GB2174210A - Airborne gravity surveying method - Google Patents

Abstract

A method is disclosed for airborne gravity surveying, in which the airborne vehicle is stabilized with respect to speed, direction of heading and altitude, and in which the gravity meter is of adequate sensitivity and provides signals that are recorded at a high sample rate on a magnetic tape, so that the aircraft position is computed using a satellite positioning system or a multi-range navigation system that is geodetically located. <IMAGE>

Description

SPECIFICATION Airborne gravity surveying method This invention relates to a method for airborne gravity surveying with which greater accuracy is obtained than has heretofore been available.

It has been proposed to use airborne vehicles for gravity surveying as pointed out in Reviews of Geophysics, Vol. 5, No. 4, November, 1967, pages 447 to 526, published by The American Geophysical Union of 2000 Florida Avenue, N.W., Washington, DC 20009 commencing at page 520 to 524 for fixed wing airborne vehicles.

A review of the airborne gravity surveying activities with respect to helicopters can be found in "Airborne Gravity Surveying, Technical Information", published March, 1981, by Carson Geoscience, Perkasie, Pennsylvania, commencing at page 1-1.

Various patents have been issued which set forth apparatus for gravity surveying.

Boitnott in U.S. Patent Nos. 3,011,347 and 3,038,338: Gustafsson U.S. Patent No.

3,180,151: and Brede U.S. Patent No.

3,447,293; and Hutchins Canadian Patent No.

652,757 disclose instruments for measuring gravity or derivatives of gravity of the earth's gravity field, but do not disclose practical systems for accurate airborne surveying.

La Coste U.S. Patent Nos. 2,293,437; 2,377,889; 2,964,948; 2,977,799; 3,474,672; Heiland U.S. Patent No.

2,626,525; Worden U.S. Patent Nos.

2,674,887 and 3,211,003; Graf U.S. Patent No. 3,019,655; Emmerich U.S. Patent No.

3,033,037; Slater U.S. Patent No. 3,062,051; Hodge et al. U.S. Patent No. 3,194,075; Ward U.S. Patent No. 3,495,460; Kuzivanov et al. U.S. Patent No. 3,501,958; Wing U.S.

Patent Nos. 3,546,943 and 3,583,225, show navigation and/or gravity meters but do not show practical systems for accurate airborne surveying.

Klasse et al. U.S. Patent No. 2,610,226; Jensen U.S. Patent No. 2,611,802; Rumbaugh et al. No. 2,611,803; and Pittman U.S. Patent No. 4,197,737 show method and/or apparatus for conducting surveys for geophysical or magnetic explorations but do not discuss or treat airborne gravity surveying.

The proposals heretofore made for airborne surveying do not provide adequate stabilization of the aircraft, with respect to speed, do not provide for level flight, do not provide accurate navigation and steering, and do not with other requirements for accurate surveying, measure the gravity, and have other shortcomings.

In accordance with the invention an improved method is provided for airborne gravity surveying in which an airborne vehicle is stabilized with respect to speed, direction of heading and altitude, its operation controlled by auto-pilot or manually, and in which a gravity meter of adequate sensitivity is used that provides signals that are recorded at a high sample rate on magnetic tape, in which the aircraft position is computed using a multirange navigation system, or by a satellite positioning system.

It is the principal object of the invention to provide an improved method for airborne gravity surveying with which greater accuracy of computed and recorded data is obtained.

It is a further object of the invention to provide an improved method for airborne gravity surveying in which the airborne vehicle is maintained at a selected level, and is stabilized as to speed and direction.

It is a further object of the invention to provide an improved method for airborne gravity surveying in which the gravity meter is controlled as to its sampling and specifically the sampling rate.

It is a further object of the invention to provide an improved method for airborne gravity surveying in which the position of the airborne vehicle is precisely known at all times.

It is a further object of the invention to provide an improved method for airborne gravity surveying in which the instruments are located and carried by the airborne vehicle at a stable temperature and preferably in a clean environment.

It is a further object of the invention to provide an improved method for airborne surveying in which a probe is located on the airborne vehicle in such a manner as to obtain accurate measurement of the static air pressure.

