Note: Descriptions are shown in the official language in which they were submitted.
CA 02595388 2007-07-31
POSITION FINDING SYSTEM AND METHOD USED WITH AN EMERGENCY
BEACON
Field of the Invention
This invention relates to locating a transmitter in general and in particular
to a
system and method for locating an emergency beacon using a single receiver
system adapted
to receive a plurality of data points relating to a signal outputted by the
emergency beacon.
Background of the Invention
In many circumstances it is necessary to locate the position of a transmitter
for
example in search and rescue (SAR) operations. In such SAR operations, an
aircraft such as a
helicopter or airplane, which has a receiver will search an area for the
location of a lost person
or object having an associated emergency beacon. The emergency beacon emits a
continuous
or intermittent radio frequency signal which is received by the receiver on
the aircraft. The
aircraft then utilizes information relating to signal emitted from the
transmitter such as the
direction of the transmitter relative to the receiver to determine the
location of the transinitter.
In most SAR operations it is necessary to locate the emergency beacon as
quickly as possible. This may be due to a time limitation for the person being
searched for
such as a critical medical emergency or fuel limitations within the SAR
aircraft. In a
helicopter rescue scenario, for example, fuel consumption is always a major
concern. The
sooner a helicopter can arrive at the target transmitter the longer it can
stay on scene to aid in
assisting or extracting the survivors. Instances have been cited where SAR
aircraft have spent
too much time homing on a beacon and had to leave the scene prior to
extracting all of the
survivors due to fuel supplies running low. An EHI01 Cormorant helicopter has
a typical fly
time of approximately 4 hours per tank of fuel. A smaller Cessna (typical
aircraft used by
civilian SAR - CASARA) can operate for 3 - 5 hours per tank. These valuable
minutes can
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easily be used up when flying a long distance to an incident and if an
inordinate amount of
time is used for homing.
Previous methods of determining the location of the transmitter using a single
receiver have been unsatisfactory. Due to some ambiguities in the data
conveyed by the
transmitter signal, single receivers have not been able to determine the exact
location of the
transmitter with a great deal of accuracy. Many of these prior methods also
require the use of
multiple receivers for receiving the signal from the transmitter where the
data received at each
of the receivers are used to triangulate the position of the transmitter.
Examples of such
systems may be found, in U.S. Patent Nos. 4,728,959, 4,799,062, 5,592,180 and
5,815,117.
For SAR purposes, it is not often possible to use multiple receivers due to
the remote location
of the emergency beacon and the lack of time to set up and co-ordinate
multiple receives
within such a remote location.
Applicant is also aware of many problems that exist with the current 121.5
MHz tracking abilities as are commonly known in the art. Examples of such
problems include
multipath and confusing Receive Signal Strength Indicators (RSSI) readings due
to motion and
antenna patterns of the transmitter and receiver.
Current direction finding techniques also have poor resolution of direction
and
indicate a heading only, not a position. Current direction finding techniques
often utilize
costly antennas. These antennas provide a direction to the beacon from the
receiver.
Successive directions at subsequent locations are used to progressively narrow
the location of
the beacon. Due to their large size these antennas can cause undesirable drag
and handling
characteristics for faster moving aircraft. Current direction finding
techniques also require a
great deal of attention and understanding from the pilot and can distract from
other mission
criteria and safety.
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What is desired is a system and method of position finding for use in the
search
and rescue community that may utilize a single receiver and be able to locate
the position of
the emergency beacon with greater accuracy and efficiency.
Summary of the Invention
The present invention is an emergency beacon receiver that calculates the
geographical position of the homing transmitter radiated from an emergency
beacon.
This invention provides a SAR receiver that determines the position of an
emergency beacon that has incorporated a homing transmitter, which is
frequency stabilized.
The receiver detects the homing signal and calculates and displays the
position of the
emergency beacon. The object is to alleviate many of the technical issues and
operation issues
that standard direction fmding receivers can cause.
According to a first embodiment of the present invention there is provided a
method of locating a transmitter. The method comprises providing a receiver
adapted to
receive signals from the transmitter and acquiring a plurality of data points
relating to the
position of the transmitter. Each of the plurality of data points corresponds
a unique location
of the receiver. The method also includes calculating the location of the
transmitter using the
plurality data points.
The plurality of data points may represent a measured frequency of a signal
transmitted by the transmitter. The plurality of data points may coinprise at
least three
measured frequencies. The plurality of measured frequencies may each include
an associated
speed and position of the receiver.
