FR2553900A1 - Apparatus and method for taking readings of gravity by an airborne vehicle - Google Patents

Abstract

The invention relates to a method and an apparatus for taking readings of gravity by an airborne vehicle, in which the airborne vehicle is stabilised as far as speed, trajectory direction and altitude are concerned, and in which is provided a gravity measuring device which possesses appropriate sensitivity and which produces signals which are recorded at a high sampling frequency on a magnetic tape, the position of the airborne vehicle being calculated by using a multiple-range navigation system which is geodisically localised.

Description

 The present invention relates to the execution of a gravity reading by airborne vehicle, and it relates more particularly to a method and to an apparatus making it possible to carry out such a reading with greater precision than that which could be obtained until now.

It has been proposed in the prior art to use airborne vehicles to carry out a gravity reading, as indicated in "Review of Geophysics",
Volume 5, No. 4, November 1967, pages 477-526, published by "The American Geophysical Union", 2000 Florida Avenue,
NW, Washington, DC 20009, starting on pages 520-524 for airborne fixed wing vehicles.

A summary of helicopter activities can be found in "Airborne Gravity
Surveying, Technical Information ", published in March 1981, by Carson Geoscience, Perkasie, Pennsylvania, starting on page 1-1.

 Different patents have been issued for devices for gravity reading.

 Boitnott, in US Patent Nos. 3,011,347 and 3,038,338, Gustafsson, in US Patent No. 3,180,151 and Brede, in US Patent No. 3,447,293, and Hutchins, in Canadian Patent No. 652,757, describe instruments for measurement of gravity or gravity derivatives of the terrestrial gravitational field, but they do not represent systems usable in practice for a precise survey by airborne vehicle.

La Coste, US patents Nos. 2,293,437, 2,377,889, 2,964,948, 2,977,799; Heiland, US Patent No. 2,626,525;
Worden, US patent Nos. 2,674 -887 and 3,211,003; Graf, US Patent No. 3,019,655, Emmerich, US Patent 3,033,037;
Slater, US Patent No. 3,062,051; Hodge et al., Patent
US. No. 3,194,075; - Yard, US Patent No. 3,495,460;
Kuzivanov et al., Patent No. 3,501,958; Swing, US patent.

Nos. 3,546,943 and 3,583-225 describe gravity measuring devices but not systems which can be used in practice for precise reading by airborne vehicle.

Klasse et al., US Patent No. 2,610,226,
Jensen, US Patent No. 2,611,802, and Rumbaugh et al., US Patent No. 2,611,803, describe methods and apparatus for performing surveys for geophysical or magnetic exploration, but they do not describe or process a gravity survey by airborne vehicle.

 The proposals made so far for an airborne vehicle survey do not provide an adequate stabilization of an airplane with respect to speed, do not establish a flight at a very precise altitude, do not provide navigation and precise direction, do not take into account other imperatives for the execution of an accurate reading and a gravity measurement and have other disadvantages.

 In accordance with the invention, there is provided an improved method and apparatus for gravity reading by airborne vehicle or the airborne vehicle is stabilized with regard to speed, flight direction and altitude, and or the device for gravity measurement has an appropriate sensitivity and produces signals which are recorded at a high sampling frequency on a magnetic tape and where the position of the airborne vehicle is calculated using a multi-range navigation system which is geodetically located, thereby that it is best to take a reading during the night hours when the air is more stable.

 The main object of the invention is to create an improved method and apparatus for carrying out a gravity reading by airborne vehicle, with the aid of which a greater precision of the calculated and recorded data can be obtained.

 The invention further aims to create an improved method and apparatus for an airborne vehicle survey which is preferably carried out when the air conditions are relatively stable, the night hours frequently establishing such stability.

 The invention also aims to create an improved method and apparatus for surveying by airborne vehicle, where the airborne vehicle is maintained at a selected altitude and is stabilized in speed and direction.

 The invention further aims to create an improved method and apparatus for gravity reading by airborne vehicle, where the gravity measuring device is controlled with regard to its sampling and specifically the sampling frequency.

