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BACKGROUND OF THE INVENTION
This invention relates to determining the position and orientation of a
remote object with respect to a reference point; and, more particularly,
to radiating an electromagnetic field from the reference point, detecting
the field at the remote object and analyzing the detected field to
determine the position and orientation of the remote object.
The use of orthogonal coils for generating and sensing magnetic fields is
well known. For example, such apparatus has received wide attention in the
area of mapping magnetic fields to provide a better understanding of their
characteristics. If a magnetic field around generating coils can be very
accurately mapped through use of sensing coils, it has also been perceived
that it might be possible to determine the location of the sensing coils
relative to the generating coils based on what is sensed. However, a
problem associated with doing this is that there is more than one location
and/or orientation within a usual magnetic dipole field that will provide
the same characteristic sensing signals in a sensing coil. In order to use
a magnetic field for this purpose, additional information must therefore
be provided.
One approach to provide the additional information required for this
purpose is to have the generating and sensing coils move with respect to
each other, such as is taught in U.S. Pat. No. 3,644,825. The motion of
the coils generates changes in the magnetic field, and the resulting
signals then may be used to determine direction of the movement or the
relative position of the generating and sensing coils. While such an
approach removes some ambiguity about the position on the basis of the
field sensed, its accuracy is dependent on the relative motion, and it
cannot be used at all without the relative motion.
Another approach that has been suggested to provide the additional required
information is to make the magnetic field rotate as taught in Kalmus, "A
New Guiding and Tracking System," IRE Transactions on Aerospace and
Navigational Electronics, March 1962, pages 7-10. To determine the
distance between a generating and a sensing coil accurately, that approach
requires that the relative orientation of the coils be maintained
constant. It therefore cannot be used to determine both the relative
translation and relative orientation of the generating and sensing coils.
U.S. Pat. No. 3,868,565, assigned to the same assignee, teaches a tracking
system for continuously determining at the origin of a reference
coordinate system the relative translation and orientation of a remote
object. The tracking system includes radiating and sensing antenna arrays
each having three orthogonally positioned loops. Properly controlled
excitation of the radiating antenna array allows the instantaneous
composite radiated electromagnetic field to be equivalent to that of a
single loop antenna oriented in any desired direction. Further control of
the excitation causes the radiated field to nutate about an axis denoted a
pointing vector. This tracking system is operated as a closed-loop system
with a computer controlling the radiated-field orientation and
interpreting the measurements made at the sensing antenna array. That is,
an information feedback loop from the sensing antenna array to the
radiating antenna array provides information for pointing the nutation
axis toward the sensing antenna array. Accordingly, the pointing vector
gives the direction to the sensing antenna array from the radiating
antenna array. The proper orientation of the pointing vector is necessary
for computation of the orientation of the remote object. The signals
detected at the sensing antenna include a nutation component. The nutating
field produces a different nutation component in each of the three
detected signals. The orientation of the sensing antenna array relative to
the radiated signals is determined from the magnitudes of these
components.
U.S. Pat. No. 4,054,881, assigned to the same assignee, teaches a magnetic
or near-field non-tracking system for determining, at a remote object, the
position of the remote object with respect to a reference coordinate
system. The orientation of the remote object can be determined, at the
remote object, with respect to the reference coordinate system by using an
iterative computational scheme. This is accomplished by applying
electrical signals to each of three mutually orthogonal radiating
antennas, the electrical signals being multiplexed with respect to each
other and containing information characterizing the polarity and magnetic
moment of the radiated electromagnetic fields. The radiated fields are
detected and measured by three mutually orthogonal receiving antennas,
having a known spatial relationship to the remote object, which produces
nine parameters. These nine parameters, in combination with one known
position or orientation parameter are sufficient to determine the position
and orientation parameters of the receiving antennas with respect to the
position and orientation of the radiating antennas.
Copending, allowed, U.S. Patent application, Ser. No. 62,140 filed July 30,
1979 entitled REMOTE OBJECT POSITION AND ORIENTATION LOCATER, and assignee
to the same assignee; and copending U.S. patent application Ser. No.
164,783, filed June 30, 1980, entitled REMOTE OBJECT POSITION AND
ORIENTATION LOCATOR and assigned to the same assignee, teach several
improvements to U.S. Pat. No. 4,054,881. In particular, two mutually
orthogonal radiating antennas each transmit electromagnetic radiation to
three mutually orthogonal receiving antennas. Alternately, three mutually
orthogonal radiating antennas each transmit electromagnetic radiation to
two mutually orthogonal receiving antennas. The first of the above noted
applications discloses a near-field system and the second of the above
noted applications discloses a far-field system. Measurement of the
transmitted signals as received by the set of orthogonal receiving
antennas produces information which, in combination with two known
position or orientation parameters, is sufficient to determine in a
non-iterative manner the position and orientation of the receiving
antennas with respect to the position and orientation of the radiating
antennas.
Copending, allowed, U.S. Patent application, Ser. No. 954,126, filed Oct.
