Hybrid radio frequency system with distributed anti-jam capabilities for navigation use

A radio frequency navigation receiver has analog and digital sections. In the analog section, a CRPA antenna module receives global navigation signals and provides two steerable nulls for jammer suppression. An analog RF module has an amplifier and a YIG filter for amplifying and filtering an antenna output signal with jammer suppression. An analog receiver receives and downconverts an RF output signal from the RF system with additional jammer suppression. The analog receiver provides broadband signal filtering with at least 30 dB rejection and narrow band signal filtering with at least 45 dB rejection. In the digital section, a digital receiver module receives and converts an analog receiver output signal to generate a digital output signal for navigation control. A configuration manager processor responds to detected jammer signals to configure the receiver modules accordingly.

BACKGROUND OF THE INVENTION 
The present invention relates to radio frequency (RF) systems used for 
navigational purposes, and, more particularly, to RF receivers which 
operate in satellite navigation systems and are structured with diverse 
anti-jam capabilities to enable receiver and navigation operations in a 
hostile jammer environment. 
Satellite navigation systems employ a plurality of satellites which 
generate respective signals for reception by an RF receiver on an airborne 
or surface vehicle or other platform for computation of the platform 
position. A navigation control can thus steer the platform in its motion 
on the basis of a stream of position calculations. The United States and 
Russia recently deployed satellite RF navigation systems respectively 
called the Global Positioning System (GPS) and Global Navigation Satellite 
System (GLONASS). For a detailed description of these navigation systems, 
reference is made to a book entitled "Understanding GPS: Principles and 
Application", edited by E. D. Kaplan, and published in 1996 by Artech 
House Publishers of Boston, Mass. 
In this disclosure, the term GPS refers generically to satellite navigation 
systems including the Global Position System, GLONASS, and any additional 
satellite navigation system which may be created in the future such as the 
system planned by the European Community. 
The satellite navigation systems, now deployed, were developed with 
civilian access built into the signal structure, and these systems are 
thus highly vulnerable to electronic attack measures. The same is likely 
to be true for any future navigation system. 
To date, the United States government has examined a number of techniques 
for ensuring RF navigation in a jamming environment. These anti-jam 
techniques have been independently developed with little or no strategic 
plan as to how, when, or where these various techniques may be used. Table 
1 provides an overview of capabilities and usage of existing RF navigation 
related anti-jam technologies. 
TABLE 1 
______________________________________ 
OFF-THE-SHELF ANTI-JAM CAPABILITIES 
Anti-Jam Typical Anti- 
Technology Jam Capability 
Use Where 
______________________________________ 
Antennas with 
-- Multipath Antenna 
Choke Rings Mitigation 
Upwards Looking 
30 to 60 dB 
Eliminate Ground 
Antenna 
Antennas Level Jammers 
Fixed Reception 
25 to 35 dB 
Fixed Jammers Antenna 
Pattern Antenna 
Fixed Nulls 
(FRPA) 
Controlled 25 to 35 dB 
Mobile Jammers 
Antenna 
Reception Steerable 
Pattern Antenna 
Nulls 
(CRPA) 
Cavity Filters 
-- Eliminate Out-of- 
Pre- 
or Bandpass band Interference 
Filtering 
Pre-filters 
Notch Filters 
20 to 30 dB 
CW Jamming RF 
Filtering 
Transversal 
20 dB (PSK) to 
CW & PSK Jamming 
IF 
Filters 30 dB (CW) Filtering 
Jammer-to-Noise 
-- Detect Jammers 
Digital 
Power Ratio Receiver 
Meters 
Non-linear A/D 
-- Limit Interference 
Digital 
Converters Receiver 
Wider Dynamic 
.about.6 dB per Bit 
Any Jammers Digital 
Range A/D Receiver 
Converters 
Anti-Spoofing 
-- Limit Access to 
Digital 
(A-S) encrypted P-code Receiver 
Y-code 
INS Aiding of 
.about.20 dB 
Coasting through 
Digital 
Code/Carrier Jamming Signals 
Receiver 
Loops 
Parallel -- Direct Y-code Digital 
Correlators Acquisition Receiver 
______________________________________ 
Even in a jammer-free and interference-free environment, the navigational 
accuracy of existing military RF receivers is inadequate for certain 
weapon-platforms. For example, such RF receivers may produce unacceptable 
vertical position errors in weapon delivery. A need thus exists for a GPS 
receiver which provides greater navigational accuracy. 
In an RF environment, interference signals may have jamming effects on an 
RF receiver even though they are not intentionally generated as jamming 
signals by combatants in a military environment or by terrorists in a 
civil or commercial environment. Jamming interference signals may 
originate, for example, in a laptop computer on an airplane, a radio or 
television tower, or any high frequency device such as a radar, a radio, 
or a cellular phone. 
Accordingly, the term "jammer signal" herein refers to both jammer and 
interference signals. In illustration, the term "jammer free" includes 
"interference-free", and the term "anti-jam capability" includes 
"anti-interference" capability. 
With the removal of selective availability, the use of various forms of 
jamming can provide a means for maintaining an edge in RF navigation in 
theaters of war. In turn, the use of electronic attack (jamming) against 
the use of GPS navigation technology leads to a need for electronic 
protection (anti-jam technology) which enables use of the RF navigation 
technology in a jamming environment. 
In state-of-the-art GPS receivers having anti-jam capability, the magnitude 
of jammer suppression has been limited by limited anti-jam capabilities. 
Moreover, existing design concepts have typically limited the kinds of 
jammers which can be suppressed in such GPS receivers. It is desirable 
that a basic GPS receiver structure be developed to provide better jammer 
suppression and to provide suppression of a wide range of different kinds 
of jammers. Further, a need exists for GPS receiver structures used in a 
variety of platforms, such as airplanes, land vehicles, cruise missiles or 
smart munitions, manportable units, ordinary munitions, etc. Accordingly, 
a wide variety of anti-jam requirements exists for the wide range of 
possible applications. 
With previously known GPS anti-jam receivers, this variety of requirements 
can be met only by a costly proliferation of GPS designs for the different 
platform applications. It is thus desirable that a basic GPS receiver 
structure be developed to enable anti-jam capabilities to be provided cost 
effectively over a wide range of platform applications. 
In one particular aspect of state-of-the-art GPS receivers, an 
analog/digital interface is placed as close as possible to the GPS 
receiver antenna to obtain an anti-jam capability principally through 
digital circuitry functioning with or without antenna nulling. This prior 
art analog/digital design concept has actually limited anti-jam 
capabilities in GPS receivers, as more fully explained subsequently 
hereinafter, and typically has employed a multitude of deep antenna nulls 
which can be substantially reduced in effectiveness by "toggling" jammers 
operating to confuse the anti-jamming operation of the deep nulls. 
Other prior art RF systems, such as Electronic Support Measures (ESM) and 
anti-radiation missile seekers (ARM) used for navigation and targeting 
purposes have also been limited in performance similar to the described 
limitations of prior art GPS RF receivers. 
Accordingly, the present invention is directed to meeting existing needs in 
the pertaining art and to overcoming the described prior art limitations. 
