U.S. patent application number 14/263820 was filed with the patent office on 2014-10-30 for systems and methods for tracking power modulation.
This patent application is currently assigned to Interstate Electronics Corporation. The applicant listed for this patent is Interstate Electronics Corporation. Invention is credited to Steven B. Alexander, Richard Redhead.
Application Number | 20140320339 14/263820 |
Document ID | / |
Family ID | 41724555 |
Filed Date | 2014-10-30 |
United States Patent
Application |
20140320339 |
Kind Code |
A1 |
Alexander; Steven B. ; et
al. |
October 30, 2014 |
SYSTEMS AND METHODS FOR TRACKING POWER MODULATION
Abstract
Apparatus and methods determine the rotational position of a
spinning object. A satellite positioning system can be used to
determine the spatial position of an object, which in turn can be
used to guide the object. However, when the object is spinning,
such as an artillery shell, then the rotational orientation should
be known in order to properly actuate the control surfaces, such as
fins, which will also be spinning.
Inventors: |
Alexander; Steven B.;
(Orange, CA) ; Redhead; Richard; (Mission Viejo,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Interstate Electronics Corporation |
Anaheim |
CA |
US |
|
|
Assignee: |
Interstate Electronics
Corporation
Anaheim
CA
|
Family ID: |
41724555 |
Appl. No.: |
14/263820 |
Filed: |
April 28, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13492447 |
Jun 8, 2012 |
8711035 |
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14263820 |
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13189962 |
Jul 25, 2011 |
8199052 |
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13492447 |
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12231315 |
Aug 29, 2008 |
7986265 |
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13189962 |
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Current U.S.
Class: |
342/357.36 |
Current CPC
Class: |
G01S 19/53 20130101;
G01S 3/22 20130101 |
Class at
Publication: |
342/357.36 |
International
Class: |
G01S 19/53 20060101
G01S019/53 |
Claims
1. (canceled)
2. An apparatus comprising: a projectile antenna up estimator
configured to: receive at least a roll phase and roll rate of an
amplitude modulation of a signal from a first space vehicle,
wherein the amplitude modulation is caused by spinning of a
projectile; receive an azimuth angle associated with the first
space vehicle; and determine a first state of a spin of the
projectile for a first time based at least partly on the roll
phase, roll rate, and the azimuth angle.
3. The apparatus of claim 2, wherein the projectile antenna up
estimator is further configured to receive a roll acceleration of
the amplitude modulation of the signal from the first space
vehicle, wherein the first state of the spin is further determined
based on the roll acceleration.
4. The apparatus of claim 2, further comprising a next up time
estimator configured to estimate a future time when the antenna of
the projectile will be pointing up based on information derived
from the first state.
5. The apparatus of claim 2, further comprising a current roll
estimate generator configured to: extrapolate a second state of the
projectile for a second time based on the first state and the first
time; and estimate a third time when the antenna of the projectile
will be pointing up based at least partly on the second state.
6. The apparatus of claim 2, wherein the projectile antenna up
estimator is further configured to: receive roll phase and roll
rate of an amplitude modulation of a plurality of signals from a
plurality of space vehicle, wherein the amplitude modulation is
caused by spinning of the projectile; receive azimuth angles
associated with the plurality of space vehicles; determine a first
plurality of states of the spin of the projectile based at least
partly on the roll phase, roll rate, and the azimuth angle for each
of the plurality of signals; estimate a plurality of future times
when the antenna of the projectile will be pointing up; and combine
at least a portion of the plurality of future times to generate an
index for flight control.
7. The apparatus of claim 6, wherein the projectile antenna up
estimator is further configured to: receive roll acceleration of
the amplitude modulation for the plurality of signals from the
plurality of space vehicles, wherein the first plurality of states
of the spin are further determined based on the roll acceleration
for each of the plurality of signals.
8. The apparatus of claim 6, wherein the projectile antenna up
estimator is further configured to combine the at least portion of
the plurality of future times by averaging.
9. The apparatus of claim 6, further comprising a current roll
estimate generator configured to extrapolate a second plurality of
states of the projectile for a current time based on the first
plurality of states, wherein the plurality of future times are
estimated based at least partly on the second plurality of
states.
10. The apparatus of claim 6, further comprising a measurement
window configured to filter out measurements such that the at least
portion of the plurality of future times that are combined
correspond to the future times corresponding to signals received
with an azimuth angle between 45 degrees and 135 degrees.
11. The apparatus of claim 6, wherein the projectile antenna up
estimator comprises a Kalman filter.
12. A method of generating spin information for flight control of a
projectile, the method comprising: receiving at least a roll phase
and roll rate of an amplitude modulation of a signal from a first
space vehicle, wherein the amplitude modulation is caused by
spinning of a projectile; receiving an azimuth angle associated
with the first space vehicle; and determining a first state of a
spin of the projectile for a first time based at least partly on
the roll phase, roll rate, and the azimuth angle.
13. The method of claim 12, further comprising: receiving a roll
acceleration of the amplitude modulation of the signal from the
first space vehicle; wherein the first state of the spin is further
determined based on the roll acceleration.
14. The method of claim 12, further comprising estimating a future
time when the antenna of the projectile will be pointing up based
on information derived from the first state.
15. The method of claim 12, further comprising: extrapolating a
second state of the projectile for a second time based on the first
state and the first time; and estimating a third time when the
antenna of the projectile will be pointing up based at least partly
on the second state.