 The invention also aims to create an improved method and apparatus for gravity reading by airborne vehicle, where the position of the airborne vehicle is known with precision at all times.

 The invention further aims to create an improved method and apparatus for gravity reading by airborne vehicle, or the airborne vehicle has an appropriate fuel supply for use in remote locations.

 The invention further aims to create an improved method and apparatus where the instruments are placed and carried by the airborne vehicle at a stable temperature and preferably in a clean environment.

 The invention also aims to create an improved method and apparatus for reading by airborne vehicle, where a probe is placed so as to obtain an accurate measurement of the static air pressure.

 The invention further aims to create an improved method and apparatus for gravity reading by airborne vehicle to provide a measure of the distance of the airborne vehicle from the ground.

 The invention also aims to create an improved method and apparatus for gravity reading by airborne vehicle, or the gravity measuring device is arranged to operate more efficiently in the environment of the airborne vehicle.

 The invention further aims to create an improved method and apparatus for gravity reading by airborne vehicle, or there is provided a magnetic digital recording system having a high degree of sensitivity, a variable sampling frequency and a possibility of reading the magnetic tape in flight after data has been recorded on it.

 The object of the invention is also to create an improved method and apparatus for gravity reading by airborne vehicle with a view to recording multiple ranges from an electronic navigation system on a magnetic tape, in order to improve the accuracy of osition.

 The invention further aims to create a method and an apparatus improved for a gravity reading by airborne vehicle to describe the data collection process so as to obtain the parameters necessary for a calculation of an accurate gravity measurement.

 Another object of the invention is to create an improved method and apparatus for gravity reading by airborne vehicle for simultaneously recording magnetic and gravity data.

 Another object of the invention is to create an improved method and apparatus for an airborne vehicle gravity reading to describe the method of prior plotting of the required flight path and to ensure that the airborne vehicle follows such a path flight.

 The invention also aims to create a cross-linked pattern of lines to be followed in flight to cover the anomaly of gravity of the region to be covered.

For this, the present invention provides an apparatus for a gravity reading by airborne vehicle, comprising an airborne vehicle, a device installed on the vehicle to control the flight of said vehicle along a pre-marked trajectory at a pre-selected level. in response to a vertical movement, comprising a mcyen to move the vehicle vertically without changing its inclination, said control device comprising a plurality of altimeters for determining the pre-selected level for correction of the vehicle trajectory and for producing signals intended to be recorded on a tape, said control device also comprising a navigation means producing signals intended to be recorded on a tape for an indication of a deviation of the flight path of the vehicle from the pre-traced trajectory vertically and horizontally and for a correction of the trajectory of the vehicle, generating means a signal comprising a gravity indicator and a magnetic means and providing signals for recording on a tape, means for digitally converting the signals from the gravity indicator with a predetermined sampling frequency and with a high order of sensitivity, a recording means comprising a tape for
recording signals from altimeters, navigation equipment, signal generators and conversion equipment, digital, and a computer to indicate the vehicle deviation by
compared to the pre-drawn trajectory.

Furthermore, the method according to 1 invention for the execution
tion of a gravity reading by airborne vehicle is characterized in that an airborne vehicle is flown along a pre-drawn trajectory, in that the stability of the airborne vehicle is maintained as regards the control of roll, pitch and yaw, and
the elevation in a range not exceeding 3 meters, the lateral deviation of the vehicle from the path traced in a range of
1500 meters and the speed in a range of 12.5 m / sec., In that a plurality of altimeters is used comprising at least one altimeter
pressure sensitive and an altimeter operating with wave propagation, in that it records data along the pre-drawn path, including simultaneous gravity and magnetic data, altimeter data and navigation data.

Other characteristics and advantages of the invention will appear on reading the following description, given by way of nonlimiting example, with reference to the single figure which represents a diagram in the form of functional blocks making it possible to better understand the 'invention
With reference to the figure, any suitable airborne vehicle can be used, in particular a fixed wing airplane, an aircraft lighter than an airplane, and helicopters.

