U.S. patent application number 15/993246 was filed with the patent office on 2020-12-31 for diversity polarization modulation.
The applicant listed for this patent is Eagle Technology, LLC. Invention is credited to Philip KOSSIN, Brett PIGON.
Application Number | 20200412439 15/993246 |
Document ID | / |
Family ID | 1000005272736 |
Filed Date | 2020-12-31 |
United States Patent
Application |
20200412439 |
Kind Code |
A1 |
KOSSIN; Philip ; et
al. |
December 31, 2020 |
DIVERSITY POLARIZATION MODULATION
Abstract
A method includes transmitting a digital code from a transmitter
to a receiver. Information is transmitted via electromagnetic waves
from the transmitter to the receiver. The transmission of the
information includes transmitting a first portion of the
information using electromagnetic waves with a first polarization
in response to a first value of the digital code, and transmitting
a second portion of the information using electromagnetic waves of
a second polarization in response to a second value of the digital
code. The first information may include a first navigational code
and the second information may include a second navigational
code.
Inventors: |
KOSSIN; Philip; (Clifton,
NJ) ; PIGON; Brett; (Grant Valkaria, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Eagle Technology, LLC |
Melbourne |
FL |
US |
|
|
Family ID: |
1000005272736 |
Appl. No.: |
15/993246 |
Filed: |
May 30, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01Q 21/245 20130101;
H01Q 15/244 20130101; H04B 1/69 20130101; H04B 7/10 20130101; H04B
2001/6904 20130101 |
International
Class: |
H04B 7/10 20170101
H04B007/10; H01Q 21/24 20060101 H01Q021/24; H01Q 15/24 20060101
H01Q015/24; H04B 1/69 20110101 H04B001/69 |
Claims
1. A method comprising: transmitting a digital code that comprises
a pseudo-random code from a transmitter to a receiver; transmitting
information including a first navigational code and a second
navigational code via electromagnetic waves from the transmitter to
the receiver, wherein transmitting the information comprises:
transmitting the first navigational code using electromagnetic
waves with a first polarization in response to a first value of the
digital code; and transmitting the second navigational code using
electromagnetic waves of a second polarization in response to a
second value of the digital code, wherein the transmitting the
first navigational code and the transmitting the second
navigational code result in polarization hopping of the first
polarization and the second polarization between different types of
polarization over time according to a random pattern of the
pseudo-random code.
2. The method of claim 1, wherein the first navigational code and
the second navigational code are transmitted consecutively in
time.
3. (canceled)
4. The method of claim 1, wherein: the first polarization comprises
one of right hand circular polarization, left hand circular
polarization, right hand elliptical polarization, left hand
elliptical polarization, and/or linear polarization; and the second
polarization comprises another of right hand circular polarization,
left hand circular polarization, right hand elliptical
polarization, left hand elliptical polarization, and/or linear
polarization.
5. The method of claim 1, wherein transmitting the digital code
from the transmitter to the receiver comprises transmitting the
digital code encrypted with an encryption key known to the
receiver.
6. The method of claim 1, wherein transmitting the information
comprises transmitting the information using a crossed dipole
antenna.
7. The method of claim 1, wherein transmitting the first
navigational code comprises transmitting the first navigational
code with a first helical antenna providing right hand circular
polarized electromagnetic waves; and wherein transmitting the
second navigational code comprises transmitting the second
navigational code with a second helical antenna providing left hand
circular polarized electromagnetic waves.
8. The method of claim 1, wherein transmitting the first
navigational code and transmitting the second navigational code
comprises transmitting the first navigational code and the second
navigational code at the same time as a combined linearly polarized
signal.
9. A method comprising: receiving, at a receiver from a
transmitter, a digital code comprising a pseudo-random code;
receiving, at the receiver, first electromagnetic waves; decoding,
using correlation, a first navigational code from the first
electromagnetic waves corresponding to a first polarization in
response to a first value of the digital code, wherein the first
polarization includes polarization hopping between different types
of the first polarization over time according to a random pattern
of the pseudo-random code, and the decoding includes determining
the different types of the first polarization using the
pseudo-random code; receiving, at the receiver, second
electromagnetic waves; and decoding, using correlation, a second
navigational code from the second electromagnetic waves
corresponding to a second polarization in response to a second
value of the digital code, wherein the second polarization includes
polarization hopping between different types of second polarization
over time according to the random pattern of the pseudo-random
code, and the decoding the second navigational code includes
determining the different types of second polarization using the
pseudo-random code.
10. The method of claim 9, wherein the first polarization comprises
one of right hand circular polarization, left hand circular
polarization, right hand elliptical polarization, left hand
elliptical polarization, and/or linear polarization; and the second
polarization comprises another of right hand circular polarization,
left hand circular polarization, right hand elliptical
polarization, left hand elliptical polarization, and/or linear
polarization.
11. (canceled)
12. The method of claim 9, wherein receiving the digital code
comprises receiving an encrypted code; and further comprising
decrypting the encrypted code with an encryption key stored at the
receiver.
13. The method of claim 9, wherein the first polarization comprises
right hand circular polarization; and wherein the second
polarization comprises left hand circular polarization.
14. The method of claim 13, further comprising determining a total
power measurement of the first electromagnetic waves combined with
the second electromagnetic waves; and determining that a jammer
signal is included in at least one of the first electromagnetic
waves and/or the second electromagnetic waves in response to the
total power measurement exceeding a predetermined threshold.
15. The method of claim 14, further comprising: making a power
measurement of a right hand circular polarized portion of the first
and second electromagnetic waves; making a power measurement of a
left hand circular polarized portion of the first and second
electromagnetic waves; comparing the power measurement of the right
hand circular polarized portion and the power measurement of the
left hand circular polarized portion; and determining a
polarization of the jammer signal in response to the comparing of
the power measurement of the right hand circular polarized portion
and the power measurement of the left hand circular polarized
portion.
16. The method of claim 15, further comprising providing anti
jamming processing in response to determining the polarization of
the jamming signal.
17. A method comprising encoding a first signal of a first channel
of a transmitter with a first navigational code; encoding a second
signal of the first channel of the transmitter with a second
navigational code; encoding a first signal of a second channel of
the transmitter with the first navigational code; encoding a second
signal of the second channel of the transmitter with the second
navigational code; phase shifting the first signal of the first
channel by 90 degrees relative to the first signal of the second
channel; phase shifting the second signal of the second channel by
90 degrees relative to the second signal of the first channel;
combining the first signal of the first channel with the second
signal of the first channel to generate a first channel combined
signal; combining the first signal of the second channel with the
second signal of the second channel to generate a second channel
combined signal; and transmitting the first channel combined signal
and the second channel combined signal such that the first signal
is transmitted with right hand elliptical polarization and the
second signal is transmitted with left hand elliptical
polarization.
18. The method of claim 17, wherein transmitting the first channel
combined signal and the second channel combined signal comprises
transmitting the first signal with right hand circular polarization
and the second signal with left hand circular polarization.
