On the use of airborne gravimetry in gravity field modelling: Experiences from the AGMASCO project

Whys.

Chem .Eurth (A), Vol. 25, No. 1,pp. l-7,2000

Pergamon

0 2000 Elsevier Science Ltd All rights reserved 1464-1895/00/$ - see front matter

HI: Sl464-1895(00)00002-8

On the use of Airborne Gravimetry in Gravity Field Modelling: Experiences from the AGMASCO project L. Bastos’, S. Cunha’, R.Forsberg’,

A. Olesen’, A. Gidskehaugj,

L. Timmen4, U. Meyer’

‘Observathio Astrorhmico, Monte da Virgem, 4430 Vila Nova de Gaia, Portugal 2KMS, Rentemestervej 8, DK-2400, Copenhagen NV, Denmark 31nstitute of Solid Earth Physics, Alle’gaten 41, N-5007 Bergen, Norway 4GeoForschungsZentrum, Telegrafenberg Al 7, D- 14473 Potsdam, Germany ‘Alfred Wegener Institut, Columbusstrasse, D-27568 Bremerhaven, Germany Received 19 August 1999; revised 27 September

1999; accepted 4 October 1999

Important areas of the earth are still not covered by accurate gravity measurements. The gravity field may be determined by using different techniques but airborne gravity surveying is becoming the most powerful tool available today. One of the main problems in airborne gravity is the separation of the vertical accelerations acting on the airborne platform from the natural gravity signal. With the advances in DGPS techniques new prospects arise for gravity field recovery which are of great importance for geodesy. geophysics oceanography and satellite airborne gmvimetric MVigi3tiOIl. Furthermore, measurements depend not only on the determination of the position but also on the attitude of the aircraft. Inertial systems can provide attitude as well as information on short-term accelerations. which are more problematic for the gravimeter. A proper integration of these systems may allow a further improvement of the whole technique where the quality of both the accelerometers and the gyros is the key sensing element. In the scope of the MAST III Project AGMASCO, an airborne geoid mapping system was successfully implemented in different aeroplanes. The characteristics of the aeroplane and the flight parameters play a major role in airborne measurements. Within AGMASCO the airborne system was applied both in a close and an open ocean (Skagerrak, Fram Strait and Azores) areas. The system proved to be a powerful tool in a variety of conditions. The results obtained showed that an accuracy better than 2mGal over 5 to 6 kilometres can be achieved. Abstract_

Correspondence

to: L. Bastes

This was proven by comparison of the airborne data with ground truth and satellite data. This accuracy makes the system interesting for use in various applications including geophysical exploitation. Different hardware installations were experienced and the methods validated. Recovery of the gravity values directly from measurements with the Lacoste & Romberg air/sea gravimeter and from measurements with the inertial sensors was analysed. The potential of these sensors to recover gravity and the experience gained within this project are reported here. 0 2000 Elsevier Science Ltd. All rights reserved

1. Introduction

Airborne gmvity methods have the advantage of allowing a quick coverage of large regions of the world with significant economy over other methods. They can be also useful in filling the gap of satellite altimetry near coastal areas. The accuracy demands vary with the specific application: from 5 to 20 mGal over 1000 km for Geotectonics applications to 0.5 a 1 mGa1 over 1 to 10 km, for Geophysics. In geodetic applications an accuracy of 3 to 5 mGal over 5 to 200 km is enough for regional geoid improvement. As a consequence of developments in GPS positioning, suflicient accuracy for airborne gravimetry can be achieved. Gravity information is obtained from differencing accelerations observed by a gravimeter (which measures gravity plus aircraft accelerations) and

