Oscar L. Colombo

Application of ionospheric tomography to real‐time GPS carrier‐phase ambiguities Resolution, at scales of 400–1000 km and with high geomagnetic activity

Theinfluenceoftheionospherecanbeoneofthe mainobstacles toGPScarrierphaseambiguityresolution in real-time,particularlyoverlongbaselines. Thisisimportant toallusersofGPSrequiringsub-decimeterpositioning, per- haps in real time, especially with high geomagnetic activity or close to the Solar Maximum. Therefore, it is desirable to have a precise estimation of the ionospheric delay in real- time, to correct the data. In this paper we asses a real-time tomographic model of the ionosphere created using dual- frequency phase data simultaneously collected with the re- ceiversofanetworkofstationsintheUSAandCanada,with separationsof400-1000km,duringaperiodofhighgeomag- netic activity (Kp=6). When the tomographic ionospheric correctionisincluded,theresolutionon-the-fly(OTF)ofthe widelanedouble-dierencedambiguitiesatthereferencesta- tions is nearly 100% successful for satellite elevations above 20 degrees, while theresolution of theL1,L2ambiguities at the rover is typically more than 80% successful.

GPS/Acoustic seafloor geodetic observation: method of data analysis and its application

We have been developing a system for detecting seafloor crustal movement by combining kinematic GPS and acoustic ranging techniques. A linear inversion method is adopted to determine the position of seafloor stations from coordinates of a moving survey vessel and measured travel times of acoustic waves in seawater. The positioning accuracy is substantially improved by estimating the temporal variation of the acoustic velocity structure. We apply our method to the ranging data acquired at the seafloor reference point, MYGI, located off Miyagi Prefecture, in northeast Japan, where a huge earthquake is expected to occur in the near future. A time series of horizontal coordinates of MYGI obtained from seven campaign observations for the period 2002–2005 exhibits a linear trend with a scattering rms of about 2 cm. A linear fit to the time series gives an intraplate crustal velocity of more than several centimeters per year towards the WNW, which implies strong interplate coupling around this region. The precision of each campaign solution was examined at MYGI and other seafloor reference points along the Nankai Trough through comparison of independent one-day subset solutions within the campaign. The resultant repeatability looks to be well-correlated with the temporal and spatial stability of the acoustic velocity structure in the seawater depending on the region as well as the season.

Feasibility of wide-area subdecimeter navigation with GALILEO and Modernized GPS

Precise corrections with a three-dimensional voxel model of the ionosphere based on Global Navigation Satellite System (GNSS) data from a wide-area network of ground receivers can help resolve differential carrier-phase ambiguities over very long baselines of hundreds of kilometers in present two-frequency systems [Global Positioning System (GPS) and Global Orbiting Navigation Satellite System (GLONASS)] or in planned three-frequency systems (GALILEO, Modernized GPS). A study based on simulated three-frequency data from a modified GNSS signal generator indicates that all the phase ambiguities could be resolved successfully more than 90% of the time. This should be useful in surveying large areas with instruments that require very precise geolocation (e.g. radar or lidar altimetry, interferometric synthetic aperture radar, interferometric sonar, etc.).

A new strategy for real‐time integrated water vapor determination in WADGPS Networks

A major issue in many applications of GPS is the real-time estimation of the Zenith Tropospheric Delays (ZTD). Several authors have developed strategies that es- timate ZTD, with a latency of one hour or more in or- der to compute Integrated Water Vapor (IWV), using lo- cal measurements of surface pressure, or to assimilate ZTD into Numerical Weather Predicition (NWP) models. These strategies require that data from a regional GPS network be processed in near real-time, using precise IGS orbits and partial orbit relaxation. Recently it has been shown that in WideAreaDierentialGPS(WADGPS)networksofseveral hundred kilometers across, double-dierenced carrier phase ambiguitiescanbecomputedon-the-fly,usingareal-timeto- mographic model of the ionosphere obtained from the same GPS data. In this work we show how ambiguity resolution can help determine in real-time the ZTD for a WADGPS network user, only 10-20% worse than those of the post- processed solutions.