16. The method of claim 12, further comprising: receiving roll
phase and roll rate of an amplitude modulation of a plurality of
signals from a plurality of space vehicle, wherein the amplitude
modulation is caused by spinning of the projectile; receiving
azimuth angles associated with the plurality of space vehicles;
determining a first plurality of states of the spin of the
projectile based at least partly on the roll phase, roll rate, and
the azimuth angle for each of the plurality of signals; estimating
a plurality of future times when the antenna of the projectile will
be pointing up; and combining at least a portion of the plurality
of future times to generate an index for flight control.
17. The method of claim 15, further comprising: receiving roll
acceleration of the amplitude modulation for the plurality of
signals from the plurality of space vehicles; wherein the first
plurality of states of the spin are further determined based on the
roll acceleration for each of the plurality of signals.
18. The method of claim 15, wherein combining comprises
averaging.
19. The method of claim 15, further comprising: extrapolating a
second plurality of states of the projectile for a current time
based on the first plurality of states; wherein the plurality of
future times for when the antenna of the projectile will be
pointing up are estimated based at least partly on the second
plurality of states.
20. The method of claim 15, wherein the at least portion of the
plurality of future times that are combined correspond to the
future times corresponding to signals received with an azimuth
angle between 45 degrees and 135 degrees.
21. The method of claim 12, wherein determining comprises using a
Kalman filter.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of U.S.
application Ser. No. 13/492,447, filed Jun. 8, 2012, now U.S. Pat.
No. 8,711,035, issued on Apr. 29, 2014, which is a continuation
application of U.S. application Ser. No. 13/189,962, filed Jul. 25,
2011, now U.S. Pat. No. 8,199,052, issued on Jun. 12, 2012, which
is a continuation application of U.S. application Ser. No.
12/231,315, filed Aug. 29, 2008, now U.S. Pat. No. 7,986,265,
issued on Jul. 26, 2011, the entireties of which are incorporated
by reference herein.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The invention generally relates to electronics. In
particular, the invention relates to a receiver for a satellite
navigation system, such as the NAVSTAR Global Positioning System
(GPS).
[0004] 2. Description of the Related Art
[0005] A Global Positioning System (GPS) satellite radiates a
spread spectrum, pseudorandom noise (PN) signal indicating the
satellite's position and time. A GPS receiver that receives signals
from a plurality of satellites can compute the distance to each
satellite and then calculate the receiver's spatial position,
spatial velocity, and time.
[0006] GPS receivers can be used in a broad variety of
environments. One application is to provide spatial position and
spatial velocity for a projectile, such as an artillery shell. This
information can then be used by a guidance system of the projectile
to guide the projectile to its intended destination.
[0007] Flight corrections can be made by manipulating fins on the
projectile. However, the projectile can be configured to spin along
an axis in its line of flight to stabilize flight. This spin
affects the flight controls for guidance, as the fins will spin
with the projectile. In order to make proper flight corrections,
the rotational orientation of the projectile should be known.
SUMMARY
[0008] Apparatus and methods determine the rotational position of
an object. A satellite positioning system can be used to determine
the spatial position of an object, which in turn can be used to
guide the object. However, for guidance of an object that spins
during flight, such as an artillery shell, then the rotational
orientation should also be known in order to properly time the
actuation of the control surfaces. Disclosed techniques ascertain
the rotational position from the satellite positioning system
signals. The disclosed techniques can be used with the
coarse/acquisition C/A code, precise P(Y) code, or both the C/A
code and the P(Y) code.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] These drawings the associated description herein are
provided to illustrate specific embodiments of the invention and
are not intended to be limiting.
[0010] FIG. 1 is a schematic diagram drawn looking down the nose of
a projectile and illustrates timing references and nomenclature
used for the projectile and space vehicles (SVs).
[0011] FIG. 2 illustrates a top-level block diagram of a GPS launch
system and a GPS projectile system according to an embodiment of
the invention.
[0012] FIG. 3 illustrates one embodiment of a GPS receiver for
determining the next time that the antenna will be facing up
t_up.
[0013] FIG. 4 illustrates an embodiment of a SV power modulation
tracking filter according to an embodiment of the invention.
[0014] FIG. 5 illustrates a phase rotator/mixer.
[0015] FIG. 6 illustrates an embodiment of a frequency-locked loop
(FLL) acquisition loop.
[0016] FIG. 7 illustrates an embodiment of a 4th order phase-locked
loop (PLL) SV modulation tracking loop.
[0017] FIG. 8 illustrates a process showing state transitions for
GPS-based "up" estimation.
[0018] FIG. 9 illustrates an aspect angle .alpha..sub.u.
[0019] FIG. 10 illustrates an azimuth angle az.sub.u.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0020] Although particular embodiments are described herein, other
embodiments of the invention, including embodiments that do not
provide all of the benefits and features set forth herein, will be
apparent to those of ordinary skill in the art.
[0021] While illustrated in the context of the NAVSTAR Global
Positioning System (GPS), the principles and advantages described
herein are applicable to other positioning systems, such as, but
not limited to, the Russian GLONASS system, the European Galileo
system, the Chinese COMPASS system, the Indian IRNSS system, or the
like.
[0022] FIG. 1 is a schematic diagram drawn looking down the nose of
a projectile 102 and illustrating timing references and
nomenclature used for the projectile 102 and space vehicles (SVs).