Calculating may comprise utilizing the plurality of measured frequencies and
their corresponding speed and position of the receiver so as to calculate the
position of the
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transmitter. Calculating may comprise assuming a transmitted frequency of the
transmitter
based upon the speed of the receiver and for each of the plurality of measured
frequencies
calculating an angle of orientation of the transmitter relative to the
receiver utilizing the
Doppler shift of the measured frequency relative to the assumed transmitted
frequency. The
angle orientation defines a conical surface. The method may further include
calculating the
intersection of the plurality of the conical surfaces to determine said
location of the transmitter.
Calculating the intersection may utilize a computer. Calculating may be
performed using
computational geometry.
The plurality of data points may represent a direction of said transmitter
relative
to the receiver at a unique location of the receiver. The receiver may
comprise a direction
finding antenna adapted to determine the angle of the transmitter relative to
the receiver. The
receiver may have a phased array antenna. The receiver may have an electronic
compass for
measuring the orientation of the receiver.
The plurality of data points may comprise at least two data points.
Calculating
may comprise calculating the intersection of the at least two directions.
According to a further embodiment of the present invention, there is provided
an apparatus for locating a transmitter. The apparatus comprises an antenna
adapted to receive
signals from the transmitter, a receiver adapted to receive the plurality of
signals from the
antenna and a memory adapted to store the plurality signals received from the
receiver. The
plurality of signals represent a plurality of data points relating to the
position of the transmitter
wherein each of the plurality of data points corresponding a unique location
of the receiver.
The apparatus fiuther includes a processor for calculating the location of the
transmitter using
the plurality data points stored in the memory and an output for outputting
the location of the
transmitter as calculated by the processor.
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The apparatus may further comprise a global positioning receiver for receiving
data representing the global position of the receiver wherein the processor
utilizes the data
representing the global position of the receiver to calculate the global
position of the
transmitter.
Brief Description of the Drawings
In drawings which illustrate embodiments of the invention,
Figure 1 is a diagrammatic view of a search and rescue helicopter having a
receiver
searching for a transmitter in an emergency beacon.
Figure 2 is a diagrammatic plan view of the aircraft and transmitter of Figure
1 during a
search and rescue operation at a first time period.
Figure 3 is the view of Figure 2 during a search and rescue operation at a
second time
period.
Figure 4 is a view of the conical surfaces of Figures 2 and 3 superimposed on
each other to
produce an intersection path of the first and second conical surfaces.
Figure 4a is a planar view of an aircraft having an antenna of Figure 4
illustrating the first
second and third angles of the location of an emergency beacon.
Figure 5 is a schematic block diagram of a receiver according to one
embodiment of the
present invention.
Figure 6 is a diagrammatic plan view of a handheld receiver system at two
positions during
a search and rescue operation to locate a transmitter.
Detailed Description of Embodiments of the Invention
The present invention uses a coinbination of Doppler tracking with Global
Navigation Satellite System (GNSS) positioning information in order to
determine the
geographical position of the emergency beacon. A receiver that is slow moving
or stationary
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may also need an antenna array to determine direction to the transmitter and
an electronic
compass to determine receiver orientation.
The Doppler principle states that the measured frequency of a wave is relative
to the motion between the source and the observer. This Doppler shift is
observed when the
receiver is in motion and the transmitter stationary. This allows the receiver
to monitor the rate
of change of the Doppler shift. The specific rate of change correlates to a
relative position of
the target transmitter. Since the receiver knows its own position through GNSS
it can convert
the relative transmitter position to absolute position coordinates.
In one application which is not intended to be limiting the receiver according
to
the present invention is mounted aboard a search and rescue airplane that is
in flight. The
receiver will continually receive the stable homing signal from the emergency
beacon,
calculate the frequency received and save this information along with its GNSS
position and
time information. These data points are continually stored in memory and used
in a calculation
that observes the Doppler shift rate of change. The Doppler shift rate of
change can only fit
one of two possible solution of the position of emergency beacon (assuming it
is on the surface
of the earth), the real solution and its image. The image can be eliminated as
a possible
solution by having the search aircraft change it's flight path. A change in
flight path will
eliminate the image by creating another real solution and a different image.
The real solution is
the only solution that will remain constant. The accuracy and resolution of
the position of the
emergency beacon will improve as the plane gets closer and as more data points
are collected.
Referring to Figure 1, in a search and rescue (SAR) operation, an aircraft 8,
such as by way of non-limiting example a helicopter or an airplane, having an
antenna 10
searches a geographic area generally indicated at 7 for a transmitter 6. The
helicopter 8 also
has a GNSS also known as a GPS antenna and receiver to provide the location of
the
helicopter at any given time. The transmitter 6 may be an emergency beacon
which may be an
Emergency Position Indicating Radio Beacon (EPIRB), an Emergency Locator
Transmitter
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CA 02595388 2007-07-31
(ELT'), or a Personal Locator Beacon (PLB) or any other emergency type beacon
that transmits
a continuous homing signal as are known in the art.
