At first, the specifications of the iGrav site will be presented, then the drift behaviour and the data processing. The drift quickly decreases to less than 0. In order to look at the stability of the iGrav over a wide time period, a FG 5 gravimeter is used for bi-monthly absolute gravity measurements and for frequent calibrations. After having discussed the instrumental and data processing points of view, preliminary results on the local karstic water storage will be presented and interpreted by combining different geophysical data.
Continuous gravity data allow to study processes at different timescale such as summer evapotranspiration or high precipitating event characteristic of the Mediterranean autumn. Superconducting gravimeter. The superconducting gravimeter was developed and applied to field measurements.
The stability of the instrument yielded the highest precision measurements of the Earth tides ever attained. It revealed unprecedented details about the effect of the atmosphere on gravity. Secular variations in gravity and the stability of the instruments were measured by comparing records from co-located instruments.
These efforts have resulted in substantial reductions in the noise level at very low frequencies so that the peak differences between two instruments at the same location can be reduced to 0. The Austrian absolute gravity base net: 27 years of spatial and temporal acquisition of gravity data. Overall more than 70 absolute gravity stations were installed in Austria and neighbouring countries and some of them have been regularly monitored. As a national metrology institute NMI the Metrology Service of the BEV maintains the national standards for the realisation of the legal units of measurement and ensures their international equivalence and recognition.
Thus the BEV maintains the national standard for gravimetry in Austria, which is validated and confirmed by international comparisons. The results of these ICAG's and especially the performance of the Austrian absolute gravimeter are reported in this presentation. We also present some examples and interpretation of long time monitoring stations of absolute gravity in several Austrian locations.
Some stations are located in large cities like Vienna and Graz and some others are situated in mountainous regions. Mountain stations are at the Conrad Observatory where a SG Superconducting Gravimeter is permanently monitoring and in Obergurgl Tyrolia at an elevation of approx. Its two gravity sensors are side by side, not one on top of another as in most earlier dual sensor installations. The second sensor uses a heavy gram sphere which gives ultra low noise and a much higher quality factor Q. We present time domain observations of the first months, and estimate drift rates after the initial exponential drift.
We have determined the transfer functions. This year old instrument is situated in the same room at a distance of 2 metres from the dual-sphere SG. Lunar surface gravimeter experiment. The lunar surface gravimeter used the moon as an instrumented antenna to search for gravitational waves predicted by Einstein's general theory of relativity.
Tidal deformation of the moon was measured. Gravitational radiation is a channel that is capable of giving information about the structure and evolution of the universe. Educational Inductive Gravimeter. A simple inductive gravimeter constructed from a rigid plastic pipe and insulated copper wire is described.
When a magnet is dropped through the vertically mounted pipe it induces small alternating voltages. These small signals are fed to the microphone input of a typical computer and sampled at a typical rate of Measurement of absolute gravity acceleration in Firenze. This paper reports the results from the accurate measurement of the acceleration of gravity g taken at two separate premises in the Polo Scientifico of the University of Firenze Italy.
In these laboratories, two separate experiments aiming at measuring the Newtonian constant and testing the Newtonian law at short distances are in progress. Both experiments require an independent knowledge on the local value of g. The only available datum, pertaining to the italian zero-order gravity network, was taken more than 20 years ago at a distance of more than 60 km from the study site.
Gravity measurements were conducted using an FG 5 absolute gravimeter , and accompanied by seismic recordings for evaluating the noise condition at the site. The absolute accelerations of gravity at the two laboratories are Other than for the two referenced experiments, the data here presented will serve as a benchmark for any future study requiring an accurate knowledge of the absolute value of the acceleration of gravity in the study region.
Assessing the precision of the iGrav superconducting gravimeter for hydrological models and karstic hydrological process identification. In this paper we present the potential of a new compact superconducting gravimeter GWR iGrav designed for groundwater monitoring. At first, 3 yr of continuous gravity data are evaluated and the performance of the instrument is investigated. With repeated absolute gravity measurements using a Micro-g Lacoste FG 5 , the calibration factor The low noise level performance is found similar to those of previous superconducting gravimeters and leads to gravity residuals coherent with local hydrology.
The iGrav is located in a fully instrumented hydrogeophysical observatory on the Durzon karstic basin Larzac plateau, south of France. Rain gauges and a flux tower evapo-transpiration measurements are used to evaluate the groundwater mass balance at the local scale. Water mass balance demonstrates that the karst is only capacitive: all the rainwater is temporarily stored in the matrix and fast transfers to the spring through fractures are insignificant in this area.
