Ground-based Reference Stations Research Articles

Comparison of long-term total ozone observations from space- and ground-based methods at Zhongshan Station, Antarctica

Total ozone errors for satellite observations at Zhongshan Station in Antarctica are characterized using their relative difference (RD) from ground-based Brewer observations during 1993–2015. All satellite total ozone observations slightly overestimated ground-based ones (with RD less than 4%). This is in contrast to conclusions drawn from global-scale validation studies, where main ground-based reference stations are located in middle latitudes. Given multiple total ozone data per day at Zhongshan Station, observed by a sun synchronous orbit satellite, measurements at the lowest solar zenith angle (SZA) show greatest consistency with Brewer ones, having an overall RD of −0.02–1.15%. Algorithm-retrieved total ozone data from the Total Ozone Mapping Spectrometer (TOMS), including Solar Backscatter Ultra Violet (SBUV), TOMS-Earth Probe (EP), Ozone Monitoring Instrument (OMI)-TOMS, show best agreement with ground-based values; followed by the Global Ozone Measurement Experiment-type Direct Fitting (GOD-FIT) algorithm for the GOME-2A, and finally the Differential Optical Absorption Spectroscopy (DOAS) —Algorithm retrieved products for satellites-detectors of Global Ozone Measurement Experiment (GOME), Scanning Imaging Absorption spectroMetr for Atmospheric CHartographY (SCIAMACHY), and OMI. Satellite total ozone RD presents some statistical characteristics, but no specific trends. DOAS and GOME-2A algorithms have values that significantly increase, when the SZA is above 60°–70°, whereas values for GOME-2A decrease, when the SZA is 80–85°. Satellite total ozone RD is a minimum, when the Brewer total ozone is 300–350 DU, with an obvious increase in RD values for DOAS- and GOME-2A, when the Brewer total ozone is 150–300 DU. Satellite total ozone RD obviously increases, as the time difference between satellite overpasses and Brewer measurements grows. Specifically, RD rises as the absolute time difference increases to more than 4 h, yielding an OMI-TOMS RD of more than 10% as this difference increases to 8 h. The DOAS- RD may be up to 15%, while GOME-2A RD does not exceed 10%. The satellite total ozone RD may reach −5%, as the distance between the satellite overpass pixel and the station become more than 100 km. Possibly because of the discrepancy in surface albedo, the TOMS-algorithm retrieved total ozone produced underestimation, when the pixel on the south-east side of the station (the Antarctica continent) is used, but overestimation on the north-west side of the station (the Indian Ocean). Consistency between space and ground-based total ozone data is least for the “ozone hole”. Typically, the RD of TOMS-algorithm retrieved total ozone is within 1%/10 yr. Thus, the SBUV and Brewer monthly averaged total ozone anomalies from 1996 to 2015 were 1%/10 yr and 0.9%/10 yr, respectively. Both indicate slight, but consistent, ozone layer recovery.

Overview of the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Forcing on the Mediterranean Climate (ChArMEx/ADRIMED) summer 2013 campaign

The Chemistry-Aerosol Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr) is a collaborative research program federating international activities to investigate Mediterranean regional chemistry-climate interactions. A special observing period (SOP-1a) including intensive airborne measurements was performed in the framework of the Aerosol Direct Radiative Forcing on the Mediterranean Climate (ADRIMED) project during the Mediterranean dry season over the western and central Mediterranean basins, with a focus on aerosol-radiation measurements and their modeling. The SOP-1a took place from 11 June to 5 July 2013. Airborne measurements were made by both the ATR-42 and F-20 French research aircraft operated from Sardinia (Italy) and instrumented for in situ and remote-sensing measurements, respectively, and by sounding and drifting balloons, launched in Minorca. The experimental set-up also involved several ground-based measurement sites on islands including two ground-based reference stations in Corsica and Lampedusa and secondary monitoring sites in Minorca and Sicily. Additional measurements including lidar profiling were also performed on alert during aircraft operations at EARLINET/ACTRIS stations at Granada and Barcelona in Spain, and in southern Italy. Remote sensing aerosol products from satellites (MSG/SEVIRI, MODIS) and from the AERONET/PHOTONS network were also used. Dedicated meso-scale and regional modelling experiments were performed in relation to this observational effort. We provide here an overview of the different surface and aircraft observations deployed during the ChArMEx/ADRIMED period and of associated modeling studies together with an analysis of the synoptic conditions that determined the aerosol emission and transport. Meteorological conditions observed during this campaign (moderate temperatures and southern flows) were not favorable to produce high level of atmospheric pollutants nor intense biomass burning events in the region. However, numerous mineral dust plumes were observed during the campaign with main sources located in Morocco, Algeria and Tunisia, leading to aerosol optical depth (AOD) values ranging between 0.2 to 0.6 (at 440 nm) over the western and central Mediterranean basins. Associated aerosol extinction values measured on-board the ATR-42 within the dust plume show local maxima reaching up to 150 Mm−1. Non negligible aerosol extinction (about 50 Mm−1) was also been observed within the Marine Boundary Layer (MBL). By combining ATR-42 extinction, absorption and scattering measurements, a complete optical closure has been made revealing excellent agreement with estimated optical properties. Associated calculations of the dust single scattering albedo (SSA) have been conducted, which show a moderate variability (from 0.90 to 1.00 at 530 nm). In parallel, active remote-sensing observations from the surface and onboard the F-20 aircraft suggest a complex vertical structure of particles and distinct aerosol layers with sea-salt and pollution located within the MBL, and mineral dust and/or aged north American smoke particles located above (up to 6–7 km in altitude). Aircraft and balloon-borne observations show particle size distributions characterized by large aerosols (> 10 μm in diameter) within dust plumes. In terms of shortwave (SW) direct forcing, in-situ surface and aircraft observations have been merged and used as inputs in 1-D radiative transfer codes for calculating the direct radiative forcing (DRF). Results show significant surface SW instantaneous forcing (up to −90 W m−2 at noon). Associated 3-D modeling studies from regional climate (RCM) and chemistry transport (CTM) models indicate a relatively good agreement for simulated AOD compared with measurements/observations from the AERONET/PHOTONS network and satellite data, especially for long-range dust transport. Calculations of the 3-D SW (clear-sky) surface DRF indicate an average of about −10 to −20 W m−2 (for the whole period) over the Mediterranean Sea together with maxima (−50 W m−2) over northern Africa. The top of the atmosphere (TOA) DRF is shown to be highly variable within the domain, due to moderate absorbing properties of dust and changes in the surface albedo. Indeed, 3-D simulations indicate negative forcing over the Mediterranean Sea and Europe and positive forcing over northern Africa.