19. The method of claim 18, wherein transmitting the first channel
combined signal comprises transmitting the first channel combined
signal via a first radiating element of a crossed dipole antenna;
and wherein transmitting the second channel combined signal
comprises transmitting the second channel combined signal via a
second radiating element of the crossed dipole antenna.
20. The method of claim 17, wherein the first channel combined
signal and the second channel combined signal are configured to be
decoded at a receiver using correlation.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to the transmission of
signals using polarization, and in particular, the transmission of
navigation codes using polarization diversity.
BACKGROUND
[0002] Global Navigation Satellite Systems ("GNSS"), Global
Positioning System ("GPS"), Galileo systems and Position,
Navigation and Timing ("PNT") systems broadcast RF energy modulated
with navigation information from spacecraft, airborne and
terrestrial platforms. Such systems are susceptible to degradation
due to multipath interference and intentional or unintentional
interference from jammers or other sources. These systems are also
susceptible to "spoofing," i.e., unauthorized transmitters which
send falsified GNSS-like signals with the intent to give the user
erroneous position, navigation, or timing estimates. GNSS systems
present challenges in designing systems that are robust and
resilient to multipath, jamming, and spoofing, while minimizing the
size, weight, and power required for the GNSS payload.
[0003] Related art anti jamming techniques assume that the
navigation signals will be right hand circularly polarized, while
the jammer signals are assumed to be linearly polarized. Other anti
jamming techniques assume that the jammer signal is radiated from
below the navigation system antenna horizon, but this also not
always true. Accordingly, new anti jamming techniques need to be
developed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is a block diagram of a transmitter configured to
provide diversity polarization modulation, according to an example
embodiment.
[0005] FIG. 2 is a block diagram of a receiver configured to
receive and demodulate a signal transmitted with diversity
polarization modulation, according to an example embodiment.
[0006] FIG. 3 is a block diagram of a first transmitter configured
to use diversity polarization modulation in order to provide a
polarization hopped signal, according to an example embodiment.
[0007] FIG. 4 is a chart illustrating different types of
polarization that may be utilized when providing diversity
polarization modulation, according to an example embodiment.
[0008] FIG. 5 is a block diagram of a second transmitter configured
to use diversity polarization modulation in order to provide a
polarization hopped signal, according to an example embodiment.
[0009] FIG. 6 is a block diagram of receiver configured to receive
and demodulate a polarization hopped signal transmitted with
diversity polarization modulation, according to an example
embodiment.
[0010] FIG. 7 is a block diagram of the signals and logic used in
conjunction with a polarization hopped signal to provide
anti-jamming techniques, according to an example embodiment.
[0011] FIG. 8 is a block diagram of a receiver configured to excise
a linearly polarized jamming signal from a diversity polarization
modulated navigation signal, according to an example
embodiment.
[0012] FIG. 9 is a block diagram of the signals and logic used in
conjunction with a polarization hopped signal to detect and
eliminate a spoof signal in a diversity polarization modulated
navigation signal, according to an example embodiment.
[0013] FIG. 10 is a block diagram of a transmitter configured to
simultaneously send first and second codes using right hand
circular polarization and left hand circular polarization,
respectively, according to an example embodiment.
[0014] FIG. 11 is a block diagram of a receiver configured to
receive and demodulate a signal containing first and second codes
using right hand circular polarization and left hand circular
polarization, respectively, according to an example embodiment.
[0015] FIGS. 12A and 12B are a block diagram of a transmitter
configured to transmit pseudo-noise codes using diversity
polarization modulation and in-phase and quadrature channels, and a
chart indicating the overall polarization of the output
electromagnetic radiation, respectively, according to an example
embodiment.
[0016] FIGS. 13A, 13B and 13C are a block diagram of a first
receiver configured to receive and demodulate a signal transmitting
pseudo-noise codes using diversity polarization modulation and
in-phase and quadrature channels, a chart indicating the overall
polarization of the received electromagnetic radiation, and a
second receiver configured to receive and demodulate a signal
transmitting pseudo-noise codes using diversity polarization
modulation and in-phase and quadrature channels, respectively,
according to example embodiments.
[0017] FIGS. 14A and 14B are a block diagram of a transmitter
configured to transmit four pseudo-noise codes using diversity
polarization, and a chart indicating the polarization of the output
electromagnetic radiation, respectively, according to an example
embodiment.
[0018] FIGS. 15A and 15B are a block diagram of a receiver
configured to receive and demodulate four pseudo-noise codes using
diversity polarization, and a chart indicating the overall
polarization of the received electromagnetic radiation,
respectively, according to an example embodiment.
DESCRIPTION OF EXAMPLE EMBODIMENTS
Overview
[0019] Described herein is a method that, according to one aspect,
comprises transmitting a digital code from a transmitter to a
receiver. This aspect further comprises transmitting information
via electromagnetic waves from the transmitter to the receiver. The
transmission of the information includes transmitting a first
portion of the information using electromagnetic waves with a first
polarization in response to a first value of the digital code, and
transmitting a second portion of the information using
electromagnetic waves of a second polarization in response to a
second value of the digital code. According to example embodiments,
the first information may include a first navigational code and the
second information may include a second navigational code.
[0020] According to another aspect of the techniques described
herein is a method comprising receiving, at a receiver from a
transmitter, a digital code. This aspect further comprises
receiving, at the receiver, first electromagnetic waves. The first
electromagnetic waves are decoded corresponding to a first
polarization in response to a first value of the digital code.
Second electromagnetic waves are received at the receiver, and the
second electromagnetic waves are decoded corresponding to a second
polarization in response to a second value of the digital code. The
first information and the second information may include a first
navigational code and a second navigational code, respectively, and
the decoding of the first navigational code and the second
navigational code may include correlating with local versions of
the first navigational code and the second navigational code,
respectively.
[0021] According to a third aspect of the techniques described
herein is a method comprising encoding a first signal of a first
channel of a transmitter with first information. A second signal of
the first channel of the transmitter is encoded with second
information. A first signal of a second channel of the transmitter
is encoded with the first information. A second signal of the
second channel of the transmitter is encoded with the second
information. Phase shifting by 90 degrees relative to the first
signal of the second channel is performed on the first signal of
the first channel. Phase shifting by 90 degrees relative to the
second signal of the first channel is performed on the second
signal of the second channel. The first signal of the first channel
is combined with the second signal of the first channel to generate
a first channel combined signal, and the first signal of the second
channel is combined with the second signal of the second channel to
generate a second channel combined signal. The first channel
combined signal and the second channel combined signal are
transmitted such that the first information is transmitted with
right hand elliptical polarization and the second information is
transmitted with left hand elliptical polarization. The first
information and the second information may include a first
navigational code and a second navigational code, respectively.
EXAMPLE EMBODIMENTS
[0022] With reference made to FIG. 1, depicted therein is a
transmitter 100 configured to provide the polarization diversity
transmission and reception techniques described herein.