FIG. 1 depicts how SVs in view are used in combination to estimate
the projectile's roll.
[0023] FIG. 1 illustrates the projectile 102 in the center of the
diagram, one or more antennas 104, and one or more fins 106. The
illustrated techniques can assess when an antenna 104 of the
projectile 102 is effectively pointing to a space vehicle.
[0024] In the diagram, the projectile 102 is shown rotating
counter-clockwise; however, the techniques disclosed herein are
applicable to rotation in either direction. The direction of the
rotation will follow the rifling of a barrel of a cannon and is
typically known before the projectile is launched from the
cannon.
[0025] Typically, the space vehicles sv.sub.1, sv.sub.2, sv.sub.3,
and sv.sub.4, are the sources for GPS signals. However, other
sources, such as beacons, can be applicable. The number of space
vehicles used by the projectile 102 can vary in a broad range, but
typically reception to at least four space vehicles is needed to
resolve spatial position.
[0026] The receiver for the projectile 102 has a plurality of
phase-locked loops (PLLs) for determining the angular orientation
of SVs based on signal power modulation. In the illustrated
embodiment, for each SV, there is a corresponding PLL of the
receiver for determining when the antenna pattern of the antenna
104 is generally pointing towards a particular SV, referred to as
"up." This "pointing towards" refers to the rotation and not to an
aspect angle. In the illustrated embodiment, these PLLs along with
line-of-sight (LOS) vector information are used to calculate the
time (t_up.sub.i) when the pattern of the antenna 104 will be
pointing "up" with respect to a particular i-th SV. In one
embodiment, this "next up time" t_up.sub.i is independently
calculated for each SV in view of the receiver, and then the
independent calculations are averaged together, which averages out
errors.
[0027] This averaged "next up time" is represented by variable
t_up. At the averaged next up time t_up, the pattern of the antenna
104 of the projectile 102 is "pointing" at the average of the
independent up times, so that the average is generally pointing
away from the center of the earth. The averaged next up time t_up
is provided as an index for flight control.
[0028] A next up time t_up.sub.i for a particular i-th space
vehicle SV.sub.i represents a time estimate of the next time that
the pattern of the antenna 104 faces towards the i-th space vehicle
SV.sub.i. Every space vehicle in view can be used to estimate a
particular next up time t_up.sub.i, but an individual estimate is
relatively noisy due to the stochastic nature of a phase-locked
loop. Averaging multiple up time estimates t_up.sub.i smoothes this
noise and provides a more accurate estimate for a reference time
t_up when the pattern of the antenna will be in a particular
orientation, in this case, up with respect to an average. In the
illustrated embodiment, equal weighting is used for the averaging;
however, an unequally weighted average can also be used. For
example, signals received from space vehicles with lower signal
power can be weighted less heavily than signals received from space
vehicles with higher signal power.
[0029] FIG. 2 illustrates a top-level block diagram of a launch
system 202 and a projectile system 204 according to an embodiment
of the invention. FIG. 2 illustrates electronic components for the
launch system 202. Other components can include, for example, a
cannon for the launch of a projectile. The launch system 202
components are typically reusable, and are used, for example, on
the ground, in a truck, in a ship, in an airplane, or the like. The
launch system 202 includes a base station GPS receiver 206 and a
destination/flight path selector 208.
[0030] The base station GPS receiver 206 receives GPS signals and
communicates information to the projectile system 204 such that the
projectile system 204 can readily acquire the GPS signals after
launch. In a GPS receiver, "acquiring" a satellite occurs when the
GPS receiver acquires the signal of a satellite. The GPS receiver
acquires the satellite by matching a code received by the GPS
receiver to a code defined for the satellite. In an enhancement,
the matching is performed to the carrier wave that is carrying the
ranging code. This code and/or carrier phase matching is termed
"correlation." For example, when the projectile system 204 is
loaded into a cannon for launch, it is typically not able to
receive normal GPS signals. Accordingly, the base station GPS
receiver 206 can preload a GPS receiver 210 with data indicating
which satellites are in the area for reception, GPS system time,
ephemeris data for the satellites, and the like. The
destination/flight path selector 208 provides information such as a
flight path to an intended destination for the projectile. In one
embodiment, an interface such as an inductive coupler is used to
preload the GPS data and the intended flight path data from the
launch system 202 to the projectile system 204 before the
projectile is launched. After the projectile is launched, the
launch system 202 and the projectile system 204 are typically not
in communication. The intended flight path data can also include
expected velocity at various points along the intended flight path.
Of course, other data can also be provided, and status
communication can also be returned from the projectile system 204
to the launch system 202.
[0031] The projectile system 204 includes the GPS receiver 210, a
guidance computer 212, and a flight control 214. For clarity, other
components of the projectile system 204, such as the components of
an artillery shell, are not shown. For example, the flight control
214 can correspond to actuators for moving or deforming fins, to
brakes, or the like. These actuators can be electromechanical,
piezoelectric, or the like. An input sgn(r'), which can be provided
by the launch system 202, indicates rotational direction of the
projectile during flight.
[0032] In the illustrated embodiment, the GPS receiver 210 and the
guidance computer 212 both receive the intended flight path. In the
illustrated embodiment, line-of-sight (LOS) vectors are calculated
using the intended flight path and time. The LOS vectors will be
described in greater detail later in connection with FIG. 3. In an
alternative embodiment, the LOS vectors are computed
conventionally, that is, using the position of the projectile as
obtained via GPS and the position of the satellites.
