THREE DIMENSIONAL RADAR SYSTEM
BACKGROUND
1. TECHNICAL FIELD
The present invention relates to the field of radar, and more particularly, to
interferometric methods using same.
2. DISCUSSION OF RELATED ART
Using radar for three-dimensional (3D) mapping is advantageous in many applications in
poor visibility conditions, such as harsh weather, limited light, underwater environments
and the like. For example, there is a need for a Helicopter Flight and Landing Radar
(Heli-FLR) to assist pilots in avoiding potential hazards for helicopter safety during flight
phase (such as terrain, antennas, trees and power transmission wires), in various weather
conditions during day and night, and, furthermore to provide near-real time information
of the landing area (such as obstacles, inclined terrain, holes) during the landing phase. A
similar need exists in neighboring applications such as mapping obstacles for submarines
and remotely piloted aircrafts (RPA).
BRIEF SUMMARY
Embodiments of the present invention provide a radar system comprising a radar unit
arranged to rotate around a rotation axis in respect to a moving platform and a processor
connected to the receiver antennas. The radar unit comprises a transmitter antenna
arranged to transmit, cyclically around the rotation axis and sequentially such as to cover
a 360° azimuth angle around the rotation axis, a plurality of radar signals, each having a
spatial angle comprising a range of off-axis angles and a range of azimuth angles; and a
first and a second receiver antennas separated along the rotation axis, and arranged to
receive, for substantially each radar signal a first and a second echo, respectively. The
processor is arranged to: co-register the second echo in relation to positional data of the
platform; generate an interferogram of the first echo and the co-registered second echo;
apply a Kalman filter to the interferogram in relation to a phase confidence map such as
to calculate an unwrapped phase difference between the first and the second echo;
calculate an off-axis angle from the unwrapped phase; derive, from the calculated offaxis
angle, height information associated with the corresponding spatial angle of the radar
signal; calculate, from the off-axis angle, a phase forward projection; and update the
phase confidence map with the phase forward projection. The processor is arranged to
initially generate the phase confidence map for each spatial angle from a specified
surface model, and sequentially update the phase confidence map every cycle in relation
to the current cycle phase forward projection and to phase forward projections of former
cycles, relating to the radar signals of each spatial angle, and considering the platform
movement from cycle to cycle.
These, additional, and/or other aspects and/or advantages of the present invention are: set
forth in the detailed description which follows; possibly inferable from the detailed
description; and/or learnable by practice of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more readily understood from the detailed description of
embodiments thereof made in conjunction with the accompanying drawings of which:
Figure 1 is a high level schematic illustration of a radar sensor, according to some
embodiments of the invention;
Figure 2 is a high level flowchart illustrating the logical flow of the interferometry
algorithm, according to some embodiments of the invention;
Figure 3 is a high level schematic illustration of the maximum allowed phase differences
from one range cell to the other due to changes in topography, according to some
embodiments of the invention;
Figure 4 is a high level flowchart illustrating the logical flow of the Kalman filter,
according to some embodiments of the invention;
Figure 5 is a high level flowchart illustrating the logical flow of the Kalman filtering step
of the algorithm, according to some embodiments of the invention;
Figure 6 is a high level schematic illustration of the geometry for phase forward
projection, according to some embodiments of the invention; and
Figures 7A and 7B are flowcharts illustrating the method, according to some
embodiments of the invention.
DETAILED DESCRIPTION
Before explaining at least one embodiment of the invention in detail, it is to be
understood that the invention is not limited in its application to the details of construction
and the arrangement of the components set forth in the following description or
illustrated in the drawings. The invention is applicable to other embodiments or of being
practiced or carried out in various ways. Also, it is to be understood that the phraseology
and terminology employed herein is for the purpose of description and should not be
regarded as limiting.
For a better understanding of the invention, the usages of the following terms in the
present disclosure are defined in a non-limiting manner:
The term "nadir" as used herein in this application, is defined as the direction pointing
directly below a particular location; that is, the direction opposite to the zenith.
