Airborne gravity surveying

Method and apparatus are 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 has adequate sensitivity and signals that are recorded at a high sample rate on a magnetic tape, in which the aircraft position is computed using a multi-range navigation system that is located geodetically.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to airborne gravity surveying and more particularly 
to method and apparatus for such surveying with which greater accuracy is 
obtained than has heretofore been available. 
2. Brief Description of the Prior Art 
It has heretofore been proposed to use airborne vehicles for gravity 
surveying as pointed out in Reviews of Geophysics, Vol. 5, No. 4, 
November, 1967, pages 477 to 526, published by The American Geophysical 
Union of 2000 Florida Ave., N.W., Washington, DC 20009 commencing at page 
520 to 524 with respect to fixed wing airborne vehicles. 
A review of the activities with respect to helicopters can be found in 
"Airborne Gravity Surveying, Technical Information", published March, 
1981, by Carson Geoscience, Perkasie, Pa., commencing at page 1-1. 
Various patents have been issued which set forth apparatus for gravity 
surveying. 
Boitnott, in U.S. Pat. Nos. 3,011,347 and 3,038,338, Gustafsson, U.S. Pat. 
No. 3,180,151 and Brede, U.S. Pat. No. 3,477,293, and Hutchins, Canadian 
Pat. No. 652,757 disclose instruments for measuring gravity or derivatives 
of gravity of the earth's gravity field but do not show practical systems 
for accurate airborne surveying. 
La Coste, U.S. Pat. Nos. 2,293,437, 2,377,889, 2,964,948, 2,977,799; 
Heiland, U.S. Pat. No. 2,626,525; Worden, U.S. Pat. Nos. 2,674,887 and 
3,211,003; Graf, U.S. Pat. No. 3,019,655; Emmerich, U.S. Pat. No. 
3,033,037; Slater, U.S. Pat. No. 3,062,051; Hodge et al., U.S. Pat. No. 
3,194,075; Ward, U.S. Pat. No. 3,495,460; Kuzivanov et al., U.S. Pat. No. 
3,501,958; Wing, U.S. Pat. Nos. 3,546,943 and 3,583,225, show gravity 
meters but do not show practical systems for accurate airborne surveying. 
Klasse et al., U.S. Pat. No. 2,610,226, Jensen, U.S. Pat. No. 2,611,802, 
and Rumbaugh et al., U.S. Pat. No. 2,611,803 show method and 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 for the aircraft, with respect to speed, do not 
provide very level flight, do not provide accurate navigation and 
steering, do not with these other requirements for accurate surveying, 
measure the gravity, and have other shortcomings. 
SUMMARY OF THE INVENTION 
In accordance with the invention an improved method and apparatus are 
provided 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 has adequate sensitivity and signals that are 
recorded at a high sample rate on magnetic tape, in which the aircraft 
position is computed using a multi-range navigation system that is located 
geodetically, it being preferable to survey during the night hours when 
the air is more stable. 
It is the principal object of the invention to provide an improved method 
and apparatus 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 and 
apparatus for airborne surveying which is preferably carried out when air 
conditions are relatively stable, the night hours frequently providing 
such stability. 
It is a further object of the invention to provide an improved method and 
apparatus for airborne 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 and 
apparatus 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 and 
apparatus 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 and 
apparatus for airborne gravity surveying in which the airborne vehicle has 
adequate fuel supply for use at remote locations. 
It is a further object of the invention to provide an improved method and 
apparatus 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 and 
apparatus for airborne surveying in which a probe is located in such a 
manner as to obtain an accurate measurement of the static air pressure. 
It is a further object of the invention to provide an improved method and 
apparatus for airborne gravity surveying to provide a measurement of the 
distance of the airborne vehicle from the ground. 
It is a further object of the invention to provide an improved method and 
apparatus for airborne gravity surveying to so construct the gravity meter 
so that it operates more efficiently in the airborne environment. 
It is a further object of the invention to provide an improved method and 
apparatus for airborne gravity surveying including and providing 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 and 
apparatus for airborne gravity surveying to record on a magnetic tape 
multiple ranges from an electronic navigation system to enhance the 
position accuracy. 
It is a further object of the invention to provide an improved method and 
apparatus for airborne gravity surveying to describe the method of data 
collection to provide the necessary parameters for computing accurate 
gravity measurement. 
It is a further object of the invention to provide an improved method and 
apparatus for airborne gravity surveying to simultaneously record magnetic 
and gravity data. 
It is a further object of the invention to provide an improved method and 
apparatus for airborne gravity surveying to describe the method of 
preplotting the required flight path and to require the airborne vehicle 
to comply with such a flight path. 
It is a further object of the invention to provide for a grid pattern of 
lines to be flown to cover the gravity anomaly of the area to be covered. 
Other objects and advantageous features of the invention will be apparent 
from the description and claims.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the FIGURE 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 helicopter preferably has 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, 
preferable to limit the elevation to .+-.10 feet 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 is preferably a clean environment. 
A combined use of inertial navigation and electronic distance measuring 
equipment provide the latitude, longitude, and speed control continuously 
for the pilot. This total navigation package allows the airborne vehicle 
to fly 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 is 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 are made to establish 
the altitude of the aircraft to within ten feet. Suitable radar equipment 
is available from Honeywell, Inc., Minneapolis, Minn. Suitable laser 
equipment is available from Spectra Physics, Inc., Mountain View, Calif. 
Suitable equipment for measuring absolute pressure is available from 
Rosemount, Inc., Minneapolis, Minn. 
Relative measurements are made and recorded of the altitude to an accuracy 
of the order of 0.5 feet. 
The pressure altimeters are of two types. One type is an absolute device 
that measures the pressure and changes that occur in the atmosphere. 
Ground based absolute altimeters 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 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, 
Mass. 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 a digital system such as the 
Lancer Electronics Model 4570, available from Lancer Electronics Corp., 
Collegeville, Pa., interfaced to a Kennedy Model 9800 tape transport 
available from Kennedy, Inc., Altadena, Calif. 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, Ariz. 
Another system for controlling the navigation of the airborne vehicle is 
known as SERIES Satellite Emission Radio Interferometric Earth Surveying 
available from Jet Propulsion Laboratory, Pasadena, Calif. and which 
formed a part of Third Annual NASA Program Review, Crustal Dynamics 
Project, Geodynamics Research, Jan. 26-29, 1981, Goddard Space Flight 
Center. An example of the system is shown in the U.S. patent to MacDoran, 
U.S. Pat. No. 4,170,776. Additionally, the iconospheric calibration 
problem recognized in the patent has been successfully addressed by a new 
technique called Satellite L-band Iconospheric Calibration (SLIC) which 
has demonstrated the ability for a single SERIES station to derive the 
total electron collumnar content by cross correlation of the two broadcast 
Globular Positioning System (GPS) channels. An important additional 
data-type is Doppler occurring at an effective wave length of 86 cms. 
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. 
Each ground station is located on a precise geodetic marker established 
using the Navy transit satellite system in the translocation mode with an 
excellent statistical sampling of good angle passes to compute 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. 
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 measurements 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 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. 
For purposes of assembling the desired information a modified three axis 
stabilized platform gravity meter available from La Coste and Romberg, 
Inc., Austin, Tex., or from Bell Aerosystems, Inc., Buffalo, N.Y., 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 
conditions. 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. 
After all the sensors have been ground calibrated, the airborne vehicle 
takes off and goes to the flight altitude of 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 raw beam 
movement, spring tension, average beam movement, cross or transverse 
acceleration, longitudinal acceleration, heading from the inertial portion 
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, simultaneously 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 monitor precisely 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 require 
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. 
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.