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International audience; In-flight assessment of the radiometric performances of space-borne instruments can be achieved by means of vicarious calibration over Pseudo-Invariant Calibration Sites (PICS). PICS are chosen for the high temporal stability of their surface optical properties combined with a high spatial homogeneity. A first list of the main desert PIC sites was identified 20 years ago for the calibration of medium/coarse spatial resolution instruments in the solar spectral range (400–2500 nm). They are located in the Saharan desert and in the Arabian Peninsula. Six of them have since been endorsed by the CEOS/WGCV/IVOS as reference Calibration/Validation test sites. In this study, we have revisited the list of desert PIC sites at the global scale with the aim of (1) assessing if these twenty PICS are still “optimal”, in terms of temporal stability and spatial uniformity, and using up-to-date multi-spectral remote sensing data, and (2) identifying new calibration sites distributed over other areas of the world. We verified that the original sites remain very relevant, although alternate locations in their close vicinity have slightly better characteristics. We proposed four additional targets with similar characteristics, some of which may offer easier logistical access. In order to support radiative transfer simulations of satellite sensor measurements over the sites, we assessed the abilities of several semi-empirical models to reproduce the spectro-directional signatures of six IVOS sites and the four new candidate sites, and we derived climatologies of the main atmospheric properties (trace gas column load and aerosol optical depth).

The consequences of GPS L1 C/A signal distortions due to payload failure on a civil aviation GPS user has received a lot of attention in the late 1990’s and early 2000 following the occurrence of a first such event in 1993. The result of the corresponding work led to the characterization and modeling of such a threat, known as Threat Model (TM), and the development of means to mitigate this threat in SBAS and GBAS, known as Signal Quality Monitoring (SQM). The proposed TM, endorsed by International Civil Aviation Organization (ICAO), is adapted to the GPS L1 C/A signal and derived from the observations of the 1993 event as well as other considerations. However, with the evolution of the payloads over the years, the multiplication of GNSSs, and the multiplication of modulations used by GNSS signals, the adaptation of this TM to other signals and systems might not be straight-forward: the a priori identification of the effect of a payload failure on the transmitted signal is a very complicated task. The objective of this paper is thus to investigate another TM, referred to as generic TM, that could cover a large threat space and that could be adapted to other signals. This generic TM could be for instance used by future Dual-Frequency Multi-Constellation SBAS to protect the use of new GNSS signals. The article thus investigates such generic TM in the specific case of Galileo E1C and its impact on an associated SQM.

International audience; Galileo E1C, the pilot component of the E1 Open Service signal (CBOC(6,1,1/11) modulation), Galileo E5a and GPS L5 (BPSK(10) modulation) are signals that will be used by civil aviation receivers for pseudorange computation. To meet stringent requirements defined for civil aviation GNSS receivers, the characterization of distortions which could affect a GNSS signal in a hazardous way is required. In particular, expected signal distortions generated at payload level are described by Threat Models (TMs). Distortions incorporated in the TM are also called Evil WaveForm (EWF).These TMs, and their associated parameter ranges, referred to as Threat Space (TS), are powerful and necessary tools to design and test the performance of Signal Quality Monitor (SQM). The SQM is a mean to detect the presence of dangerous signal distortions and is necessary to protect users with high requirements in terms of integrity, accuracy, availability, and continuity (for example civil aviation users). Nowadays, this monitoring task is performed by GBAS and SBAS reference stations for GPS L1 C/A to warn the user in a timely manner. In this paper, SQMs for Galileo E1C and Galileo E5a will be designed and compared using a new representation introduced in [1]. Using this representation, different SQMs are compared and an optimized SQM is proposed to monitor signal distortions on Galileo E5a and Galileo E1C signals.

International audience; This paper analyses two ways to investigate nominal distortions of GNSS signals. Firstly, these perturbations can be observed from the correlation function point of view and secondly, from the CDO (Chip domain observable) which is a way to extract the shape of such distortions directly from the digitized signal samples using the code periodicity. This method is also known as the Vision Correlator (NovAtel). [1]These two techniques are compared in terms of capacity to observe and characterize GNSS signal nominal deformations. The CDO has the advantage to process only one specific part of the signal whereas the correlation function observable is less affected by the noise and is directly linked with the pseudorange estimation. As a conclusion, this publication proposes a new way to perform the Signal Quality Monitoring based on the Chip Domain Observable.

International audience; On 12 August 2003, the four Cluster spacecraft crossed the mid-altitude cusp one after the other a minute or two apart. Shortly after the cusp crossing, two of the Cluster observed three structures poleward of the cusp that appeared and grew in successive satellite passes. In these structures, high fluxes of low-energy magnetosheath-like ions and electrons are observed. The analysis of particle and magnetic field data reveals that it is the cusp region that moved back and forth over the spacecraft. We show that the cusp reacts extremely fast to rotations of the interplanetary magnetic field (IMF) and that each of the three northward turnings of the IMF is accompanied by a poleward displacement of the cusp. The latitudinal component of the cusp velocity at ∼5 Re altitude is estimated to be of the order of 30 km/s.