The term "off-nadir" as used herein in this application, is defined as the angle a line to a
certain point forms to the nadir.
Figure 1 is a high level schematic illustration of a radar sensor 100, according to some
embodiments of the invention. Radar sensor 100 may comprise a radar electronics unit
110 mounted below a flying platform (not shown, e.g., a helicopter) rotating around the Z
axis. Three antennas are attached to electronics unit 110. A transmitter 120 is arranged to
transmit a signal 125 to ground and the other two - a first receiver sensor 130 and a
second receiver sensor 140, separated vertically in different height positions, are arranged
to receive echoes 135, 145 (respectively) from the ground. These two receiving channels
are processed interferometrically for evaluation of the elevation angle of the incoming
echoes and hence determination of the position of the reflection point on ground. The
rotation time may comprise about 2 seconds allowing a highly dynamic update of the
three dimensional terrain model.
In embodiments, a one dimensional (ID) interferometric Radar algorithm for generating a
three dimensional (3D) model of the landscape beneath a flying platform (e.g., a
Helicopter) is provided. Radar 100 comprises transmitter antenna 120 and two receiver
antennas 130, 140 that are separated in height, along the rotation axis (e.g., vertically
when the rotation axis is the z axis). All antennas 120, 130, 140 are looking down at an
angle to the ground and are rotating 360° around the z axis, at a specified speed selected
to cover the entire area beneath the platform within a specified short time. Antennas 120,
130, 140 may comprise fan beams with small beams, e.g. smaller than 5 degrees in
azimuth and wide beams, e.g., larger than 50 degrees in elevation, for covering a large
area during each rotational scan. Radar signals 125 are transmitted towards the ground,
reflected from landscape and obstacles (135, 145) and are received in parallel by both
receiver antennas 130, 140. The phases of the two received signal chains 135, 145 are
processed by interferometric methods in order to derive the vertical (off-nadir) angle to a
reflector on ground allowing to estimate its height. The distance of the reflector to radar
100 is measured by the round trip delay time and the azimuth position according to the
pointing direction of antennas 120, 130, 140.
Radar sensor 100 may be housed in a RADOM 150 (a radar-transparent covering dome),
e.g., having a half spherical form.
Radar sensor 100 may comprise a transmit-receive switch (e.g., a circulator) connecting
one of receiving antennas 130, 140 with transmitter antenna 120 for routing transmitted
pulse 125 to transmitter antenna 120 and the corresponding one of received echoes 135,
145 from the corresponding one of receiving antennas 130, 140.
Transmitter antenna 120 may be positioned between receiver antennas 130, 140 as
illustrated in Figure 1, or aside from receiver antennas 130, 140, to minimize a
separation distance between the first and the second receiver antennas. Transmitter
antenna 120 may be a long transmitter antenna with a small azimuth beamwidth and may
be located close to the upper end (mounting point of the flying platform). For example,
transmitter antenna 120 may be 1.5 to 2 times longer and with a smaller azimuth
beamwidth in respect to receiver antennas 130, 140. Receiver antennas 130, 140 may be
smaller with a larger azimuth beamwidth. One of receiver antennas 130, 140 may be
located at the lower position of the rotating sensor 100 such as to minimize the volume of
the required spherical RADOM 150. The azimuth resolution may result from the twoway
beamwidth of the combined transmitter 120 and receiver 130, 140 antennas.
Figure 2 is a high level flowchart illustrating the logical flow of the interferometry
algorithm, according to some embodiments of the invention.
In a first step, the radar data of both sensors are pulse compressed 203, 204. Then, the
pulse-compressed data of second sensor 140 is co-registrated 205 to the data of first
sensor 130. Because of the geometry, the resolution and the small baseline, difference
between both data sets is very limited. Nevertheless, co-registration 205 is performed in
order to reduce phase errors and is realized by multiplying second signal 145 with a
phase shift:
in frequency domain, where tShift represents the time difference between both received
data with respect to sensor positions and flat earth geometry.