SUMMARY OF THE INVENTION 
A radio frequency (RF) navigation receiver, usable in military and 
commercial mobile platforms, is thus structured to comprise an analog 
section which includes an antenna system for receiving global navigation 
signals and an analog RF system having an amplifier and preferably a 
filter for amplifying and filtering an antenna output signal which 
includes the navigation signals and any received jammer signals. 
Further, an analog receiver receives and downconverts an RF output signal 
from the analog RF system. 
In a digital system of the receiver, a digital receiver system receives and 
converts an analog receiver output signal to generate a digital output 
signal for navigation control. 
In one aspect of the invention the analog receiver preferably has means for 
filtering by adaptive cancellation to provide a first capability for 
suppressing received jammer signals. The adaptive cancellation filtering 
is preferably performed for broadband signals with at least 30 dB 
rejection and/or for narrow band signals with at least 45 dB rejection. At 
least one of the digital receiver, the antenna system, and the RF system 
preferably has means for suppressing received jammer signals to provide a 
second jammer suppression capability, or the second and additional jammer 
suppression capabilities. 
In another aspect of the invention, the analog RF system employs an 
adaptive filter, which provides a first means for suppressing received 
jammer signals, and the antenna system and/or the analog receiver and/or 
the digital receiver may selectively include one or more additional jammer 
signal suppressing means. 
Accordingly, the invention enables GPS and other RF navigation receivers to 
be structured flexibly, and preferably modularly, to provide improved 
anti-jam capability suitable for the use to which each receiver is to be 
applied.

DESCRIPTION OF THE INVENTION 
The present invention is directed to RF systems in which jammer suppression 
and navigation signal enhancement technologies are employed in a manner 
which generally provides better, more reliable RF system performance in 
military navigation and targeting applications and commercial navigation 
applications, while specifically enabling such RF systems to be flexibly 
and economically configured with specified performance. 
The invention generally employs distributed, and preferably modularly 
distributed, jammer suppression and navigation signal enhancement in an RF 
system which is provided at the system level with a hybrid analog/digital 
architecture. In application of the invention to GPS RF receivers, a 
diversity of anti-jam (AJ) technologies can be employed to defeat, with 
greater effectiveness, a wide variety of jammer threat configurations 
(number, disposition, power, and modulation) that might otherwise jam the 
use of navigation to be provided with use of GPS receivers. 
In accordance with the invention, an AJ GPS receiver is preferably 
organized into six modules which can be combined in different ways to 
enable AJ GPS receiver products to be configured with different levels of 
AJ performance according to specified needs. 
A first module is a receiver antenna module which preferably incorporates a 
distributed upwardly directed antenna array enabling nulls to be steered 
against possible jammers in an upper hemisphere of coverage. 
A second module is a receiver protector preferably embodied as a PIN diode 
switch or a similar type of device which protects sensitive electronics 
from high power microwave jamming which might be used to try to disable 
the GPS receiver. 
A third module is an adaptive RF filter based on YIG filter technology. If 
desired, the receiver protector and the adaptive RF filter may be combined 
in a single module. 
A fourth module is an analog receiver which contains two types of adaptive 
IF filters (continuous look-through filters) for suppressing broad band 
spread spectrum and narrow band CW jamming signals while having little 
impact on a GPS signal. 
A fifth module is a miniature, low power clock which provides an atomic 
time standard. The atomic clock module provides a highly stable frequency 
source for maintaining absolute time and for generating low phase noise 
clock signals throughout the GPS receiver. By closely coupling the atomic 
clock within the GPS receiver, the AJ GPS receiver is capable of: (1) 
achieving on average factor of three improvement in vertical positioning, 
(2) achieving on average a 10% improvement in horizontal positioning, (3) 
performing direct Y-code acquisition orders of magnitude faster with a 
small number of correlators, and (4) improving the AJ capability by 
typically two orders of magnitude through reduction in the time and 
frequency uncertainty in searching for the satellite signals. 
Finally, a sixth module is a digital GPS receiver. In this module, AJ 
capability is preferably provided through: (1) increased dynamic range 
provided by higher precision A/D conversion, and (2) increased processing 
gain through parallel processing provided by numerous digital correlators. 
The hybrid organization of the invention significantly contributes to its 
described benefits. The modular structure of the invention enables a GPS 
receiver to be economically configured for the level of performance needed 
in a military or commercial platform in which the GPS receiver is to be 
applied. Such military or commercial platforms may include, for example, 
ground-based vehicles, ships, missiles, and airplanes. 
The modular structure for an AJ GPS receiver allows a family of GPS 
receivers to be designed with a variety of levels of AJ capability ranging 
from as low as 25 dB to over 120 dB. From the six described basic modules, 
low cost, AJ GPS receivers can be tailored for munitions and man-portable 
applications; moderate cost, AJ GPS receivers can be made for expendable 
UAVs, cruise missiles, land vehicles, and smart munitions; and high cost, 
AJ GPS receivers can be made for aircraft, surface vessels, complex UAVs, 
and satellites. 
More particularly, FIG. 1A is a schematic representation of a hostile 
environment in which RF navigation is to be performed by blue forces using 
the principles of the invention for a plurality of platforms 30, each to 
be navigated with use of a GPS RF receiver 32. Typical platforms are 
indicated by text and associated icons. Red forces use a plurality of 
platforms 31 navigated by respective conventional GPS RF receivers 33. 
Current satellite navigation systems typically employ twenty-four 
satellites with up to eleven satellites being in sight of a 
terrestrially-based navigable platform, but only four satellites 34, 36, 
38, and 39 are shown since a minimum of four satellite signals are 
required for navigation computations. A plurality of blue and red aerial 
and/or surface jammers 11, 12 (blue) and 20 (red) generate signals 
intended to penetrate the indicated red and blue GPS receivers 32, 33 and 
interfere with navigation of the associated red and blue 30, 31 platforms. 
As shown in FIG. 1C, the amplitude of GPS satellite signals (such as -133 
dB) is below the ambient noise amplitude (such as -106 dB). The satellite 
signals are thus difficult to find for direct jamming purposes, but can be 
identified by a GPS receiver through signal correlation techniques based 
on knowledge of the pattern of the satellite signals. Such identification 
is made possible when jammer signals, such as the one shown at -14 dB, are 
adequately suppressed. 
A combined land, sea, and air engagement is shown in FIG. 1A wherein 
various forces are attempting to use RF signals from the satellites 34, 
36, 38, and 39 to navigate within a battlefield. The battlefield area is 
being actively jammed by the stand-off jammers 11, the airborne jammers 12 
and the land mobile jammers 20. Blue forces are using RF navigation 
signals to guide cruise missiles 16 into targets such as ground radars 18. 
Likewise, red forces are trying to eliminate blue assets such as ships 10 
and are using C/A code receivers to guide their missiles 14 into the 
targets. The greatest benefit of RF navigation, to the blue forces, is 
reduced fratricide resulting from accurate placement of blue troops 28 and 
29 and assets such as tanks 20, 24 on a desert or other featureless 
battlefield. A red tank 22 and red troops 26, 27 are also shown. The 
forces with better tools for operating in this hostile RF environment will 
have a better chance of survival and greater likelihood of success. 