24, 1978, assigned to the same assignee and entitled METHOD AND APPARATUS
FOR TRACKING OBJECTS, now U.S. Pat. No. 4,298,874, teaches a tracking
system for: (a) determining at the origin of a first body coordinate
reference frame the relative position and orientation of a second body,
and (b) determining at the origin of a second body coordinate reference
frame the relative position and orientation of the first body. The
separation distance between the bodies is not limited to the near field.
Each body of the tracking system includes at least two independently
oriented stub dipoles for radiating and sensing electromagnetic fields.
Properly controlled excitation of the radiating antenna allows the
radiated field to nutate about an axis denoted a pointing vector. The
first body receives radiation transmitted from the second body and
establishes the pointing angles to the second body with respect to the
first body coordinate reference frame. The processing which determines the
pointing angles is dependent on the fact that no modulation or nutation
components exist in the radial direction. The field received by the first
body can include information defining the second body's pointing angles to
the first body with respect to the second body's coordinate reference
frame and the relative roll about their mutually aligned pointing axes.
This information is sufficient for determining the orientation of the
first body relative to the second. This process is then repeated with the
second body receiving radiation transmitted from the first body. Further,
information can be transmitted from the first body to the second body
which established a vector from the second body to a third body, thus
defining the location of the third body at the second body.
However, in the context of a non-tracking far-field system there still
remains a need to determine the position of a source of electromagnetic
radiation relative to a remote object in the case where the source is of
unknown structure and orientation.
SUMMARY OF THE INVENTION
According to the present invention these and other problems in the prior
art are solved by provision of a multicomponent radiating means of unknown
structure and orientation having components centered about the origin of a
source or transmitter. The source includes means for applying to the
plurality of radiating means electrical signals which generate a plurality
of electromagnetic fields. The signals are formatted or multiplexed such
that the electromagnetic fields are distinguished from one another. A
multicomponent receiving means is disposed on a remote object. The
receiving means is provided with at least three orthogonal components for
detecting and measuring components of the electromagnetic fields
transmitted by the radiating means. The three components of the receiving
means are centered about the origin of a reference coordinate frame
associated with the remote object. The radiating means and receiving means
are specifically adapted for operation at a separation distance sufficient
to insure that the far-field components of the electromagnetic fields
received by the receiving means are substantially greater in magnitude
than the near-field components of the electromgnetic fields received by
the receiving means. Analyzing means is associated with the receiving
means for converting the received components of the electromagnetic fields
into source position relative to the remote object, and the relative
orientation of the remote object, without a priori knowledge of the
orientation of the source or the relative orientation of its components.
The analyzing means operates open-loop with respect to the radiating means
and determines source positon relative to the remote object with at least
one ambiguous combination of orientation or position.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partly block, side elevational view of a landing aid system in
accordance with an embodiment of this invention;
FIG. 2 is a graphical representation showing the relationship between
electric field strength and distance from a radiator;
FIG. 3 is a simplified representation of a electric field associated with a
current-carrying electric dipole;
FIG. 4 is a graphical representation of the location coordinate system of
the remote object with respect to the location of the origin of the
reference coordinate frame;
FIG. 5 is a graphical representation of the orientation coordinate system
of the remote object with respect to the reference coordinate frame;
FIG. 6 is a graphical representation of the amplitude of the signals
applied to the transmitting antennas, with respect to time, in the case
where the signals are frequency division multiplexed;
FIG. 7 is a block diagram of a portion of the receiver in accordance with
an embodiment of this invention;
FIG. 8 is a graphical representation of the far-field electromagnetic
coupling of a three axis sensor to a three axis source; and
FIG. 9 is a graphical representation of the far-field electromagnetic
coupling of a three axis sensor with a three axis source of unknown
orientation and structure;
FIG. 10 is a flow chart for the computations carried out in a three-state
power solution for remote object position and orientation;
FIG. 11 is a flow chart of the computations carried out in a two-state
power and dot product solution for remote object position and orientation;
FIG. 12 is a graphical representation of the signals applied to the
transmitting antennas, with respect to time, in the case where the signals
are time division multiplexed;
FIG. 13 is a schematic representation of a transmitter employed in a time
division multiplexed system;
FIG. 14 is a schematic representation of a transmitter employed in a
frequency division multiplexed system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
APPARATUS
The invention is described herein in the context of a system for
determining the position and orientation of a remote object relative to a
source of known structure. When encountering a source of unknown structure
the system is provided with means for determining the position of the
source relative to a reference coordinate frame centered at the remote
object.