After co registration, multiplying first signal 135 with the complex conjugated of second
signal 145 is used to generate the interferogram (stage 210).
In a pre-processing step, phase gradients are estimated out of the interferogram and serve
as driving model for the Kalman filter process (stage 230). For single pass interferometry
the height error s depends on the phase noise s , wavelength l, distance R, look angle Q
and the perpendicular baseline length Bperp according to:
The maximum detectable height difference (Height of Ambiguity) can be calculated from
equation 2 by setting s y= 2p. After sampling of the Radar signals, the maximum allowed
phase differences from one range cell to the other due to changes in topography (Figure
3) can be expressed as:
The Tc path is identical (one separate Tx antenna 120). RAI and RA2 are the distances
from a point A to the receiving antennas Rxl 130 and Rx2 140. RBI and RB2 are the
distances from a point B that is located in an adjacent range cell. To be able to unwrap
the phases unambiguous, the magnitude of the phase difference introduced by the
topography has to be less than p. Due to the extreme near field geometry, high and steep
objects lead to height-ambiguity problems (2p jumps). To overcome this problem a phase
confidence map is generated (stage 225) by forward projection (stage 220) of old
measured data and current platform-position, velocity and attitude data. This phase
confidence map (225) is used as an input of the Kalman filtering (230).
The result of the Kalman filtering 230 is the unwrapped phases 240. Finally, off-nadir
angles 250 can be calculated. On one hand, they are used to estimate the appropriate
height information (255), and on the other hand, they serve as input to the phase forward
projection (220) to compute the phase confidence map (225).
The distance between the receiver antennas 130, 140 may be selected according to a
relation between expected motion characteristics of the moving platform and an expected
topographical variability of the height information. For example, the distance may be
adapted to a scenario of an helicopter flying above an urban landscape, with the
corresponding flight characteristics (e.g. flight height) and topographical characteristics
of the landscape (e.g. height and spacing between the buildings).
Figure 4 is a high level flowchart illustrating the logical flow of the Kalman filter,
according to some embodiments of the invention. The interferometry processing
algorithm is based on the Unscented Kalman filter to unwrap the interferometric phases.
The Kalman filter and all its derivates employ a prediction/correction structure based on a
state-space model 310 and an observation model 320 to estimate the true state (340) of
the measured variable (the interferometric phase in this case). It also incorporates noise
into the state-space model as well as the observation model. By this, the data are treated
as measurements of a hidden random process of a state variable.
The function /describes state-space model 310 in terms of:
time step, i = 0,...,n-l.
Xi = x(ti): State of the ¾ time step to be estimated.
U = u(ti): known input of the ¾ time step.
driving error covariance of the ¾ time step.
The function h describes observation model 320 in terms of:
Z = z(ti): given measurements of the time step
i = v(ti): driving error covariance of the time step
The original Kalman filter assumes that the state-space model and the observation model
are linear and the noise is Gaussian with zero mean.
In the Unscented Kalman filter, these assumptions are relaxed and it allows nonlinear
functions f and h. The predicted error covariance is approximated as a Gaussian density
by a set of deterministically chosen sample points. These are projected forward (330) by
state-space model 310 and describe the estimated mean and covariance.
For our case, the state to be estimated is the unwrapped interferometric phase. The
observations of this state are the two parts of the complex interferogram, the real and the
imaginary part. Observation model 320:
The state-space model 310 is the linearization of the phase, so the "known" inputs are the
phase gradients. State-space model 310:
The gradients of the true phase are not known exactly. They are estimated out of the
noisy measured data.
Mode extraction (350, Figure 5). The phase gradients can be observed as frequency shift
of the autocorrelation function of the interferogram (370, Figure 5). For a time interval of
length the interferogram s is : with a phase
function f and an amplitude function a.
This can be rewritten as:
The integral part is a dynamic frequency variation. The linear part describes the
phase slope with a constant frequency
Autocorrelating the signal gives:
The power spectral density of the signal therefore can be seen as a time shifted version of
the power spectral density of the function y. If the power spectral density is unimodal and
approximately symmetric, the frequency f o can be extracted as the mode of the power
spectral density.