FIG. 1B depicts threats which an AJ GPS receiver is likely to face and the 
impact of such threats on signal processing. A robust AJ GPS receiver must 
be designed to withstand six major types of threats: CW (narrow band, 
fixed frequency continuous waveforms), AMCW (amplitude modulated narrow 
band, fixed frequency continuous waveforms), Swept CW (narrow band, swept 
frequency continuous waveforms), Pulse CW (pulsed power, narrow band, 
fixed frequency continuous waveforms), PSK (phase shift keyed, broad band 
20.46 MHz bandwidth! waveforms) and Noise (broad band 20.46 MHz 
bandwidth! Guassian and near Guassian! white noise). Any number of these 
threats may occur in any combination. 
The maximum anticipated threat environment in which the GPS receiver is to 
operate is one in which integrated power levels or jammer to satellite 
signal (J/S) levels are greater than 120 dB. The threats can be located 
anywhere in space; that is, the threat disposition is 4 pi steradians. The 
example illustrated on the right-hand side of FIG. 1B is based on an 
environment with an integrated power level of 103 dB, i.e., an integrated 
jammer power of -30 dBm. The typical satellite signal level is -133 dBm 
which is 27 dB below the noise floor (-106 dBm) of a typical GPS receiver. 
Since the GPS receiver is processing a spread spectrum signal for which the 
spreading function is known, receiver correlators are able to provide 
processing gain on the order of 55 dB which means that an effective 
satellite signal (post-processing) is on the order of 28 dB above the 
noise floor (-78 dBm). The GPS receiver only needs to remove jammer 
signals above the effective satellite signal level, and, therefore, the 
effective jammer to signal level (J/S).sub.eff is usually much smaller (48 
dB) than the integrated power level (103 dB) as shown in FIG. 1B. 
As shown in FIG. 2A, a GPS navigation system 40 implements the invention 
with the AJ GPS receiver 32 of FIG. 1A, and, in higher performance 
systems, an atomic clock 42. A navigation computer 44 operates on inputs 
from the GPS receiver 32, the atomic clock 42, and an inertial navigation 
system (INS) 46 to generate navigation parameters, such as location, 
velocity, time, attitude, and acceleration. 
A GPS RF receiver 50, illustrated in FIG. 2B, is an embodiment of the 
invention which is hybridized at the system level and preferably modularly 
structured to provide high level navigation signal enhancement and AJ 
capacity for improved GPS navigation. The GPS receiver 50 has a modular 
structure including an AJ antenna module 52, an AJ RF module 54, an AJ 
analog receiver module 56, and a digital receiver module 58 with 
distributed AJ and signal enhancement capabilities which enable improved 
GPS receiver performance in navigation operations. 
In designing a particular GPS receiver, the modular structure of the 
invention enables a large variety of GPS receivers to be configured with 
an AJ capability needed for each GPS receiver design. As more fully 
considered subsequently herein, use of all modules provides maximum GPS 
receiver accuracy and greatest jammer suppression. In contrast, use of 
only the RF module 54, with jammer suppression filtering, the analog 
receiver module 56, without jammer suppression filtering, and the digital 
receiver module 58 (four-channel) provides specified jammer suppression in 
a low cost/low prime power GPS receiver. 
With the RF filter module used to provide a core jammer suppression 
capability, one or more additional modules can selectively have additional 
jammer suppression capability (capabilities) thereby enabling a variety of 
RF receivers to be configured economically to meet a variety of user AJ 
needs with improved performance. 
In another aspect of the invention, the analog receiver module can have a 
core jammer suppression capability with one or more other modules 
selectively providing additional jammer suppression enabling a variety of 
improved RF receivers to be configured according to user AJ needs. 
In the GPS receiver 50, the system analog/digital interface is located 
between the analog and digital receiver modules 56 and 58. In this manner, 
advanced AJ capability is enabled through effective analog filtering in 
the analog receiver module 56. 
Generally, the antenna module 52 is capable of rejecting, by 25 dB or more, 
all signals originating below a horizontal plane through the GPS receiver 
platform, and is additionally preferably capable of generating two 25 dB 
deep steerable nulls which can be pointed in any direction above the 
horizontal plane. 
As previously indicated, conventional antenna arrays (CRPA) employ multiple 
steerable nulls intended to achieve most or all of the jammer suppression 
capability of the conventional GPS receiver. Such antennas employ 
attenuators, phase shifters, and variable controlled amplifiers to provide 
phase and amplitude control for steering nulls toward jammers. Typical 
CRPA arrays having six steerable nulls are large, expensive, and as 
subsequently explained, generally have a poor return on investment. 
Blinking or terrain bouncing of signals can easily defeat jammer 
suppression in the conventional GPS receiver design with CRPA antenna 
based AJ. Further, conventional GPS receiver design cannot apply jammer 
suppression using expensive CRPA antennas to man-portable receivers. 
The AJ antenna module 52 may be omitted from the GPS receiver 50 and 
replaced by a conventional antenna, but it is included when higher level 
GPS receiver performance is specified. The module 52 employs an adaptive 
array antenna 60 (typically illustrated in FIG. 3C) which is an upwardly 
directed analog apparatus classifiable as a controlled reception pattern 
antenna (CRPA). The antenna 60 has a ground plane and choke rings which 
provide a 35 to 60 dB sector null below the ground plane, while producing 
a desirable antenna gain in the upper hemisphere. The CRPA antenna has up 
to seven antenna patches 61-1 through 61-7 which provide the capability of 
steering up to six 25 to 35 dB nulls toward jammers. These antenna patches 
are spaced at approximately a half wavelength apart. 
However, as shown in FIG. 3A, the antenna 60 preferably uses two 25 dB 
steerable nulls which combine to form one 50 dB null. A digital 
configuration manager processor 62 employs feedback to provide null 
steering and minimize the amount of jammer energy present in a signal sent 
from the antenna 60 to the digital receiver 58. The configuration manager 
62 also operates to identify and correctly process blinking jammers which 
might otherwise tax the capability of the CRPA antenna from performing its 
mission. 
The typical adaptive array antenna 60 employs a manifold for the signals 
from up to seven patch antennas rather than carrying the signals in up to 
seven analog channels. This arrangement significantly simplifies the 
analog part of the system. The illustrated seven element adaptive array 
antenna with its 7:1 manifold requires a single 14 bit A/D converter 90 to 
steer the nulls as efficiently as 2-bit A/D converters used with each 
antenna element in the equivalent digital CRPA. 
In the preferred invention embodiment, as few as three antenna patches are 
used. The two-null analog CRPA antenna of the invention is approximately 
25% smaller in diameter (50% smaller in area) and has only three patch 
antennas. The 3:1 manifold requires a 6-bit A/D converter to steer the 
nulls as efficiently as a 2-bit A/D converter for each antenna element in 
the equivalent two-null digital CRPA. The reduction from seven elements to 
three elements, and the associated reduction in the A/D converter 
requirements for the preferred embodiment, leads to a simpler, less costly 
digital receiver 58. 