It is a further object of the invention to provide an improved method for airborne gravity surveying wherein accurate measurement of the distance of the airborne vehicle from the ground is obtained.

It is a further object of the invention to provide an improved method for airborne gravity surveying wherein the gravity meter is constructed so that it operates more efficiently in the airborne environment.

It is a further object of the invention to provide an improved method for airborne gravity surveying which provides a magnetic digital recording system with a high degree of sensitivity, variable sampling rate, and a capability of reading the magnetic tape in flight after data has been recorded thereon.

It is a further object of the invention to provide an improved method for airborne gravity surveying which records multiple ranges on a magnetic tape from an electronic navigation system for enhancement of the position accuracy.

It is a further object of the invention to provide an improved method for airborne gravity surveying wherein the operation of the aircraft is controlled by an auto-pilot.

It is a further object of the invention to pm- vide an improved method for airborne gravity surveying wherein the vehicle is guided by signals received from a global satellite system, which can be linked to an auto-pilot for operation.

It is a further object of the invention to provide an improved method for airborne gravity surveying which describes the method of data collection, to provide the necessary parameters for computing accurate gravity measurement.

It is a further object of he invention to provide an improved method for airborne gravity surveying wherein simultaneous recording of magnetic and gravity data is obtained.

It is a further object of the invention to provide an improved method for airborne gravity surveying which describes the method of preplotting the required flight path, and which requires the airborne vehicle to comply with such preplotted flight path.

It is a further object of the invention to provide a method for airborne gravity surveying which provides for a grid pattern of lines to be flown, which covers the gravity anomaly of the area to be surveyed.

Other objects and advantageous features of the invention will be apparent from the description and claims.

The nature and characteristic features of the invention will be more readily understood from the following description taken in connection with the accompanying drawings forming part hereof in which: FIG. 1 is a block diagram for purposes of understanding the invention; FIG. 2 is a pictorial view of a satellite system used with the method of the invention; and FIG. 3 is a pictorial view illustrating an airborne vehicle and a portion of the satellites of the system of FIG. 2.

It should, of course, be understood that the description and drawings herein are illustrative merely and that various modifications and changes can be made in the structure disclosed without departing from the spirit of the invention.

Like numerals refer to like parts throughout the several views.

Referring now more particularly to FIGS. 1 to 3 of the drawings any suitable airborne vehicle may be employed including fixed wing aircraft, lighter than air aircraft, and helicopters.

If a helicopter is employed one suitable helicopter is a Sikorsky model 61, which is preferably equipped with internal fuel tanks of a capacity of up to about 8 hours of flight. The Sikorsky helicopter is preferably provided with a uniquely tuned automatic flight control system that uses collective lift to control the vertical movement of the vehicle during flight without changing the pitch.

For any airborne vehicle it is essential that it have a flight control system that controls the vertical movement of the vehicle during flight and preferably to limit the elevation to + 10 feet or less in thirty seconds of time, from a selected predetermined level.

It is preferred to employ an environmental chamber on the vehicle which is maintained at a stable temperature and provides a clean environment.

A combined use of inertial navigation and electronic distance measuring equipment may be used to provide the latitude, longitude, and speed control continuously for the pilot. This total navigation package allows the airborne vehicle to be manually flown within a speed range of five knots and along a predetermined flight path to within a few hundred meters.

If a helicopter is employed the rotor blades are precisely tracked and aligned for smoothness of flight.

A probe for measurement only of static air pressure and not subject to ram pressure may be provided to aid in measuring the elevation of the plane, and is so located that only static air pressure is measured, and for a helicopter it may be on a retractable probe located in front of the helicopter or a few feet above the helicopter blades at the center of rotation of the blades. The probe is in communication with the environmental chamber.

In order to determine the altitude of the airborne vehicle a combination of radar or laser and sensitive pressure measurements can be used to establish the altitude of the aircraft to within ten feet. Suitable radar equipment is available from Honeywell, Inc., Minneapolis, Minnesota. Suitable laser equipment is available from Spectra Physics, Inc., Mountain View, California.