Moreover, the upper part of the karst around the observatory appears to be representative of slow transfer of the whole catchment. Indeed, slow transfer estimated on the site fully supports the low-flow discharge at the only spring which represents all groundwater outflows from the catchment.
In the last part of the paper, reservoir models are used to characterize the water transfer and storage processes. Particular highlights are done on the advantages of continuous gravity data compared to repeated campaigns and on the importance of local accurate meteorological data to limit misinterpretation of the gravity observations. The results are complementary with previous studies at the basin scale and show a clear potential for continuous gravity time-series assimilation in hydrological simulations, even.
Atom chip gravimeter. Atom interferometry has developed into a tool for measuring rotations , accelerations , and testing fundamental physics . Gravimeters based on laser cooled atoms demonstrated residual uncertainties of few microgal [2,4] and were simplified for field applications . Atomic gravimeters rely on the interference of matter waves which are coherently manipulated by laser light fields. The latter can be interpreted as rulers to which the position of the atoms is compared. At three points in time separated by a free evolution, the light fields are pulsed onto the atoms.
First, a coherent superposition of two momentum states is produced, then the momentum is inverted, and finally the two trajectories are recombined. Depending on the acceleration the atoms experienced, the number of atoms detected in the output ports will change. Consequently, the acceleration can be determined from the output signal. The laser cooled atoms with microkelvin temperatures used in state-of-the-art gravimeters impose limits on the accuracy .
Therefore, ultra-cold atoms generated by Bose-Einstein condensation and delta-kick collimation [6,7] are expected to be the key for further improvements. These sources suffered from a low flux implying an incompatible noise floor, but a competitive performance was demonstrated recently with atom chips .
In the compact and robust setup constructed for operation in the drop tower  we demonstrated all steps necessary for an atom chip gravimeter with Bose-Einstein condensates in a ground based operation. We will discuss the principle of operation, the current performance, and the perspectives to supersede the state of the art.
Absolute -gravity stations are an important part of the geodetic infrastructure of the Antarctic. They provide accurate starting values for gravity surveys performed e. The time variation in gravity determined from repeated absolute -gravity measurements provides insights into the Glacial Isostatic Adjustment GIA and into solid Earth deformation due to variation in contemporary ice load. Given sufficient joint coverage with International Terrestrial Reference Frame ITRF sites, gravity rates in high latitudes could in principle provide an independent check of the geocentricity of the z-dot velocities in the direction of the rotation axis of the Earth of the ITRF.
All these bases have continuous GNSS stations. Numerous supporting measurements have been made at the sites: microgravity networks, levelling and GNSS ties to excentres etc. At some sites, nearby glacier elevations were surveyed to monitor the attraction of the variable close-field snow and ice masses.
We give a description of the sites and the measurements performed at them. A highly accurate absolute gravimetric network for Albania, Kosovo and Montenegro. The objective of this project is to establish a basic gravity network in Albania, Kosovo and Montenegro to enable further investigations in geodetic and geophysical issues.
Therefore the first time in history absolute gravity measurements were performed in these countries. The gravity measurements are funded by Kartverket. Laser and clock of the absolute gravimeter were calibrated before and after the measurements. The absolute gravimetric survey was carried out from September to October Finally all 8 scheduled stations were successfully measured: there are three stations located in Montenegro, two stations in Kosovo and three stations in Albania. The stations are distributed over the countries to establish a gravity network for each country. The vertical gradients were measured at all 8 stations with the relative gravimeter Scintrex CG5.
The high class quality of some absolute gravity stations can be used for gravity monitoring activities in future. The measurement uncertainties of the absolute gravity measurements range around 2. In Montenegro the large gravity difference of MilliGal between station Zabljak and Podgorica can be even used for calibration of relative gravimeters. Rather new superconducting and spring gravimeters , iGrav and gPhone are collocated with a superconducting gravimeter , TT 70 at the Mizusawa VLBI Observatory of the National Astronomical Observatory of Japan, since the end of September, in order to evaluate those performances before field deployment planned in Calibration of iGrav was carried out by collocation with an absolute gravimeter FG 5 of the Earthquake Research Institute, University of Tokyo Okubo, , personal comm.
Based on the scale factors of iGrav obtained by the calibration and of gPhone provided by the manufacturer Micro-g LaCoste, Inc. The high drift rate of gPhone , however, is well approximated by a quadratic function at present and can be removed. The detrended time series of gPhone shows good agreement with iGrav time series in the overall feature: gravity fluctuations with amplitudes of about a few micro-Gal and with durations of a few days, which may be due to variations in the moisture content of the topmost unsaturated sedimentary layer and the water table height.