FIG. 3B is based on an existing CRPA quoted as having 50 dB deep steerable 
nulls and graphically illustrates why multiple, deep-null GPS receivers of 
the prior art have a poor return on investment. Basically, such receivers 
are expensive yet relatively easily defeated. In FIG. 3B, the CRPA has 
five 50 dB deep nulls. If five jammer threats exist, the total J/S is 
reduced by 50 dB. However, if six threats exist, only 83% of the threats 
are attacked by the CRPA, and the overall effective suppression is 9 dB 
which is only 2 dB better than the performance achievable with a CRPA 
having five 10 dB deep nulls. Normally, a null requires about 10 
microseconds of computer processing to be created, and during this 
processing time the receiver is vulnerable to various jamming threats. It 
is thus at least questionable whether the considerable cost and 
computational time for creating the 50 dB nulls is justifiable. 
CRPAs which use a random search for null location placement are highly 
vulnerable to multiple jammer threats, and further are even vulnerable to 
only two jammers if they are separated by more than one null-width, and 
are repeatedly toggled on-off. Dependent on the repetition rate, the null 
processor may never catch up with the toggled jammers and properly place 
the nulls on the jammers. 
Generally, the AJ RF module 54 receives a signal from the antenna module 52 
to generate a filtered output for downconverting. A conventional RF 
preamplifier 57 amplifies the signal to a level set by the configuration 
manager 62. By setting the signal level high, the configuration manager 62 
forces RF filtering to operate in a linear region, thereby passing the 
input signal through the RF module 54 virtually (ignoring insertion 
losses) unaltered. By setting the level low, the configuration manager 62 
forces the RF filtering to operate over both linear and saturated regions, 
causing the input signal to be significantly attenuated for signal levels 
above an intrinsic filtering threshold value. 
In the AJ RF module 54 (FIG. 2B), a receiver protector 55 is included as a 
sub-module which provides protection against microwave transmissions 
intended to "burn-out" the GPS receiver 32. The receiver protector 55 is 
omitted if it is not needed in a particular product. The receiver 
protector 55 operates on the signal input from the antenna module 52 in 
providing this additional AJ feature of the invention. The receiver 
protector 55 is a limiter device which either reflects or absorbs 
essentially all incident RF power above a certain level. 
Preferably, a PIN diode limiter is employed as the receiver protector 55 
because its insertion loss can be as small as a few tenths of a dB. The 
PIN diode works as a receiver protector by having its bias level, and 
hence its shunt impedance, a function of incident RF power. 
In the AJ RF module 54, an adaptive RF filter 59 (FIG. 4) provides an 
additional AJ feature of the invention. As previously indicated, the AJ RF 
module 54 is preferably included as a core jammer suppression module in RF 
receiver products of the invention. However, the AJ RF module 54 can be 
omitted and replaced by a conventional RF module (not shown) containing 
only a preamplifier and, if desired, a band pass filter in products having 
no need for the AJ capability of the AJ RF module 54. 
Unlike conventional GPS notch filters which use tunable lumped element 
circuits requiring processing time, the adaptive RF filter 59 is embodied 
as a magneto-static surface wave structure 70 which operates at the 
molecular level. A magnetic material 72, preferably yttrium, iron, and 
garnet (YIG), is mounted on stripline conductors 74, 76. When 
appropriately biased by an external magnetic field, this configuration 
functions as a variable impedance transmission line which uses a spin wave 
to absorb excess energy above a threshold value from incoming signals. The 
threshold for this effect is tunable through film thickness. This AJ 
suppression technique is excellent for mitigating single and multiple high 
power CW threats. 
The molecular level adaptive RF filter 70 operates as a frequency or, more 
precisely, a power selective limiter which suppresses high power jamming 
signals above a threshold level by absorbing excess energy within a spin 
wave. The preferred YIG filter is made adaptive through a signal feed back 
from the configuration manager 62 which controls the gain of amplifier 57 
and hence the signal level injected into the YIG filter. 
The YIG filter 70 preferably provides 20 dB of jammer suppression, and is 
cascadable allowing higher levels of jammer suppression (at a cost of 
greater insertion loss) if required. The primary targets for the adaptive 
RF filter 59 are CW jammers that try to eliminate Y-code operation in 
military receivers by precise placement of a jamming signal on either or 
both sides of the C/A-code. 
In the prior art, various filters have been used for RF AJ filtering. For 
example, notch filters have been employed, but such filters tend to be 
bulky and are not susceptible to miniaturization. Further, adaptive notch 
filters have typically been easily defeated by agile jammers, because they 
do not operate at the molecular level. 
In contrast, the adaptive RF filter of the invention is based on molecular 
level filtering to lend itself to miniaturization, operates to handle 
multiple jammers, and operates with agility to follow agile jammers. 
Although the adaptive RF filter of the invention will not handle noise 
jamming, it is powerfully effective against CW, swept CW, and pulse CW 
jammers. Moreover, this RF filter is relatively low cost and can be 
cascaded for multiplied suppression of jamming signals. 
The following publication provides further information on YIG filters and 
is hereby incorporated by reference: 
Frequency Selective Limiters for High Dynamic Range Microwave Receivers 
published in IEEE Transactions on Microwave Theory and Techniques, Vol. 
41, No. 12, December 1993, pps. 2227-2231, by J. Douglas Adam and Steven 
N. Stitzer. 
The following United States patents provide additional information 
regarding YIG filters are hereby incorporated by reference: 
U.S. Pat. No. 4,595,889 entitled FREQUENCY SELECTIVE SIGNAL-TO-NOISE 
ENHANCER/LIMITER APATUS, issued to S. N. Stitzer et al. on Jun. 17, 
1986, and assigned to The United States of America as represented by the 
Secretary of the Air Force; 
U.S. Pat. No. 4,845,439 entitled FREQUENCY SELECTIVE LIMITING DEVICE, 
issued to S. N. Stitzer et al. on Jul. 4, 1989, and assigned to the 
present assignee; and 
U.S. Pat. No. 4,980,657 entitled COPLANAR WAVEGUIDE FREQUENCY SELECTIVE 
LIMITER, issued to S. N. Stitzer et al. on Dec. 25, 1990, and assigned to 
the present assignee. 
The AJ analog receiver module 56 (FIG. 2B) is structured principally with 
analog circuitry, and is thus included, at the system level, in the analog 
portion of the analog/digital hybrid GPS receiver 50. The analog receiver 
module 56 is configured with amplifiers, filters, and mixers to function 
as a superheterodyne receiver with two or more stages of downconversion. A 
first frequency mixer 82 downconverts a signal from the RF module to a 
first intermediate frequency (IF) level, and a second frequency mixer 84 
provides further downconverting of the signal to a second IF level. 
Amplifiers are connected as shown in the embodiment of FIG. 2B with 
automatic gain control. 
A multi-stage IF filter arrangement operates by adaptive cancellation in 
the first and second IF levels to provide additional jammer suppression in 
the GPS receiver 50. This filter arrangement can be variably arranged 
adaptively identifies and eliminates narrow-band CW jammer signals and 
wide-band spread spectrum signals from the signals passed through them. In 
the modularity aspect of the invention, the AJ analog receiver module 56 
can be a core AJ module in embodying the invention in various products as 
previously described. If the RF module 54 is the core module of a group of 
receiver products, the analog receiver module 56 is included in its jammer 
suppression capability as needed by the user. Otherwise, the module 56 can 
be used without its AJ filters. Various configurations of the analog 
receiver module may differ in the number and placement of IF AJ filters. 
