required to investigate the quality and accuracy of scanner points cloud and the 3D geometric models derived from laser scanner. So this paper investigates the precision of creation three dimensional structural model resulted from terrestrial laser scanner observations. The paper also presented the ability to create 3D model by structural faces depending on the plane equation for each face resulted from coordinates of several observed points cover this face using reflector less total station observations. Precision comparison for the quality of 3D models created from laser scanner observations and structure faces is also presented.The results of the practical measurements, calculations and analysis of results are presented.

The spatial sampling often used to process and represent bathymetric data are of fixed grid resolution where the least depth value is stored in each grid cell. This results in Digital Elevation Models (DEMs) that are used to depict the underlying features of the seafloor. With the discretion of the user, the resulting DEMs used may either be of coarse resolution or a very fine resolution surface which provides as many details as possible. However, depending on the resolution of the data collected and the variability of the seafloor, the arbitrary user defined grid resolution is not the best option. Hence we address the problem of finding an optimal grid resolution for representing and processing the bathymetric data for the application of bathymetric risk assessment whilst maintaining computational efficiency. Here we adopt the quadtree decomposition approach. In addition, the research suggests the optimal criteria and standard deviation threshold, σth{\sigma _{th}} values for this particular application. These suggestions are still flexible and can be optimized for this application depending on the end user requirements. Previous studies have focused only on the splitting criteria or the constrained criteria to ensure that there is homogeneous accuracy over the entire dataset. However, an investigation into the threshold selection for the standard deviation, σth{\sigma _{th}} which describes the variability in the dataset is one of the most important splitting criterion, that is still lacking. Also, a new approach to store the depths in the grid in a time ordered approach for each epoch is shown. By optimizing the criteria for the quadtree decomposition and time series algorithm, the approaches shown in this paper provide the adaptive, accurate DEM which makes optimal use of the available bathymetric data for the Netherlands Continental Shelf (NCS) as the study area. This data preparation step forms the basis for developing a probabilistic approach to assigning hydrographic resurvey frequencies in the NCS.

Leveling is a traditional geodetic surveying technique that has been used to realize a vertical datum. However, this technique is time consuming and prone to accumulate errors, where it relies on starting from one station with a known orthometric height. Establishing orthometric heights using Global Navigation Satellite Systems (GNSS) and a geoid model has been suggested [14], but this approach may involve less precisions than the direct measurements from leveling. In this study, an experimental study is presented to adjust the highly accurate leveling observations along with orthometric heights derived from GNSS observations and a geoid model. For the geoid model, the National Geodetic Survey’s gravimetric geoid model (TxGEOID16B) and hybrid geoid model (GEOID12B) were applied. Uncertainties in the leveled height differences, GNSS derived heights, and the geoid models were modeled, and a combined adjustment was implemented to construct the optimal combination of orthometric, ellipsoidal, and geoid height at each mark. As a result, the discrepancy from the published orthometric heights and the CSM (Corrector Surface Model) based adjusted orthometric heights with GEOID12B showed a mean and RMS of -8.5 mm and 16.6 mm, respectively, while TxGEOID16B had a mean and RMS of 28.9 mm and 34.6 mm, respectively. It should be emphasized that this approach was not influenced by the geodetic distribution of the stations where the correlation coefficients between the distance from the center of the surveying network and the discrepancy from the published heights using TxGEOID16B and GEOID12B are 0.03 and 0.36, respectively.