In addition to leveling, long period GPS, and defelction of vertical observations, absolute gravity and vertical gravity gradients were measured at bench marks approximately 1. The same relative instruments were also used to measure two-tier linear vertical gravity gradients at the A10 sites. In the current work - becuase of the rapidly changing terrain in the Rocky Mountains - it was decided to employ the A10 at all bench marks, and acquire three-tier quadratic gradients at every bench mark using the new Scintrex CG-6 Autograv relative gravimeter.
Using these results, we will provide a real worldsummary of the CG-6's behavior by examining noise levels, repeatability, and acquisition rates.
In addition, the coincident A10 absolute data set allows us to evaluate the CG-6's accuracy, and allows us to simulate and discuss various relative gravity survey designs. Laser interferometry method for absolute measurement of the acceleration of gravity. Gravimeter permits more accurate and precise absolute measurement of g without reference to Potsdam values as absolute standards. Device is basically Michelson laser beam interferometer in which one arm is mass fitted with corner cube reflector. Time stability of spring and superconducting gravimeters through the analysis of very long gravity records.
Long gravity records are of great interest when performing tidal analyses. Indeed, long series enable to separate contributions of near-frequency waves and also to detect low frequency signals e. In addition to the length of the series, the quality of the data and the temporal stability of the noise are also very important. We study in detail some of the longest gravity records available in Europe: 3 data sets recorded with spring gravimeters in Black Forest Observatory Germany, , Walferdange Luxemburg, and Potsdam Germany, and several superconducting gravimeters SGs data sets, with at least 9 years of continuous records, at different European GGP Global Geodynamics Project sites Bad Homburg, Brussels, Medicina, Membach, Moxa, Vienna, Wettzell and Strasbourg.
The long term stability of the tidal observations is also dependent on the stability of the scale factor of the relative gravimeters. Therefore we also check the time stability of the scale factor for the superconducting gravimeter C installed at the J9 Gravimetric Observatory of Strasbourg France , using numerous calibration experiments carried out by co-located absolute gravimeter AG measurements during the last 15 years.
The reproducibility of the scale factor and the achievable precision are investigated by comparing the results of different calibration campaigns. Finally we present a spectrum of the 25 years of SG records at J9 Observatory, with special attention to small amplitude tides in the semi-diurnal and diurnal bands, as well as to the low frequency part. PubMed Central. Gravimeters are used to measure density anomalies under the ground. They are applied in many different fields from volcanology to oil and gas exploration, but present commercial systems are costly and massive. A new type of gravity sensor has been developed that utilises the same fabrication methods as those used to make mobile phone accelerometers.
In this study, we describe the first results of a field-portable microelectromechanical system MEMS gravimeter. The stability of the gravimeter is demonstrated through undertaking a multi-day measurement with a standard deviation of 5. It is then demonstrated that a change in gravitational acceleration of 4.
Finally, the device is demonstrated to be stable in a more harsh environment: a 4. These initial field-tests are an important step towards a chip-sized gravity sensor. Double diffraction in an atomic gravimeter. We demonstrate the realization of a scheme for cold-atom gravimetry based on the recently demonstrated use of double-diffraction beam splitters [T. Leveque, A. Gauguet, F. Michaud, F. Pereira Dos Santos, and A. Landragin, Phys. Such pulses are used to realize an interferometer insensitive to laser phase noise and some of the dominant systematics.
This approach reduces the technical requirements and would allow the realization of a simple atomic gravimeter. A sensitivity of 1. Absolute marine gravimetry with matter-wave interferometry. Measuring gravity from an aircraft or a ship is essential in geodesy, geophysics, mineral and hydrocarbon exploration, and navigation. Today, only relative sensors are available for onboard gravimetry. This is a major drawback because of the calibration and drift estimation procedures which lead to important operational constraints.
Atom interferometry is a promising technology to obtain onboard absolute gravimeter. But, despite high performances obtained in static condition, no precise measurements were reported in dynamic. Here, we present absolute gravity measurements from a ship with a sensor based on atom interferometry. The atom gravimeter was also compared with a commercial spring gravimeter and showed better performances.
This demonstration opens the way to the next generation of inertial sensors accelerometer, gyroscope based on atom interferometry which should provide high-precision absolute measurements from a moving platform.
Publications - Airborne Geophysics|NRG
Preliminary results of the traverse gravimeter experiment successfully performed during the Apollo 17 mission are discussed. An earth-moon gravity tie was established. Free-air and Bouguer corrections were applied to the gravity data. The resultant Bouguer anomaly, analyzed with a two-dimensional approximation, shows a relative gravity maximum of about 25 to 30 mgal over the Taurus-Littrow valley.