Referring now to Figure 2, antenna 10 is shown at a first position 42 having a
first forward orientation 41 while searching for transmitter 6. The antenna 10
receives the
radio frequency (RF) signal transmitted by the transniitter. In the example
shown in Figure 2
antenna 10 is mounted to an aircraft 8 having a forward direction of movement
parallel to the
first forward orientation 41 of the antenna 10.
It is known that a signal transmitted by a stationary transmitter 6 that is
received by a moving antenna 10 will experience a shift in the measured
frequency from the
transmitted frequency known as the Doppler shift. According to the Doppler
shift, it is also
known that the frequency of the measured signal at the antenna will be higher
than the
transmitted signal while the antenna is moving towards the transmitter.
Conversely, the
measured signal will be lower than the transmitted signal when the antenna is
moving away
from the transmitter. In addition, it is also well known that the rate of
change of the Doppler
shift from positive to negative as the antenna approaches and then moves away
from the
transmitter can be utilized to determine the angle of the transmitter from the
forward direction
of movement of the antenna.
As illustrated in Figure 2, the antenna 10 measures the Doppler shift rate of
change of the signal transmitted by the transmitter 6. By the method and
apparatus as
described fiuther below, the first angle 40 of the transmitter 6 from the
antenna 10 direction 41
is calculated for the first position 42. It will be seen in Figure 2, that
although the angle from
the antenna is known which direction this angle 40 is oriented relative to the
antenna.
Accordingly the location of the transmitter 6 relative to the antenna 10 is
known to be located
on a conical surface 44 having an axis concentric with the first forward
orientation 41 of the
antenna at the first position 46.
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Turning now to Figure 3, the antenna 10 is shown at a second position 52
having a second forward orientation 51 while searching for transmitter 6. The
antenna 10
receives the RF signal transinitted by the transmitter. As illustrated in
Figure 3, the antenna 10
measures the Doppler shift rate of change of the signal transmitted by the
transmitter 6. By the
method and apparatus as described further below, the second angle 50 of the
transmitter 6 from
the antenna 10 direction 51 is calculated for the first position 52. As
described above, the
location of the transmitter is therefore calculated to be located upon a
conical surface. As was
illustrated in Figure 2, again as illustrated in Figure 3, although the angle
from the antenna is
known, it is not known which direction this angle 50 is oriented relative to
the antenna.
Accordingly the location of the transmitter 6 relative to the antenna 10 is
known to be located
on a conical surface 54 having an axis concentric with the first forward
orientation 51 of the
antenna at the second position 52.
Turning now to Figure 4, it is illustrated that the fist and second conical
surfaces 44 and 54 may be superimposed upon each other utilizing the known
positions and
speed of the antenna 10 at the first and second positions 42 and 52. When the
first and second
conical surfaces 44 and 54 are mathematically combined, it will be appreciated
that the
resulting solution plotting the points common to both surfaces 44 and 54 is an
ellipse
illustrated as line 58 in Figure 4. It will be appreciated that solving using
the first and second
conical surfaces 44 and 54 will therefore define an elliptical path 58 upon
which the
transmitter is located. A third reading may therefore be taken with the
antenna 10 at a third
position (not shown) to determine a third conical surface. When the first,
second and third
conical surfaces are mathematically combined, it will be appreciated that the
point location of
the transmitter 6 is the single intersection of all three of these conical
surfaces. This single
intersection is therefore the location of the transmitter 6. Further
measurements and
calculations will accordingly also intersect all previous conical surfaces at
the same point.
As illustrated in Figure 4a, an aircraft 30 is shown having a flight path 38 a
first
second and third positions 42, 52 and 62, respectively. Each of the first
second and third
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positions has an associated forward orientation or velocity vector. Using the
above method, a
first second and third conical surface 44, 54, and 64 respectively is
determined at first second
and third angles 40, 50 and 60, respectively from it's associated velocity
vector for each of the
first second and third positions 42, 52 and 62. The solution of the
intersection of the first
second and third conical surfaces 44, 54 and 64 may then be solved to locate
the position of
the target 36.
The above method will be useful where the exact frequency of the transmitter
is
known. However, in practices the transmitting frequency of the transmitter is
not known
precisely or may vary slightly. This is due to emergency broadcasting signals
being
designated within a specified band or inaccuracies in the emergency beacon. In
such a
circumstance, the measured frequency at the antenna is Doppler shifted from
the transmitted
frequency by an unknown amount. It is however known that for each possible
transmitted
frequency related to the measured frequency, a unique conical surface defines
the location of
the transmitter. Accordingly, where the transmitted frequency is not known, at
least three
Doppler shift rate of change measurements of the signal are measured as
described above. An
assumed transmission frequency is provided and the at least three conical
surfaces are
determined for the at Ieast three measurements. The convergence of the
intersection of these
three conical surfaces is calculated for the assumed frequency. Where the
unique intersection
of at least three conical surfaces is not achieved with the assumed frequency
it is known that
the actual broadcast frequency of the transmitter is not the assumed
frequency. A different
assumed frequency is then utilized with the same at least three measured
Doppler shift rates of
change to converge on the unique intersection of the at least three conical
surfaces.
