Coastal zones are highly dynamical systems affected by a variety of natural and anthropogenic forcing factors that include sea level rise, extreme events, local oceanic and atmospheric processes, ground subsidence, etc. However, so far, they remain poorly monitored on a global scale. To better understand changes affecting world coastal zones and to provide crucial information to decision-makers involved in adaptation to and mitigation of environmental risks, coastal observations of various types need to be collected and analyzed. In this white paper, we first discuss the main forcing agents acting on coastal regions (e.g., sea level, winds, waves and currents, river runoff, sediment supply and transport, vertical land motions, land use) and the induced coastal response (e.g., shoreline position, estuaries morphology, land topography at the land-sea interface and coastal bathymetry). We identify a number of space-based observational needs that have to be addressed in the near future to understand coastal zone evolution. Among these, improved monitoring of coastal sea level by satellite altimetry techniques is recognized as high priority. Classical altimeter data in the coastal zone are adversely affected by land contamination with degraded range and geophysical corrections. However, recent progress in coastal altimetry data processing and multisensor data synergy, offers new perspective to measure sea level change very close to the coast. This issue is discussed in much detail in this paper, including the development of a global coastal sea-level and sea state climate record with mission consistent coastal processing and products dedicated to coastal regimes. Finally, we present a new promising technology based on the use of Signals of Opportunity (SoOp), i.e., communication satellite transmissions that are reutilized as illumination sources in a bistatic radar configuration, for measuring coastal sea level. Since SoOp technology requires only receiver technology to be placed in orbit, small satellite platforms could be used, enabling a constellation to achieve high spatio-temporal resolutions of sea level in coastal zones.

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Requirements for a Coastal Hazards Observing System

Autonomous orbit determination is the ability of navigation satellites to estimate the orbit parameters on-board using inter-satellite link (ISL) measurements This study mainly focuses on data processing of the ISL measurements as a new measurement type and its application on the centralized autonomous orbit determination of the new-generation Beidou navigation satellite system satellites for the first time The ISL measurements are dual one-way measurements that follow a time division multiple access (TDMA) structure The ranging error of the ISL measurements is less than 025 ns This paper proposes a derivation approach to the satellite clock offsets and the geometric distances from TDMA dual one-way measurements without a loss of accuracy The derived clock offsets are used for time synchronization, and the derived geometry distances are used for autonomous orbit determination The clock offsets from the ISL measurements are consistent with the L-band two-way satellite, and time–frequency transfer clock measurements and the detrended residuals vary within 05 ns The centralized autonomous orbit determination is conducted in a batch mode on a ground-capable server for the feasibility study Constant hardware delays are present in the geometric distances and become the largest source of error in the autonomous orbit determination Therefore, the hardware delays are estimated simultaneously with the satellite orbits To avoid uncertainties in the constellation orientation, a ground anchor station that “observes” the satellites with on-board ISL payloads is introduced into the orbit determination The root-mean-square values of orbit determination residuals are within 100 cm, and the standard deviation of the estimated ISL hardware delays is within 02 ns The accuracy of the autonomous orbits is evaluated by analysis of overlap comparison and the satellite laser ranging (SLR) residuals and is compared with the accuracy of the L-band orbits The results indicate that the radial overlap differences between the autonomous orbits are less than 150 cm for the inclined geosynchronous orbit (IGSO) satellites and less than 100 cm for the MEO satellites The SLR residuals are approximately 150 cm for the IGSO satellites and approximately 100 cm for the MEO satellites, representing an improvement over the L-band orbits

Initial results of centralized autonomous orbit determination of the new-generation BDS satellites with inter-satellite link measurements

Aiming at regional services, the space segment of COMPASS (Phase I) satellite navigation system is a constellation of Geostationary Earth Orbit (GEO), Inclined Geostationary Earth Orbit (IGSO) and Medium Earth Orbit (MEO) satellites. Precise orbit determination (POD) for the satellites is limited by the geographic distribution of regional tracking stations. Independent time synchronization (TS) system is developed to supplement the regional tracking network, and satellite clock errors and orbit data may be obtained by simultaneously processing both tracking data and TS data. Consequently, inconsistency between tracking system and TS system caused by remaining instrumental errors not calibrated may decrease navigation accuracy. On the other hand, POD for the mixed constellation of GEO/IGSO/MEO with the regional tracking network leads to parameter estimations that are highly correlated. Notorious example of correlation is found between GEO’s orbital elements and its clock errors. We estimate orbital elements and clock errors for a 3GEO+2IGSO constellation in this study using a multi-satellite precise orbit determination (MPOD) strategy, with which clock error elimination algorithm is applied to separate orbital and clock estimates to improve numerical efficiency. Satellite Laser Ranging (SLR) data are used to evaluate User Ranging Error (URE), which is the orbital error projected on a receiver’s line-of-sight direction. Two-way radio-wave time transfer measurements are used to evaluate clock errors. Experimenting with data from the regional tracking network, we conclude that the fitting of code data is better than 1 m in terms of Root-Mean-Square (RMS), and fitting of carrier phase is better than 1 cm. For orbital evaluation, difference between computed receiver-satellite ranging based on estimated orbits and SLR measurements is better than 1 m (RMS). For clock estimates evaluation, 2-hour linear-fitting shows that the satellite clock rates are about 1.E-10 s/s, while receiver clock rates are about 1×10−13–1×10−12 s/s. For the 72-hour POD experiment, the average differences between POD satellite clock rates estimates and clock measurements based on TS system are about 1×10−13 s/s, and for receiver clock rates, the differences are about 1×10−15 s/s.

Orbit determination and time synchronization for a GEO/IGSO satellite navigation constellation with regional tracking network

Time synchronization of new-generation BDS satellites using inter-satellite link measurements

Improvement of orbit determination accuracy for Beidou Navigation Satellite System with Two-way Satellite Time Frequency Transfer

A new precise orbit determination (POD) strategy based on the combination of satellite laser ranging (SLR) and C-band transfer ranging for geostationary satellites (GEO) is presented. Two approaches to calibrate ranging biases of the C-band ranging system are proposed, namely the two tracking system co-location comparison and the combined POD method, with calibration accuracies estimated to be 0.5 ns and 1 ns respectively. Using data from a C-band tracking network in China, POD experiments indicate that meter-level POD accuracy is achievable for GEO. Root-mean-square (RMS) of the post-fit C-band ranging data is about 0.205 m. The radial component errors of POD are evaluated with SLR data from a station in Beijing, with residual RMS of 0.133 m. Orbital overlapping experiments show the total orbit error is a few meters. Computations of SLR residuals also suggest that for 2-hour prediction, the predicted radial error is about 0.373 m.

Bo Tang

Papers

Precise orbit determination for geostationary satellites with multiple tracking techniques

Precise orbit determination and rapid orbit recovery supported by time synchronization