This maximum is interpreted in terms of a 1-km-thick block of basalt flow with a positive density contrast of 0. Limits on Lorentz violation in gravity from worldwide superconducting gravimeters. We have investigated Lorentz violation through analyzing tides-subtracted gravity data measured by superconducting gravimeters. At the level of precision of superconducting gravimeters , we have brought up and resolved an existing issue of accuracy due to unaccounted local tidal effects in previous solid-earth tidal model used.
Specifically, we have taken local tides into account with a brand new first-principles tidal model with ocean tides included, as well as removed potential bias from local tides by using a worldwide array of 12 superconducting gravimeters. Compared with previous test with local gravimeters , a more accurate and competitive bound on space-space components of gravitational Lorentz violation has been achieved up to the order of The ability to measure tiny variations in the local gravitational acceleration allows, besides other applications, the detection of hidden hydrocarbon reserves, magma build-up before volcanic eruptions, and subterranean tunnels.
All of these devices can observe the Earth tides: the elastic deformation of the Earth's crust as a result of tidal forces.
This is a universally predictable gravitational signal that requires both high sensitivity and high stability over timescales of several days to measure. All present gravimeters , however, have limitations of high cost more than , US dollars and high mass more than 8 kilograms. We use it to measure the Earth tides, revealing the long-term stability of our instrument compared to any other MEMS device. MEMS accelerometers--found in most smart phones--can be mass-produced remarkably cheaply, but none are stable enough to be called a gravimeter.
Our device has thus made the transition from accelerometer to gravimeter. The small size and low cost of this MEMS gravimeter suggests many applications in gravity mapping. For example, it could be mounted on a drone instead of low-flying aircraft for distributed land surveying and exploration, deployed to monitor volcanoes, or built into multi-pixel density-contrast imaging arrays.
Relativistic theory of the falling retroreflector gravimeter. We develop a relativistic treatment of interference between light reflected from a falling cube retroreflector in the vertical arm of an interferometer, and light in a reference beam in the horizontal arm. Coordinates that are nearly Minkowskian, attached to the falling cube, are used to describe the propagation of light within the cube. Relativistic effects such as the dependence of the coordinate speed of light on gravitational potential, propagation of light along null geodesics, relativity of simultaneity, and Lorentz contraction of the moving cube, are accounted for.
The calculation is carried to first order in the gradient of the acceleration of gravity. Analysis of data from a falling cube gravimeter shows that the propagation time of light within the cube itself causes a significant reduction in the value of the acceleration of gravity obtained from measurements, compared to assuming reflection occurs at the face.
An expression for the correction to g is derived and found to agree with experiment. Depending on the instrument, the correction can be several microgals, comparable to commonly applied corrections such as those due to polar motion and earth tides. Work of the US government, not subject to copyright. Microelectromechanical system gravimeters as a new tool for gravity imaging. Middlemiss, Richard P. A microelectromechanical system MEMS gravimeter has been manufactured with a sensitivity of 40 ppb in an integration time of 1 s.
This sensor has been used to measure the Earth tides: the elastic deformation of the globe due to tidal forces. No such measurement has been demonstrated before now with a MEMS gravimeter. Since this measurement, the gravimeter has been miniaturized and tested in the field. Measurements of the free-air and Bouguer effects have been demonstrated by monitoring the change in gravitational acceleration measured while going up and down a lift shaft of These tests demonstrate that the device has the potential to be a useful field-portable instrument.
The development of an even smaller device is underway, with a total package size similar to that of a smartphone. Superconducting- gravimeter measurements are used to test the local Lorentz invariance of the gravitational interaction and of matter-gravity couplings. The best laboratory sensitivities to date are achieved via a maximum-reach analysis for 13 Lorentz-violating operators, with some improvements exceeding an order of magnitude. It is shown that the physically based approach of direct loading calculations performs better in both residual minimization and ET retrieval.
Finally, gravity residuals are briefly analyzed at two distinct time scales: rapid up to a few days and seasonal. At the rapid time-scale, it is shown that ET retrieval is hardly achievable given shelter size and state-of-the-art atmospheric corrections. Still, mean values retrieved from this study are in accordance with known values of potential ET and lateral flow. Direct comparison of gravity changes with hydrological data neutron probe monitoring and water table levels show some discrepancies, particularly for the hydrological year of , for which all hydrological data show a deficit, but SG and FG 5 data do not.