THREE AXIS TRANSMISSION AND THREE AXIS SENSING WITH FREQUENCY DIVISION
MULTIPLEXING
Although the invention may have utility in a number of environments, only
an embodiment relating to a long distance landing system is described in
detail. Referring to FIG. 1, a landing aid system 10 includes ground based
components 30 for radiating an electromagnetic field and airborne
components 20 for receiving the electromagnetic field and determining the
position and orientation of airborne components 20 with respect to ground
based components 30. Ground based components include a signal generator 31
coupled in parallel to power amplifiers 32, 33 and 34. A ground antenna
array 40 includes orthogonal electric dipole antennas 41, 42, and 43
(denoted X, Y, Z) coupled to power amplifiers 32, 33, and 34,
respectively. The dipole antennas 41, 42 and 43 are short relative to the
wave length of the carrier frequency so that they each produce an electric
dipole-field pattern providing spatial component data unique to each
antenna. A monitor receiver 44 is coupled to signal generator 31, spaced
from ground antenna array 40 and has an orthogonal antenna array 45 for
receiving electromagnetic radiation from ground antenna array 40. The
separation distance of monitor receiver 44 from the ground antenna array
40 is such that the electromagnetic field has a far-field component
substantially in excess of the near-field component. Monitor receiver 44
provides a means of verifying the electromagnetic transmission from ground
antenna array 40. Airborne components 20 include a three-axis receiving
antenna consisting of mutually orthogonal elements (21, 22 and 23) and
analyzing means for converting the received components of the
electromagnetic fields into remote object position and orientation
comprising three identical channels of amplification (25, 26 and 27),
frequency translation (55, 56 and 57), and signal processing (58, 59 and
60). The analyzing means also includes the computer 50 which receives the
outputs of the three signal processors and calculates position and
orientation for display at 51. More specifically, antenna array 21
includes receiving dipole antennas 22, 23 and 24 (denoted U, V, W) coupled
sequentially to signal amplifiers 25, 26 and 27, respectively, frequency
translators 55, 56 and 57, respectively, and signal processors 58, 59 and
60 respectively.
Landing aid system 10 operates "open loop" in that the only communication
between airborne components 20 and ground based components 30 is the
radiated electromagnetic field from ground based components 30. There need
be no communication from airborne components 30 to ground based components
30 in order to establish the position and orientation of receiving antenna
array 21 with respect to ground antenna array 40. Further, landing aid
system 10 allows simultaneous use by any number of remote users. In
addition to providing the capability for measuring position and
orientation, the signals radiated by ground antenna array 40 can provide a
one-way data link from ground based components 30 to receiving antenna
array 21. The link can carry information such as transmitter
identification, transmitter power, field distortion corrections, locations
of nearby obstacles, the location of the landing site relative to ground
antenna array 40 and wind direction.
Referring to FIG. 2, the field produced by excitation of a dipole antenna
can be separated into two components referred to as the near-field and the
far-field components. According to the present invention, the separation
distance of the remote object from the transmitting means is limited to
far-field conditions. The far-field component of the transmitted
electromagnetic radiation decreases linearly as the distance between the
remote object and the transmitter increases. The intensity of the
far-field depends on the relative size of the antenna and the wave length
of the excitation frequency. For electrically short antennas, as the wave
length of the excitation frequency is shortened, or the excitation
frequency is increased, the strength of the far-field component increases.
The far-field component of electromagnetic radiation is generally used for
long distance communications and navigation. On the other hand, the
near-field component of electromagnetic radiation decreases with the cube
of the distance from the antenna preventing its detection at large
distances. The intensity of the near-field is not a function of frequency
and it can be quite high at short distances or low excitation frequencies
which reduce field distortion. When using the far-field component, some
additional field distortion occurs because of surrounding objects. The
amount of distortion resulting from surrounding objects depends on the
conductivity and permeability of these objects and their size and location
relative to the receiving and transmitting antennas. It is possible to
predict and compensate the distortion caused by nearby fixed objects and
hence essentially remove position and orientation errors caused by these
objects.
Ground based components 30 generate a far-field landing aid signal. Signal
generator 31 generates the electrical signals which excite each of
antennas 41, 42 and 43. The signal must be multiplexed so receiving
antenna array 21 can distinguish the electromagnetic radiation from each
of the antennas 41, 42 and 43. Although the list is not exhaustive, the
electromagnetic radiation transmitted from each of the antennas 41, 42 and
43 may be distinguished by using time division multiplexing, frequency
multiplexing, phase multiplexing and spread spectrum multiplexing.
Additionally, the electrical signal may contain information characterizing
the phase of the electromagnetic radiation. A simple example would be to
include a timing pulse whenever the signal goes positive. Alternatively,
if frequency multiplexing is used, the excitation to each of antennas 41,
42 and 43 is advantageously coherent. That is, periodically all of the
signals go positive simultaneously (see FIG. 6). Additionally, the data
frequency determines the spacing between the carrier frequencies, and is
thus the basic reference frequency of signal generator 31. The data
frequency is labeled f.sub.o in FIG. 6. Advantageously, the reference
frequency will be derived from a temperature compensated crystal
oscillator in the 10 MHz range and frequency selection will be in 10 kHz
steps.
The three power amplifiers 32, 33 and 34 boost the outputs of signal
generator 31 to a level sufficient to produce the desired power with the
given antenna. To make efficient use of the power available, a switching
power amplifier may be used. For example, either class D (carrier
frequency switching) with a class S (high frequency switching) modulator
can be used. An RFI filter i | | |