A narrow band IF filter 86 preferably handles only one notch at a time and 
extends the capability of the adaptive RF filter 59, if the latter is 
modularly included in applying the invention, as in the case of the GPS 
receiver 50. The narrow-band IF filter 86 is structured to provide at 
least 45 dB rejection of narrow-band jammer signals. A broadband IF filter 
88 is structured to suppress broadband jammer signals with a rejection of 
at least 30 dB. 
The narrow band IF filter 86 is preferably embodied as a filter 
configuration known commercially as a COLT C filter. The broad band IF 
filter 88 is preferably embodied as a filter configuration known 
commercially as a Colt SS filter. These IF filters can be structured as 
disclosed in the following patents assigned to the present assignee and 
incorporated herein by reference: 
U.S. Pat. No. 5,263,191 entitled METHOD AND CIRCUIT FOR PROCESSING AND 
FILTERING SIGNALS and issued to R. W. Dickerson on Nov. 16, 1993; 
U.S. Pat. No. 5,339,456 entitled METHOD AND CIRCUIT FOR NON-COOPERATIVE 
INTERFERENCE SUPPRESSION OF RADIO FREQUENCY SIGNALS and issued to R. W. 
Dickerson on Aug. 16, 1994; 
U.S. Pat. No. 5,355,533 entitled METHOD AND CIRCUIT FOR RADIO FREQUENCY 
SIGNAL DETECTION AND INTERFERENCE SUPPRESSION and issued to R. W. 
Dickerson on Oct. 11, 1994, and 
U.S. Pat. No. 5,428,834 entitled METHOD AND CIRCUIT FOR PROCESSING AND 
FILTERING SIGNALS and issued to R. W. Dickerson on Jun. 27, 1995. 
Generally, as shown in FIGS. 5B and 5C, the COLT filters are adaptive 
signal cancellers which identify and remove any high power jammer signal 
from the circuitry. The COLT C filter has been tested to establish that 
the COLT C provides 40 to 45 dB jammer suppression against CW jamming, 12 
to 17 dB jammer suppression against BPSK jamming, and 15 to 20 dB of 
jammer suppression against QPSK jamming. For comparison, the prior art 
Amplitude Domain Processing (ADP) technique only provides 30 dB jammer 
suppression against CW jamming and 20 dB against PSK jamming. The COLT 
filter technology has the further comparative advantage of being suitable 
for miniaturization. 
The narrow band signal canceller 86 illustrated in FIG. 5B breaks up an 
incoming composite signal at A into two parts. It drives a voltage 
controlled oscillator (VCO) with an error signal K and forms direct B and 
quadrature D signals proportional to the largest incoming signal (the 
jammer or interference signal). Through the use of appropriate high pass 
filters F and G and low pass filters H and I, all product signals are 
eliminated except the product of the interference signal and the signal of 
interest from the two signal paths. Finally, the resultant signal is 
recombined with direct C and quadrature E copies of the interference 
signal to form remaining (smaller) direct and quadrature signals of 
interest which are recombined into a single signal at an output J. 
The wide band signal canceller illustrated in FIG. 5D breaks up an incoming 
composite signal at A into two parts. It drives a log limiting amplifier 
which compresses the incoming signal L and forms direct B and quadrature D 
signals proportional to the largest incoming signal (the jammer or 
interference signal). Through the use of appropriate high pass filters F 
and G and low pass filters H and I, all product signals are eliminated 
except the product of the interference signal and the signal of interest 
from the two signal paths. Finally, the resultant signal is recombined 
with direct C and quadrature E copies of the interference signal to form 
remaining (smaller) direct and quadrature signals of interest which are 
recombined into a single signal at an output J. 
FIG. 5A illustrates an AJ analog receiver module 56 embodied in 
miniaturized form, i.e., with semiconductor chips #1, #2, #3, and #4. The 
#1 chip embodies the first IF downconversion stage with a bandpass filter 
90. The #3 chip similarly embodies the second IF downconversion stage with 
a bandpass filter 92. 
The #4 chip provides a reference oscillator 94 and a frequency translator 
96 which generates mixer frequencies LO1 and LO2. The #2 chip can embody, 
for example, a one, two or three (shown) stage Colt filter connected 
between the first and second IF stages. Other variants of this miniature 
analog receiver module can be realized from the combination of chips. 
Respective bypass switches 83, 85, and 87 are controlled by the 
configuration manager 62 to bypass any combination of COLT filtering 
stages, such as the third stage, or the second and third stages, or all 
three stages. 
The AJ analog receiver module 56 applies an analog IF output signal to an 
analog/digital (A/D) converter 90 at the input of the AJ digital receiver 
module 58 (FIGS. 2B and 6A). In the module 58, the configuration manager 
processor 62 preferably is a microcontroller programmed to respond to an 
output signal from the A/D converter 90 and apply feedback control to the 
adaptive antenna array 60 and the adaptive RF filter 59 as shown in FIG. 
2B. The digital receiver module has AJ capabilities which also may be 
separately or collectively deselected in an RF receiver product if a user 
has no need for them. 
As previously indicated, the configuration manager 62 is structured to 
steer antenna nulls for minimized jammer signal content in the digital 
signal output from the A/D converter 90 and to control the threshold level 
of the RF filter, i.e., to control the threshold level of the preferred 
YIG filter by varying its input signal level. 
The A/D converter 90 (FIGS. 2B, 6E) is preferably structured with a wide 
range to provide greater dynamic range in searching for a satellite 
navigation signal relative to a jammer signal(s). FIG. 6E shows a typical 
2-bit A/D converter with a J/N meter used in some state-of-the-art GPS 
receivers. Since each bit added to the A/D converter 90 widens the dynamic 
range by approximately 6 dB, a GPS receiver with an 8 bit A/D converter 
has a processing gain improved by at least 36 dB over the typical GPS 
receiver. In AJ GPS receivers provided with adaptive array antennas in 
accordance with the invention, the A/D converter 90 is at least an 
n.times.2 bit converter where n is the number of patch antennas used to 
get the minimum dynamic range. For the three element CRPA chosen for the 
preferred embodiment, the minimum bit width of the A/D converter 90 is at 
least 6 bits for full effectiveness. 
Hardware is preferably used in the A/D converter 90 for detection of noise, 
CW and PSK jamming. The presence or absence of a jammer is determined by a 
J meter also referred to as a J/N meter as illustrated in FIG. 6E. Once a 
jammer has been detected, the signal is sent to a device 91 such as a Fast 
Fourier Transform (FFT) to determine whether this jammer signal is 
characterized by a single frequency (CW), multiple frequencies (PSK), or 
otherwise (Noise). This is a coarse hardware method for implementing 
signal support for the configuration manager 62. Other more sophisticated 
techniques can be used for more sophisticated receiver designs. In other 
applications of the invention, jammer signal detection can be embodied by 
software, such as in the configuration manager processor 62. 
A plurality of signal channels 92 are provided in the AJ digital receiver 
module 58 for separate processing of the multiple satellite navigation 
signals contained in the output from the A/D converter 90. The total 
number of GPS satellite signals can vary based on the number of carriers 
broadcast by the GPS satellites and the number of variant satellite 
systems in orbit. In the case where a total of sixteen GPS satellite 
signals are to be processed, a total of 16 channels for processing of 
satellite signals can be provided, i.e., four channels on each of four 
channel boards 94, 96, 98, and 100. 