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L. Bastes et al,: Use of Airborne Gravimetry in Gravity Field Modelling: Experiences from the AGMASCO project

platform accelerations derived from GPS observations which do not include gmvity. Relevant works from Brozena (1992. 1993) Forsbcrg and Kenyon (1994) Kingele et al. (1995) with scalar gravimetry and from Schwarz et al. (1992, 1996) and Salychev et al. (1995) with vector gravimetry should be mentioned The pioneering adaptation of marine gravimetry methods for airborne use started with Brozena whose results have proved the feasibility of the use of airborne gravity and altimetry for wide area geoid determination. Exploitation of INS (Inertial Navigation System) techniques for airborne gravimetry was started by the University of Calgary and the Inertial Technology Centre in Moscow. First results using INS techniques for recovering the gravity field using the RISG concept are from in 1996. Within the scope of the Airborne Geoid M&ping System for Coastal Oceanography (AGMASCO) project the development of a new airborne gravimetry/altimetry system was accomplished The main goal of AGMASCO was the development of a combined airborne gravity/airborne altimetry system for geoid modelling and sea-surface topography (SST) determination The system was designed to be especially suited for application to coastal and shelf regions. The primary areas of interest defined for this project were: the Skagerrak area in the North Sea and the Azores region in the North Atlantic. Experiences from this project and the main results obtained in the Azores area are reported here. The feasibility and accuracy of the airborne gravity method is discussed.

An Qptech 501 SX laser altimeter. acquired by Kort & Matrikelstyrelsen; A data logging unit, developed within an industry contract of GFZ (GeoForschungsZentrum); A data logging unit, developed at the Astronomical Observatory of the University of Porto; A strapdown accelerometer system constituted by three QA-3000-030 accelerometers, produced by AlliedSignal Aerospace. (The mounting device including the electronics has been developed at the Bavarian Academy of Sciences/BEK). In order to meet the established goals, the demands concerning the accuracy to be achieved for position, acceleration and orientation of the different sensors were the following: 0.00 lcm& for the vertical acceleration of the gravimeter, 10 cm for the position of the a&meters for SST determination and better than 1 meter for the gravimeterJ better than 0.1 degree for the attitude determination. The system was successfully installed in different aeroplanes: a Dormer 228 from the Alfred Wegener Institute in Bremerhaven for the Skagerrak survey in September 1996 (For&erg et. al. 19%); a CASA Aviocar C2 12, from the Portuguese Airforce for the Azores survey in October 1997 (Figure I). Furthermore the basic system was also instakd in a Twin - Otter (OY-PQF) aeroplane from Greenlandair for a survey done in GreenIand in the summer of 1997 (For&erg et. al. 1998).

2. The AGMASCO system One important aspect of the AGMASCO system is its modularity and the versatility in order to allow easy installation and use with different hinds of aeroplanes. Hardware and software developments were done to accomplish these goals and prototype devices were combined with some off the shelf equipment. The hardware installation for the Azores campaign involved the following devices: A gravimeter system based on a LaCoste & Romberg marine gravimeter (S-56) belonging to the Alfred Wegener Institute in Bremerhaven, modified for airborne use by ZLS corporation; A navigation system based on three GPS receivers (two Trimble 4000 SSI and one Ashtech 212) and a Litton 200 IMU (Inertial Measurement Unit) from the Astronomical observatory of the University of Porte; A radar altimeter. a prototype developed by the University of Stuttgart, Germany;

Radar altimeter

Fig. 1. Hardware installation in the CASA

L. Bastes et al.: Use of Airborne Gravimetry in Gravity Field Modelling: Experiences from the AGMASCO project QFZ central data logging unit. time frame 1s6PS time 1 _. _-. _._-..__. __-, . .. “__IIx, ___^-__l___ _... -L ~” .( ..“_..-..I.. OPS ! .._...”__-_ I....... v”4”

Trlmbk 4Mx)SSi

INS LN72

Tumble 4oM) ss

Trimble 4000 SSI

IL

Ashtech 212

Fig. 2. AGM.4SCO system architedure

3. The

INS LN72

II

IMU LN200

for the Azores campaign

Azores CAMPAIGN

The Azores airborne survey was done with a CASA Aviocar C212. from the Portuguese Airforce. The airborne campaign was carried from October 3 to 20, 1997 with a total flight time of 70 hours. The profiles to be observed were chosen taking into account the direction of the main fault crossing the Archipelago. the Mid-Atlantic Ridge, and the location of some known submarine features. For the comparison of the airborne and the satellite measurements, the direction of the tracks of the EBS2 and TOPEX satellites during the observation period were also flown.

