George S. Vergos

Use of MODIS satellite images for detailed lake morphometry: Application to basins with large water level fluctuations

Abstract Lake morphometry is essential for managing water resources and limnetic ecosystems. For reservoirs that receive high sediment loads, frequent morphometric mapping is necessary to define both the effective life of the reservoir and its water storage capacity for irrigation, power generation, flood control and domestic water supply. The current study presents a methodology for updating the digital depth model (DDM) of lakes and reservoirs with wide intra and interannual fluctuations of water levels using satellite remote sensing. A time series of Terra MODIS satellite images was used to map shorelines formed during the annual water level change cycle, and were validated with concurrent Landsat ETM+ satellite images. The shorelines were connected with in-situ observation of water levels and were treated as elevation contours to produce the DDM using spatial interpolation. The accuracy of the digitized shorelines is within the mapping accuracy of the satellite images, while the resulting DDM is validated using in-situ elevation measurements. Two versions of the DDM were produced to assess the influence of seasonal water fluctuation. Finally, the methodology was applied to Lake Kerkini (Greece) to produce an updated DDM, which was compared with the last available bathymetric survey (1991) and revealed changes in sediment distribution within the lake.

First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities

This work presents the first calibration results for the SARAL/AltiKa altimetric mission using the Gavdos permanent calibration facilities. The results cover one year of altimetric observations from April 2013 to March 2014 and include 11 calibration values for the altimeter bias. The reference ascending orbit No. 571 of SARAL/AltiKa has been used for this altimeter assessment. This satellite pass is coming from south and nears Gavdos, where it finally passes through its west coastal tip, only 6 km off the main calibration location. The selected calibration regions in the south sea of Gavdos range from about 8 km to 20 km south off the point of closest approach. Several reference surfaces have been chosen for this altimeter evaluation based on gravimetric, but detailed regional geoid, as well as combination of it with other altimetric models. Based on these observations and the gravimetric geoid model, the altimeter bias for the SARAL/AltiKa is determined as mean value of −46mm ±10mm, and a median of −42 mm ±10 mm, using GDR-T data at 40 Hz rate. A preliminary cross-over analysis of the sea surface heights at a location south of Gavdos showed that SARAL/AltiKa measure less than Jason-2 by 4.6 cm. These bias values are consistent with those provided by Corsica, Harvest, and Karavatti Cal/Val sites. The wet troposphere and the ionosphere delay values of satellite altimetric measurements are also compared against in-situ observations (−5 mm difference in wet troposphere and almost the same for the ionosphere) determined by a local array of permanent GNSS receivers, and meteorological sensors.

Permanent facility for calibration/validation of satellite altimetry: GAVDOS

An absolute sea-level monitoring and altimeter calibration permanent facility has been established on the isle of Gavdos, 50 km south of the island of Crete, Greece. This calibration/validation facility has been chosen because Gavdos is under a crossing point of the ground-tracks of Jason-1 satellite, and adjacent to an Envisat pass. Satellite altimeter missions are evaluated at that site using external measurements from tide gauges, GPS, a DORIS beacon, meteorological sensors, wave-height sensors, airborne campaigns for gravity and sea-surface topography, water-vapour radiometry, solar atmospheric spectrometry, GPS buoys, altimeter transponder, Satellite Laser Ranging, etc. The mean sea level and the earths tectonic deformation field in the region have also been determined accurately. Comparison over the cycles 70 to 77 of the Jason-1 satellite indicate that its absolute mean bias for the sea-surface heights is 13 mm ± 20 mm. The GAVDOS project has started in December 2001 and has been in the context of an international calibration/validation effort of the Jason-1 Science Working Team.

Recent developments for the estimation of the altimeter bias for the Jason-1 and 2 satellites using the dedicated calibration site at Gavdos

The dedicated calibration site for satellite radar altimeters in Gavdos has been operational as of 2004. The small island of Gavdos is located along a repeating ground track of Jason satellites (crossover point No 109 ascending and No. 18 descending pass and adjacent to Envisat), and where the altimeter and radiometer footprints do not experience significant land intrusion. The purpose of such permanent Cal/Val facility is to calibrate the sea-surface height and ancillary measurements made by the satellite as it passes overhead, by using observations from tide gauges, GPS, DORIS and other sensors directly placed under the satellite ground tracks. The successful launch of Jason-2 satellite (20 June, 2008) initiated its calibration-validation phase. This was achieved having the two satellites flying with less than one minute difference and in the same orbit. Using the Gavdos calibration facility the following have been determined: (1) the absolute altimeter bias of Jason-1 satellite for the cycles 209-259; using GDR-C data; (2) the absolute altimeter bias of Jason-2 satellite for the cycles 2-28 using GDR-A data ; (3) the inter- mission bias for the period July 2008 - January 2009. The expansion of the Gavdos Cal/Val facilities with the deployment of a new site in the south of Crete and along pass No. 109 is also presented in this work.