The number of channel boards provided in any particular GPS receiver 
configuration depends on the desired navigation accuracy. For example, a 
single channel board provides the least accuracy, whereas all four channel 
boards can provide the highest accuracy. 
Each navigation signal carries a satellite signature, i.e., orbit 
parameters, the time at which the signal was sent and other information 
useful to accurate location and time determination. The signals are 
applied from the channels 92 to a receiver processor 102 which is 
conventionally programmed to respond to signal data for at least four 
satellites and compute the location of the platform carrying the GPS 
receiver 50. The receiver processor applies an output to a navigation 
processor 104 to compute navigation parameters for motion control of the 
GPS receiver platform. 
An on-board clock 106 corresponds to a sub-module within the AJ digital 
receiver module and generates a timing signal which enables calculation of 
the time of arrival of each satellite signal at the GPS receiver 50. The 
signal arrival times are used in the platform position computation 
described above. A counter 108 is driven by the clock 106 to generate a 
timer signal which provides greater RF navigation system availability 
since there is no need to compute time from the satellite signals. The 
clock 106 operates as a common frequency source for the analog receiver 
module and the digital receiver module to avoid errors which are otherwise 
possible. 
To obtain additional AJ suppression and enhanced navigation signal 
reception, the clock 106 is preferably a phase stable oscillator which 
generates an output with precise timing, i.e., an atomic clock with 
1.times.10.sup.- 11 or better stability and with low prime power input 
(preferably about 1/2 watt or less). More particularly, the atomic clock 
is preferably a cesium cell clock which is relatively low in cost, small 
in size (25 cubic centimeters), and low in power requirements (about 0.3 
watts). 
The cesium cell clock 106 can be embodied in accordance with the following 
United States patents which are assigned to the present assignee, which 
discloses more detailed information on a miniaturized atomic frequency 
standard, and which are hereby incorporated by reference: 
U.S. Pat. No. 5,192,921 entitled MINIATURE ATOMIC FREQUENCY STANDARD and 
issued to P. J. Chantry et al. on Mar. 9, 1993. 
U.S. Pat. No. 5,327,105 entitled GAS CELL FOR A MINIATURIZED ATOMIC 
FREQUENCY STANDARD and issued to I. Liberman et al. on Jul. 5, 1994. 
U.S. Pat. No. 5,442,326 entitled ATOMIC TIME STANDARD WITH PIEZOELECTRIC 
STABILIZATION OF DIODE LASER LIGHT SOURCE and issued to I. Liberman on 
Aug. 15, 1995. 
The following publications provide further information on cesium clocks and 
are hereby incorporated by reference: 
A Low-Cost Atomic Clock: Impact on the National Airspace and GNSS 
Availability published in Proceedings of ION GPS-94, pps. 1329-1336, in 
Sep. 20-23, 1994, by John Murphy and Dr. Trent Skidmore; and 
Laser Pumped Cesium Cell Miniature Oscillator published in Proceedings of 
the 52nd Annual Meeting Navigation Technology for the Third Millenium, 
pps. 731-739, in Jun. 19-21, 1996, by Peter J. Chantry, Irving Liberman, 
William R. Verbanets, Carlo F. Petronio, Robert L. Cather, and William D. 
Partlow. 
When the atomic clock 106 is modularly included in the GPS receiver 50, it 
provides significant additional AJ suppression and improved position 
accuracy based on better time accuracy. The atomic clock 106 also provides 
direct Y-code acquisition with as few as 64 correlators in less than one 
second, and further, for signal acquisition times of 100 seconds can 
improve processing gain by as much as 20 dB through reduced time 
uncertainty (as illustrated in FIG. 6C). As compared to conventional 
receivers in which crystal oscillators are employed, the GPS receiver 50, 
tightly coupled with the atomic clock 106, on average improves the 
intrinsic vertical accuracy by a factor of three and horizontal accuracy 
by 10%. 
FIG. 6B shows a top level diagram of a miniature Cesium cell-based 
embodiment of the atomic clock 106. Fundamentally, this atomic clock is a 
highly stable frequency source which is achieved by an 852 nm laser 101 
pumping the cesium atoms in a cesium cell 103 to an elevated state which 
can be probed with a 9,192,631,770 Hz microwave as it undergoes a 
hyperfine transition back to the ground state. 
To establish the microwave frequency, light from the laser 101 is sent 
through the cesium cell 103 to an optical detector 105. By measuring the 
optical transmission through the cell 103 while it is being probed by the 
microwave frequency, digital control electronic circuitry accurately 
determine when the hyperline transition occurs, thereby accurately 
defining the microwave frequency to be 9,192,631,770 Hz. 
The highly stable microwave frequency source operates in a phased locked 
loop to drive the output of a crystal oscillator (XO) at 10 MHz (or any 
other frequency of choice) with the same level of stability (on the order 
of 1 part in 10.sup.11). The output of the crystal oscillator (XO) is sent 
to the counter 108 to count the number of cycles from some initial point 
in time and maintain a time reference. 
In the digital receiver 58 (FIG. 2A), conventional correlation techniques 
are employed to identify the satellite signals for processing through the 
channels 92 for navigation purposes. The configuration manager processor 
62 responds to signals from the A/D converter 90 to configure the 
available AJ resources of the GPS receiver 50 for suppression of detected 
jammer threats. 
In the preferred embodiment, the correlation process is the similar for C/A 
code and P code/Y code receivers. Such receivers employ different code 
generators (with decryption included for the Y code correlation). 
Conventional hardware 85 (FIG. 2B) is used in the digital receiver 58 to 
perform the correlation process. If desired, software can be used to 
perform the correlation process. 
The digital signal provided to the correlator 85 is divided into in-phase 
(I) and quadrature (Q) components by multi-bit multiplication with the 
sine and cosine functions, respectively. The correlator 85 receives a copy 
of the appropriate code (C/A code or P code/Y code) from a code generator 
circuit. The correlator 85 searches this code space for a match with the 
incoming signal. When a match occurs, the correlator 85 is locked to the 
signal. The following description applies to a typical correlation process 
used in a GPS receiver. 
Through the use of a shift register, the correlator 85 generates early, 
prompt and late copies of the appropriate single bit code. The correlator 
spacing between the early/prompt/late codes is set within the correlator 
85 to be some fraction of a chip (the fraction may be as low as 0.1 during 
signal acquisition or as high as 0.5 during parallel code or carrier 
tracking) dependent upon the state (signal acquisition, parallel code or 
carrier tracking) of the correlator. For C/A carrier code, the chipping 
rate is 1.023 MHz while the P carrier code/Y carrier code chipping rate is 
10.23 MHz. In this example, the correlation process is a multi-bit 
exclusive or between the single bit carrier codes and the multi-bit I and 
Q samples. 