Specifically, the techniques described herein provide one or more
of the following, which will be described in greater detail below:
[0023] Polarization hopping Transmission and Reception Systems;
[0024] Polarization Diverse Global Positioning and/or Global
Navigation Satellite Systems; [0025] Simultaneous Dual Polarization
Code Transmission and Reception Systems; and [0026] Diverse
Circular Polarization Transmission and Reception Systems; among
others.
[0027] The specific example embodiment of transmitter 100 will be
described in the context of a navigation code transmitter, such as
a Global Position System ("GPS") transmitter, that provides
polarization hopping, though the techniques described herein are
applicable to other applications.
[0028] Transmitter 100 includes a navigational data unit ("NDU")
102 which, when used in a navigation system, generates navigation
codes 103a-h. NDU 102 may also add encryption and timing
information to the codes. Digital waveform generators ("DWGs") 104a
and 104b, which are illustrated as separate elements but may be
embodied as a single DWG, combines the codes to form baseband
signals 105a and 105b. As will be explained in more detail below,
and depending on the implementation, the baseband signals 105a and
105b may be the same or different signals. Similarly, codes 103a-d
may be the same as codes 103e-h, respectively, or the codes may be
different. The baseband signals 105a and 105b are then provided to
phase shifters 106a and 106b. The phase shifters 106a and 106b may
be controlled via a control signal 107. Depending on the
embodiment, the control signal 107 may be used to provide
polarization hopping and/or control a desired polarization for the
one or more signals transmitted by the transmitter 100.
Specifically, by altering the relative phase of signals 105a and
105b, the polarization or the electromagnetic signals transmitted
by transmitter 100 may be controlled.
[0029] The output of the phase shifters 106a and 106b are input to
digital-to-analog converters ("DACs") 108a and 108b, respectively.
The DACs 108a and 108b convert the signals from baseband to a
transmission frequency, such as a radio frequency signal. According
to this specific embodiment, DACs 108a and 108b upconvert the
signals from baseband to a signal with an operating frequency
between 1-2 GHz, also known as an "L-Band" frequency. The analog,
transmission frequency signals are then processed by an antenna
assembly 110a or 110b, in which the signal is processed by
amplitude/phase weight adjuster 112a or 112b, high power amplifier
("HPA") 114a or 114b, isolator 116a or 116b, band pass filter 118a
or 118b, and are ultimately radiated by one of antenna elements
120a or 120b. As illustrated in FIG. 1, antenna elements 120a and
120b are embodied as the radiating elements of a crossed dipole
antenna, though other types of antennas may be used. For example,
separate left and/or right hand polarized antennas that share a
common phase center, crossed-dipole Yagi-Uda arrays, dual
polarization microstrip patch antennas, slot antennas, parabolic
reflector antennas with dual polarization feeds or arrays, and
others, may be used without straying from the concepts disclosed
herein.
[0030] Through the use of such an arrangement, right-hand or
left-hand circular polarization sense can be controlled for
transmitter 100. Polarization sense may be alternated over time
according to a pseudo-random pattern known only to registered
users, providing "polarization hopping." This pseudo-random pattern
may be provided to transmitter 100 though control signal 107. Such
a transmitter may provide improved signal-to-noise ratio in
jamming. Through the polarization diversity provided by transmitter
system 100, the receiver or user of the transmitted signal may
perform combined correlation of orthogonal linearly polarized
samples to eliminate a jamming signal. Such a system may also be
used to identify spoof signals. For example, the receiver or user
of the transmitted signal may perform combined correlation of
orthogonal linearly polarized samples to identify the spoofed
signal.
[0031] Furthermore, such a system may be backwards compatible with
existing related art receivers or user systems. For example,
transmitter system 100 may be configured to provide information
sent as right hand circular polarized information, which may be
decoded by related art systems, while simultaneous providing
additional information in left hand circularly polarized
information.
[0032] With reference now made to FIG. 2, depicted therein is a
receiver configured according to the techniques described herein.
Receiver 200 includes, for example, two orthogonal antenna elements
202a and 202b of a crossed dipole antenna. Though, like the antenna
of the transmitter 100 of FIG. 1, the antenna of receiver 200 may
be embodied as separate left and/or right hand polarized antennas
with a shared phase center, crossed-dipole Yagi-Uda arrays, dual
polarization microstrip patch antennas, slot antennas, parabolic
reflector antennas with dual polarization feeds or arrays, and
others known to those skilled in the art.
[0033] From antenna elements 202a and 202b, horizontal or "X"
signal 203a and vertical or "Y" signal 203b are received,
respectively. From signals 203a and 203b, right hand circularly
polarized signal 204 and left hand circularly polarized signal 206
may be determined. Specifically, by providing a positive 90.degree.
phase shift (which is also described as a complex component "j")
through phase shifters 208a and 208b, respectively, the right hand
circularly polarized signal 204 and the left hand polarized signal
206 sent by, for example, transmitter 100 of FIG. 1 may be
determined. Based upon these values, receiver processing unit 212
may perform one or more of polarization diversity processing, anti
jamming processing, polarization hopping processing, Controlled
Radiation Pattern Antenna ("CRPA") nulling processing, and other
processing according to the techniques described herein. The
processing performed by processing unit 212 may include processing
performed in conjunction with a control signal 214. Control signal
214 may be, for example, a signal that corresponds to control
signal 107 of FIG. 1, and that indicates the polarization state of
the transmitter and transmitted signal so that the received signal
may be appropriately processed. Once appropriately processed, the
received signals may be sent to other functional units, such as
functional unit 216, so that additional processing may be performed
on the signal. This additional processing may include correlation
such as complex correlation, discrimination, navigational code
processing, and other functions in accordance with the techniques
disclosed herein, such as processing that accounts for a random
rotation ambiguity between antennas 202a and 202b of the receiver
200 and the antennas of the transmitter or jammer. For example,
complex correlation will detect the codes and the time of receipt
in spite of an offset between the alignment of crossed dipole
antennas of a transmitter and a receiver. The correlation detection
and peak will be found in the magnitude of the complex
correlations, while the rotation offset will appear as part of the
phase of the complex correlation, along with the phase error.
[0034] With reference now made to FIG. 3, the polarization hopping
embodiment of the present techniques will be described. Illustrated
in FIG. 3 is a portion of a transmitter 300, such as a portion of
transmitter 100 of FIG. 1. Included in transmitter 300 is a signal
302 to be transmitted according to the polarization hopping
described herein, and according to a specific example embodiment,
signal 302 may be embodied as corresponding to a navigational code
signal. This signal 302 is sent to feeds for a first radiator or
antenna element 304a of a crossed-dipole antenna and a second
radiator or antenna element 304b of the crossed-dipole antenna
which is orthogonally oriented relative to antenna element 304a. As
illustrated, a first version 302a of signal 302 is sent to antenna
element 304a, and a second version 302b of signal 302 is sent to
antenna element 304b. A control signal 307 is used to induce a
phase difference between signal 302a and 302b.