3

much as possible. Flight velocities varied between 200 and 270 km/h with altitudes from 150 to 250 meters over the profiles. The use of the autopilot was essential to guarantee a smooth flight with almost constant speed and orientation of the airplane. Aircraft banking angles were limited to a maximum of 10” during turns mainly to optimise the gravimeter operational conditions. In order to have ground truth measurements and also for the calibration of the airborne measurements. land and sea surveys were also conducted: land surveys took place from October 3 to 30 and a marine survey from October 17 to 30. The land measurements included the establishment of GPS and gravity ties to tide-gauges and absolute gravity stations existing in the islands. To support the positioning of the aeroplane by DGPS. five reference stations were used for this campaign: reference GPS stations were located at Flores. Faial, Terceira and S. Miguel islands. For the calibration of the altimetry data flights over the runway and the apron were performed. A kinematic DGPS smvey of the apron was carried out and a detailed map of airfield heights produced with these data. All gravity flights in Azores were processed with gravity base values determined by land gravimeter ties to the absolute station in Faial, (Bastos et al. 1996). The GPS coordinates refer to the ITRF-96 geodetic reference system (Fernandes et al. 1998). The shipbome measurements, over some of the profiles flown, were surveyed with the oceanographic vessel from the University of Bergen, “R/V Hakon Mosby”. The primary purpose of the shipbome measurements was to provide marine gravimetry for evaluating the airborne gravity measurements. The validation of the results is done by direct comparison of the data from the different sensors as well as comparison with ground truth data and satellite altimetry.

4. Data processing

Fig. 3. Profiles obsevzd

Flight conditions have not always been ideal, as there were several days with unstable atmospheric conditions both over the ocean and over land As a consequence the flight characteristics were adapted to avoid turbulence as

4.1 GPS GPS baseline lengths in the Azores exceed 300 km. To achieve the necessary accuracy, advanced DGPS processing methods must be applied. Due to the different requirements for the determination of positions, heading and accelerations of the different sensors onboard the GPS data collected from the three antennas installed in the aircraft (two antennas along the fuselage - AirF and AirB - and one on the wing - AirW) were processed using several strategies. AirB was processed against one or more reference stations on ground for computing positions and

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L. Bastes et al.: Use of Airborne Gravimetry

in Gravity Field Modelling: Experiences

instantaneous velocities. Difficulties arise from large atmospheric perturbations due to the long baselines in the Azores campaign. To overcome these problems precise satellite orbits and good models for the ionospheric and tropospheric perturbations are needed. The use of data from different reference stations can be very useful to improve the tropospheric model. Multiple antenna/receiver enables aircraft GPS measurement errors to be detected and eliminated. The inertial sensor, giving attitude with high precision and good relative positions for medium time spans, was used to support GPS calculations. This is particularly helpful in critical situations like cycle-slips occurring simultaneously in several satellites or during satellite constellation changes when the aircraft is at a long distance from the reference station, (Ctmha et al. 1999). 4.2 Inertial The inertial data is necessary to refer the tilted altimeter measurement path to the vertical and to correct the lever arm effect due to the position offset of the sensors. For this purpose the Euler angles were obtained from integration of the IMU LN200 inertial data, stabilized by GPS measurements in a Kalman filter. The height above ellipsoid is needed and the heading estimates obtained from GPS are very helpful because heading and position errors are weekly coupled in smooth flights. To keep the Schuler effect to a minimum it is necessary to have absolute position and velocity measurements. This is also needed for the continuous alignment of the inertial platform. Position and velocity absolute readings also enable the estimation of inertial sensors biases and vertical gravity anomaly. One application of the multi-antenna configuration is the determination of an estimate of the attitude angles. In this particukn case, using the two antennas mounted on the body of the aircra& the 3D direction from one antenna to the other was determined (with ambiguity fixing) and used in the continuous alignment of the IMIJ. A good heading control can be achieved by computing the relative baselines between the three antennas installed in the aircraft therefore AirB to AirF, AirB to AirW and AirF to AirW baselines were computed. Doppler readings are needed to report measurements to the same instant. In this case ionospheric and tropospheric perturbations are irrelevant and precise satellite orbits are not needed. The processing algorithm is based on a Kalman Filter. In order to obtain better estimates for the state vector and to eliminate the response delay of the Kalman filter, especially for the state variables with slow dynamic behavior, smoothing is needed. The smoothing algorithm uses all measurements (past and future) to estimate the state at each instant. Details of the