Suitable equipment for measuring absolute pressure is available from Rosemount, Inc., Minneapolis, Minnesota.

Relative measurements are made and recorded of the altitude to an accuracy of the order of 0.5 feet.

Two types of pressure altimeters are disclosed, and since it is important to achieve extreme accuracy of measurement, multiple altimeters may be used to provide a means to check each other and to measure acceleration.

One type of pressure altimeter is an absolute device that measures the pressure and changes that occur in the atmosphere. Ground based absolute altimeters will record the changes at ground level, and all of these measurements are combined to establish and record pressure surface changes in the survey area.

The second type of pressure altimeter comprises two bi-directional narrow range pressure transducers which are temperature stabilized in the environmental chamber, and are used to measure and record minute changes in the aircraft altitude. Such transducers are available from Setra Systems, Inc., Natick, Massachusetts. A static air pressure source of non-turbulent air is provided to these sensors through the pressure probe, that is constructed to measure no ram pressure, only static air pressure.

In order to provide a record of the accumulated data, all data is recorded at a one second or other desired interval on magnetic tape. All the analog data channels are recorded at a sensitivity of the order of 0.0001 volts. Suitable equipment for this purpose comprises digital system such as the Lancer Electronics Model 4570, available from Lancer Electronics Corp., Collegeville, Pennsylvania, interfaced to a Kennedy Model 9800 tape transport available from Kennedy, Inc:, Altadena, California. The information is read after write on the tape and displayed on a paper tape reader. A digital voltmeter is available to visually monitor any channel of data.

In order to control the navigation of the airborne vehicle a line of sight electronic distance measuring system using multiple ground stations may be employed. One suitable type of such a control system is the Motorola Miniranger, available from Government Electronics Division, Motorola, Inc., Scottsdale, Arizona.

Another system which may be used for operating and controlling the navigation of the airborne vehicle is known as the NAVSTAR Global Positioning System, as described in detail in Microwave System News, November 1984, Volume 14, Number 12, at pages 5459, 62, 65, 67, 68, 70, 75-78, and 83. The NAVSTAR system uses 18 satellites plus 3 spares to cover the globe with three, six unit orbits. The satellites rotate every 12 hours and appear once every 24 hours. The satellites are precisely mathematically positioned so that one desiring to obtain a position location can always receive four separate signals at one time for precise location. The four separate signals are longitude, latitude, elevation, and exact time.

A grid pattern of equally spaced lines in two directions is selected to allow a multiple number of intersections that are data check points for all of the measurements to be made by the aircraft. These lines can provide calibration information, equipment verifications and data validity certification and each of these lines is to be flown with data therealong recorded as hereinafter pointed out.

If it is desired to use the line of sight system then each ground station is located on a precise geodetic marker using the Navy transit satellite system in the translocation mode, with an excellent statistical sampling of good angle passes which computes a position to less than 1 meter in latitude, longitude and elevation.

Each transponder, one of which is located at each of the ground stations is adjusted to measure a calibrated distance on a known range before being installed at the ground station.

If desired the NAVSTAR system can be used to precisely locate the ground transponders.

After all the ground stations are in action the airborne vehicle is flown across the centerpoint between two stations to check the base line distance. Several passes along each base line are made before the survey begins.

These calibrations and meaurements are made so that the computed position will be known to an accuracy of the order of a circle of three meters diameter.

After an area of survey has been selected, a plot of lines to be flown is made. A computer listing of the grid forming the beginning and ending points of the lines, and all of the intersection points of any two lines is made. This listing is entered into the computer on the aircraft.

At least three unique ranges are measured every second to determine the aircraft position. An onboard computer calculates the aircraft position and supplies the data to the navigator plot board and to a pilot display on the flight panel. This collected data is compared to a predetermined flight path that is located in the memory of a computer in the airborne vehicle and the airborne vehicle is guided down the required path.

The use of the satellites as seen in FIG. 3 enables the airborne vehicle operator to precisely determine the elevation of the airborne vehicle due to the angular relationship between the vehicle and the satellites.