Accordingly, what is to be solved for the above problem is the actual
broadcast frequency of
the transmitter 6 and the position of the transmitter given the knowledge of
the at least three
Doppler shift rate of change measurements and their associated known positions
and speeds.
Mathematical methods of solving this problem are well known and include
computational
geometry as well as other methods. Accordingly it will be observed that by
measuring at least
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three Doppler shift rates of change at three unique speeds and locations, both
the actual
broadcast frequency and the location of the transmitter may be calculated.
It will be appreciated that although the above method may be utilized with
only
three frequency measurements, inaccuracies in these measurements or
fluctuations in the
broadcast frequency may hinder the calculation of a precise location of the
transmitter from
three measurements alone. In such a case, solving the above matheinatical
model will produce
a location range of the transmitter. Further measurements may thereafter be
incorporated into
the mathematical model to converge the location range to the position of the
transmitter.
In a further embodiment of the above method, the measured frequency of the
first measurement may be utilized to narrow the frequency scanning band for
subsequent
measurements. Based upon the first frequency measurement, it is known, due to
knowing the
instantaneous speed of the aircraft what the maximum Doppler shifted frequency
of the
measured frequency is from the broadcast frequency. This narrowed range of
frequencies is
provided by calculating the actual frequencies assuming that the aircraft is
flying both directly
towards and away from the transmitter. These two extremes will provide the
narrowed range
of frequencies of the actual broadcast frequency. Further measurements may
therefore be
limited to this narrowed band.
Turning now to Figure 5, an exemplary non-limiting system utilizing the above
method is shown for determining the location of a transmitter. The system may
include a
master oscillator 24 which provides a very stable reference and
synchronization to the radio
receiver 14, analog to digital converter (ACD) 16 and digital signal
processing (DSP)
controller 22. The antenna 10 is connected to the radio receiver 14. The
antenna 10 may be a
single monopole antenna or alternatively multi-antenna array such as for
example a phased
array antenna. The radio receiver 14 is a standard superhetrodyne type
receiver that down
converts the received signal to an intermediate frequency (IF). This IF signal
is sampled by the
ADC 16 and converted into digital information. The DSP controller 22 processes
the sampled
CA 02595388 2007-07-31
digital information to very accurately deternzine the exact frequency of the
received homing
signal. The measured frequency along with the position from the GNSS Receiver
18 is time
stamped and stored in memory 26 for further processing. The DSP controller 22
later retrieves
the stored data to calculate the 'best fit' positional solution for the target
emergency
transmitter. This position solution is sent to the display unit 20. The system
may also include
an electronic compass 28 for measuring the orientation of the receiver
antenna. The DSP
controller 22 uses the orientation of the antenna 10 as provided by the
electronic compass 28
to determine the direction to the target.
Turning to Figure 6, a further embodiment of the present invention is
illustrated
in which a receiver system 70 having a directional antenna 71, such as, for
example, as
described above is handheld in a ground search scenario for locating a
transmitter 6. The
receiver 70 will continually or periodically receive the distress signal from
the emergency
beacon 6 to determine the direction to the transmitter and it's own relative
orientation and save
this information along with its GNS9. position information. The receiver 70
will, for example
measure the bearing to the transmitter from the directional antenna 71 from at
least two
positions 72 and 74 as determined by the GNSS information. The first and
second bearings 76
and 78 corresponding to the first and second positions 72 and 74 may then be
utilized to
calculate the position of the emergency beacon transmitter 6. The accuracy and
resolution of
the position of the emergency beacon will improve as the search personnel gets
closer and as
more data points are collected.
The current invention is not intended to be limited to the embodiments
described. As will be appreciated, the above method may be utilized with a
plurality of
different data received at the antenna. Examples of such data may include
frequency of the
received signal, direction of the received signal, time of arrival of the
received signal, position
of the receiver, velocity of the receiver, vector of the receiver and
orientation of the receiving
antenna. Utilization of a system as set out above of any of these or other
types of data received
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at the single receiver at a plurality of positions may stored in memory of the
receiver system
and used to calculate the unique location of the transmitter.
While specific embodiments of the invention have been described and
illustrated, such embodiments should be considered illustrative of the
invention only and not
as limiting the invention as construed in accordance with the accompanying
claims.
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