from the AGMASCO

project

implementation of this algorithm can be found in Tome et al (1999). Preliminary results from the altimetric and gravimetric survey point out to an estimation accuracy of 0.01’ of roll and pitch angles. In non-turbulent flights and during tracks, the heading accuracy is mainly determined by the GPS differential vector observations smoothed by the vertical gyro which is better than 0.05“. This IMU/GPS integration exhibits good performance in aircralt attitude determination. 4.3 Gravimeter For the processing of the gravity data, for the computation of the geoid, standard methods of physical geodesy (collocation and spherical FFT) were applied. Atmospheric correction was applied and EGM96 was used for remove/restore technique. The basic quantity needed for geoid computation is the gravity anomaly (free-air anomaly), which can be measured at altitude by a combination of gravity and GPS measurements. The airborne gravity data is evaluated in comparison with ground truth data upward continued to 1200 A elevation (flight level) by using FFI methods after griding the available ship and land data. For the attenuation of normal gravity with altitude, a constant gradient of 0.3086 mGal/m was used. This can be justified by the low flight elevation (below 1200 ft) used in the AGMASCO Azores campaign. Vertical accelerations of the airplane are separated from gravitational accelerations by standard geodetic kinematic DGPS positioning methods (Xu et al. 1997). The disagreement between laser and GPS accelerations varied from 1.1 to 2.3 mGal after filtering. In the Skagenak campaign these figures were 0.4 to 1.2 mGal (Olesen et. al. 1997). The less good agreement in the Azores campaign can be expected in an open ocean area were long waves can generate a signal in the altimetry data, which was not removed by the filter. Nevertheless this is a preliminary result which is expected still to be improved with a more careful processing of the altimeter data.

Units: mGa1

R.m.s. cross-over

Max. absohrte

error

difkmce

Before adjustment

6.1

17.4

Afkr airborne only

3.5

7.0

4.1

9.3

bias adjustment After joint marine/ airborne bias adjustment Table 1. Results of cross-over

analysis,

(58 cross-avers).

I... Bastos ef al.: Use ofkrb‘orne

Gravimetry in Gravity Field Modelling:

Experiences

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The final track data were subjected to a bias-only cross-over adjustment, where lines with two or more crossings are adjusted to minimise the crossing difference. The results are given in Table 1. 5. Gravity

anomalies from gravimeter data

The comparison of airborne gravity at cross-over points is done by free-air anomalies rather than actual gravity. since anomalies to first order will be independent of the actual flight elevation. Due to noise in both-gravity and GPS measurements. all quantities must be suitably lowpass filtered. Lowpass filtering plays a fundamental role in airborne gravity processing. The objective of the filtering is both to account for the difference in filtering inherent from the data, and to remove the high frequency noise masking the gravity anomaly signal. The gravimeter data acquisition system uses a 1 sec. boxcar filter on internal 200 Hz data, whereas the inherent filtering of the accelemtions derived from the GPS positions depends on the GPS processing software. and the algorithm applied for differentiation. This difference in filtering has little impact on the linear terms in our processing algorithm because of the heavy final filtering. But the nonlinear terms, mainly represented by the tilt correction. are quite sensitive to the initial filtering. A second order Butterworth filter was used with a fullwidth (half-wavelength) resolution of approx. 100 sec. corresponding to an along-track resolution of approx. 6 km. In the conventional bias adjusted data set. some of the lines were biased when compared to the marine survey. This was due to both uncertain spring tension synchronization for some lines and a weak survey geometry concerning cross-over analysis. To rectify those problems the marine AGMASCO gravity data were used in a joint marine/airborne bias adjustment of the final airborne data set. Main error sources that affect the quality of the gravimetric results are due to: accelerometer bias; scale factor; resolution; vehicle dynamics: sensor orientation (stabilization); misalignment; separation of gravitational and non-gravitational forces. Long term stability due to the LaCoste & Romberg gravimeter has proven to be operational under the most common weather conditions. The slightly worst Azores results can be due to: spring tension synchronization and a weak survey geometry concerning cross-over analysis: meteo effects and open ocean conditions: the gravimeter itself

Fig. 5. Azores airborne pvity,

free air anomaly. Countor interval is 10

mGal.