A New Independent GPS-Free System for Geo-Referencing from Space

A new system’s geo-referencing from space is entirely free from any GNSS (GPS or equivalent) systems. The system addresses to various strategic and economic applications such as in remote clock synchronism, aircraft and balloon navigation, missile and smart bombs tracking, satellite orbital determination and remote target geo-positioning. The new geometry concept corresponds to an “inverted GPS” configuration, utilizing four ground-based reference stations, synchronized in time, installed at well known geodesic coordinates and a repeater in space, carried by an aircraft, balloon, satellite, etc. Signal transmitted by one of the reference bases is retransmitted by the transponder, received back by the four bases, producing four ranging measurements which are corrected for the time delays undergone in every retransmission. A minimization function was derived to compare the repeater’s positions referred to at least two groups of three reference bases, to correct for the signal transit time at the repeater and propagation delays, and consequently to provide the accurate repeater position for each time interaction. Once the repeater’s coordinates are known, the other determinations and applications become straightforward. The system solving algorithm and process performance has been demonstrated by simulations adopting a practical example with the transponder carried by an aircraft moving over bases and a target on the ground. Effects produced by reference clock synchronism uncertainties at the four bases on the measurements are reviewed.

Performance of ZTD models derived in near real-time from GBAS and meteorological data in GPS fast-static positioning

This paper presents accuracy analyses of two near real-time tropospheric delay estimation models, IGGHZG and IGGHZM, in fast-static precise positioning. The first model is based on the processing of GPS data collected from ground-based reference stations, and the second one uses metrological data fed into the Saastamoinen model. These two methods were developed within the ASG+ project by researchers from Wroclaw University of Environmental and Life Sciences. The accuracy of these models was evaluated by comparison to tropospheric zenith total delays (ZTDs) obtained from the ASG-EUPOS network final solution and the UNB3m model. The applicability of these models to precise positioning was studied by their application to ultra-fast-static positioning, in which the resulting coordinates' accuracy and ambiguity resolution performance were analysed. Baselines from 65 to 72 km, with height differences from 39 to 379 m, were processed. The obtained results show that the GPS-based model can be successfully used in precise positioning, and the meteo-based model still requires improvements, mostly related to the interpolation technique.

Signal Transceiver Transit Times and Propagation Delay Corrections for Ranging and Georeferencing Applications

The accuracy of ranging measurements depends critically on the knowledge of time delays undergone by signals when retransmitted by a remote transponder and due to propagation effects. A new method determines these delays for every single pulsed signal transmission. It utilizes four ground‐based reference stations, synchronized in time and installed at well‐known geodesic coordinates and a repeater in space, carried by a satellite, balloon, aircraft, and so forth. Signal transmitted by one of the reference bases is retransmitted by the transponder, received back by the four bases, producing four ranging measurements which are processed to determine uniquely the time delays undergone in every retransmission process. A minimization function is derived comparing repeater’s positions referred to at least two groups of three reference bases, providing the signal transit time at the repeater and propagation delays, providing the correct repeater position. The method is applicable to the transponder platform positioning and navigation, time synchronization of remote clocks, and location of targets. The algorithm has been demonstrated by simulations adopting a practical example with the transponder carried by an aircraft moving over bases on the ground.

Status and development of GLONASS

This chapter focuses on various satellite-based navigation systems. The most well-known and widespread system currently used for localization of a radio-receiving terminal is the global positioning system (GPS). All GPS satellites share the same frequency bands, making use of the code division multiple access (CDMA) technique. The transmitted signal on each sub-band is a low-rate binary phase shift keying (BPSK) digital signal containing the so-called navigation (NAV) data that are used by the receiver to perform ranging. Galileo is the European Global navigation satellite systems (GNSSs) providing a global positioning service under civilian control. It has to be interoperable with GPS and GLONASS, the American and Russian GNSSs, respectively. Like GPS and Galileo, GLONASS is based on several ground stations that monitor and control the satellites and their signals. The GPS and Galileo stations are spread on the globe, to guarantee a better control of the constellation and its signals. Differential GPS (DGPS) is an enhancement to GPS that uses a network of fixed, ground-based reference stations to broadcast the difference between the position of a receiver located at each station, as derived instant by instant using the GNSS, and the known fixed positions of the stations themselves.