This preliminary analysis both provides a basis and call for further hydro-gravity modeling, to comprehensively investigate the water-cycle at the Djougou station. Easy Absolute Values? The authors teach a problem-solving course for preservice middle-grades education majors that includes concepts dealing with absolute -value computations, equations, and inequalities. Many of these students like mathematics and plan to teach it, so they are adept at symbolic manipulations.
Getting them to think differently about a concept that they…. High precision tide spectroscopy. Diurnal and long period earth tides were measured to high accuracy and precision with the superconducting gravimeter. The results provide new evidence on the geophysical questions which have been attacked through earth tide measurements in the past. In addition, they raise new questions of potential interest. Slow fluctuations in gravity of order 10 micron gal over periods of 3 to 5 months were observed and are discussed. A network of superconducting gravimeters detects submicrogal coseismic gravity changes.
With high-resolution continuous gravity recordings from a regional network of superconducting gravimeters , we have detected permanent changes in gravity acceleration associated with a recent large earthquake. Detected changes in gravity acceleration are smaller than 10 -8 meters seconds -2 1 micro-Galileo, about 10 -9 times the surface gravity acceleration and agree with theoretical values calculated from a dislocation model. Superconducting gravimetry can contribute to the studies of secular gravity changes associated with tectonic processes.
This article is part of a discussion meeting issue 'The promises of gravitational-wave astronomy'. Inertial navigation systems and gravimeters are now routinely used to map the regional gravitational quantities from an aircraft with mGal accuracy and a spatial resolution of a few kilometers. However, airborne gravimeter of this kind is limited by the inaccuracy of the inertial sensor performance, the integrated navigation technique and the kinematic acceleration determination.
As the GPS technique developed, the vehicle acceleration determination is no longer the limiting factor in airborne gravity due to the cancellation of the common mode acceleration in differential mode. A new airborne gravimeter taking full advantage of the inertial navigation system is described with improved mechanical design, high precision time synchronization, better thermal control and optimized sensor modeling.
Apart from the general usage, the Global Positioning System GPS after differentiation is integrated to the inertial navigation system which provides not only more precise altitude information along with the navigation aiding, but also an effective way to calculate the vehicle acceleration. Design description and test results on the performance of the gyroscopes and accelerations will be emphasized. Analysis and discussion of the airborne field test results are also given. Absolute gravimetry for monitoring geodynamics in Greenland.
Here are presented the preliminary results of the absolute gravity measurements done in Greenland by DTU Space with their A10 absolute gravimeter the A The purpose, besides establishing and maintaining a national gravity network, is to study geodynamics.
The first measurements were conducted in and a few sites have been re-visited. As of present is there a gravity value at 18 GNET sites. There are challenges in interpreting the measurements from Greenland and several signals has to be taken into account, besides the geodynamical signals originating from the changing load of the ice, there is also a clear signal of direct attraction from different masses.
Here are presented the preliminary results of our measurements in Greenland and attempts explain them through modelling of the geodynamical signals and the direct attraction from the ocean and ice. Gravity can be measured in many ways, from static-point observations to dynamic measurement using land vehicles, ships, aircrafts and satellites.
This system is the first strapdown gravimetry system in China. The test contained 30 survey lines in the west-east direction and four groups of repeat lines in four directions; the evaluation of the repeatability was based on repeat lines. The measurement and in-movement alignment algorithm for strapdown gravimeters in the sea are discussed, and the results and analysis of this test are presented.
These results indicate that the strapdown gravimetry system SGA-WZ can be used in marine gravimetry and the moving base alignment method can improve the computation efficiency greatly when using the strapdown gravimetry system. Lunar Surface Gravimeter Experiment. The lunar surface gravimeter which was emplaced on the moon by the Apollo 17 flight is described and a schematic diagram of the sensor is provided.
The objective of the lunar surface gravimeter is to use the moon as an instrumented antenna to detect gravitational waves. Another objective is to measure tidal deformation of the moon. Samples of signals received during lunar sunrise activity and during quiet periods are presented in graph form based on power spectrum analysis. Relative gravimeter prototype based on micro electro mechanical system. This research to make gravity measurement system by utilizing micro electro mechanical system based sensor in Gal order. System design consists of three parts, design of hardware, software, and interface.
The design of the hardware include of designing the sensor design to measure the value of a stable gravity acceleration. The design of the instrumentation system the next stage by creating a design to integrate between the sensor, microcontroller, and GPS. The design of programming algorithm is done with Arduino IDE software.