 When a helicopter is used, a suitable helicopter is the Sikorsky model 61, which is preferably equipped with internal fuel tanks with a range of up to about 8 hours flight time. The helicopter preferably includes an unequivocally tuned automatic flight control system which uses collective upward force to control the vertical movement of the vehicle during flight without changing the incline.

For any airborne vehicle, it is
essential to have a flight control system that ensures the
control of the vertical movement of the vehicle during flight, preferably to limit the elevation to + 3 meters in 30 seconds from a selected predetermined altitude.

 It is preferable to use in the vehicle an environmental chamber which is maintained at a stable temperature and which is preferably in a clean environment.

 A combined use of inertial navigation equipment and electronic distance measurement equipment allows the pilot to provide continuous control of latitude, longitude and speed.

This combined navigation system allows the airborne vehicle to fly within a speed range of 2.5 m / sec. and following a predetermined flight path with an accuracy of less than a few hundred meters.

 If using a helicopter, the rotor blades are precisely mounted and aligned to achieve uniformity of flight.

 A probe for measuring only the static air pressure and not subjected to an aerodynamic pressure is provided to facilitate the measurement of the altitude of the airborne vehicle, and it is placed so that only a static air pressure is measured; for a helicopter, it can be a retractable probe placed in front of the helicopter or a few tens of centimeters above the blades of the helicopter at the center of rotation of the blades. The probe is in communication with the environment chamber.

To determine the altitude of the airborne vehicle, a combination of sensitive pressure measurements and radar or laser measurements are performed to establish the altitude of the airborne vehicle with an accuracy of less than three meters. Appropriate radar equipment is available from Honeywell, Inc.,
Minneapolis, Minnesota. Appropriate laser equipment is available from Spectra Physics, Inc., Mountain View,
California.

 Appropriate absolute pressure measurement equipment is available from Rosemont, Inc., Minneapolis, Minnesota.

 Relative measurements are made and recorded for the altitude with an accuracy of the order of 15 cm.

 There are two types of pressure sensitive altimeters.

One type is an absolute measuring device that measures pressure and variations occurring in the atmosphere. Ground-based absolute measurement altimeters record variations at ground level and all of these measurements are combined to establish and record area variations in pressure in the survey area.

 The second type comprises two bidirectional narrow range pressure transducers which are temperature stabilized in the environment chamber and which are used to measure and record small variations in altitude of the airborne vehicle. Such transducers are available from Setra Systems, Inc., Natick, Massachusetts. A non-turbulent air source establishing a static air pressure is supplied to these detectors via the pressure probe which is arranged so as not to measure an aerodynamic pressure but only a static air pressure.

To obtain a record of accumulated data, all data is recorded in one second, or another desired interval, on a magnetic tape. All analog data channels are recorded with a sensitivity on the order of 0.0001 volts. Appropriate equipment for this purpose includes a digital system such as the Model 4570,
Lancer Electronics, available at Lancer Electronics
Corp., Collegeville, Pennsylvania, interfaced with a 9800 Kennedy belt conveyor, available from Kennedy Inc., Altadena, California.

The information is read after writing to the tape and is displayed on a paper tape reader.

A digital voltmeter is available to visually monitor a data channel.

To control the navigation of the airborne vehicle, an electronic line of sight distance measurement system using multiple ground stations can be used. A suitable type of such a control system is the Motorola Miniranger, available from Government
Electronics Division, Motorola, Inc., Scottsdale,
Arizona.

Another system for controlling airborne navigation is known under the designations "SERIES Satellite Emission Radio Interferometric Earth Surveying", it is available at Jet Propulsion
Laboratory, Pasadena, California, and it formed part of Third Annual NASA Program Review, Crustal
Dynamics Project, Geodynamics Research, January 26-29, 1981, Goddard Space Flight Center. An example of the system is described in US Patent No. 4,170,776 to MacDoran.