Monitoring water quality parameters in the marine area of Danube Delta using satellite remote sensing: preliminary results

The aim of this work was to produce water quality parameter maps for the marine area of the Danube Delta using remotely sensed data and to validate the results with in-situ measurements. For this reason, satellite images from ENVISAT/MERIS and Aqua/MODIS were used along with collocated in-situ measurements. The latter were in-sync with the satellite images acquisition so that rigorous and validation could be performed. Chlorophyll-a concentration and total suspended matter were estimated using the CASE-II algorithm and MERIS satellite images, while sea surface temperature was estimated from MODIS Ocean Team products. The results show that the satellite images covered the study area completely, with some data gaps due to cloud coverage. Comparisons show a good correspondence with in-situ measurements. Thus, the time series of satellite images that was produced suggests that it is possible to monitor the biological changes on an operational basis. The produced maps described a detailed spatial pattern of chlorophyll-a and total suspended matter that could not have been identified from the sparse in-situ measurements.

Fifteen Years of Cal/Val Service to Reference Altimetry Missions: Calibration of Satellite Altimetry at the Permanent Facilities in Gavdos and Crete, Greece

Satellite altimetry provides exceptional means for absolute and undisputable monitoring of changes in sea level and inland waters (rivers and lakes), over regional to global scales, with accuracy and with respect to the center of mass of the Earth. Altimetry system’s responses have to be continuously monitored for their quality, biases, errors, drifts, etc. with calibration. Absolute calibration of altimeters is achieved by external and independent to satellite facilities on the ground. This is the mainstay for a continuous, homogenous, and reliable monitoring of the earth and its oceans. This paper describes the development of the Permanent Facility for Altimetry Calibration in Gavdos/Crete, Greece, as of 2001 along with its infrastructure and instrumentation. Calibration results are presented for the reference missions of Jason-1, Jason-2, and Jason-3. Then, this work continues with the determination of relative calibrations with respect to reference missions for Sentinel-3A, HY-2A, and SARAL/AltiKa. Calibration results are also given for Jason-2 and Jason-3 altimeters using the transponder at the CDN1 Cal/Val site on the mountains of Crete, with simultaneous comparisons against sea-surface calibration and during their tandem mission. Finally, the paper presents procedures for estimating uncertainties for altimeter calibration to meet the Fiducial Reference Measurement standards.

Accuracy assessment of the SRTM 90m DTM over Greece and its implications to geoid modelling

With the realization of the Shuttle Radar Topographic Mission (SRTM) and the distribution of the 3 arcsec (90 m) data over Europe, a high-resolution digital terrain model (DTM) became available for Greece. Until today, high-resolution DTMs for Greece were generated by the Hellenic Military Geographic Service (HMGS) only and present variable resolutions with the finest one being set to 100 m. These DTMs were usually determined by digitizing topographic maps and are thus of variable and sometimes unknown accuracy. When a high-resolution, e.g., 0.5 - 1 arcmin geoid is needed, it is absolutely necessary to employ a very high resolution DTM to compute the terrain effects to gravity and the indirect effect to the geoid. If this information is not available and a coarser DTM is used, then the topographic effects computed are aliased, due to the insufficient resolution of the topographic data used. The scope of this work is twofold. First, a validation and accuracy assessment of the SRTM 90 m DTM over Greece is performed through comparisons with existing global models, like GLOBE and GTOPO30, as well as with the Greek 450 m DTMs delivered by HMGS. Whenever a misrepresentation of the topography is identified in the SRTM data, it is “corrected” using the local 100 m DTM. This processes resulted in an improved SRTM DTM called SRTMG, which was then used to determine terrain and RTM effects to gravity field quantities. Then, all available DTMs were used to compute terrain effects on both gravity anomalies and geoid heights at variable spatial resolutions. From the results acquired, the performance of the SRTMG model with respect to geoid modeling was assessed and conclusions on the effect of the DTM resolution were drawn.