Reduced-Dynamic Precise Orbit Determination of Haiyang-2B Altimetry Satellite Using a Refined Empirical Acceleration Model

The Haiyang 2B (HY-2B) satellite requires precise orbit determination (POD) products for geodetic remote sensing techniques. An improved set of reduced-dynamic (RD) orbit solutions was generated from the onboard Global Positioning System (GPS) measurements over a 14-month period using refined strategies and processing techniques. The key POD strategies include a refined empirical acceleration model, in-flight calibration of the GPS antenna, and the resolution of single-receiver carrier-phase ambiguities. In this study, the potential periodicity of empirical acceleration in the HY-2B POD was identified by spectral analysis. In the along-track direction, a noticeable signal with four cycles per revolution (CPR) was significant. A mixed spectrum was observed for the cross-track direction. To better understand the real in-flight environment, a refined empirical acceleration model was used to cope with the time variability of empirical accelerations in HY-2B POD. Three POD strategies were used for the reprocessing for superior orbit quality. Validation using over one year of satellite laser ranging (SLR) measurements demonstrated a 5.2% improvement in the orbit solution of the refined model. Reliable correction for the GPS antenna phase center was obtained from an over-420-day dataset, and a trend in radial offset change was observed. After application of the in-flight calibration of the GPS antenna, a 26% reduction in the RMS SLR residuals was achieved for the RD orbit solution, and the carrier phase residuals were clearly reduced. The integer ambiguity resolution of HY-2B led to strong geometric constraints for the estimated parameters, and a 15% improvement in the SLR residuals could be inferred compared with the float solution.

Circularly Polarized Dual-Band LoRa/GPS Antenna for a UAV-Assisted Hazardous Gas and Aerosol Sensor

UAV assisted wireless sensor networks play a key role in the detection of toxic gases and aerosols. UAVs can be used to remotely deploy sensor nodes and then collect gas concentration readings and GPS positioning from them to delimit an affected area. For such purpose, a dual-band communication system is required, supporting GPS reception, and sensor reading data transfer, which is chosen to be at 2.4 GHz using LoRa physical layer. In this work we propose a switched-beam antenna subsystem for the sensor nodes capable not only of satisfying the dual band requirements but also of maximizing communication range or energy consumption through a good antenna polarization match and improved antenna gain. This antenna subsystem is built using dual-port, dual-band, circularly polarized antenna elements, whose design and experimental validation is carefully detailed. A low profile microstrip stacked structure has been used to obtain return loss in both bands better than 15 dB, axial ratios below 1.5 dB, and wide −3 dB beamwidths around 90° and 75° for GPS and 2.4 GHz bands, respectively, thus limiting the gain reduction of the switched-beam system in critical sensor orientations. Special attention has been paid to reduce the coupling between both ports through the optimization of the relative placement of both patches and their feeding points. The measured coupling is below −30 dB.

Copernicus POD Service - Orbit reprocessing for Copernicus Sentinel-1 satellites

<p>The Copernicus Sentinel-1 SAR (Synthetic Aperture Radar) mission consists of two satellites A and B launched in April 2014 and April 2016, respectively. The Copernicus POD (Precise Orbit Determination) Service is responsible for the generation of precise orbital products of the mission requiring a high orbit accuracy of 5 cm in 3D RMS in the comparison to external processing facilities.</p><p>The operational POD setup at the Copernicus POD Service has passed through several updates during the last years. For instance the ITRF update from ITRF08 to ITRF14 at the end of January 2017, the fundamental background model update in May 2020, and the switch to updated GPS antenna reference point coordinates together with the introduction of carrier phase ambiguity fixing at the end of July 2020 have been done to mention just the major changes in the processing. To provide a homogeneous and up-to-date orbit time series for the two satellites a reprocessing of the full mission period is done.  </p><p>The quality control of the reprocessed Copernicus Sentinel-1 orbits is done by analysing processing metrics and by comparing the results to orbits, which were independently reprocessed by members of the Copernicus POD Quality Working Group (QWG).</p><p>Results from the full Copernicus Sentinel-1 POD reprocessing campaign are presented together with the accuracy and quality assessment of the orbits.</p>

UAV-based aeromagnetic gradient measurements and inversion

<p>Rotary wing UAV’s are used in aeromagnetic measurements for UXO detection. That way, contaminated areas can be mapped fast and with high resolution. Until today, only the total magnetic intensity (TMI) is evaluated, even when a three axis fluxgate magnetometer is flown. In this project, we use two three component fluxgate sensors, an inertial measurement unit (IMU) and a GPS antenna. The IMU allows for a projection of the magnetic data into the geographic coordinate system as well as the calculation of the sensor positions relative to the GPS antenna. With this system, it is possible for the first time to evaluate the component gradients between the magnetometers.</p><p>The sensors are attached to the UAV via a versatile, T-shaped boom hanging below the UAV with the sensors positioned in a horizontal distance of 50 cm. The total mass of the flight system is about 5 kg with an air time of 15 minutes.</p><p>For the inversion, we use a dipole model which calculates the magnetic data for all sensor positions. Because the sources of the magnetic anomalies are unknown as a general rule, there is no distinction between induced and remanent magnetisation. Instead, the three components of the magnetic moment are fitted alongside the positions of the anomaly sources. The number of dipoles to be fitted and their initial parameters are arbitrary. For the inversion, the TMI and component gradients between the sensors are considered.</p><p>In order to analyse the accuracy of the complete system, we conducted surveys over a test field of 100 x 20 m, separated into four sections with varying anomaly configurations. As anomaly sources, we used neodymium magnets which we characterised in laboratory measurements. For optimal coverage and to compare flight directions, the test field was surveyed both lengthways and crossways with a sensor height of 1.5 m above ground. Inversion results show that when component gradients are used, overlapping anomalies can be separated and parameterised. The mean errors of the derived anomaly positions are 5 cm, the total magnetic moment can be determined with an accuracy of 0.35 Am<sup>2</sup>, whereby the errors in direction (declination and inclination) are 4 ° and 2 °, respectively.</p>

Brief Communication: Update on the GPS reflection technique for measuring snow accumulation in Greenland

GPS interferometric reflectometry (GPS-IR) is a technique that can be used to measure snow accumulation on ice sheets. The footprint of the method (∼1000 m2) is larger than that of many other in situ methods. A long-term comparison with hand measurements yielded an accuracy assessment of 2 cm. Depending on the placement of the GPS antenna, these data are also sensitive to firn density. The purpose of this short note is to make public GPS-IR measurements of snow accumulation for four sites in Greenland, compare these records with in situ sensors, and make available open-source GPS-IR software to the cryosphere community.

IOT based Speed Monitoring System based on Location of the Vehicle

Nowadays there is an increasing amount of road accidents happening around the world. The amount of road accidents is especially high in metropolitan cities. The main reason for road accidents is almost always due to the negligence on part of the driver. Maximum number of Road Accidents occur due to over-speeding and drunk driving. This system helps detect over-speeding vehicles by directly implementing the particular hardware inside the vehicle itself and depending on the GPS position of the car and regional speed limits alert the authorities based on the violation. This technique helps us to reduce costs which need to be incurred to install speed cameras at roads at regular intervals. The system will detect the speed of the car, and in case of a violation the license plate number along with the registration details and the photo of the driver will be directly sent to the concerned authorities. This system aims to decrease the amount of accidents that happen across the globe and help conserve human lives. Road accident still remain one of the highest contributors to the loss of human life. For experimental results, we used raspberry pi and the GPS antenna for detection of over speed and alert the authorities on violation.