Ionospheric tomography helps resolve GPS ambiguities on the fly at distances of hundreds of kilometers during increased geomagnetic activity

We describe a procedure for resolving ambiguities in real time with the help of very precise ionospheric corrections calculated tomographically using data from several reference stations that are part of a local area automatic control network. We report successful results during a period of high geomagnetic activity (Kp=6) in the region of the Pacific Northwest (in Washington and British Columbia), using a set of continuously operating IGS stations with separations of between 400 and 1000 km. While the data were collected before the test, our calculations carefully emulated those made in a real-time situation. All GPS receivers were dual-frequency. This is a proof-of-concept for the use of such a technique in the operation of large regional networks of automatic stations, capable of supporting sub-decimeter surveying and navigation in real time anywhere within their area.

Airborne Gravimetry, Altimetry, and GPS Navigation Errors

Proper interpretation of airborne gravimetry and altimetry requires good knowledge of aircraft trajectory. Recent advances in precise navigation with differential GPS have made it possible to measure gravity from the air with accuracies of a few milligals, and to obtain altimeter profiles of terrain or sea surface correct to one decimeter. These developments are opening otherwise inaccessible regions to detailed geophysical mapping. Navigation with GPS presents some problems that grow worse with increasing distance from a fixed receiver: the effect of errors in tropospheric refraction correction, GPS ephemerides, and the coordinates of the fixed receivers. Ionospheric refraction and orbit error complicate ambiguity resolution. Optimal navigation should treat all error sources as unknowns, together with the instantaneous vehicle position. To do so, fast and reliable numerical techniques are needed: efficient and stable Kalman filter-smoother algorithms, together with data compression and, sometimes, the use of simplified dynamics.

Evaluation result of new seafloor mirror transponder and AUV observation system in seafloor geodetic observation

Intending to capture sea-bottom crustal deformations with high accuracy, we have been developing new technologies for seafloor geodetic observations with an AUV. This has led to the development of a small high-performance geodetic observation system and an ideal spherical acoustic transducer, both to be mounted on an AUV, and the tuning and evaluation tests of them have yielded good results.

Fundamental Developments of New Generation Seafloor Geodetic Observation System Based on AUV Technology

Institute of Industrial Science, University of Tokyo (IIS) has started a project to develop the fundamental technologies for constructing new-generation seafloor geodetic observation system. The current observational method using research vessel cannot help being subjected to annual cruise schedule of research vessels. It has been difficult for us to change the cruise schedule as appropriate according to weather and sea condition, GPS satellite distribution and so on. The new system, which we are developing, based on AUV technology will give us opportunities for observation with choosing favorable conditions of sea and GPS satellite distribution, much more frequent observations and flexible planning of observation in response to sudden geodetic events. Trial models of the sea surface and seafloor units were finished. We conducted several performance evaluation experiments in the sea and dam site, in order to bring the observation system to completion.

Mapping the Earth’s Gravity Field with Orbiting GPS Receivers

The orbit of an artificial satellite is shaped primarily by the force of terrestrial gravitation. If the earth were a perfect and homogeneous sphere, the orbit would be a simple Keplerian ellipse. The planet is closer to a flattened ellipsoid than to a sphere; the pull of its equatorial bulge causes the orbit to turn within its plane at a constant rate (precession of the perigee), and also slowly rotates the plane of the orbit about the earth’s axis (precession of the line of nodes). But the actual shape and mass distribution is much more complex than for a flattened, homogeneous ellipsoid. Departures from this simpler body cause irregularities (anomalies) in the gravity field that in turn produce “wiggles” or perturbations in the orbit. These departures from a precessing ellipse about a flattened ellipsoid can be sensed with tracking systems that measure the instantaneous distance or velocity of the satellite along the line of sight to the tracker.

Precise GPS orbits for geodesy

The Global Positioning System (GPS) has become, in recent years, the main space-based system for surveying and navigation in many military, commercial, cadastral, mapping, and scientific applications. Better receivers, interferometric techniques (DGPS), and advances in post-processing methods have made possible to position fixed or moving receivers with sub-decimeter accuracies in a global reference frame. Improved methods for obtaining the orbits of the GPS satellites have played a major role in these achievements; this paper gives a personal view of the main developments in GPS orbit determination.