In addition, the problem of iconospheric calibration cited in the patent has been successfully resolved by a new technique called "Satellite L-band
Iconospheric Calibration (SLIC), which demonstrated the possibility for a single SERIES station to obtain the total electronic content in column by a cross-correlation of the two broadcasting channels of a "Globular Positioning System (GPS)". An additional important type of data is based on the Doppler effect which occurs for an effective length of 86 cm.

 A cross-linked pattern of lines equally spaced in two directions is selected to obtain a number of multiple intersections that constitute data control points for all measurements to be made by the airborne vehicle. These lines can provide calibration information, equipment checks, and data validity authentication, and each of these lines should be followed in flight with data logging along the line as will be defined in the after.

 Each ground station is placed on a precise geodetic benchmark established using the Navy's transit satellite system or others in the transposition mode with excellent statistical sampling of good angular trajectory to calculate a position with an accuracy of less than 1 meter in latitude, longitude and elevation.

 Each transponder which is placed in each of the ground stations is adjusted to measure a calibrated distance within a known range before being installed in the ground station.

 After all the ground stations are in action; the airborne vehicle performs a flight on the central path between two stations to check the distance on the baseline. Several passes along each baseline are made before the survey begins. These calibrations and measurements are carried out in such a way that the calculated position is known with an accuracy of the order of a circle of 3 meters in diameter.

 After a survey region has been selected, a line drawing to follow in flight is performed. A list of the grid forming the start and end points of the lines and all the intersection points of any two lines is established by computer. This listing is entered into the computer mounted on the airborne vehicle.

 At least three unique ranges are measured every second to determine the position of the airborne vehicle. A computer installed on board calculates the position of the airborne vehicle and transmits the data to a recording panel for the navigator and to a screen placed on the pilot's dashboard. This collected data is compared with a predetermined flight path which is placed in the memory of a computer located in the airborne vehicle, and this airborne vehicle is guided along the required path.

For the assembly of the desired information, a gravity measuring device with stabilized platform with three axes is used and available at
La Coste and Romberg, Inc. Austin, Texas, or a
Bell Aerosystems, Inc., Buffalo, New York. The gravity measuring device is modified so that the data is recorded with only 1.5 seconds of filtering. Another modification is made for the mounting of a stabilizing control which sets the output of the amplifiers to zero so that the gravity device can be stabilized in a short period of time.

 All the parameters of the measuring device and its platform are recorded every second on a magnetic tape. The total acceleration measurement provided by the modified gravity device is recorded with little or no filtering.

The stabilization time of the measuring device is therefore very short because the output signal is maintained in a electronically zero state until the airborne vehicle is in a stable flight condition.

The measuring device is then authorized to accumulate total accelerations measured by the gravity measuring device.

 All important output elements are monitored on graphic recorders so that the operating details of the gravity measuring device can be observed and corrected if necessary. Among these output elements, one can cite cross-coupling corrections, ie corrections of intrinsic faults and imperfections. These corrections are basically corrections concerning a slight difference in level of the measuring device and a deflection of the mechanical components of the measuring device under the effect of an acceleration. This is specified in more detail in the publication La Coste, previously cited, on pages 501 to 505.

 We will now describe the operating mode.

 After all the detectors have been calibrated on the ground, the airborne vehicle takes off and takes the flight altitude of the survey. A reference altitude provided by the radar or laser altimeter is preferably established above a known elevation zone, such as a lake or an aerodrome.

 All data is controlled in flight by analog graphic recorders using common time events. The analog recordings coming from the gravity device relate to the rough pendulum movement, the spring tension, the mean pendulum movement, the transverse acceleration, the longitudinal acceleration, an advance with respect to the inertia part of gravity measurement, and they are recorded on the tape.

 Analog recordings are also available for altimeter detectors, distances measured by radar or laser, absolute pressure measurement and relative pressure measurement, and they are recorded on the tape.

 Analog recordings are also received from the navigation system for each range measurement and they are recorded on the tape for the navigation system that is used.