The correlator 85 includes a delay lock loop discriminator process which 
allows the selection of "early and late", "early-minus-late", and 
"punctual" correlation. The "early and late" correlation process is used 
during signal acquisition. The "early-minus-late" correlation process is 
used during optimal parallel code tracking. The "punctual" correlation 
process is used during carrier tracking. A detailed description of these 
discriminator functions is provided in "Understanding GPS: Principles and 
Applications" edited by E. D. Kaplan. 
The correlator 85 is coupled to accumulators (not shown) which accumulate 
(or time average) the correlated samples over some period of time. The 
accumulation process results in an output signal if full correlation is 
achieved, or it results in no output signal when the signal is 
uncorrelated (i.e., the signal components are still pseudorandom over the 
averaging time) with the internally generated carrier codes. Once the 
correlators are locked to the signal, the signal is channeled to the 
receiver processor 102 for extraction of the data message (50 Hz 
information modulated on top of the carrier signal) from the signal. 
The configuration manager processor 62 employs hardware/software to control 
the use or non-use of the variety of AJ technologies built into the GPS 
receiver 50. The configuration manager processor 62 can be 
designed/preprogrammed with knowledge of the receiver configuration or it 
can dynamically determine the receiver configuration from signals provided 
to it. The configuration manager logic is modular corresponding to the 
modular construction of the GPS receiver 50. However, the configuration 
manager processor 62 can be configured utilizing a minimal set of logic 
only appropriate to the receiver hardware configuration for which it is 
designed or it can be made utilizing a maximal set of logic which is 
adaptable to any possible configuration that might be implemented. The 
following description is based on a non-adaptable configuration manager 
processor which manages a system containing all AJ resources described 
herein. 
The configuration manager 62 employs external hardware in the A/D converter 
90 to examine the signal levels present before the signals are sent to the 
digital receiver processor 102. The hardware compares these signal levels 
with the level of the noise floor to determine if there is any jamming 
needing to be removed by the anti-jam features of the GPS receiver. If a 
jamming threat exists, the hardware then does a frequency analysis such as 
a simple Fourier transformation of the signal to determine its frequency 
characteristics. 
From these measurements the configuration manager 62 begins to assign 
anti-jam resources against the threats. The first anti-jam resource is the 
antenna module 52. The configuration manager 62 places one null on a 
target by searching in space for a single null antenna pattern which 
minimizes the impact of the jammer signal. 
If the null brings the signal level down to the noise floor, only one noise 
threat needed to be removed and was removed. Otherwise, the configuration 
manager 62 places two nulls on the two largest targets by searching in 
space for the second null pattern which minimizes the impact on the jammer 
signal. 
Next, the configuration manager 62 assesses the existence of a CW threat 
using frequency analysis. If a CW threat exists, the configuration manager 
62 sets the injection level for the RF filter such that the RF filter 
attacks the CW threat. Otherwise, it sets the injection level for the RF 
filters to pass the signal through to the next stage in the receiver 
analog section. 
The configuration manager 62 then reassesses the existence of a CW threat 
using frequency analysis. If a CW threat exists, the configuration manager 
62 sets the analog receiver module to use the COLT-C IF filter to remove 
the remaining CW threat; otherwise, it bypasses the COLT-C IF filter. 
Next, the configuration manager 62 determines whether a broadband or PSK 
threat exists using the latest frequency analysis of the signal waveform 
going to the digital receiver 58. 
If a PSK threat exists, the configuration manager 62 sets the analog 
receiver module to use the COLT-SS IF filter to remove the PSK threat; 
otherwise, it bypasses the COLT-SS filter. 
Through the use of the configuration manager 62, an optimal signal level to 
jammer level can be presented to the digital receiver 58 for correlation 
processing. The configuration manager 62 operates with the ability to 
detect the presence or absence of a jamming signal. By detecting the 
absence of a jamming signal, the configuration manager 62 provides the 
digital receiver with the largest IF signal level which has not been 
processed by any of the analog front-end anti-jam features. 
The logic operation of the configuration manager processor 62, in 
performing AJ configuration management, is represented by the flow diagram 
shown in FIG. 6D. Antenna module logic is performed as illustrated in 
dotted box 120 if the AJ antenna module 60 is included in the GPS receiver 
50, as it is here. If a noise threat is detected by block 122, a 50 dB 
antenna null is directed to the target if only one threat is present. If 
two or more threats are present, block 124 directs two 25 dB nulls against 
the two largest threats. 
In RF filter logic indicated by dotted box 125, test block 126 next 
determines whether CW threats are present and, if so, the adaptive RF 
filter 59 is activated by block 128 (when, as here, the filter 59 is 
included in the GPS receiver). The RF filter 59 is bypassed by block 130 
if no CW threats are present. 
IF filter logic is next performed as indicated in dotted boxes 132 and 134. 
If test block 136 determines that more CW threats are present, the Colt-C 
filter 89 is activated as indicated by block 138 in the first IF stage. 
Otherwise, the Colt-C filter 59 is bypassed by block 140. 
Test block 142 then determines whether a PSK threat is present. If so, the 
Colt-SS filter 88 is activated by block 144. Otherwise, block 146 directs 
the Colt-SS filter 88 to be bypassed and a return is made for the logic to 
be repeated in the next cycle of operation. 
The digital receiver module 58 has the following advantages when it is 
structured in accordance with the invention: 
A/D Increased Bit Improves Signal(s) Capture 
Results in Reduced J/S up to 36 dB 
Flexibility-Minimum (4) Satellites for Low-Tech Platforms 
Maximum (16) Satellites for Prime Platforms 
Configuration Manager Controls Threat Prioritization 
Atomic Clock Allows Fewer Correlators.about.60 
Receiver Processor with 50% Excess Capacity-Growth 
Data Rate Adaptable to Platform Requirements 
FIG. 8 graphically illustrates the manner in which the invention provides 
enhanced reception of satellite signals, even in the absence of jammers. 
FIG. 9 shows how suppression is applied to jammer threats in accordance 
with the operation of the configuration manager 62. 
The following apply to FIG. 9: 
##EQU1## 
In FIG. 10, an exemplary multiple jammer threat scenario is shown for a 
surface platform. FIG. 11 graphically illustrates how the GPS receiver 50 
handles the multiple jammer threats for the surface platform. 
FIG. 13 similarly illustrates the operation of the GPS receiver 50 on an 
airborne platform in a multiple threat scenario shown in FIG. 12. In this 
scenario, a blue cruise missile 150 is attacking a ground target 152 
protected by extensive red jammer resources. The combined jammer signal 
level at the platform is approximately -14 dBm, resulting in a J/S of 119 
dB. This scenario includes a very large range of threat power levels, 
modulation types, and geometric dispositions. 
The top of FIG. 13 shows the progression of these signals through the GPS 
receiver 50. The bottom of FIG. 12 shows the effective satellite signal 
strength due to the improved digital Rx/Proc performance. For this 
scenario, the resultant J/S is -2.5 dB, and the GPS receiver wins, that is 
the GPS receiver is able to complete its mission of calculating its 
location. 
This example shows how careful consideration needs to be given to the 
determination of AJ effectiveness. Here, a total suppression of 155 dB is 
applied to the incoming jammers to reduce the total effective J by 46.5 
dB. The GPS receiver always needs much more AJ capability than can 
actually be applied in most scenarios in order to win. 