[0035] As illustrated, control signal 307 is a pseudo random
digital signal via which the transition of the code from a "1" to a
"0" and vice versa, is used to alter the polarization of the signal
sent by antenna elements 304a and 304b. Specifically, pseudo random
code 307 serves as a control signal for phase shifters 308a and
308b, respectively. In the example of FIG. 3, control signal 307 is
defined as a digital function p(n). This signal is used to induce a
90.degree. phase shift into either of signals 302a or 302b, which
results in either left or right hand polarization, depending on
which signal receives the phase shift. According to the specific
example of FIG. 3, phase shifter 308a and 308b receive conjugate
versions of the control signal. Accordingly, when phase shifter
308a receives a "1" from the control signal, a phase shift is
introduced into signal 302a, and at the same time, phase shifter
308b receives a "0" and no phase shift is introduced into signal
302b. Alternatively, phase shifters 308a and 308b may be embodied
as multipliers, with multiplier 308a multiplying signal 302a by a
signal mathematically described as:
? , ? indicates text missing or illegible when filed
##EQU00001##
and, multiplier 308b multiplying signal 302b by a signal
mathematically described as:
? . ? indicates text missing or illegible when filed
##EQU00002##
[0036] As control signal 307 transitions between 1s and 0s, the
polarization of the transmitted signal transitions from right hand
circular polarization to left hand circular polarization, and vice
versa. When used in conjunction with a navigation code transmitter,
the time "T_hop" for control signal 307 may be set to the same rate
at which navigational symbols are transmitted. In other words, the
polarization of the transmitted signal may change on a per-symbol
or code basis. T_hop may also be set to other values, including
transitions within navigation symbols or codes, transitions after a
predetermined number symbols or codes, and even a completely random
time between transitions. Accordingly, control signal 307 may be
used to alter the polarization of the signals transmitted via
transmitter 300 in a manner that will appear random to receivers
that are not themselves aware of control signal 307. As will be
discussed with reference to FIGS. 6-9, when a receiver also has
knowledge of the control signal used to randomly alter the
polarization of the transmitted signal, the receiver may perform
processing on the received signal that allows the receiver to
eliminate jamming and/or spoofing signals from the received
signal.
[0037] The phase shifts illustrated as taking place in phase
shifters 308a and 308b may be introduced in the digital domain via
a DWG, such as DWGs 104a and 104b of FIG. 1, or through digital
phase shifters 106a and 106b of FIG. 1.
[0038] An embodiment like the transmitter illustrated in FIG. 3 may
be advantageous in that it allows for the transmission of a variety
of differently polarized signals. Such transmitted signals allow
for benefits in addition the jamming and spoofing applications
described below with reference to FIGS. 7 and 9. These additional
applications will be described in conjunction with FIGS. 10-15,
below. The embodiment of FIG. 3 may also be advantageous as it may
be used to provide other forms of polarization other than left and
right hand circular polarization. For example, if phase shifters
308a and 308b are not used to introduce phase shifts to either of
signals 302a and 302b, crossed-dipole elements 304a and 304b will
transmit a linearly polarized signal whose orientation is dependent
on the relative amplitudes of signals 302a and 302b. If signals
302a and 302b have the same amplitude, the transmitted signal will
have a 45.degree. linear polarization.
[0039] FIG. 4 provides a chart 400 of non-limiting examples of the
types of polarization that may be achieved through the techniques
described herein. As noted above, if signals 302a and 302b have the
same amplitude, and the same orientation, the transmitted signal
will have a 45.degree. linear polarization, as illustrated in row
405 of FIG. 4. If the signals 302a and 302b have equal magnitude
but opposite orientations, the transmitted signal will have a
135.degree. linear polarization, as illustrated in row 410 of FIG.
4. If signal 302 is provided as only signal 302a (i.e., signal 302b
has an amplitude of zero), the transmitted signal will be linearly
polarized in an "X" direction (sometimes referred to as "horizontal
polarization" in the context of terrestrial transmission systems"),
and if signal 302 is provided as only signal 302b (i.e., signal
302a has an amplitude of zero), the transmitted signal will be
linearly polarized in a "Y" direction (sometimes referred to as
"vertical polarization" in the context of terrestrial transmission
systems"). Though, in example embodiments, the absence of either of
signals 302a or 302b will result in a decreased power signal.
Accordingly, when providing linear polarized signals, the linearly
polarized examples illustrated in rows 405 and 410 of FIG. 4 may be
the preferred way of providing linearly polarized signals.
Different relative amplitude values will cause the orientation of
the polarization to rotate more towards the "X" orientation if the
amplitude of signal 302a is greater than that of 302b (e.g., the
current associated with signal 302a leads the current associated
with signal 302b by 90.degree.), and the orientation will rotate
more towards the "Y" orientation if the amplitude of 302b is
greater than that of 302a (e.g., the current associated with signal
302b leads the current associated with signal 302a by 90.degree.).
If amplitude differences are introduced in conjunction with
90.degree. phase shifts, the transmitted signal may be transmitted
with either right or left hand elliptical polarization. If the
amplitudes of the signals are the same, and the signals are sent
with 90.degree. phase shifts, the transmitted signals will have
either right or left hand circular polarization, a special case of
the more general elliptical polarization. Furthermore, the
amplitudes and/or phases of signals 302a and 302b may be altered in
order to alter or correct the axial ratio of the transmitted
signal. For example, the amplitudes and/or phases of signals 302a
and 302b may be altered in order to adjust the axial ratio of the
transmitted signal to correct for nonidealities in the antenna or
elsewhere in the transmitter radiofrequency chain.
[0040] With reference now made to FIG. 5. Depicted therein is
another embodiment of a transmitter 500 configured to transmit
signals according to the techniques described herein. Like
transmitter 300 of FIG. 3, transmitter 500 provides polarization
diversity, but utilizes two helical antennas 504a and 504b instead
of the crossed dipole radiating elements 304a and 304b of FIG. 3.
For ease of illustration, antenna 504a and antenna 504b are
illustrated as separate antennas, but in example embodiments,
antenna 504a and antenna 504b may be the right and left hand
radiating windings of a helical antenna that share the same core,
and therefore, share the same phase center. The radiating winding
504a is wound in a direction opposite to that of the radiating
winding 504b. Accordingly, windings 504a and 504b are configured to
radiate with circular polarization with opposite orientations. Also
included in transmitter 500 is switch 508. Switch 508 is configured
to switch between feed 506a and feed 506b in response to control
signal 507. Like control signal 307 of FIG. 3, control signal 507
is a pseudo random digital signal. Switch 508 is configured to
respond to control signal 507 such that when control signal 507 is
a "1," switch 508 connects signal 502 to feed 506a. When control
signal 507 is a "0," switch 508 connects signal 502 to feed 506b.