The interface design uses a 20x4 LCD display to display the gravity acceleration value and store data on the storage media. The system uses a box made of iron and plate leveling to minimize measurement errors. The sensor test shows the ADXL sensor has a more stable value. The system is examined by comparing with gravity measurement of gravimeter A results in Bandung observation post.
The result of system test resulted the average of system correction value equal to 0. The system is expected to use for mineral exploration, water supply analyze, and earthquake precursor. Field test of the superconducting gravimeter as a hydrologic sensor. We report on a field test of a transportable version of a superconducting gravimeter SG intended for groundwater storage monitoring. The test was conducted over a 6-month period at a site adjacent to a well in the recharge zone of the karstic Edwards Aquifer, a major groundwater resource in central Texas.
The purpose of the study was to assess requirements for unattended operation of the SG in a field setting and to obtain a gravimetric estimate of aquifer specific yield. The experiment confirmed successful operation of the SG, but water level changes were small A gyro-stabilized platform leveling loop for marine gravimeter. The instrument was developed for regional surveys in deep ocean areas where high-resolution gravity measurements with accuracy 1 mGal are required.
Horizontal accelerations in the surge and sway directions are suppressed about 60 dB in the frequency range 0. This typically improves the quality of the gravity data before any processing corrections. The time required for stabilizing the platform at the beginning of a survey line or course change is about 3 min, which improves the data collection efficiency.
Sixteen traverses were run in the South China Sea to evaluate the loop performance. Platform motion tracks and gravity data from the survey were of satisfactory quality. According to analyses of 16 sets of calculated errors, the root mean square repeatability of the pitch and roll off-level angles were less than 10 and 20 arc sec, respectively, with a horizontal acceleration of about 50 Gal.
Errors derived from the inability of the platform to maintain perfect sensor leveling during the survey cruise were less than 0. Superconducting gravimeters reveal unprecedented details of changes related to volcanic processes. Continuous gravity measurements have been successfully carried out at a number of volcanoes around the world using spring gravimeters.
Nevertheless, these instruments do not provide reliable measurements when used in continuous mode for weeks or more, because they are influenced by environmental factors and are subject to instrumental drift. Accordingly, most studies of continuous gravity at active volcanoes have focused on the analysis of changes over time-scales of minutes to a few days. An alternative to spring gravimeters for continuous measurements is given by superconducting gravimeters SGs that feature a much higher precision and stability than spring gravimeters.
However, even the most portable SGs e. Indeed, they require AC power at the installation site and some kind of hut or vault to house the instrumentation. At Mt. Etna, the installation of a mini-array of three SGs distances of 3. To our knowledge, these are the first SGs ever installed on an active volcano. Signals from these instruments show hydrologically-induced components superimposed on small a few microGal gravity changes that are related to volcanic processes. Such changes, occurring over periods of minutes to weeks, would not be observable by spring gravimeters due to their intrinsic limitations regarding precision and long-term stability.
These errors would be reduced by further filtering at the expense of losing resolution. More investigation is ongoing in order to identify the sources of these systematic errors and feasible methods for compensation. A gravity anomaly estimation better than 2 mGal for a half wavelength at 6. Results show that it is a very appropriate device for airborne gravimetric surveys in a standard geophysical survey aircraft. More flight tests should be carried out to test the tolerance of the aircraft dynamics to prove its ability and accuracy for drape flights.
Preliminary analysis of the systematic error demonstrates that the gravity sensor drift in dynamic flights is the dominant error source. Other errors are the effect of the earth magnetic field, the misalignment estimation and the error model of the accelerometers. The authors are grateful to Shaokun Cai, Xian Li and Bing Luo for their constructive suggestions and insightful comments. Acknowledgment is also given to Yuanxin Wu and Tao Li for their pleasant and fruitful talks on related topics.
National Center for Biotechnology Information , U. Journal List Sensors Basel v. Sensors Basel. Published online Jul 6. Find articles by Yangming Huang. Find articles by Meiping Wu. Find articles by Kaidong Zhang. Author information Article notes Copyright and License information Disclaimer.
This article has been cited by other articles in PMC. Abstract Inertial navigation systems and gravimeters are now routinely used to map the regional gravitational quantities from an aircraft with mGal accuracy and a spatial resolution of a few kilometers. Keywords: airborne gravimeter, strapdown inertial navigation system, differential GPS. Introduction The Earth's gravity is one of the basic forces that affects everything on the earth.
Principle of Strapdown Airborne Gravimetry Before the introduction, several reference frames will be presented and their relationship is shown in Figure 1. Open in a separate window. Figure 1. Gravimeter Overview In this section, an overview of the SGA-WZ is given with four subsections including system components, sensor performance, static test and road test. Figure 2.