[0033] The GPS receiver 210 provides the spatial position of the
projectile to the guidance computer 212 so that the guidance
computer 212 can make adjustments to the actual flight path. Such
adjustments can be one-dimensional (range only) as encountered with
brakes, or can be two-dimensional.
[0034] The GPS receiver 210 also provides the guidance computer 212
with a reference time to indicate rotation. In the illustrated
embodiment, the reference time t_up is referenced to GPS system
time, and indicates when the projectile is expected to be "up,"
that is, would have a particular angular position. The particular
angular position can be used as an index by the guidance computer
212. In the illustrated embodiment, the particular angular position
is the average of "up" positions to the satellites in view. While
this average can vary over time, it is fixed enough to be useful
for guidance. In an alternate embodiment, the GPS receiver 210 can
provide a different type of indicator, such as, for example, an
edge on a clock signal, a data register indicating approximate
angle, or the like. One embodiment of the GPS receiver 210 will be
described in greater detail later in connection with FIG. 3.
[0035] As described earlier, both the GPS receiver 210 and the
guidance computer 212 receive the intended flight path. The
guidance computer 212 also receives the spatial position from the
GPS receiver 210 and determines whether or not to adjust the actual
flight path. If the projectile is spinning and is guided by
manipulating fins, then the fins will typically be spinning with
the projectile. The guidance computer 212 uses the rotation
indicator sgn(r') to determine how to actuate the flight controls
to adjust the actual flight path.
[0036] FIG. 3 illustrates a system diagram for one embodiment of
the GPS receiver 210 for estimating an averaged next up time t_up.
Initialization data 302, such as data coupled from the launch
system 202 (FIG. 2), can include space vehicle ephemeris data, GPS
system time, both as determined by the base station GPS receiver
206, and an intended trajectory or nominal trajectory for the
projectile. A dashed line separates components for a positioning
processor 350 and components for a roll processor 360. In the
illustrated embodiment, the positioning processor 350 portion of
the GPS receiver 210 is conventional.
[0037] Various components can be implemented in hardware, in
software/firmware, or in a combination of both hardware and
software/firmware. In one embodiment, the hardware correlators 310
and the code/carrier numerically controlled oscillators 316 are
implemented by hardware, and the other components of the GPS
receiver 210 are implemented in software/firmware. For example,
executable instructions can be stored in a computer-readable
medium, such as RAM or ROM memory, and executed by a processor,
such as a microprocessor. For example, the instructions can be
embedded into flash ROM of the GPS receiver 210. In another
example, the instructions can be loaded into RAM from the launch
system 202.
[0038] In the illustrated embodiment, after initialization from the
launch system 202, an independent timer 304 maintains a "current
time," which is denoted with variable t.sub.curr. The current time
t.sub.curr is independent of GPS signals received by the GPS
receiver 210 and independent of GPS system time after launch. In
one embodiment, the timer 304 is maintained by
software/firmware.
[0039] In the illustrated embodiment, the positioning processor 350
is conventional. For example, the illustrated positioning processor
350 includes hardware correlators 310, a deep integration filter
312, a navigation Kalman filter 314, and code/carrier numerically
controlled oscillators 316 arranged in a tracking loop. Received
GPS signals are provided as an input to the HW correlators 310. The
HW correlators 310 correlate the received GPS signal with the
receiver's best estimate of a replica signal to generate in-phase
(I) and quadrature-phase data (Q) that contain projectile position
error measurements and SV power measurements. In one embodiment,
the navigation Kalman filter 314 has 12 states and computes
position, velocity, acceleration, and time. For clarity, antennas
and front-end components of the GPS receiver 210 are not shown in
FIG. 3. An output of the positioning processor 350 is the ECEF
position, velocity, and time, which is provided as an input to the
guidance computer 212.
[0040] In the illustrated embodiment, for calculating LOS vectors,
an LOS estimator 306 estimates its location using the current time
t.sub.curr and the intended or nominal trajectory (preloaded), and
estimates the location of a space vehicle using the current time
t.sub.curr and the SV ephemeris data (preloaded). The LOS estimator
306 can be implemented in hardware or in software/firmware. In the
illustrated embodiment, the LOS estimator 306 is implemented in
software/firmware. Advantageously, the LOS vectors can be computed
even when the GPS receiver 210 encounters interference. This can
improve tracking under certain circumstances and reduce computation
time in flight.
[0041] In FIG. 3, subscript n indicates a particular navigation
channel, subscript u indicates a particular upfinder channel,
subscript c indicates the set of all channels, .DELTA..phi..sub.u
indicates phase compensation. LOS data can include, for example,
the aspect angle .alpha..sub.u to a u-th space vehicle (see, for
example, FIG. 9, SVs projected on the vertical plane through the
longitudinal axis of the projectile 102), and the azimuth angle
az.sub.u in an up-starboard plane to the u-th upfinder source (see,
for example, FIG. 10, SVs projected on a horizontal plane). As
illustrated in FIG. 9, the aspect angle .alpha..sub.u is between
the longitudinal axis of the u-th space vehicle SV.sub.u (projected
rearward to the projectile 102) and the line of sight to the
projectile 102 measured from the tail of the u-th space vehicle
SV.sub.u.