Additional data is also recorded on the tape and it concerns the number of lines, the time, the observed gravity, the digital measurement by radar, the magnetic properties observed, the total correction, the cross coupling, the mean pendulum movement for different filtering levels, five different transverse couplings concerning a transverse angle acceleration, a vertical angle acceleration, a vertical transverse coupling, a longitudinal transverse coupling and a transverse acceleration; gyroscope outputs
East and North, the azimuth gyroscope, the inertial navigation direction, the pressure altimeter output with additional filtering, the signal base, simultaneously signals are digitally converted and sampled in a sampling interval of one second and they are recorded on the tape.

 Before, during and after each flight, all information is printed on a strip of paper to ensure that data is being collected. Analog recorders continuously monitor all important signal parameters.

During the flight, the user of the gravity measuring device is able to modify the sensitivity of the data loggers so as to
accurately monitor system performance. Of
this way he is able to check the level of
platform and the pendulum position so very
precise. The pendulum is an internal component of the
gravity measuring device from La Coste. The
pendulum acts as a lever between the mass in the
gravity device and the pivot point of the
spring tension measurement screw. The spring of
zero length provided for in the measuring device
gravity is fixed on the mass which is supported by the
pendulum.The pendulum position is an im
bearing in mind that the automatic cancellation circuit
of the gravity measuring device requires that it be
close to zero or that it spreads the spring tension
of the value needed for the best readings.

If this spring is offset from zero, the
measurement requires 10 to 30 minutes to stabilize completely for recording precise values;
The beginning of trajectories requires concentration
and complete coordination between the user of the
gravity device, browser, and driver for
to prevent deviations in altitude, trajectory or
speed - which would affect the position of the pendulum.

In regions with strong gradients of gravity or
coarse topography, the initial zeroing of the
gravity measuring device requires a crew
trained.

The computer and the navigation plotter
installed on board ensure continuous monitoring of the
flight path of the airborne vehicle. Meadows
track spacing records
proposed are established and entered into a computer
navigation already mentioned. After one of the trajectories
pre-registration was followed, the aero vehicle
carried is brought back to the beginning of the next pre-recording trajectory, which is followed with available and recorded data as before.

 The computer directs the pilot to the start of the flight path and calculates the ground speed. If the flight path starts to deviate from the pre-recorded line, slight changes in trajectory are then made by the pilot.

 At the end of the flight, the airborne vehicle returns to the known reference elevation on the lake or the aerodrome and calibrates the elevation before landing.

Claims (34)