Sensor Performance Airborne gravity has stringent requirements for the performance of the sensors. Table 1. The performance of the RLGs. Table 2. The performance of the accelerometers. Static Test Test is a necessity to validate the performance after the triad is put together. Figure 3. Road Test Airborne flight test is pretty expensive and time consuming whereas road test is an efficient and cheap approach to test the performance of the SGA-WZ in a relative low dynamic cases. Table 3. System Damping In strapdown cases, all kinds of dynamic motions as well as the aircraft engines cause high frequency noise, which consequently affects the accuracy of the gravity sensors severely.
Integrated Navigation Technology The error of the measurement of the specific force comes mainly from the accelerometers and the attitude misalignment. Figure 4. Figure 5. Figure 6. Figure 7. References 1. Torge W. Verdun J. The alpine Swiss-French airborne gravity survey. Lacoste L. LaCoste and romberg stabilized platform shipboard gravity meter. Berzhitsky V. Elieff S. Boedecker G. In: Flury J. Observation of the Earth System from Space. Olesen A. In: Siderius M. Volume Jekeli C. The effect of Earth's gravity on precise short term, 3D free inertial navigation.
Bruton A. Deurloo R. University of Porto; Porto, Portugal: Forsberg R. In: Sideris M. Schwarz K. Support Center Support Center. The repeat line internal RMSE can be used as the principle to illustrate the performance of certain airborne gravimeter and the cross point internal RMSE can evaluate the quality of the large area airborne gravimetry task.
Another method, the external RMSE, can be applied to compare the data from certain airborne gravimeter with reference data to testify the performance of gravimeter in a different way from the internal RMSE. However, when the reference data or extra flights are not available, all these methods cannot directly evaluate the quality of single survey line as well. The flow chart of strapdown airborne gravimeter data processing is shown in Figure 1 [ 9 ]. The strapdown airborne gravimeter can be regarded as a multiple inputs single output control system. The inputs of this control system are from inertial sensors and GPS, whereas the output is the gravity disturbance.
As a control system, the strapdown airborne gravimeter also has a limited measurement ability and is related to the noise level of the system inputs. In the Equation 5 , the acceleration information of vehicle comes from GPS. The specific force and the transformation matrix rotating from b-frame to n-frame could be obtained from SINS. The accuracy of GPS was able to meet the requirement of airborne gravimetry in current condition [ 1 , 2 , 3 ], therefore the main error in strapdown airborne gravimeter comes from the inertial sensors.
Considering the differential form of Equation 5 and only keeping the items related with SINS, the error model in three directions is shown in Equation 6 [ 23 ]. Only considering the error in down direction, Equation 6 has shown that the accuracy of attitude measurement and specific force measurement are the key factors in strapdown airborne gravimetry.
The error model of three-axis accelerometers unit in b-frame is shown in Equation 7 , in which the error factors impacts the measurement through specific force inputs. In addition, gyro error factors influence the attitude measurement in a similar way with accelerometers error factors impacting the specific force measurement. So, larger input means larger measurement error in accelerometers when most of the error factors stabilize at certain levels, which is the same in the gyros.
Furthermore, larger specific force errors from accelerometers and larger attitude rate errors from gyros will lead larger specific force errors and attitude errors in navigation-frame, which will eventually negative impact the airborne gravimetry result. In particular, airborne gravimetry mainly measures the change of gravity disturbance during the flight and is a relative measurement way, the bias of which is corrected by base point correction. Therefore, establishing the model between stability of airborne gravimeter inputs and gravimetry quality is a feasible and reliable way to estimate single line quality of airborne gravimeter.
A case is that the airborne gravimeter performs better in the smooth airflow condition than in the turbulence airflow condition. Totally speaking, stability of roll angle, pitch angle, north specific force, east specific force and vertical specific force not only can describe the stability of flight but also have obvious relationship with gravimetry quality which can be regarded as the determinants in the estimation model.
To estimate the airborne gravimetry quality by the five determinants, the repeat line flight is also needed to establish the assessment model. However, the purpose of repeat line flight here is different from the common flight, which aimed at building the connection between airborne gravimetry quality and the determinants. The standard of quality assessment is established by repeat line flight, which is like the use of high precision three-axis turntable in SINS calibration. Once the estimation model between determinants and internal RMSE were established from repeat line flight, it can be used in other survey tasks to get estimated quality of airborne gravimetry.