If desired, the NAVSTAR system can be used to provide longitude, latitude, elevation, and exact time of the airborne vehicle. This sytem enables the operator to go into an area which may be mountainous or have other undesirable features for transponder location, and to set up and operate without the use of the ground transponders.

In addition, less personnel and equipment are needed using the NAVSTAR system which does away with the need for support airborne vehicles and personnel to set up the ground transponders, thereby providing greater economy and more precision operation then is available from the ground transponder approach.

The satellite data may be received on board the airborne vehicle and the radio signals from the satellites linked to an auto-pilot to automatically provide very precise control of the vehicle's flight path along the preplotted path with an elevational accuracy of the order of six inches.

For purposes of assembling the desired gravity information, a modified three axis stabilized platform gravity meter available from La Coste and Romberg, Inc., Austin, Texas, or from Bell Aerosystems, Inc. Buffalo, New York, is used. The gravity meter is modified so that the data is recorded with only 1.5 seconds of filtering. A further modification is made to provide a shorting switch, that zeros the output from the amplifiers, so that the gravity meter can be stabilized in a short period of time.

All parameters of the meter and its platform are recorded every one second on magnetic tape. The gravity meter output of the total acceleration measurement as modified is recorded with little or no filtering. The stabilization time of the meter is therefore very short, as the output is kept in null state electronically until the aircraft is in stable flight condition.

The meter is then allowed to accumulate the total accelerations measured by the gravity meter.

All important outputs are monitored on strip chart recorders so that the details of gravity meter operation can be observed and corrected when required. Among these outputs are the cross coupling corrections, i.e., inherent and imperfection types. These corrections are basically corrections to the meter for being slightly off level and for the mechanical components of the meter flexing under acceleration. This is set forth in more detail in the La Coste publication previously referred to at pages 501 to 505.

The mode of operation will now be pointed out.

The blades of the helicopter are precisely tracked and aligned for smoothness of flight.

All the sensors are ground calibrated after which the airborne vehicle takes off and goes to the flight altitude selected for the survey. A reference altitude from the radar or laser altimeter is preferably made over a known elevation such as a lake or airfield.

All data is monitored in flight by analog strip chart recorders with common time events. The analog recordings are from the gravity meter of the raw beam movement, spring tension, average beam movement, cross or transverse acceleration, longitudinal acceleration, heading from the inertial position of the gravity meter, and are recorded on the tape.

Analog recordings are also available from the altimeter sensors, and of the radar or laser distance, the absolute pressure reading, and the relative pressure movement, and are recorded on the tape.

Analog recordings are also received from the navigation system as to each range measurement, and are recorded on the tape for whichever navigation system is employed.

Additional data is also recorded on the tape and includes the line number, the time, the observed gravity, the digital radar measurement, the observed magnetics, the total correction, cross coupling, the average beam movement at different levels of filtering; five different cross couplings including cross acceleration squared, vertical acceleration squared, vertical cross coupling, longitudinal cross coupling and cross acceleration; east and north gyroscope outputs, the azimuth gyroscope, the inertial navigation heading, pressure altimeter output with additional filtering, the signal ground, and the simultaneous signals are digitized and sampled at a one second sampling interval and put onto the tape.

Before, during and after each flight, all information is printed on paper tape to provide assurance that data are being collected. Analog recorders continuously monitor all important signal parameters.

During, flight, the operator of the gravity meter is able to change the sensitivity of the data recorders in order to precisely monitor the system performance. In this manner, he is able to check the platform level and the beam position very accurately.

The beam is an internal component of the La Coste gravity meter. The beam acts as a lever between the mass in the gravity meter and the fulcrum point of the spring tension measuring screw. The zero length spring in the gravity meter is attached to the mass that is supported by the beam.

The beam position is an important measurement because the automatic nulling circuit of the gravity meter requires it to be near zero, or it will drive the spring tension away from the value necessary for the best readings. If this spring is driven away from null, the meter requires 10 to 30 minutes to fully stabilize for accurate readings to be recorded. The beginning of lines to be flown requires concentration and a full coordination between the operator of the gravity meter, the navigator, and the pilot to prevent any elevation, course, or speed changes that would affect the beam position. In areas of steep gravity gradients or rough topography, the initial nulling of the gravity meter requires a skilled flight crew.