With the normalized power spectral density, we can also extract the spectral bandwidth
of the signal as the second central moment, which relates to the driving error covariance
. via
Figure 5 is a high level flowchart illustrating the logical flow of the Kalman filtering step
of the algorithm, according to some embodiments of the invention.
The extracted modes 350 are bound by the bandwidth of the system. If the spectral
bandwidth, and thus the driving error covariance are too high, extracted modes 350 are
not reliable, due to wrap around effects of the Fast Fourier Transform (FFT).
In this case, the phase gradients would be replaced by the local slope 355 of the true
interferometric phases corresponding to the flat earth signal. These can be pre-generated
out of the current position and antenna pointing information of the flying platform (e.g., a
helicopter).
As modes 350 are extracted from the noisy data, the real phase gradients are only
estimated by modes 350. Wrong modes can lead the filter to insert wrong 27i-phase jumps
into the estimated phase, which must be corrected.
Out of the previously computed flat earth true interferometric phases and the forward
projected known phases, a map of offsets is generated which give the integer multiple of
p of the phase corresponding to every time step. This is called the phase confidence map
360. If the integer multiple of p of the estimated interferometric phase deviates more than
2p off offset map 260 corresponding to the flat earth signal and the projected phases, the
difference between the two will be added (correction 375). By this, we can bind the
estimated interferometric phase to a realistic height profile.
Figure 6 is a high level schematic illustration of the geometry for phase forward
projection, according to some embodiments of the invention. In the near field geometry,
high and steep objects lead to height-ambiguity problems (2p jumps) whereas the phase
of the same object in far range can be unwrapped without problems. To make use of this
behavior the phase forward projection was introduced.
As an initializing step the flat earth phase is used.
Descending angle b and distance Rplatform are known from platform movement. Off-nadir
angles 0 meas are calculated from unwrapped phases, using the following formula:
where:
is the measured off-nadir angles; is the unwrapped phase; l is the
wavelength; is the angle of baseline from horizon; Ri is the slant range sampling; and B
is the baseline length.
The forward projected range can be calculated by:
and then the forward projected off-nadir angles by:
By solving Equation 10 for the unwrapped phases f and replacing and Ri with
and Rproj we get the forward projected phases that have to be resampled and
interpolated to the current slant range sampling.
Using the forward projected phases, phase confidence map 360 can be calculated by:
This phase confidence map 360 then is used during Kalman filtering to track phase jumps
introduced by the topography or objects.
To summarize, the objective of the algorithm is to derive a 3D model of a scene under a
Radar 100 rotating around a vertical axis transmitting with one antenna 120 and receiving
the echoes with two antennas 130, 140 located at different height positions. The two
antennas 130, 140 receive the echoes of the terrain and process a 3D model out of them
based on a one dimensional interferometric principle.
The new algorithm for this processing makes use of known procedures from two
dimensional interferometric processing and applies it to the one dimensional case that is
often mentioned in the literature as reference for the two dimensional case, but was never
realized in its original form. The one dimensional interferometric processing suffers from
effects (ambiguities, phase unwrapping uncertainties, etc.) which can easily be solved in
the two dimensional case, but need new procedures to be solved in the one dimensional
one.
The method takes the complex echoes 135, 145 in digital form. After preprocessing of the
single echoes for reduction of noise and statistical fluctuations (speckle), an appropriate
filtering allows the evaluation of the height of each reflection point on ground from the
phase difference in the echo data. The problem of phase confidence is solved by Kalman
filtering and applying a feedback loop for phase forward projection and generation of a
phase confidence map for further use in the Kalman filtering process.