Accordingly, based on pseudo random control signal 507, the
polarization of the transmitted signal will "hop" between the
circular polarization provided by winding 504a, and the opposite
direction circular polarization provided by winding 504b. According
to other example embodiments, an array of eight helical antennas
may be arranged in a circle. By using four right hand circularly
polarized antennas alternately arranged with four left hand
polarized antennas, polarization may be provided such that a phase
center for the transmission remains in a fixed location. By
switching between each set of four antennas, transmissions with a
constant phase center may be provided for use in applications, such
as GPS with centimeter level accuracy.
[0041] With reference now made to FIG. 6, depicted therein is a
receiver 600 configured to determine the polarization of a jammer
signal within a polarization hopped navigation signal, excise or
blank the jamming frequency, and correlate the signal to determine
the time of receipt of the transmitted navigation codes. More
specifically, receiver 600 receives a signal from a transmitter
like those illustrated in FIGS. 1, 3 and/or 5, and utilizes a
control code corresponding to control codes 107, 307 and/or 507 of
FIGS. 1, 3 and/or 5, respectively, to identify and excise a jamming
signal or spoof signal from the received navigation signal.
[0042] Signals 602a and 602b represent the signals received from,
for example, a crossed dipole antenna or other dual polarization
antenna systems. According to the present example, signal 602a
represents the signal received from the "X" oriented element of a
crossed dipole antenna and signal 602b represents the signal
received from a "Y" oriented element of the crossed dipole antenna.
Signals 602a and 602b have been down converted to baseband. Phase
shifters 604a and 604b receive signals 602a and 602b, respectively,
and will phase shift either signal 602a or 602b, as appropriate, so
that the receiver may correctly decode the received signal. For
example, phase shifters 604a and 604b provide a positive 90.degree.
shift to signals 602a or 602b, depending on the current state of
control signals 603a and 603b. Similar to the discussion above with
reference to FIG. 3, phase shifters 604a and 604b may be embodied
as multipliers, with multiplier 604a multiplying signal 602a by a
signal mathematically described as:
? . ? indicates text missing or illegible when filed
##EQU00003##
Similarly, multiplier 604b may multiply signal 602b by a signal
mathematically described as:
? . ? indicates text missing or illegible when filed
##EQU00004##
[0043] According to the example of FIG. 6, control codes 603a and
603b are the inverse of the control codes that were used to provide
the polarization hopped signal received by receiver 600. For
example, if the signals 602a and 602b were received from receiver
300 of FIG. 3, controls codes 607a and 607b would be the inverse of
control codes 307a and 307b, respectively. If the received signal
was generated by phase shifting the feed signal for an "X"
radiating element of a crossed dipole transmitter antenna, phase
shifter 604a induces a phase shift into signal 602a that is equal
in magnitude but in the opposite direction of the phase shift
applied by the transmitter. If the received signal was generated by
phase shifting the feed signal for a "Y" oriented radiating element
of the crossed dipole transmitter antenna, phase shifter 604b
induces a phase shift into signal 602b that is equal in magnitude
but in the opposite direction of the phase shift applied by the
transmitter. In other words, if a pseudo random digital code (e.g.,
control code 107, 307 and/or 507 from FIGS. 1, 3 and 5,
respectively) is used to induce a phase shift in the transmitted
signal, the inverse of that codes is used at the receiver. Control
codes 603a and/or 603b may be communicated to receiver 600 by the
transmitter during a training sequence sent to the transmitter.
Control codes 603a and/or 603b may also be transmitted to the
receiver via an out-of-band signal. To ensure the security of the
codes 603a and/or 603b, the codes made may be transmitted using an
encryption key known only to trusted users of the navigation
system. The phase shifted signals are then combined by combiner 606
to form signal 608.
[0044] Signal processing unit 610 then processes signal 608 to
determine whether or not there is a jammer signal included in
signal 608. If a jammer signal is detected, the jammer signal is
excised or blanked, generating signal 612. The processing performed
in signal processing unit 610 will be described in greater detail
with reference to FIG. 7. Signal 612 is then correlated by
correlator 614, and the correlator output corresponding to the
transmitted navigation codes are output as signal 616, so that
ranging and timing information may be derived.
[0045] With reference now made to FIG. 7, depicted therein are the
logic and signals utilized to detect and excise or blank a jammer
signal based upon a polarization-hopped navigation signal. For
example, the signals and logic illustrated in FIG. 7 may be
utilized in receiver processing unit 212 of FIG. 2 and/or
processing unit 610 of FIG. 6. Before a jammer signal may be
excised or blanked, a determination is made whether or not a jammer
signal is present. Accordingly, first processing 702 is performed
in order to determine whether or not a jammer signal is present. In
processing 702, the total power of the received signal is measured
and compared to a predetermined threshold. Specifically, the power
722 of samples received from the "X" element of crossed-dipole
antenna is combined with the power 724 of samples from the "Y"
element of the crossed-dipole antenna in combiner 703 to measure
the amount of total power being received. If the total signal power
measured in power measurement 704 is greater than this threshold,
it may be determined that a jammer signal is present in the
received signal. This threshold value may be set based upon the
expected power in the signal received from a satellite or platform,
including aircraft and terrestrial platforms. Accordingly, any
significant amount of signal power over this expected value may be
indicative of a jammer signal.
[0046] Once a jammer signal is detected, processing 705 is
performed. Specifically, comparison 714 is performed in which the
received right hand circular polarized signal is compared to power
in the left hand circular polarized signal. More specifically, to
determine the power in the right polarized circular signal, the
samples received from the "X" element of the crossed-dipole antenna
are combined with 90.degree. phase-shifted samples from the "Y"
element of the crossed-dipole antenna to measure the amount of
power being received as the right hand circular polarized signal
706. In the example of FIG. 7, the samples from the "Y" element of
the crossed-dipole antenna are phase shifted by multiplying them by
a signal having a mathematical description of e{circumflex over (
)}(i .pi./2). Similarly, the samples received from the "Y" element
of crossed-dipole antenna are combined with 90.degree.
phase-shifted samples from the "X" element of the crossed-dipole
antenna to measure the power in the received signal as the left
hand circular polarized signal 708. Power measurements 710 and 712
are then taken of the right hand circularly polarized signal 706
and the left hand circularly polarized signal 708, respectively.
These power measurements are then evaluated by logic 714. If the
power 710 in right hand circularly polarized signal 706 is
approximately equal to the power 712 in the left hand circularly
polarized signal 708, it may be determined that the jammer signal
is linearly polarized, and signal 716 is generated. If the power
710 in right hand circularly polarized signal 706 is greater than
the power 712 in the left hand circularly polarized signal 708, it
may be determined that the jammer signal is right hand circularly
polarized, and signal 718 is generated. If the power 712 in left
hand circularly polarized signal 708 is greater than the power 710
in the right hand circularly polarized signal 706, it may be
determined that the jammer signal is left hand circularly
polarized, and signal 720 is generated.
[0047] The logic illustrated in FIG. 7 may take place at the same
frequency with which the polarization hopping changes (e.g., the
operations illustrated in FIG. 7 may repeat at a time interval of
T_hop as illustrated in FIG. 3). Accordingly, even if the jammer is
performing polarization hopping, the jamming will only "match" the
polarization of the transmitting signal, on average, 50 percent of
the time because the jammer will not know the hopping sequence, and
due to the correlation techniques used to retrieve the navigation
codes, the navigation system may still operate successfully.