For the intuitive idea that the quality of airborne gravimetry should be decided by the gravimeter itself, a quality assessment method for strapdown airborne gravimetry is developed. The actual data used in method demonstration was collected during a regional gravimetry by SGA-WZ02 airborne gravimeter. The following part will introduce the survey conditions in detail. Three GPS receivers were used for the differential kinematic positioning, one of the receivers was located on the airplane and the other two were located on the roof as ground stations.
The gravimetry system was mounted on a Y aircraft, which was a fixed-wing aircraft without an autopilot. The detail characters of flight were shown in Table 2. The Y is shown in Figure 3. The survey area was about km 2 , in which the longitude range was 0.
Download Gt 1A Inertial Gravimeter System Results Of Flight Tests 2002
Each survey line was about 40 km, whose ends were extended 5 km to eliminate the boundary effect of low-pass filter. In this regional airborne gravimetry task, the maximum distance between survey area and airport was less than km. The flight path from airport to survey area was fixed to collect repeat line data, which could be called quality assessment line.
Figure 4 is the path of certain flight and path of quality assessment line is red highlight. To cover the survey area, there were 82 north-south lines and 8 east-west cross lines. The interval of north-south lines was m and the interval of east-west lines was m. In addition, there were also several experimental flights, for example undulated flights, during the survey. So, this survey totally contained 21 separate flights. Based on the cross-point internal quality assessment, the total accuracy of this survey was 2. After cross-point adjustment, the accuracy could reach 1.
Figure 5 shows the gravity disturbance and cross-point differences in the survey area after adjustment. The use of cross point internal RMSE could evaluate the truly accuracy of system and the total survey quality. However, as mentioned, cross point internal RMSE needs extra flight and more extra flights mean more comprehensive evaluation. When the cross points are not available, how to assess the single line quality should be considered, which means the quality assessment method is quite needed. Let assume that such a situation, the survey lines were added outside the arranged area where cross-over points were unavailable.
In addition, single line quality assessment method is also helpful in the airborne gravimetry quality control. In the next section, the relationships between quality assessment determinants and internal RMSE in this regional airborne gravimetry task were given by linear fitting. What needs to be noted is that the model was specially established for SGA-WZ02 while the method could be also generalized to other strapdown gravimeter. The error in strapdown airborne scalar gravimetry was mainly originated from the specific force errors in three directions and two horizontal attitudes errors.
To analyze the relationship between the stability of five determinants and gravimetry quality, standard deviation of five determinants and gravity disturbance internal RMSE in quality assessment lines should be calculated. For the reason of flight path conflict, several flights did not finish the original plan, so the data from 21 quality assessment lines in 13 flights was collected and applied in the model establishment which was shown in Table 3. From Table 3 , we can see that larger standard deviations of five determinants usually mean worse gravimetry quality, such as line and , and small standard deviations usually mean better gravimetry quality, such as line and The gravimetry results of 21 quality assessment are shown in Figure 6.
Most of quality assessment lines show good results and internal RMSE of these lines is 0. The correlation between the stability of five factors and internal RMSE is shown in Figure 7 , where x -axis is the standard deviation of determinant and y -axis is internal RMSE. The red line in each subfigure is obtained by linear fitting and the correlation coefficients are also given. Thus, it is reasonable to estimate the quality of survey line through the standard of five determinants. Based on linear fitting results in Figure 7 , relationship model between the five factors and internal RMSE is built by weighted method, which is shown in Equation 8.
The model residuals are shown in Figure 8. We can see that quality assessment model has shown good performance and the maximum residual is 0. However, the universality and applicability of model may be influenced by flight direction. It was because the stabilities of two horizontal specific force were related to the flight direction. So the north specific force and east specific force can be combined as one horizontal specific force.
Equation 9 has shown how we got horizontal specific force. Relationship between the horizontal specific force standard deviation and internal RMSE is shown in Figure 9. The correlation between horizontal specific force standard deviation and internal RMSE is also significant. Based on new linear fitting results, the estimation model is rebuilt, which is shown in Equation The residuals of new model are shown in Figure Combining the north specific force with east specific force, the applicability of this model is promoted and also has good performance.
Based on the strong correlation, the assessment model in SGA-WZ02 is obtained by weighted linear fitting. After fusion the north and east specific force, the effect of flight direction is eliminated to a large extent in quality assessment.
The model is specially established for SGA-WZ02 gravimeter assessing the gravimetry quality, but a similar model can also be obtained by other strapdown gravimeter during a gravimetry task if the same method is used. In the next section, data from other repeat lines will be used to testify the quality assessment model performance.