6. Gravity anomalies from &MUdata Due to the fact that during the processing of the IMU data the vertical gravity anomaly was estimated by the Kalman filter. in order to improve mainly the position estimates. it was found out that interesting results could be obtained with the low cost Litton LN-200 device for recovering values of the gravity field. This situation was explored and the results are presented here.

-_

L. Bastes et al.: Use of Airborne Gravimetry in Gravity Field Modelling: Experiences from the AGMASCO project

DiUuemebelwmGPS+MJmdULRAircm~ Dillrace ti GPS*LILI and L&R Ship

.:

*___:

37.5 Ladhuh (da@

3a

1 38.5

Fig. 8. Diferences between the gravity anomaly solutions obtained with GPS+INS and the gravimdas

installed on board the ship and the

airuafl.

Ladhh (dsn)

b) Fig. 6. Gravity anomaly estimates obtained with GPS+INS without

The IMU measured gravity anomalies obtained with this method show a good agreement with marine results. IMU sensors perturbations (biases, scale factors and temperature dependencies) need to be further studied. This is needed to improve IMU ability to measure gravity anomaly more accurately. Considering the suitability of the inertial sensor to measure the shorter wavelengths (limited by GPS accuracy), combination of the IMU with the gravimeter (which exhibits better stability over long periods of time) is an issue that can contribute for the improvement of the final geoid solution.

filtering (a) and filtered with a 200 set second-order But&worth low pass filter (b).

30

7. Comments

7

I --

GPS+MU LKZW Lmsh&Ruhrg-tip

‘1 ,,

i__

i 37

Fig. 7. Comparison btzween the soluticms obtained with GPS+INS and the La~wste & Romberg gravimekrs installed respedively on the ship and onboard the airaatl.

From the experience gained within the AGMASCO project, some key issues which are fundamental for the performance of the system can be mentioned: precise velocity and acceleration derived from GPS; platform stability; keep alignment errors and gyro drifts stable; location of the GPS and INS (small arms); synchronization; stable flight with low speed and altitude; separation of flight tracks; smooth atmospheric conditions. Some of problems that must be solved to improve the whole method: - Filtering periods below 100 seconds are needed to achieve the necessary accuracy with low aircratt velocities. Low pass filtering only works if the period is longer than the phugoid motion therefore knowledge of the phugoid motion of the aircraft, which generates a

L. Bastos

et al.: Use of Airborne

Grawmetry

in Grawty

quasi-systematic horizontal, acceleration is necessary (typical periods are < 100 sec.). - Measurement noise in the high frequencies masks the low-amplitude gravity signal and is the main limiting factor for short wavelength resolution; - Aircraft dynamics; - Long term stability of the gravity sensor: - Sensing small short-periodic signals of the gravity field (below a few tenths of mGa1).

8. Conclusions and future prospects The AGMASCO system allows determination of geoid and sea surface height/topography. Surveys can be done over sea or over land. Plight pattern and flight parameters can be selected although one should be aware that the flight conditions are a key element for the quality of the final product Precise DGPS positioning is a determinant factor for the performance of this system. IMU/GPS integration shows a good performance in the determination of the aircraft attitude. The results obtained with the LN 200 are very promising. lmproved regional geoid is obtained by combining the aerogravity data with existing geoid. With geoid and GPS/altimetry sea surface topography can be computed. Those results will provide the geometrical/physical base for the validation and improvement of sea surface models. Combination of satellite and airborne measurements will allow the worldwide determination of the geoid with a few centimetres accuracy.

Acknowledgements.

AGMASCO

was a proj&

funded by the European

Commission

within the frame of the MAST III (MArine

Technology)

program in the period 1996

MAS3

Science and

1999 under contract number

CTOOl4195.