The algorithm uses a Kalman filter for phase unwrapping. The platform's position,
velocity and attitude are used for estimating phase signals of the illuminated scene after
each radar cycle. A phase forward projection 330 for each new (¾) phase signal is
estimated in respect to the position, velocity and attitude of the platform at the current (¾)
and at the former (¾- ΐ ) radar cycle. Phase confidence map 360 is generated using the
phase forward projection 330. Phase confidence map 360 is further used to solve
ambiguity effects and uncertainties created by noise in the signals. The estimated phases
from previous scans are used to update and correct the results of phase unwrapping
during the Kalman filter process. The corrected phases are used together to generate a
three dimensional position determination of radar reflections from the surface leading to
the three dimensional terrain model.
Figures 7A and 7B are flowcharts illustrating the method, according to some
embodiments of the invention. The method comprises the following stages: defining a
rotation axis in respect to a moving platform, a range of off-axis angles in respect to the
rotation axis, and a range of azimuth angles, in respect to the rotation axis (stage 400);
transmitting, cyclically around the rotation axis and sequentially such as to cover a 360°
azimuth angle around the rotation axis, a plurality of radar signals, each having a spatial
angle comprising the range of off-axis angles and the range of azimuth angles (stage
405). For substantially each radar signal: receiving a first and a second echo (stage 410);
co-registering the second echo in relation to positional data of the platform (stage 415);
generating an interferogram of the first echo and the co-registered second echo (stage
420); applying a Kalman filter to the interferogram in relation to a phase confidence map
such as to calculate an unwrapped phase difference between the first and the second echo
(stage 425); calculating an off-axis angle from the unwrapped phase difference (stage
430); deriving, from the calculated off-axis angle, height information associated with the
corresponding spatial angle of the radar signal (stage 435) (e.g., derivation may be
completed during a single cycle of the transmitter antenna); calculating, from the off-axis
angle, a phase forward projection (stage 440); and updating the phase confidence map
with the phase forward projection (stage 445).
The phase confidence map is initially generated for each spatial angle from a specified
surface model, and sequentially updated every cycle in relation to the current cycle phase
forward projection and to phase forward projections of former cycles, relating to the radar
signals of each spatial angle, and considering the platform movement from cycle to cycle.
In embodiments, the method may further comprise, for substantially each radar signal,
the following stages: estimating a distance to a source of the echoes by measuring a
round trip delay time of the radar signal (stage 450); estimating an azimuth of the source
from the spatial angle of the radar signal (stage 455); and deriving a three dimensional
(3D) model from the height information, the distances and the azimuths (stage 460).
The transmitter antenna and the first and second receiver antennas may be aligned such
that the interferogram is one dimensional (ID).
The moving platform may be a flying platform, the rotation axis is a nadir axis, and the
derived 3D model is of a landscape below the flying platform.
In embodiments, the method may further comprise positioning the transmitter antenna
between the receiver antennas such as to maximize a separation distance between the first
and the second receiver antennas (stage 465) and/or positioning the transmitter antenna
and the receiver antennas such as to maximize a separation distance between the receiver
antennas and to maximize a length of the transmitter antenna under specified RADOM
size and form limitations (stage 470).
In embodiments, the method may further comprise selecting a distance between the first
and the second receiver (receiving the first and the second echo 410 being carried out by
a first and a second receiver antenna respectively) according to a relation between
expected motion characteristics of the moving platform and an expected topographical
variability of the height information stage 407, see Figure 3).
In embodiments, the method may further comprise generating alerts relating to the height
information (stage 475), e.g., alerts from nearing objects. The method may further
comprise visualizing the 3D model (stage 480).
In embodiments, the method may further comprise preprocessing the first and the second
echoes after receiving them to reduce noise and statistical fluctuations (stage 490), and
co-registering the first echo as well as the second echo and wherein the interferogram is
generated from the co-registered first and second echo (stage 485).
The procedure compensates for movements of the flying platform by using information
from a navigation unit measuring the speed vector and the attitude of the platform. This
new method allows generating a terrain model within one cycle of the antenna assembly
azimuth rotation and improves the accuracy of this model with every new rotation cycle
lasting in the order of a few seconds, for example.