[0042] The I and Q data for the positioning processor 350
(subscript n) or for the roll processor 360 (subscript u)
corresponds to a correlated in-phase output and a quadrature-phase
of the HW correlators 310. In one embodiment, the roll processor
360 is entirely implemented in software/firmware, but can also be
implemented in hardware. In the illustrated embodiment, the roll
processor 360 tracks each SV in view using a phase-locked loop for
each SV, but typically uses less than all the trackable SVs to
compute the aggregate up time t_up as will be discussed in greater
detail in the following.
[0043] A SV power modulation tracking filter 320 incorporates both
a frequency-locked loop (FLL) and a phase-locked loop (PLL). A
mode/state.sub.u status indicates whether the SV power modulation
tracking filter 320 is operating in FLL mode or in PLL mode, and
whether or not the loop is locked (for the u-th SV). The
mode/state.sub.u information is provided as an input to a
measurement enabler 328. Further details of the SV power modulation
tracking filter 320 will be described later in connection with FIG.
4. In one embodiment, the SV power modulation tracking filter 320
is implemented by software/firmware. Preferably, each SV in view is
tracked by the SV power modulation tracking filter 320.
[0044] The SV power modulation tracking filter 320 isolates the
power modulation on the I/Q data that is a result of the
projectile's spin. The SV power modulation tracking filter 320
phase locks to the amplitude modulation of the GPS signal caused by
the projectile's spin. This phase information (angle) is then
combined with LOS information from the LOS estimator 306. In the
illustrated embodiment, the combination of the phase information
and the LOS information is performed in a projectile antenna up
estimator 324. In the illustrated embodiment, the LOS information
is derived from the position, velocity, and time (PVT) information
based on preloaded information from the timer 304 and the
initialization data 302, that is, information that is not obtained
from GPS signals by the positioning processor 350 during the flight
of the projectile 102.
[0045] The line-of-sight (LOS) vectors from the LOS estimator 306
are provided as an input to the navigation Kalman filter 314 and to
the projectile antenna up estimator 324. In the illustrated
embodiment, the LOS vectors used by the navigation Kalman filter
314 are based only on preloaded initialization data 302 (ephemeris
and trajectory) and on the time maintained by the timer 304.
However, in an alternative embodiment, the LOS vectors can also be
computed in a conventional manner.
[0046] Aspect angles .alpha..sub.u are provided as an input to the
roll compensator 322 and to the measurement enabler 328. The roll
compensator 322 is optional. Azimuth angles az.sub.u are provided
as an input to the projectile antenna up estimator 324. In an
alternative embodiment, the LOS vectors are computed in another
manner, such as the conventional practice of determining the
position of the GPS receiver 210 via GPS signal tracking and
determining the position of a space vehicle from received ephemeris
data.
[0047] As the projectile 102 (FIG. 1) spins during flight, the
pattern of the antenna 104 (FIG. 1) will at times point toward a SV
and at times away from the SV. This rotation induces a power
modulation on the received GPS signal that is detectable as an
amplitude modulation having a modulation rate (frequency) that is
the same as the spin of the projectile 102. In the illustrated
embodiment, the SV power modulation tracking filter 320 only phase
locks to the amplitude modulation (due to rotation) of a GPS signal
from a SV and does not monitor phase modulation (due to rotation).
Typically, the lobes of the antenna 104 point outwards from the
projectile 102. Preferably, the antenna 104 is configured such that
its main lobe points normal to the flight trajectory; however, it
will be understood that depending on the angle that a particular SV
has with respect to the projectile 102, it may not be the main lobe
that has the strongest overall power. In addition, it should be
noted that it can be difficult to detect the amplitude modulation
of the GPS signal when a SV is too far in the direction of the nose
or the tail of the projectile 102, for example, directly in front
of or behind the projectile 102. In one embodiment, SVs that make
an angle within 45 degrees of the nose or 50 degrees from the tail
are not used for up determination, that is, are not part of the "u"
set of SVs, and an up time t_up.sub.u for those SVs is not
computed. However, these SVs may still be used for obtaining
positioning information, that is, may be used as one of the "n" set
of SVs.
[0048] The roll compensator 322 receives the aspect angle
.alpha..sub.u as an input, and generates phase compensation
.DELTA..phi..sub.u as an output. The phase compensation
.DELTA..phi..sub.u provides a correction factor used by the
projectile antenna up estimator 324 to compensate for the antenna
pattern. In one embodiment, the roll compensator 322 is implemented
with a lookup table (LUT). In one embodiment, the roll compensator
322 is optional. For example, if the antenna pattern gain and phase
is relatively symmetric over +/-180 degrees, then the roll
compensator 322 can be omitted. In one embodiment, the roll
compensator 302 further receives an estimate of the expected roll
rate (not shown) from the initialization data 302 and the computed
roll rate .phi.'.sub.u from the SV power modulation tracking filter
320 as a check of the computed roll rate .phi.'.sub.u. The expected
roll rate typically varies over the intended trajectory. If the
expected roll rate and the computed roll rate .phi.'.sub.u do not
agree to within a threshold, then the computed roll rate
.phi.'.sub.u can be determined to be untrustworthy, and the roll
and roll rate estimate for the particular SV can be discarded as
invalid. For example, the threshold can be predetermined, such as a
threshold of +/-20 Hz.
