GNSS transpolar earth reflectometry exploriNg system (G-TERN): mission concept

The global navigation satellite system (GNSS) Transpolar Earth Reflectometry exploriNg system (G-TERN) was proposed in response to ESA's Earth Explorer 9 revised call by a team of 33 multi-disciplinary scientists. The primary objective of the mission is to quantify at high spatio-temporal resolution crucial characteristics, processes and interactions between sea ice, and other Earth system components in order to advance the understanding and prediction of climate change and its impacts on the environment and society. The objective is articulated through three key questions. 1) In a rapidly changing Arctic regime and under the resilient Antarctic sea ice trend, how will highly dynamic forcings and couplings between the various components of the ocean, atmosphere, and cryosphere modify or influence the processes governing the characteristics of the sea ice cover (ice production, growth, deformation, and melt)? 2) What are the impacts of extreme events and feedback mechanisms on sea ice evolution? 3) What are the effects of the cryosphere behaviors, either rapidly changing or resiliently stable, on the global oceanic and atmospheric circulation and mid-latitude extreme events? To contribute answering these questions, G-TERN will measure key parameters of the sea ice, the oceans, and the atmosphere with frequent and dense coverage over polar areas, becoming a “dynamic mapper”of the ice conditions, the ice production, and the loss in multiple time and space scales, and surrounding environment. Over polar areas, the G-TERN will measure sea ice surface elevation (<;10 cm precision), roughness, and polarimetry aspects at 30-km resolution and 3-days full coverage. G-TERN will implement the interferometric GNSS reflectometry concept, from a single satellite in near-polar orbit with capability for 12 simultaneous observations. Unlike currently orbiting GNSS reflectometry missions, the G-TERN uses the full GNSS available bandwidth to improve its ranging measurements. The lifetime would be 2025-2030 or optimally 2025-2035, covering key stages of the transition toward a nearly ice-free Arctic Ocean in summer. This paper describes the mission objectives, it reviews its measurement techniques, summarizes the suggested implementation, and finally, it estimates the expected performance.Peer ReviewedPostprint (published version

Real time targeting for GPS guided weapons using the on-board systems of the F-14D Super Tomcat

Precision Strike has been the central doctrine of the Air Warfare Commanders of the U.S. Navy since this capability was demonstrated and proven during the month long air campaign of Desert Storm. Effectiveness analysis immediately following that conflict showed that natural and man made obscurations of targets, such as clouds and smoke, made precision targeting from the air impossible with laser guided munitions during an alarming percentage of attack missions. In order to attack a target with a laser guided precision weapon, the striking aircraft had to maintain an unobstructed line of sight until weapon impact in order to provide continuous laser energy on the target. To solve this dilemma, a requirement was set forth that demanded an all weather through the clouds precision attack capability. This requirement is being fulfilled by a bevy of new generation weapon systems that are collectively known as GPS guided weapons. These weapons are programmed with target location coordinates and navigate autonomously to the impact point after the aircraft release by using on board inertial navigation computers aided by Global Positioning Satellite technology. Subsequent military operations have employed these new generation GPS guided weapons with great success against fixed targets A significant deficiency has arisen however with GPS guided weapons in attacking relocatable targets. These targets include mobile missile systems, command and control vehicles, and troop convoys, and usually make up more than seventy percent of the overall target list. While the Navy is currently upgrading the F-14D Super Tomcats with the capability to employ GPS guided weapons, the aircraft does not have the capability of using its own sensors as a source of target coordinates. Therefore, if the intended target moves between the time it is located and the time that it is attacked, the GPS guided weapons will miss their mark. This study summarizes the F-14D weapon system and its capabilities and deficiencies in order to form a basis for improved GPS guided weapon targeting. It proposes three possible sources of accurate targeting information that the F-14D can provide to the GPS guided weapons, and outlines a test and evaluation procedure to verify the integrity and airworthiness of proposed avionics and software modifications, as well as a method to employ a systems approach to determine the capability of the F-14D precision strike system to accurately self-target for GPS guided weapons

SETHI / RAMSES-NG: New performances of the flexible multi-spectral airborne remote sensing research platform

International audienceSETHI is an airborne SAR/GMTI system developed by the French Aerospace Lab. ONERA, and integrating various sensors. In 2016 ONERA invested in upgrade and improvement of all SETHI components. The microwave ones cover from VHF-UHF to X Band, full polarimetric and very high resolution, along track and cross track interferometry and very high precision multi-baseline capacity for interferometry and tomography applications. The optronic sensors offer very high spatial resolution visible images and fine spectral scene analysis in VNIR and SWIR bands. This paper presents the upgrade and new performances of this flexible platform and the qualification campaign results with various sensor configurations

Wind-Wave induced velocity in ATI SAR Ocean Surface Currents: First experimental evidence from an airborne campaign

Conventional and along-track interferometric (ATI) Synthetic Aperture Radar (SAR) sense the motion of the ocean surface by measuring the Doppler shift of reflected signals. Measurements are affected by a Wind-wave induced Artefact Surface Velocity (WASV) which was modelled theoretically in past studies and has been estimated empirically only once before with Envisat ASAR by Mouche et al., (2012). An airborne campaign in the tidally dominated Irish Sea served to evaluate this effect and the current retrieval capabilities of a dual-beam SAR interferometer known as Wavemill. A comprehensive collection of Wavemill airborne data acquired in a star pattern over a well-instrumented validation site made it possible for the first time to estimate the magnitude of the WASV, and its dependence on azimuth and incidence angle from data alone. In light wind (5.5 m/s) and moderate current (0.7 m/s) conditions, the wind-wave induced contribution to the measured ocean surface motion reaches up to 1.6 m/s upwind, with a well-defined 2nd order harmonic dependence on direction to the wind. The magnitude of the WASV is found to be larger at lower incidence angles. The airborne WASV results show excellent consistency with the empirical WASV estimated from Envisat ASAR. These results confirm that SAR and ATI surface velocity estimates are strongly affected by WASV and that the WASV can be well characterized with knowledge of the wind knowledge and of the geometry. These airborne results provide the first independent validation of Mouche et al., 2012, and confirm that the empirical model they propose provides the means to correct airborne and spaceborne SAR and ATI SAR data for WASV to obtain accurate ocean surface current measurements. After removing the WASV, the airborne Wavemill retrieved currents show very good agreement against ADCP measurements with a root mean square error (RMSE) typically around 0.1 m/s in velocity and 10° in direction