The onboard navigation computer and plotter provides a continuous monitor for the flight path of the airborne vehicle. Preplots of the proposed line spacings are made and fed into a navigational computer heretofore identified. After one of the lines of the preplot has been followed the airborne vehicle is returned to the start of the next line of the preplot, which is followed with data available and recorded as before.

The computer vectors the pilot to the beginning of the flight line and computes the ground speed. If the flight path begins to deviate from the preplotted line, then slight course changes are made by the pilot if the vehicle is being flown manually.

If desired, the NAVSTAR Satellite system can be used to control the operation of the airborne vehicle, this can be accomplished by recording the altitude, longitude, elevation, and time as such data is obtained from the NAVSTAR satellites and the information fed into the onboard navigation computer.

The vehicle can be flown manually by the pilot as described above. If, however, it is desired to operate the vehicle by auto-pilot, the NAVSTAR satellite signals can be linked to the auto-pilot to control its operation, to insure that the aircraft flight path precisely follows the preplotted line, with course changes being automatically made by the auto-pilot.

At the end of the flight the airborne vehicle returns to the known reference elevation over the lake or airfield and calibrates the elevation before landing.

It will thus be seen that a method of airborne gravity surveying has been provided in accordance with the objects of the invention.

Claims (19)

1. The method of airborne gravity surveying which comprises flying an airborne vehicle along a preplotted path, maintaining the stability of the airborne vehicle with respect to roll, pitch and yaw control and the elevation within a range of one half to ten feet, the sidewise deviation of the vehicle from the preplotted path within a range of 5000 feet and the speed within a range of twenty-five knots, receiving signals to locate the position of said airborne vehicle with respect to longitude, latitude, elevation and real time and, recording data along the preplotted path including simultaneous gravity and magnetic data, altimeter data, and navigation data.

2. The method as defined in claim 1 in which the airborne vehicle is a helicopter.

3. The method as defined in claim 2 in which stability is maintained within a range not exceeding ten feet, sidewise deviation is maintained within a range of 500 feet and speed is maintained within a range of five knots.

4. The method as defined in claim 2 in which the blades of the helicopter have been precisely tracked and aligned for smoothness of flight.

5. The method as defined in claim 1 in which at least some of the received signals which locate the airborne vehicle position are obtained by employing a plurality of altimeters including at least one pressure sensitive altimeter, and an altimeter operating upon wave propagation.

6. The method as defined in claim 1 in which changes in the aircraft altitude are measured by bi-directional narrow range temperature stabilized pressure transducers.

7. The method as defined in claim 6 in which signals from said pressure transducers are supplied for recording.

8. The method as defined in claim 1 in which one of the altimeters is a radar altimeter.

9. The method as defined in claim 1 in which one of the altimeters is a laser altimeter.

10. The method as defined in claim 1 in which the pressure sensitive altimeter has an input probe located so as to be subject only to static air pressure.

11. The method as defined in claim 9 in which bi-directional temperature stabilized pressure transducers measure changes in the aircraft altitude and to which said input probe is connected and which provide signals for recording.

12. The method as defined in claim 1 in which the recording is of digitalized signals at intervals of the order of one second.

13. The method as defined in claim 1 in which a line of sight electronic distance measuring device is employed at at least three ranges within a predetermined time interval and provides signals for recording.

14. The method as defined in claim 1 in which said gravity meter can be brought to a null position.

15. The method as defined in claim 1 in which signals are provided by a gravity meter at a predetermined rate.

16. The method as defined in claim 1 in which signals are simultaneously provided of magnetic and gravity data and are recorded.

17. The method as defined in claim 1 in which said position locating signals are supplied from a satellite positioning system.

18. The method as defined in claim 17 in which said airborne vehicle has an auto-pilot, and said received position locating signals are linked to said auto-pilot to control the operation thereof.

19. A method of airborne gravity surveying substantially as herein described with reference to and as shown in the accompanying drawings.