Specifically, losing half of the hops is equivalent to a 3 dB loss
in signal power, whereas a conventional receiver would be degraded
by much more than this amount in jamming. When the jammer is on the
opposite polarization, its energy will be rejected by a degree
commensurate with the cross-polarization isolation ("XPI"). For a
typical circular polarized antennas, XPI is between 20 to 25 dB.
Accordingly, on average, receiver performance will have improved by
the XPI-3 dB, resulting in a net anti-jamming improvement of 17 to
22 dB.
[0048] Returning for FIG. 6, if it is determined in the operations
illustrated in FIG. 7 that the jammer signal is linearly polarized,
then the signal 612 may be selected as the right hand circularly
polarized signal with the jammer signal excised therefrom, as will
be detailed in FIG. 8. As illustrated in FIG. 8, the left hand
circularly polarized signal 802 received by receiver 800 is shifted
by phase shifter 804 such that the jammer power in the
phase-shifted left hand circularly polarized signal 806 is shifted
180.degree. from the jammer power in the right hand circularly
polarized signal 808. Signal 808 is then combined with signal 806
to generate signal 810 from which the jammer signal has been
eliminated. Signal 810 is then provided to correlator 812.
[0049] Once again returning to FIG. 6, if processing unit 610
determines that the jammer signal is right hand circularly
polarized, then processing unit 610 provides the left hand
circularly polarized portion of the signal received at the
transmitter 600 as the signal 612. The right hand circularly
polarized portions of the signal received at transmitter 600 are
blanked. The left hand circularly polarized portion of the received
signal are properly received because the jamming energy is on the
other polarization and is rejected. Similarly, if processing unit
610 determines that the jammer signal is left hand circularly
polarized, then right hand circularly polarized portion of the
signal received at transmitter 600 is provided to correlator 614 as
signal 612, and the left hand circularly polarized portion of the
signal received at the transmitter 600 is blanked by processing
unit 610. Even though the blanked portions of the signal are not
utilized, and this will happen on average, up to 50% of the time
for a circular polarized jammer, due to the use of correlation to
determine the received signal, receiver 600 may still operate
effectively because this corresponds to only a 3 dB reduction in
signal power, whereas a conventional system would suffer a
degradation in jamming of more than 30 dB.
[0050] As noted above, the techniques described herein may provide
an anti jamming improvement of more than 17 dB compared to related
art techniques. This improvement may be added to jammer rejection
provided by a controllable receive pattern array antenna ("CRPA"),
increasing the power level of the transmitter, and other means so
that a total solution may be provided to combat jamming. More
advanced, higher precision antennas, or those with electronic
corrections to axial ratio, may achieve higher levels of XPI
especially if they can be pointed in the direction of the jammer.
In such example embodiments, levels of XPI can be 30 to 35 dB,
meaning that the anti jamming improvements from the techniques
described herein would increase commensurate with the improvement
in XPI. For such antennas, the anti jam improvements may be between
27 and 32 dB.
[0051] With reference now made to FIG. 9, depicted therein is an
illustration of the processing performed by receiver 900 in order
to identify a spoofing signal using the polarization hopping
techniques described herein, which is analogous to that performed
to identify a jammer, as illustrated in FIG. 7. Specifically,
comparison 914 is performed in which the received right hand
circular polarized signal is compared to power in the left hand
circular polarized signal. More specifically, to determine the
power in the right polarized circular signal, the samples received
from the "X" element of crossed-dipole antenna are combined with
90.degree. phase-shifted samples from the "Y" element of the
crossed-dipole antenna to measure the amount of power being
received as the right hand circular polarized signal 906. In the
example of FIG. 9, the samples from the "Y" element of the
crossed-dipole antenna are phase shifted by multiplying them by a
signal having a mathematical description of e{circumflex over (
)}(i.pi./2). Similarly, the samples received from the "Y" element
of crossed-dipole antenna is combined with 90.degree. phase-shifted
samples from the "X" element of the crossed-dipole antenna to
measure the power received as the left hand circular polarized
signal 908. Power measurements 910 and 912 are then taken of the
right hand circularly polarized signal 906 and the left hand
circularly polarized signal 908, respectively. These power
measurements are then evaluated by logic 914. If the power 910 in
right hand circularly polarized signal 906 is approximately equal
to the power 912 in the left hand circularly polarized signal 908,
it may be determined that the spoofer signal is linearly polarized,
and signal 916 is generated. If the power 910 in right hand
circularly polarized signal 906 is greater than the power 912 in
the left hand circularly polarized signal 908, it may be determined
that the spoofer signal is right hand circularly polarized, and
signal 918 is generated. If the power 912 in left hand circularly
polarized signal 908 is greater than the power 910 in the right
hand circularly polarized signal 906, it may be determined that the
spoofer signal is left hand circularly polarized, and signal 920 is
generated. Based upon signals 916, 918 or 920, the receiver may
identify spoof signals and remove them from the correlation of the
signals through a process analogous to that described with
reference to FIGS. 6-8 for excising a jammer signal.
[0052] With reference now made to FIG. 10, depicted therein is an
example embodiment of a transmitter 1000 through which polarization
diversity is used to provide simultaneous transmission of right
hand circularly polarized signals and left hand circularly
polarized signals. This embodiment is backwards compatible with
related art systems while simultaneously providing polarization
diversity. These techniques also allow for the transmission of
additional codes using right and left hand polarized signals in one
transmission signal.
[0053] Illustrated in FIG. 10 is the simultaneous transmission of
two codes, C1 and C2, with opposite circular polarization sense on
the same transmission signal and the same antenna. As illustrated,
a first channel 1002 feeds the "X" element 1006 of a crossed dipole
antenna, and a second channel 1004 feeds the "Y" element 1008 of
the crossed dipole antenna. Each of channels 1002 and 1004 contains
an adder 1010 and 1012, respectively. The adder performs the sum of
the two inputs. The results are complex terms in which C1 is the
real part of X, which is the in-phase component of the signal, and
C2 is the imaginary or quadrature component of the signal output by
adder 1010. Similarly C1 is the quadrature component of the output
of adder 1012, C2 is the in-phase component of the output of adder
1012. In channel 1002, the signal feeding C1 to combiner 1010 is
not phased shifted, while in channel 1004, the signal feeding code
C1 to combiner 1012 is phase shifted by phase shifter 1016 a
positive 90.degree. relative to the C1 in channel 1002. This will
result in left hand circular polarization transmission of code C1.
Code C2, on the other hand, is fed to combiner 1010 with phase
shifter 1014 providing a positive 90.degree. phase shift relative
to the signal providing C2 to combiner 1012 in channel 1004.
Accordingly, C2 is transmitted with right hand circular
polarization.
