Stelios P. Mertikas

The GAVDOS Mean Sea Level and Altimeter Calibration Facility: Results for Jason-1

The location of the GAVDOS facility is under a crossing point of the original ground-tracks of TOPEX/Poseidon and the present ones for Jason-1, and adjacent to an ENVISAT pass, about 50 km south of Crete, Greece. Ground observations and altimetry comparisons over cycles 70 to 90, indicate that a preliminary estimate of the absolute measurement bias for the Jason-1 altimeter is 144.7 ± 15 mm. Comparison of Jason microwave radiometer data from cycles 37 and 62, with locally collected water vapor radiometer and solar spectrometer observations indicate a 1–2 mm agreement.

Statistical Models and Latest Results in the Determination of the Absolute Bias for the Radar Altimeters of Jason Satellites using the Gavdos Facility

The dedicated calibration site for satellite radar altimeters in Gavdos, Greece, has been operational as of 2004. The island of Gavdos is located along a repeating ground track of Jason satellites, adjacent to Envisat, where the altimeter and radiometer do not experience significant land intrusion. In this article, the models and techniques for calculating the satellite altimeter bias, as well as the software tool called “TUCalibrit,” are presented. In summary, over cycles 209–259 for Jason-1 and cycles 1–40 for Jason-2, the altimeter biases have been estimated as B(J1) = +103.6 mm ± 4.7 mm and B(J2) = +181.9 mm ± 6.7 mm, respectively.

Automatic and Online Detection of Small but Persistent Shifts in GPS Station Coordinates by Statistical Process Control

This paper describes algorithms to detect sudden and persistent changes of small magnitude in the GPS baseline solution vectors or station coordinates. The aim of the algorithms is to monitor and control the quality of GPS measurements for critical real-time and crustal deformation applications using statistical process control. Quality control is a major problem in GPS, especially where real-time, automatic, and reliable positioning is needed, such as in the case of the GPS crustal deformation-monitoring network (GEONET) of Japan.To monitor the quality of baseline solutions the statistical techniques of Shewhart and cumulative sum charts are implemented. GPS data from GEONET, analyzed by the National Research Institute for Earth Science & Disaster Prevention (NIED), are used to test and evaluate the process control algorithms.

A New Method of Precise Jason-2 Altimeter Calibration Using a Microwave Transponder

This study presents a new method of precise altimeter absolute calibration using a dedicated microwave transponder, which acts as an altimeter signal repeater that can be deployed at any sub-track position both in coastal regions and inland. The Austrian Academy of Sciences operates an altimeter transponder at the Gavdos calibration/validation facility located beneath a Jason cross-over point. We discuss the capabilities and strengths of the transponder technique in general and in the particular case of a dedicated calibration campaign carried out in 2011. For the accomplishment of this campaign, including 26 ascending and descending Jason-2 passes, the onboard Poseidon-3 altimeter had to be switched to the DIODE/DEM mode for every overflight. Four different methods have been developed to analyze the transponder generated waveforms, provided by S-IGDR and S-GDR products, respectively. The resulting biases of the altimetric ranges have proven to be stable (3 mm rms) and agree to a large extent among the proposed algorithms. However, the absolute bias value of 25.8 ± 0.3 cm derived from the transponder calibration technique shows a significant deviation of several centimeters compared with the ones resulting from conventional techniques. Possible causes of this behavior are under further investigation.

Ascending and Descending Passes for the Determination of the Altimeter Bias of Jason Satellites using the Gavdos Facility

This paper presents the improvements made on the calibration methodology conducted at the Gavdos calibration/validation facility along with the latest altimeter calibration results for Jason-1 and Jason-2 satellite missions. Calibration results are presented, for the first time, for both ascending and descending passes of Jason satellites. The altimeter bias for Jason-2 has been estimated to be +173 ± 4 mm for Pass No. 109 and +171 ± 5 mm for Pass No. 018 over cycles 1–79. In tandem mission, the difference between Jason-1 and Jason-2 has been determined to be 72mm (Pass No. 109) and 68 mm (pass No. 018) and over cycles 2–20.

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.

A High-Resolution Geoid for the Establishment of the GAVDOS Multi-Satellite Calibration Site

The Topex/Poseidon (T/P) follow-on mission JASON-1 is planned to be launched by the end of 2000. A new multi-satellite calibration site has been proposed for the isle of Gavdos, south of the island of Crete, Greece. Part of the multi-satellite calibration experiment is the detailed computation of a high resolution geoid. The computed geoid is based on altimeter-derived, surface and shipborne gravity and height data. A currently available multi-satellite-based (GEOSAT-GM, ERS1-GM, ERS1-ERM, ERS2, T/P) altimetric geoid combined with newly available gravity data are used in the final model. New methods for the efficient combination of heterogeneous data are employed and special emphasis is paid to the prediction error estimates. We present the evaluation of the approximated accuracy estimates and the effect of the geoid error on the stability and reliability of the calibration site results. We will also elaborate on the assimilation of future measurements that are planned under the proposed project for the establishment of the calibration site.

Real‐time failure detection in the carrier phase measurements of GPS by robust and conventional kalman state estimates

Among the fastest‐growing applications of high‐precision GPS positioning are those which are kinematic in nature. Carrier phase‐based GPS positioning of a moving antenna—for example, attached to a ship, an aircraft, or a land vehicle—is now commonplace. Recent software innovations make use of advanced ambiguity resolution “on the fly” and real‐time kinematic data processing algorithms to emulate the ease of operation of conventional differential GPS (DGPS) based on transmitted pseudo‐range corrections. However, as much higher accuracy must now be assured compared to DGPS, greater attention must be focused on the quality control aspects of GPS positioning. This study describes two methods for detecting failures or changes of small magnitude in real time in GPS measurements. Examination of the overlap or disjointedness of robust and conventional confidence intervals and studentized normal variates have been used as failure detection tools. These methods are based on testing the performance of the difference...

Description of accuracy using conventional and robust estimates of scale

At first, the term “accuracy measure”; is described for the one‐dimensional case. Then, the limitations under which conventional methodology of accuracy expressions is useful and the effects of various degrees of assumption failures are examined. Next the criteria of appropriateness, completeness, resistance, and efficiency are set forth as the generic requirements of an investigation into properly selecting scale estimators. Finally, reliable and efficient accuracy measures are proposed for expressing one‐dimensional position uncertainty.

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.