[0049] In one embodiment, the projectile antenna up estimator 324
is a Kalman filter. The Kalman filter smoothes the up estimate for
each SV tracked. The projectile antenna up estimator 324 uses the
amplitude modulation information from the SV power modulation
tracking filter 320, the direction of the spin sgn(r') (either 1 or
-1), and the phase compensation .DELTA..phi..sub.u to generate an
estimate of the u-th SV's angular position, angular velocity, and
angular acceleration relative to the projectile 102. This
information is summarized in vector x. In one embodiment, the
projectile antenna up estimator 324 is implemented in
software/firmware. In the illustrated embodiment, the projectile
antenna up estimator 324 performs the computations expressed in
Equations 1 and 2. Equation 1 describes a roll estimate vector x
describing rotation with respect to a particular u-th SV. The
components of the roll estimate vector x include phase or angle r,
angular velocity r', and angular acceleration r''. In the
illustrated embodiment, the phase or angle r is referenced to the
particular u-th SV, with 0 angle being with the antenna 104 (FIG.
1) pointing towards the u-th SV,
x = [ r r ' r '' ] u = [ az - ( .phi. + .DELTA..phi. ) sgn ( r ' )
.phi. ' sgn ( r ' ) .phi. '' sgn ( r ' ) ] u ( Eq . 1 )
##EQU00001##
[0050] Equation 2 illustrates a covariance matrix R for a Kalman
filter for the noise estimated by the FLL/PLLs.
R u = [ .sigma. .phi. 2 0 0 0 .sigma. .phi. ' 2 0 0 0 .sigma. .phi.
'' 2 ] u ( Eq . 2 ) ##EQU00002##
[0051] The measurement enabler 328 controls a measurement window
326 for monitoring the roll estimate vector x output of the
projectile antenna up estimator 324. In the illustrated embodiment,
the measurement window 326 is open for a measurement associated
with a u-th SV when the following conditions are true: (a) the SV
power modulation tracking filter 320 is in PLL mode; (b) the PLL is
locked; and (c) aspect angle az.sub.u to the u-th SV is between 45
degrees and 135 degrees. Other applicable aspect angles will be
readily determined by one of ordinary skill in the art and can
depend on the antenna pattern. Beyond the selected range for the
aspect angle az.sub.u, the amount of amplitude modulation caused by
the spinning of the projectile 102 is deemed to be relatively
small. Thus, while the SV power modulation tracking filter 320
preferably tracks all the SVs in view, the operation of the
measurement window 326 limits the SVs used to generate the
aggregate up time t_up to a smaller subset. The SVs in the smaller
subset can change over time as the projectile 102 travels and its
aspect angle changes. The measurement window 326 can be embodied in
software/firmware by, for example, inspecting a limited range of
data.
[0052] A current roll estimate generator 330 generates a current
roll estimate vector x.sub.curr based on the current time
t.sub.curr, the roll estimate vector x, and a previous up time
t.sub.u, that is, when the antenna 104 was last pointing at the
u-th SV. The foregoing data is combined with a transition matrix
.PHI. (see Eq. 3). The current roll estimate vector x.sub.curr has
the same dimensions as the roll estimate vector x of Equation 1.
The current roll estimate vector x.sub.curr can include an estimate
of the angle r, angular velocity r', and optionally an angular
acceleration r'' of the antenna 104 relative to the SV for the
current time t.sub.curr. In one embodiment, the current roll
estimate generator 330 is embodied by software/firmware.
.PHI. = [ 1 .DELTA. t 1 2 .DELTA. t 2 0 1 .DELTA. t 0 0 1 ] ( Eq .
3 ) ##EQU00003##
[0053] The next up time estimator 332 determines an estimate (in
time) of the next time that the pattern of the antenna 104 will be
pointing toward the u-th SV. The information can be computed as
illustrated within next up time estimator 332 based on the current
time t.sub.curr and the angle (r) and the angular velocity (r') of
the current information vector x.sub.curr. A particular up time
output of next up time estimator 332 corresponds to the next up
time t_up.sub.u for the u-th upfinder channel.
[0054] The various next up times t_up.sub.u are then aggregated to
form the next up time t_up for the projectile 102 as a whole. For
example, as discussed earlier in connection with FIG. 1, the
aggregated next up time t_up can be formed by calculating an
average of the next up times from the set of up times for the u
SVs.
[0055] FIG. 4 illustrates an embodiment of the SV power modulation
tracking filter 320 according to an embodiment of the invention.
The SV power modulation tracking filter 320 includes a phase
rotator/mixer 402, a frequency-locked loop (FLL) acquisition loop
404, a 4th-order phase-locked loop (PLL) tracking loop 406, an FLL
lock detector 408, a mode selector 410, and a phase lock detector
412. There should be a separate SV power modulation tracking filter
320 for each SV that is tracked. In one embodiment, there are at
least 12 separate SV power modulation tracking filters 320. When
implemented in software/firmware, the same routine can be used for
each SV that is tracked. When implemented in hardware, preferably,
each of the SV power modulation tracking filters 320 is identical
to each other. However, when implemented in hardware, some
components, such as the FLL acquisition loop 404, the FLL lock
detector 408 or selected components thereof, can be shared among
two or more SV power modulation tracking filters 320. For example,
after phase locking by the PLL tracking loop 406, the FLL
components are typically not used in operation, and can be used for
acquisition to a different SV. In addition, the roll frequency of
the projectile 102 is caused by the rolling of the projectile 102
itself, and thus, the power modulation should have the same
frequency (the roll frequency) among the various SVs. Thus, a FLL
acquisition loop 404 for one SV can be used for pulling in a PLL
tracking loop 406 for another. When implemented in
software/firmware, the routine implementing the FLL would not need
to be called after the PLL tracking loop 406 is locked.