[0054] Because the signals will be correlated at the receiver end,
with one correlation being performed on the right hand circular
polarization portion of the signal and a second correlation being
performed on a left hand circular polarization portion of the
signal, the two codes may be retrieved from the same transmission
signal as values transmitted with opposite polarization sense. A
receiver 1100 performing these correlations is illustrated in FIG.
11. Even though the right hand circular polarization receiver
correlation will detect C2 only and not C1, and the left hand
circular polarization receiver correlation will detect C1 only and
not C2, as would be the case with right hand circular polarization
transmission or left hand circular polarization transmission, the
aggregate transmitted energy from the transmitter antenna is
actually linearly polarized, jumping between the polarizations
illustrated in rows 405 and 410 of FIG. 4 above. This linear
polarization is nevertheless transparent to each of the circular
polarized receivers. Each circular polarized receiver will find the
energy of its respective signal (e.g., a pseudo noise code,
described in more detail below) on the correct circular polarized
sense as though it were transmitted with a conventional right hand
or left hand circular polarized antenna because each of these right
hand and left hand circular polarized components is part of the
aggregate signal.
[0055] As illustrated in FIG. 11, two correlations are performed on
the signals received from "X" and "Y" crossed dipole element 1102
and 1104, respectively. Right hand circular polarization
correlation is performed by correlator 1106 while left hand
circular polarization correlation is performed by correlator 1108.
Specifically, the "X" and "Y" signals are fed to adder 1110 with
the "Y" signal receiving a phase shift from phase shifter 1114 that
is the opposite of the shift provided to code C2 by phase shifter
1014 of FIG. 10. Accordingly, when the sample from the "Y" element
1104 is added to the sample from the "X" element 1102 and
correlated by correlator 1106, correlator 1106 detects code C2 and
provides a correlation peak for this code. This right hand circular
polarization processing isolates the C2 code and reduces the noise
produced by the C1 code transmitted on the opposite sense, by the
XPI level of 20 dB or more.
[0056] Similarly, the "X" and "Y" signals are fed to adder 1112
with the "X" signal receiving a phase shift from phase shifter 1116
that is the opposite of the phase shift provided to code C1 by
phase shifter 1016 of FIG. 10. Accordingly, when the sample from
the "Y" element 1104 is added with the sample from the "X" element
1102 and correlated by correlator 1108, the correlator 1108 detects
code C1 and provides a correlation peak for this code. This left
hand circular polarization processing isolates the C1 code and
reduces the noise produced by the C2 code transmitted on the
opposite sense, by the XPI level of 20 dB or more.
[0057] As would be understood by the skilled artisan, the equations
shown in the FIG. 11 are simplified for the case with no relative
rotation shift between the transmitter antenna and the "X" and "Y"
elements 1102 and 1104 of FIG. 11. The equations also assume no
rotation of signal polarization from the transmission channel.
However, the processing of FIG. 11 also works when there is a
rotation shift of the polarization. In that case, the right hand
circular polarization and the left hand circular polarization
output will have an added phase term due to the above-described
rotation shift. The complex correlation performed in correlators
1106 and 1108 will extract this phase and the final correlated
output is the magnitude of the correlation which is not affected by
the rotation term.
[0058] It is noted that correlation provided by correlator 1106 is
essentially a related art right hand circular polarization
receiver. Accordingly, code C2 could be received by a receiver only
configured with a single channel. Therefore, a transmitter as
illustrated in FIG. 10 can provide backwards compatibility with
legacy systems--transmitting code C2 using right hand circular
polarization for legacy receivers, and also transmitting C1 using
left hand circular polarization for a new or different group of
receivers. The transmitter of FIG. 10 may also transmit C2 using
right hand circular polarization for legacy users, while
transmitting C1 using the polarization diversity techniques
described herein. Furthermore, the techniques illustrated in FIGS.
10 and 11 are particularly applicable to correlation systems as it
is the correlation of the "X" and "Y" signals that allows the two
different codes C1 and C2 to be retrieved from the same
transmission signal.
[0059] With reference now made to FIG. 12, depicted therein is a
transmitter 1200 that allows for the transmission of signals
according to the techniques described here. For example,
transmitter 1200 may be configured to transmit three independent
pseudo noise codes ("PN codes"), two of which are transmitted with
right hand circular polarization and left hand circular
polarization, respectively, and a third is transmitted via the
polarization sense of the transmitted signal. Specifically, the two
codes PN_1 and PN_2 are transmitted on the in-phase and quadrature
channels of the output signal such that the values C1 and C2 are
transmitted with opposite polarization sense, respectively. At the
same time, the overall polarization sense of the transmitted signal
(positive or negative linear polarization, right hand polarization
or left hand polarization) depends on the value of K. By altering
the value of K, the polarization sense of the overall signal will
change as illustrated in FIG. 12B. For example, by giving K a value
of "1", the polarization of the transmitted signal will be linear
polarization in which both the X and Y transmitted portions are
positively oriented. By giving K a value of "-1", the polarization
of the transmitted signal will be linear polarization in which both
the X and Y transmitted with an opposite sign. By giving K an
imaginary value of "j", the polarization of the transmitted signal
is left hand polarized. By giving K an imaginary value of "-j", the
polarization of the transmitted signal is right hand polarized.
[0060] Accordingly, the changes in polarization of the overall
signal may serve as a third code, with the diversity circular
polarization modulation ("DCPM") navigational code defined as an
aggregate of the in-phase value C1 transmitting the code PN_1, the
quadrature value C2 transmitting the code PN_2, and the third code
PN_3 being transmitted through the polarization sense of the
transmitted signal. In other words, the series of values comprising
"K" are the third PN code, PN_3.
[0061] As illustrated in FIG. 12, the code C1, PN code PN_1, and
code C2, PN code PN_2 are fed to circular polarization phase shift
controller 1202. The circular polarization phase shift controller
is controlled by phase shift control code K, which serves as PN_3.
The outputs from the circular polarization shift controller 1202
are provided to quadrature up converters which up-convert the
baseband signals to the Radio Frequency ("RF") carrier signal 1208
to form modulated RF signals 1210 and 1212. The modulated RF
signals 1210 and 1212 are amplified by amplifiers 1214 and 1216,
respectively, and transmitted by crossed dipole radiating elements
1218 and 1220, respectively.
[0062] Transmitter 1200 may also be configured to transmit PN codes
with polarization hopping, as described above with reference to
FIG. 1 or FIG. 3. In such an example embodiment, K is the pseudo
random value that switches the polarization sense of the
transmitter between, for example, right and left hand polarization.
While this value is described above in, for example, FIG. 1 as a
binary code that hops the signal between left and right hand
polarization, FIG. 12 illustrates how the code may have four values
(1, -1, j, -j) that hop the signal between two orthogonal linear
polarizations, left hand circular or elliptical polarization and
right hand circular or elliptical polarization.
[0063] With reference now made to FIG. 13A-C, depicted in FIG. 13A
is a receiver 1300a which may decode a polarization hopped signal
received from a transmitter, such as transmitter 1200 of FIG. 12A.