References Bastes. I,., Osono. J.. Hein. G., Makimn. J.. Kakkuri, J., Torrcs. A., Kol, H.. Forjaz V., Alves. J., AIves. M., Absolute Gravity measurernmts in mainland Portugal and the Azores islands in 1992

Field

Modelling:

Experiences

from the AGMASCO

project

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and 1994. Symposium G4 of the 2lst General Assembly of the EGS. The Hague, Holland, 6-10 May 1996. Brozena. J. M.. M. Peters and R. Forsberg Dirti measurement of absolute sea-surface height Tom an aircraft. Gecphysical Research Letters. vol. 20.. no. 9, pp. 875-878. 1993. Brozena, J.: The Greenland Acrogeophyscis Expcrimcnt: Airborne Gravity, Topographic and Mat@ic Mapping of an entire Continent. In: Colombo (ed.): From Mars to GreenIanmd: Charting Gravity with Space and Airborne hrstruments. IAG Symposium Series 110, pp. 203-214, Springer Valag 1992. Cunha. T., Tome, P.. Cunha, S.. Bastes, L.(1999), Long Base Real Time Positioning of Airborne Sensors. Proceedings of VISION 2010 present and Future. San Diego. Calit&nia. January. 25-27. 1999. Fernandes R., Bastes L., Osorio J.. Baptista P., Hein G.(1998), Recent uustal dynamics in the Azores archipelago derived horn repeated GPS observations. Paper presented at the EGS General Assembly. Nice. France. April, 20-24. 199X. Forsberg R., Olescn, A. Timmen. L., Nesemann, M., Su, G.. Bastes. L. Cunha. S.. Gidskehaug A.. Meyer. I!.. Boebel, T.. Hehl. 6.. Solheim. D., Airborne Gravity in SkagaTak and elsewhere: the AGMASCO project and a Nordic outlook. Proceedings of the Nordic Geodetic Commission General Assembly. Gavle. May 1998 (In Prtss). Forsbag. R. K. HehI, L. Bastes. A Gidskehaug, 1:. Meyer. Development of an Airborne Geoid Mapping System for Coastal Oceanography (AGMASCO). In: J. Segaua. II. Fujimoto and S. Okubo (ed.): Gravity, Geoid and Marine geodesy. IAG Symposium Series 117. pp. 163-170. Springer Verlag lYY6. Forsberg R.. Iienyon. S.. Evaluatmnand downward continuation of airborne gravity data the Greenland example. Proc. Int. Symp. on Kinematic Systems in Gecdesy. Geomatics and Navigation. Banff. Canada, Ugust 30-September 2, pp.531-538.1994. Klingclc E. and II. Halliday. Airborne Gravity Survey of Switzerland. in: Proc. L\G Symposium G4, Boulder. Co., Special Report 60010, Dept. of Geomatics Engineering University of Calgary, pp. 109-115.1995. Olesen A 1.. R For&erg A Gidskrhaug, Airborne gravimetry using tbr IaCoste & Rombcrg gravim&r an error analysis. In: M.E. Cannon and G. Iachapelle (eds.). Proc. Int. Symp. on Kinematic Systems in Geodesy, Geomatics and Navigation. Banff Canada. June 3-6,1997. PublUniv. of Calgary. pp. 613-618,1997. ScbwarG K. P., Airborne gravimtiry and the boundary value problem. Ledure Notes, International Summer School on Mathematical Geodesy, Coma, Italy. 1996. Scbwarz E. P.. 0. Colombo, G. H&r. E. Knickmyer, Requiremrllts for airborne vector gravimetry. In: Colombo (ed.): From Mars to Greenland: Charting Gravity with Space and Airborne Instrummts. IAG Symposium Series 110, pp. 203-214. Springer Valag 1992. Salychevl S., Voronov V., Airbcme gravim&ry application ofthe ITC2 inatiai survey system. UCGE rep. 30012. Department of geomatics Fngineering IJnivasity ofCalgary. 1995. Tome. P.. Cunha. S., Cunha, T., Bastes. L.(1999). Integrating Multiple GPS Receivers With A Low Cost IMU For Aircraft Attitude D&rmination. Proceedings of VISION 2010 present and Future. San Diego. California. January, 25-27. 1999. Su. G. Bastes. L.. Timmen. I,.. GPS Kmematic Positioning in .-\GMASCO Campaigns - Strategic Goals and Numerical Results. Proceedings of ION 97, Kansas City. Septtnnbcr 19Y7.