[0056] In the illustrated embodiment, outputs of the 4th-order PLL
tracking loop 406 include roll phase .phi..sub.u, roll rate
.phi.'.sub.u, roll acceleration .phi.''.sub.u, which are available
from the loop filter 704 (FIG. 7). In one embodiment, standard
deviations of each are also computed and depicted as
.sigma..sub..phi.u, .sigma..sub..phi.'u, .sigma..sub..phi.''u.
[0057] One embodiment of the phase rotator/mixer 402 will be
described in greater detail later in connection with FIG. 5. One
embodiment of the frequency-locked loop (FLL) acquisition loop 404
will be described in greater detail later in connection with FIG.
6. One embodiment of the 4th-order phase-locked loop (PLL) tracking
loop 406 will be described in greater detail later in connection
with FIG. 7.
[0058] The phase rotator/mixer 402 utilizes mixers to phase rotate
the input I.sub.CORR and Q.sub.CORR to generate I.sub.ROT and
Q.sub.ROT as outputs. The I.sub.CORR and the Q.sub.CORR signals
correspond to the particular I.sub.U and Q.sub.U signals for the
u-th SV. The phase rotation is controlled by the roll phase
estimate. The FLL acquisition loop 404 determines the roll rate
(frequency) to assist the PLL tracking loop 406 to pull in to
achieve phase lock. After the PLL tracking loop 406 achieves phase
lock, then the PLL tracking loop 406 is used to track the roll of
the projectile 102. The FLL lock detector 408 is used for
acquisition of the signal modulation and generates a roll rate
error (frequency). Based on the roll rate error from the FLL lock
detector 408 and the pull-in range of the PLL tracking loop 406,
the mode selector 410 selects between the FLL acquisition loop 404
or the PLL tracking loop 406. The phase lock detector 412
determines whether or not PLL tracking loop 406 is phase
locked.
[0059] FIG. 5 illustrates one embodiment of the phase rotator/mixer
402. In the illustrated embodiment, the phase rotator/mixer 402 is
embodied in software/firmware. However, the phase rotator/mixer 402
can alternatively be embodied in hardware. The spin of the
projectile 102 causes a power modulation on the received GPS
signals. When the main lobe of the antenna (assuming a symmetrical
antenna pattern) of the projectile 102 faces a particular SV, then
maximum power for the received GPS signal is received. When the
antenna faces away from the SV, minimum power is received. This
power modulation is similar to a sine wave over time, and the PLL
tracking loop 406 tracks the phase of that amplitude modulated
pattern.
[0060] The phase rotator/mixer 402 includes a magnituder 502, a
magnitude lossy integrator 504, a summer 506, a sine function block
508, a cosine function block 510, an I-phase mixer 512, a Q-phase
mixer 514, an I-phase lossy integrator 516, a Q-phase lossy
integrator 518.
[0061] The magnituder 502 generates a raw magnitude signal that has
the magnitude of the I.sub.CORR and Q.sub.CORR signals. The
magnitude lossy integrator 504 is a low-pass filter. In one
embodiment, the time constant of the magnitude lossy integrator 504
is about 128 milliseconds. The time constant should be relatively
long relative to the spin interval of the projectile 102, which is
typically around 4 milliseconds, but varies within a wide range.
The time constant can vary in a very broad range and other
applicable time constants will be readily determined by one of
ordinary skill in the art. The output of the magnitude lossy
integrator 504 contains the DC component of the raw magnitude
signal, and the DC component is subtracted from the raw magnitude
signal by the summer 506 to generate a signal referred to as a real
input signal.
[0062] A sine and cosine of a roll phase estimate are generated by
the sine function block 508 and by the cosine function block 510,
respectively. The sine function block 508 and the cosine function
block 510 can be implemented by, for example, a lookup table or by
a function call. When the SV power modulation tracking filter 320
is phase locked, the sine and cosine of the roll phase estimate are
phase-locked to the reference inputs I.sub.CORR and Q.sub.CORR. In
one embodiment, the roll phase estimate is an output of the mode
selector 410 (FIG. 4). The I-phase mixer 512 and to the Q-phase
mixer 514 form a phase detector. The sine and cosine of the roll
phase estimate are provided as inputs to the I-phase mixer 512 and
to the Q-phase mixer 514, respectively, which mixes the sine and
cosine with the real input signal for phase detection. The outputs
of the I-phase mixer 512 and to the Q-phase mixer 514 represents a
difference in phase between the reference inputs I.sub.CORR and
Q.sub.CORR and the sine and cosine of the roll phase estimate. The
outputs of the I-phase mixer 512 and the Q-phase mixer 514,
respectively, are provided as inputs to the I-phase lossy
integrator 516 and to the Q-phase lossy integrator 518,
respectively.
[0063] The I-phase lossy integrator 516 and the Q-phase lossy
integrator 518 are low-pass filters that remove sidebands from the
outputs of the I-phase mixer 512 and the Q-phase mixer 514,
respectively, to generate the outputs I.sub.ROT and Q.sub.ROT of
the phase rotator/mixer 402. The I-phase lossy integrator 516 and
the Q-phase lossy integrator 518 form a complex low pass filter. In
the illustrated embodiment, the 2-sided bandwidth (2BW) point is
100 Hertz (50 Hz each). Other applicable bandwidth specifications
are applicable and will be readily determined by one of ordinary
skill in the art. However, it should be noted that the 2-sided
bandwidth for the complex low-pass filter formed by the I-phase
lossy integrator 516 and the Q-phase lossy integrator 518 should be
much wider than the 2-sided bandwidth for the loop.