Advantageously, in some embodiments, the invention satisfies the need for a Helicopter
Flight and Landing Radar (Heli-FLR) to assist pilots in avoiding potential hazards for
helicopter safety during flight phase (such as terrain, antennas, trees and even wires), in
various weather conditions during day and night, and furthermore, to provide near-real
time information of the landing area (such as obstacles, inclined terrain, holes) during the
landing phase. The invention uses an advanced one dimensional innovative
interferometric method and intuitive visualization. Landing and flight modes may be
combined into a light weight and low cost radar-based system. This new system may
prevent accidents and casualties and improve the service of helicopter operation
companies by increasing the flight capabilities in difficult conditions and the availability
of the helicopters.
In embodiments, the system is able to detect obstacles such as terrain, trees, antennas,
wires, etc., in flight mode up to 2000m thus providing the pilot 30 seconds alert prior to
the collision (even in degraded weather conditions, the system may still provide and alert
at least 10 seconds ahead). In landing mode, the system may be effective from 500 feet
(150 meters), detect obstacles and holes with a resolution of 30 cm during the approach
as well as measuring the slope angle in the landing area. The angular coverage of
detecting obstacles in the landing area may be 360 degrees with a refresh rate lower than
2 seconds. The information may be presented to the pilot on a multi-function display in
an intuitive human factors concept to allow a reduction of pilot workload and enhancing
situational awareness. Vocal alerts may inform the pilot in case of danger.
During landing mode, the radar system may realize a 360° mapping of the terrain below
the helicopter. High elevation beamwidth of the antennas may guarantee coverage of
large areas. The inventors have discovered that the 3D-mapping is exceptionally well
realized by transmitting a 36 GHz signal through one antenna 120 and receiving the
return from two different other antennas 130, 140. This frequency allows using the
interferometric methods with minimal hardware component size. A one dimensional
interferometric data-processing may then permit to retrieve the information from the
signals 135, 145.
The 3D-mapping may be realized in quasi real-time, during the rotation of the system. A
full rotation and therefore the complete landing zone may be covered in less than 2
seconds.
Data-base modeling integrating real-time and pre-loaded information: In traditional data
base systems, the algorithms and the display are based on pre-loaded information which
does not allow pilots to view real-time information (such as a new obstacle) on the same
system. Innovative algorithms that are implemented in the system allow integration of
pre-loaded data (such as Digital Terrain Elevation Data and obstacles) and real-time data.
The design concept is based on comparison of each new detected obstacle with the data
base and updating it if needed. The integration of the real-time and pre-loaded data shall
increase the level of confidence in the real-time data and enhance the situational
awareness of the pilots, allowing them to view all the relevant information on the same
display.
In recent years, systems became more sophisticated and the amount of information
needed to be displayed has become enormous. In order to reduce the pilot workload, to
enhance situational awareness and to shorten training time, a display combining intuitive
symbology, terrain and relevant obstacles information may be implemented based on
techniques such as color coding, display complexity reduction and data fusion.
In the above description, an embodiment is an example or implementation of the
inventions. The various appearances of "one embodiment", "an embodiment" or "some
embodiments" do not necessarily all refer to the same embodiments.
Although various features of the invention may be described in the context of a single
embodiment, the features may also be provided separately or in any suitable combination.
Conversely, although the invention may be described herein in the context of separate
embodiments for clarity, the invention may also be implemented in a single embodiment.
Reference in the specification to "some embodiments", "an embodiment", "one
embodiment" or "other embodiments" means that a particular feature, structure, or
characteristic described in connection with the embodiments is included in at least some
embodiments, but not necessarily all embodiments, of the inventions.
It is to be understood that the phraseology and terminology employed herein is not to be
construed as limiting and are for descriptive purpose only.
The principles and uses of the teachings of the present invention may be better
understood with reference to the accompanying description, figures and examples.
It is to be understood that the details set forth herein do not construe a limitation to an
application of the invention.