FIG. 14 graphically illustrates the operation of the invention, as embodied 
in the GPS receiver 50, against various combinations of an airborne CW 
threat, a ground CW threat, and airborne or ground broadband threats. 
FIG. 7 illustrates some of the ways the modular structure of the invention 
can be arranged to meet the needs of various applications. Prime platforms 
such as aircraft, cruise missiles, surface platforms, complex UAVs and 
spacecraft may require significant anti-jam protection. In most of these 
applications, all six modules are likely to be required. These platforms 
may require simultaneous tracking of up to 8 satellites using both L1 and 
L2 carrier signals. Receiver 8 of Table 3 (ahead) is an example of a 
receiver which was built according to this model. 
Secondary platforms such as expendable UAVs, low cost cruise missiles, and 
smart munitions like JDAM and JSOW may require less anti-jam protection. 
Features like the adaptive antenna array might be replaced by a single 
antenna, and the receiver protector may be eliminated to reduce cost and 
weight. These platforms may require simultaneous tracking of only 4 
satellites using both L1 and L2 carrier signals. Receiver 7 of Table 3 is 
an example of a receiver built according to this model. 
Ultra light, low cost receivers are at the other end of the application 
spectrum for use in artillery shells or in man portable devices. In these 
cases, features like the atomic clock and the RF adaptive filter may be 
eliminated and simultaneous tracking of 4 satellites with the L1 carrier 
signal may only be required. Receiver 4 of Table 3 is an example of a 
receiver built according to this model. 
In operation, the GPS receiver 50 provides better AJ capability at reduced 
cost as compared to conventional GPS receivers. The improved capability is 
based on better jammer suppression and better GPS satellite signal 
enhancement. 
Table 2 summarizes the improved performance achieved with application of 
the invention (as compared to -40 dB AJ capability of typical prior art 
GPS receivers): 
TABLE 2 
______________________________________ 
GPS PERFORMANCE 
J/S REDUCTION THROUGH A COMBINATION 
OF J REDUCTION AND S ENHANCEMENT 
Threat Function At Capacity 
______________________________________ 
Distributed Low 
Antenna Sector Null; 
25 dB 
Power Ground Threats 
High Power Noise 
Antenna Adaptive 
25-50 dB 
Nulls; (2) @ 25 dB 
High Power CW RF Filter 20 dB 
(Notch/FSL); 
Any CW IF Filter (Colt C); 
45 dB 
Any Broadband (PSK) 
IF Filter (Colt SS); 
30 dB 
All A/D Improvement; 
30 dB 
(2 to 8 - bit) 
All Atomic Clock 25 dB 
Stability; 
______________________________________ 
As previously indicated, the invention has application to GPS receivers as 
well as other RF receivers employed for global navigation purposes. These 
include an Electronic Support Measures (ESM) receiver 180 which is 
structured in accordance with the invention as shown in FIG. 15 to detect 
and locate jammers operating in the environment. 
The ESM receiver 180 uses a non-linear array 183 of antenna elements to 
establish an unambiguous angle of arrival of a jammer signal. The line 
array antenna 183 feeds GPS receiver elements 184, 185, and 186 of the 
invention for detection of a GPS-like signal. The antenna 183 also feeds a 
processor 187 which detects time difference of arrival (TDOA) and the 
angle of arrival of the signal used in range determination. The ESM 
receiver 180 can be used as a stand alone unit synthetically determining 
the TDOA over a long base line based on motion of the ESM receiver 180, or 
it can be used with other ESM receivers to determine TDOA over a 
physically long base line for ESM receivers that are fixed in space. With 
use of an atomic clock module 189, a very accurate TDOA capability is 
achieved with an attendant increase in passive ranging accuracy. 
Among other applications, the invention can also be embodied in 
anti-radiation missile seekers (ARM) as illustrated by an ARM 182 shown in 
FIG. 16. The missile seeker receiver uses a track array antenna 190 plus 
receiver modules 192, 194, and 196 of the invention to guide the seeker in 
on a jammer. The output of the analog receiver 194 is fed to both the 
digital receiver 196 and a navigation computer 198. The navigation 
computer 198 homes in on the jammer (HOJ) signal as long as the jammer 
signal is present. When the jammer signal is turned off, the navigation 
computer 198 flies to a fixed location based on the last known location of 
the jammer signal using the RF navigation signals to locate the target in 
space. 
In summary, the invention employs a hybrid/modular structure and a 
diversity of anti-jamming and signal enhancement features to achieve the 
described improvement in performance in each of a variety of modular 
receiver configurations employed for global navigation purposes. As 
applied to AJ GPS receivers, the invention enables creation of a product 
line of receivers which have degrees of anti-jam capability tailored to 
the type of jamming environment expected and the type of platform on which 
it is to be deployed. 
The following Table 3 illustrates the breadth of capability that can be 
built within a modular AJ GPS receiver architecturally structured in 
accordance with the invention. 
TABLE 3 
__________________________________________________________________________ 
THE BREADTH OF MAXIMUM J/S CAITIES FOR 
REPRESENTATIVE MODULAR ANTI-JAM GPS RECEIVERS 
Maximum 
Number 
Jammer J/S 
Receiver 
C/A 
P- Atomic 
Dual COLT 
COLT or Suppression 
Processing 
Capacity 
Example 
Code 
Code 
Clock 
Frequency 
C SS FSL 
CRPA 
Channels 
(dB) Gain (dB) 
(dB) 
__________________________________________________________________________ 
1 X 4 0 43 43 
2 X X 4 0 73 73 
3 X X 4 45 43 68 
4 X X X 4 75 43 118 
5 X X X 4 45 73 118 
6 X X X X 4 75 73 148 
7 X X X X X 8 95 73 168 
6 X X X X X X X 16 170 73 243 
__________________________________________________________________________ 
Receiver example number 1 can be sold commercially to the non-military 
market. Receiver examples numbers 2 through 6 can be sold to the military 
for incorporation in munitions/shells and manportable units. Receiver 
example number 7 can be sold to the military for use in expendable UAVs, 
cruise missiles and smart munitions. Receiver example number 8 can be used 
by the military aboard high valued assets such as aircrafts, cruise 
missiles, surface platforms, land vehicles, complex UAVs, and satellites. 
The maximum J/S capacity indicated in this table is a figure of merit for 
the corresponding receiver example. The actual jammer performance depends 
on the level of signals, the spatial origin of the signals, and the types 
of signals (CW, AM CW, FM CW, Pulsed CW, Swept CW, BPSK, QPSK, Noise, GPS, 
etc.) which comprise the signal environment being sampled by the antenna 
and processed by the receiver. The complex interaction of these parameters 
with the anti-jam technology used in the GPS receiver determines the 
amount of jammer suppression and signal enhancement that can be physically 
realized. Analysis has shown that, dependent on the scenario, these 
receivers can realize J/S ratios of anywhere from 25 to over 120 dB. 
The foregoing description of the preferred embodiment has been presented to 
illustrate the invention without intent to be exhaustive or to limit the 
invention to the form disclosed. In applying the invention, modifications 
and variations can be made by those skilled in the pertaining art without 
departing from the scope and spirit of the invention. It is intended that 
the scope of the invention be defined by the claims appended hereto, and 
their equivalents.