 1. Device for a gravity reading by  airborne vehicle, characterized in that it comprises:  - an airborne vehicle,  - a device installed on the vehicle  to control the flight of said vehicle along a  pre-drawn trajectory at a pre-selected level in  response to vertical movement, comprising means for moving the vehicle vertically without  change its inclination,  - said control device comprising a plurality of altimeters for determining the pre level  selected for trajectory correction  of the vehicle and to produce signals intended for  be recorded on a tape,  - said control device comprising a navigation means producing signals intended for  be recorded on a tape for an indication of a deviation from the vehicle's flight path by  reports the pre-drawn trajectory vertically and  horizontally and for a correction of the trajectory  of the vehicle,  - means of generating compressed signals  providing a gravity measurement indicator and magnetic means and providing signals intended to be  recorded on a tape,  - a means to digitally convert  signals from the gravity indicator  with a predetermined sampling frequency and  with a high order of sensitivity,  - a recording means comprising a  tape for recording signals from  altimeres, navigation, generators  signals and digital conversion means, and  - a computer to indicate the deviation of the vehicle from the pre-drawn trajectory.  2. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that said airborne vehicle is a helicopter.  3. Apparatus for a gravity reading by airborne vehicle according to claim 2, characterized in that the blades of the helicopter are mounted and aligned with precision for uniformity of flight.  4. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that said plurality of altimeters comprises at least one altimeter sensitive to a pressure, and an altimeter operating with a propagation of waves.  5. Apparatus for a gravity survey by airborne vehicle according to claim 4, characterized in that one of said altimeters is a radar altimeter.  6. Apparatus for a gravity reading by airborne vehicle according to claim 4, characterized in that one of said altimeters is a laser altimeter.  7. Apparatus for a gravity reading by airborne vehicle according to claim 4, characterized in that there is provided for the pressure sensitive altimeter a probe positioned so as to be subjected only to a static air pressure.  8. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that the analog data are recorded with a sensitivity of the order of 0.0001 volts.  9. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that the recording means ensures the recording of digitally converted signals at intervals of the order of one second.  10. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that the navigation means is an electronic device for measuring distance with line of sight, and in that at least three ranges are measured in a predetermined time interval.  11. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that the pre-drawn trajectory comprises precise geodesic elements obtained from positions of satellites in the transposition mode.  12. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that the navigation means uses interferometric means of radio transmissions by satellites.  13. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that said gravity measuring device produces signals at a predetermined frequency.  14. Apparatus for gravity reading by airborne vehicle according to claim 13, characterized in that at least two bidirectional narrow range pressure transducers are mounted inside said chamber to which air is supplied by a probe placed so as to be subjected only to static air pressure.  15. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that the gravity measuring device can be stabilized in a short period of time.  16. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that there is provided a protected environmental chamber inside which is enclosed the gravity device.  17. Apparatus for a gravity reading by airborne vehicle according to claim 1, characterized in that magnetic and gravity data are simultaneously recorded on a tape.  18. A method of carrying out a gravity reading by airborne vehicle, characterized in that an airborne vehicle is made to fly along a predetermined trajectory, in that the stability of the airborne vehicle is maintained as regards the control of roll, pitch and yaw, and the elevation in a range not exceeding 3 meters, the lateral deviation of the vehicle from the trajectory traced in a range of 1500 meters and the speed in a range of 12, 5 m / sec., In that a plurality of altimeters are used comprising at least one pressure-sensitive altimeter and an altimeter operating with wave propagation, in that data are recorded along the pre-flight path -plotted, in particular simultaneous gravity and magnetic data, altimeter data and navigation data.  19. The method of claim 18, characterized in that the airborne vehicle is a helicopter copter.  20. The method of claim 19, characterized in that the stability is maintained in a range not exceeding 3 meters, in that the lateral deviation is maintained in a range of 150 meters, and in that the speed is maintained in a range of 12.5 m / sec.  21. The method of claim 19, characterized in that the blades of the helicopter have been mounted and aligned with precision for uniformity of flight.  22. Method according to claim 18, characterized in that bidirectional pressure transducers stabilized in temperature and with a narrow range measure variations in the altitude of the airborne vehicle.  23. The method of claim 22, characterized in that signals from the pressure transducers are provided for recording.  24. The method of claim 18, characterized in that one of the altimeters is a radar altimeter.  25. The method of claim 18, characterized in that one of the altimeters is a laser altimeter.  26. The method of claim 18, characterized in that the pressure-sensitive altimeter comprises an inlet probe positioned so as to be subjected only to a static air pressure.  27. The method of claim 26, characterized in that bidirectional temperature stabilized pressure transducers measure variations in the altitude of the airborne vehicle, are connected to said input probe and provide signals for recording.  28. The method of claim 18, characterized in that one carries out a recording of digitally converted signals at intervals of the order of one second.  29. Method according to claim 18, characterized in that an electronic line of sight distance measurement device is used in at least three ranges in a predetermined time interval and provides signals for recording.  30. The method of claim 18, characterized in that said gravity measuring device can be brought into a zero position.  31. Method according to claim 18, characterized in that signals are supplied by a gravity measuring device at a predetermined frequency.  32. Method according to claim 18, characterized in that signals are simultaneously supplied with regard to magnetic and gravity data and are recorded.  33. Method according to claim 18, characterized in that signals are supplied by an interferometer of radio transmissions by satellites for recording.  34. The method of claim 18, characterized by bidirectional temperature stabilized and narrow range transducers providing signals for recording.