Furthermore, it is to be understood that the invention can be carried out or practiced in
various ways and that the invention can be implemented in embodiments other than the
ones outlined in the description above.
It is to be understood that the terms "including", "comprising", "consisting" and
grammatical variants thereof do not preclude the addition of one or more components,
features, steps, or integers or groups thereof and that the terms are to be construed as
specifying components, features, steps or integers.
If the specification or claims refer to "an additional" element, that does not preclude there
being more than one of the additional element.
It is to be understood that where the claims or specification refer to "a" or "an" element,
such reference is not to be construed that there is only one of that element.
It is to be understood that where the specification states that a component, feature,
structure, or characteristic "may", "might", "can" or "could" be included, that particular
component, feature, structure, or characteristic is not required to be included.
Where applicable, although state diagrams, flow diagrams or both may be used to
describe embodiments, the invention is not limited to those diagrams or to the
corresponding descriptions. For example, flow need not move through each illustrated
box or state, or in exactly the same order as illustrated and described.
Methods of the present invention may be implemented by performing or completing
manually, automatically, or a combination thereof, selected steps or tasks.
The term "method" may refer to manners, means, techniques and procedures for
accomplishing a given task including, but not limited to, those manners, means,
techniques and procedures either known to, or readily developed from known manners,
means, techniques and procedures by practitioners of the art to which the invention
belongs.
The descriptions, examples, methods and materials presented in the claims and the
specification are not to be construed as limiting but rather as illustrative only.
Meanings of technical and scientific terms used herein are to be commonly understood as
by one of ordinary skill in the art to which the invention belongs, unless otherwise
defined.
The present invention may be implemented in the testing or practice with methods and
materials equivalent or similar to those described herein.
Any publications, including patents, patent applications and articles, referenced or
mentioned in this specification are herein incorporated in their entirety into the
specification, to the same extent as if each individual publication was specifically and
individually indicated to be incorporated herein. In addition, citation or identification of
any reference in the description of some embodiments of the invention shall not be
construed as an admission that such reference is available as prior art to the present
invention.
CLAIMS
What is claimed is:
1. A radar system comprising:
a radar unit arranged to rotate around a rotation axis in respect to a moving
platform, the radar unit comprising:
(i) a transmitter antenna arranged to transmit, cyclically around the
rotation axis and sequentially such as to cover a 360° azimuth
angle around the rotation axis, a plurality of radar signals, each
having a spatial angle comprising a range of off-axis angles and
a range of azimuth angles; and
(ii) a first and a second receiver antennas, separated along the rotation
axis, and arranged to receive, for substantially each radar signal
a first and a second echo, respectively; and
a processor connected to the receiver antennas, and arranged to:
(a) co-register the second echo in relation to positional data of the
platform;
(b) generate an interferogram of the first echo and the co-registered
second echo;
(c) apply a Kalman filter to the interferogram in relation to a phase
confidence map such as to calculate an unwrapped phase difference
between the first and the second echo;
(d) calculate an off-axis angle from the unwrapped phase;
(e) derive, from the calculated off-axis angle, height information
associated with the corresponding spatial angle of the radar signal;
(f) calculate, from the off-axis angle, a phase forward projection; and
(g) update the phase confidence map with the phase forward projection,
wherein the processor is arranged to initially generate the phase confidence map for
each spatial angle from a specified surface model, and sequentially update the phase
confidence map every cycle in relation to the current cycle phase forward projection and
to phase forward projections of former cycles, relating to the radar signals of each spatial
angle, and considering the platform movement from cycle to cycle.
2. The radar system of claim 1, wherein the moving platform is a flying platform and the
rotation axis is vertical to the platform.
3. The radar system of claim 1, wherein the transmitter antenna is positioned between
the first and the second receiver antennas, to maximize a separation distance between
the first and the second receiver antennas.
4. The radar system of claim 1, wherein the transmitter antenna and the first and second
receiver antennas are aligned to yield a one dimensional interferogram.
5. The radar system of claim 1, wherein the transmitter antenna is positioned aside of
the first and the second receiver antennas.