According to the example of FIG. 13A, the receiver is aware of the
value K from FIG. 12A as the phase shift control code of a
polarization hopping embodiment. As illustrated, signals received
from the "X" and "Y" oriented antenna elements 1302 and 1304,
respectively, of a crossed dipole antenna are amplified by low
noise amplifiers 1306 and 1308, respectively. The signals from the
from the "X" and "Y" oriented antenna elements 1302 and 1304,
respectively, are then downconverted to baseband by downconverters
1310 and 1312, respectively. The amplified and demodulated signal
from the "X" element of the crossed dipole antenna is fed to
combiner 1314, while the signal received from the "Y" element 1304
undergoes phase shifting, from phase shifter 1316 and 1318, with
the phase shift provided by phase shifter 1318 being controlled by
the value of K, essentially "undoing" the phase shift implemented
by the value K in FIG. 12. The phase shifted signal from the "Y"
element is then fed to the combiner 1314. The combiner output
consisting of the sum of the phase shifted "Y" signal, and the
un-shifted "X" signal, are fed to the complex correlator 1320. The
complex correlator correlates the combiner output with local values
for the first and second navigational codes representing by values
C1 and C2. The real part of the local code C1 is PN_1 and the
imaginary part of the local code C2 is PN_2. The magnitude 1322 of
the correlator output is the correlation peak used for detection
and ranging measurement, for this aggregate code.
[0064] Depicted in FIG. 13C is a receiver 1300c configured to
decode three navigational codes, a code C1 transmitted with right
hand circular polarization sense, a code C2 transmitted with left
hand circular polarization sense, and a third code sent via the
polarization sense of the overall combined signal used to transmit
codes C1 and C2. In other words, receiver 1300c is configured to
decode a signal sent from the three-code embodiment described above
with reference to FIG. 12A.
[0065] The processing performed by receiver 1300c begins like that
of receiver 1300a of FIG. 13A with signals received from the "X"
and "Y" oriented antenna elements 1302 and 1304, respectively, of a
crossed dipole antenna are amplified by low noise amplifiers 1306
and 1308, respectively. The signals from the "X" and "Y" oriented
antenna elements 1302 and 1304, respectively, are then
downconverted to baseband by downconverters 1310 and 1312,
respectively.
[0066] Unlike FIG. 13A, the outputs of downconverters 1310 and 1312
are fed to right hand circular polarization processing 1324 and
left hand circular polarization processing 1326, which correspond
to the processing illustrated in right hand circular polarization
processing and left hand circular polarization processing of FIG.
11. The outputs of each of the right hand circular polarization
processing 1324 and the left hand circular polarization processing
1326 are fed to combiner 1328, which serves an analogous function
to combiners 1110 and 1112 of FIG. 11. Complex correlators 1332a
and 1332b correlate the combiner output with local values for the
first and second navigational codes represented by values C1 and
C2. The magnitudes 1334a and 1334b of the correlator outputs are
the correlation peaks used for detection and ranging measurement,
for codes C1 and C2.
[0067] To process the third code corresponding to K from
transmitter 1200 of FIG. 12A, the outputs of downconverters 1310
and 1312 are also fed to vector cross product calculator 1338. The
value of the vector cross product of the output of downconverters
1310 and 1312 will change sign with changes in polarization sense
of the overall transmitted signal. Accordingly, the sign of the
output of vector cross product calculator 1338 will change with the
value of K. Complex correlator 1332c can correlate these changes in
sign with K. The magnitude 1334c of the correlator output is the
correlation peak used for detection and ranging measurement for the
code corresponding to K.
[0068] With reference now made to FIGS. 14A and 14B, depicted
therein is a transmitter 1400 that generalizes the polarization
diversity transmission techniques described herein. Specifically,
illustrated in FIG. 14A is a transmitter 1400 that is configured to
transmit four PN code sequences C1, C2, C3 and C4 which will be
received at a receiver and decoded using correlation at the
receiver. Each of these values may be either a positive 1 or
negative 1 value over a symbol period, and based on the values of
C1, C2, C3 and C4, the polarization output from the crossed dipole
antennas will be as illustrated in FIG. 14B.
[0069] The transmitter 1400 also simplifies to the polarization
hopping example as illustrated in, for example, FIGS. 3 and/or 12,
if constraints are placed on the values of C1, C2, C3 and C4 as
illustrated in, for example, FIGS. 3 and 12. Transmitter 1400
similarly simplifies to transmitter 1200 of FIG. 12A by placing
other constraints on the values of C1, C2, C3 and C4, such that the
phase shift between the values controls the polarization sense such
that a third PN code is sent via the changes in polarization
sense.
[0070] Specifically, codes C1 and C2 are provided to baseband
upconverter 1402a in which C2 is phase shifted relative to C1, the
signals are combined and are upconverted to carrier signal 1404,
which in this case is a radio frequency (RF) carrier signal. The
modulated radio frequency signal is amplified by amplifier 1406a
and transmitted via the X element 1408a of a crossed dipole
antenna. Codes C3 and C4 are provided to baseband upconverter 1402b
in which C4 is phase shifted relative to C3, the signals are
combined and are upconverted to RF carrier signal 1404. The
modulated radio frequency signal is amplified by amplifier 1406b
and transmitted via the Y element 1408b of the crossed dipole
antenna.
[0071] FIG. 14B illustrates the overall polarization of the signal
output from the crossed dipole antenna as dependent on the values
of C1, C2, C3 and C4.
[0072] With reference now made to FIGS. 15A and 15B, depicted
therein is a general receiver 1500 configured to receive and
demodulate signals received from a receiver like receiver 1400 of
FIG. 14A. Specifically, signals are received over the X element
1502a and the Y element 1502b of a crossed dipole receiver antenna.
These signals are downcoverted to baseband from the radio frequency
carrier signal.
[0073] As illustrated, the complex baseband signals received from
the X antenna element 1502a and Y antenna element 1502b are
combined in combiner 1504 after the Y element signal is phase
shifted by the inverse of K. K is known at the receiver 1500 based
upon the local values of C1, C2, C3 and C4 known by the receiver
1500, as illustrated in FIG. 15B. The local codes C1 and C2 are
correlated with the output of combiner 1504 from the X element
1502a in complex correlator 1508a. Similarly the local codes C3 and
C4 are correlated with the output of combiner 1504 from Y element
1502b in complex correlator 1508b. Because the output of Y element
1502b was multiplied by the inverse of K, the local versions of C3
and C4 are similarly multiplied by the inverse of K in operation
1509. The magnitudes 1510a and 1510b of the correlator outputs are
the correlation peaks used for detection and ranging measurement,
for codes C1, C2, C3 and C4, respectively.
[0074] The above description is intended by way of example only.
Although the techniques are illustrated and described herein as
embodied in one or more specific examples, it is nevertheless not
intended to be limited to the details shown, since various
modifications and structural changes may be made within the scope
and range of equivalents of the claims.
* * * * *