[0064] FIG. 6 illustrates an embodiment of the FLL acquisition loop
404. The FLL acquisition loop 404 includes a phase differentiator
602, a loop filter 604, and a roll-phase integrator 606. The phase
differentiator 602 generates a roll rate error as an output. The
roll rate error output of the phase differentiator 602 is provided
as an input to the FLL lock detector 408 (FIG. 4) and as an input
to the loop filter 604. In the illustrated embodiment, the loop
filter 604 has a roll acceleration integrator and a roll rate
integrator. An output of the loop filter 604 is provided as an
input to the roll phase integrator 606, which generates a roll
phase as an output. The FLL acquisition loop 404 assists the PLL
tracking loop 406 (FIG. 4) to achieve phase lock. For example, the
PLL tracking loop 406 is typically unable to acquire phase lock to
the reference inputs I.sub.CORR and Q.sub.CORR unless the frequency
error or roll rate error is within a certain range, typically a few
hertz for the illustrated embodiment of the PLL tracking loop 406.
After the PLL tracking loop 406 is locked, the operation of the FLL
acquisition loop 404 is typically not needed.
[0065] The FLL lock detector 408 (FIG. 4) senses when the roll rate
error is within the pull-in range of the PLL tracking loop 406. In
one embodiment, the FLL lock detector 408 is configured to receive
the roll rate error as an input to a lossy integrator to generate a
filtered roll rate error, to take the absolute value of the
integrated roll rate error, to compare the integrated roll rate
error to a threshold, such as a few hertz, and then to determine
that the FLL acquisition loop 404 is locked if the roll rate is
below the threshold and unlocked if otherwise. The state of
locked/unlocked can be used to determine whether or not the roll
rate error is within the pull-in range of the PLL tracking loop
406.
[0066] FIG. 7 illustrates an embodiment of the PLL tracking loop
406 implemented as a 4-th order PLL tracking loop. The PLL tracking
loop 406 has a phase detector 702, a loop filter 704, and a
roll-phase integrator 706. A roll phase error output of the phase
detector 702 is provided as an input to the loop filter 704. A roll
rate output of the loop filter 704 is provided as an input to the
roll-phase integrator 706, which generates a roll phase output in
cycles. The phase lock detector 412 (FIG. 4) detects when the PLL
tracking loop 406 is phase locked to the reference inputs
I.sub.CORR and Q.sub.CORR.
[0067] In one embodiment, the phase lock detector 412 is configured
to low-pass filter the I.sub.ROT and Q.sub.ROT outputs of the phase
rotator/mixer 402 with lossy integrators. The bandwidth of these
lossy integrators should be less than the bandwidth of the PLL
tracking loop 406. While phase locked, most of the power should be
in the filtered I.sub.ROT signal and relatively little should be in
the filtered Q.sub.ROT signal. This relationship can be used to
determine whether the PLL tracking loop 406 is locked. In one
embodiment, the phase lock detector 412 is configured to take the
absolute value of the filtered Q.sub.ROT signal, and then multiply
the filtered Q.sub.ROT signal by a lock threshold value. In one
embodiment, the lock threshold value is around 1.5. However, the
lock threshold value can vary in a relatively broad range and other
applicable values will be readily determined by one of ordinary
skill in the art. The multiplied and filtered Q.sub.ROT signal is
then compared with the filtered I.sub.ROT signal. In one
embodiment, if the multiplied and filtered Q.sub.ROT signal is less
than the filtered I.sub.ROT signal, then the phase lock detector
412 determines that the PLL tracking loop 406 is locked. Otherwise,
the phase lock detector 412 determines that the PLL tracking loop
406 is unlocked. The status mode/state.sub.u can be provided as an
input to the measurement enabler 328.
[0068] FIG. 8 illustrates a process showing state transitions for
GPS-based "up" estimation for control of the FLL acquisition loop
404 and PLL tracking loop 406. The illustrated process can be used
with either a software/firmware implementation or a hardware
implementation. In the illustrated embodiment, software/firmware is
used.
[0069] The process begins in an idle state 802. When commanded to
track the rotation relative to a previously untracked SV, the
process proceeds to an acquisition state 804 to phase lock to the
SV. The process begins by closing the feedback loop of the SV power
modulation tracking filter 320 (FIG. 3) around the FLL acquisition
loop 404 (FIG. 4). The process stays in the acquisition state 804
until the frequency error is within the pull-in range of the PLL
tracking loop 406 (FIG. 4).
[0070] When the frequency error is within the pull-in range of the
PLL tracking loop 406, the process advances to a phase state 806,
in which the PLL tracking loop 406 phase locks to the spin
modulation on the signal from the SV. The process stays in the
phase state 806 unless a new SV is commanded to be tracked, in
which case, the process returns from the phase state 806 to the
idle state 802, or if a loss of phase lock is detected, in which
case, the process returns from the phase state 806 to the
acquisition state 804.
[0071] Various embodiments have been described above. Although
described with reference to these specific embodiments, the
descriptions are intended to be illustrative and are not intended
to be limiting. Various modifications and applications may occur to
those skilled in the art.
* * * * *