6. The radar system of claim 1, wherein the transmitter antenna and the first and the
second receiver antennas are fan beams with relatively smaller beams in azimuth and
wider beams in elevation, for covering a large area during each rotational scan.
7. The radar system of claim 1, wherein the processor is further arranged to calculate a
distance of a reflector from a round trip delay time and an azimuth position of the
antennas.
8. The radar system of claim 1, enclosed within a half spherical RADOM.
9. The radar system of claim 8, wherein the transmitter antenna and the receiver
antennas are positioned to maximize a separation distance between the receiver
antennas and to maximize a length of the transmitter antenna under specified
RADOM size and form limitations.
10. The radar system of claim 8, wherein the transmitter antenna is positioned at an upper
third of the radar system and one of the receiver antennas is positioned in a lower
third of the radar system, and wherein the antennas are configured to minimize the
volume of the required spherical RADOM.
11. The radar system of claim 1, further comprising a transmit-receive switch connecting
one of the receiver antennas with the transmitter antenna and arranged to route a
transmitted signal to the transmitter antenna and a corresponding one of received
echoes to the connected receiver antenna.
12. The radar system of claim 1, wherein the transmitter antenna is elongated with a
smaller azimuth beamwidth and the receiver antennas are relatively with a larger
azimuth beamwidth in respect to the transmitter antenna.
13. The radar system of claim 1, wherein the distance between the receiver antennas is
selected according to a relation between expected motion characteristics of the
moving platform and an expected topographical variability of the height information.
14. A method comprising:
defining a rotation axis in respect to a moving platform, a range of off-axis angles
in respect to the rotation axis, and a range of azimuth angles, in respect to the rotation
axis;
transmitting, cyclically around the rotation axis and sequentially such as to cover
a 360° azimuth angle around the rotation axis, a plurality of radar signals, each having a
spatial angle comprising the range of off-axis angles and the range of azimuth angles;
for substantially each radar signal:
receiving a first and a second echo;
co-registering the second echo in relation to positional data of the
platform;
generating an interferogram of the first echo and the co-registered second
echo;
applying a Kalman filter to the interferogram in relation to a phase
confidence map such as to calculate an unwrapped phase difference between the
first and the second echo;
calculating an off-axis angle from the unwrapped phase difference;
deriving, from the calculated off-axis angle, height information associated
with the corresponding spatial angle of the radar signal;
calculating, from the off-axis angle, a phase forward projection; and
updating the phase confidence map with the phase forward projection,
wherein the phase confidence map is initially generated for each spatial angle from
a specified surface model, and sequentially updated every cycle in relation to the current
cycle phase forward projection and to phase forward projections of former cycles,
relating to the radar signals of each spatial angle.
15. The method of claim 14, further comprising initially generated the phase confidence
map for each spatial angle from a specified surface model, and sequentially updating
every cycle in relation to the current cycle phase forward projection and to phase
forward projections of former cycles, relating to the radar signals of each spatial
angle, and considering the platform movement from cycle to cycle.
16. The method of claim 14, further comprising:
estimating a distance to a source of the echoes by measuring a round trip delay
time of the radar signal;
estimating an azimuth of a source from the spatial angle of the radar signal; and
deriving a three dimensional (3D) model from the height information, the
distances and the azimuths.
17. The method of claim 14, further comprising generating alerts relating to the height
information.
18. The method of claim 14, further comprising visualizing the 3D surface model.
19. The method of claim 14, further comprising preprocessing the first and the second
echoes after receiving them to reduce noise and statistical fluctuations.
20. The method of claim 14, further comprising co-registering the first echo as well as the
second echo and generating the interferogram from the co-registered first and second
echo.
21. The method of claim 14, wherein the receiving the first and the second echo is carried
out by a first and a second receiver antenna respectively, and further comprising
selecting a distance between the first and the second receiver according to a relation
between expected motion characteristics of the moving platform and an expected
topographical variability of the height information