Miniaturized GPS Antenna Array Technology and Predicted Anti-Jam Performance

Abstract

NAVSYS has developed a miniaturized GPS antenna array technology that reduces the size of the antenna elements and the array dimensions. This technology enables GPS controlled reception pattern antenna arrays (CRPAs) with anti-jamming capability to be installed on vehicles where their size has previously prohibited their use. This includes aircraft where size and weight constraints resulted in fixed reception pattern antenna (FRPA) installations instead of CRPAs and munitions where space and surface area are at a premium. BOEING has developed AGHAST™ (Another GPS High Anti-jam Simulation Tool), which allows the anti-jam performance of CRPAs and their null-steering electronics to be predicted. Using detailed antenna pattern and coupling data collected in the instrumented anechoic chamber at Boeing, AGHAST™ can accurately predict the expected installed performance of anti-jam hardware in a jamming environment. In this paper, the miniaturized antenna array technology is described, along with test results and an AGHAST™ evaluation of the mini-array anti-jam performance.

 A GPS Receiver Designed For Carrier-Phase Time Transfer

Abstract

The development of time transfer techniques using GPS carrier-phase observations promises the capability to deliver sub-nanosecond time transfer capabilities. Testing to date has shown that conventional GPS receivers introduce significant time offset in the carrier phase. NAVSYS have developed a GPS receiver capable of making observations with high phase stability. Test data is presented in this paper showing the accuracy of the code and carrier phase observations from this receiver for time transfer applications.

Effect of Antenna Performance on the GPS Signal Accuracy

Abstract

The effect of antenna performance on the carrier phase measurement accuracy is described. The antenna performance characteristics in the presence of multipath are analyzed in terms of an antenna Axial Ratio and an antenna Noise Figure. In this analysis, the multipath is assumed to originate from a single point located on a perfectly conducting ground. The analysis focuses on the carrier phase sensitivity with respect to various parameters such as antenna Axial Ratio, Noise Figure, ground reflection coefficient, etc. The analytical simulations show that Axial Ratio has significantly more influence on the carrier phase accuracy in presence of multipath than antenna Noise Figure.

Four-Element Adaptive Array Evaluation for United States Navy Airborne Applications

Abstract

This paper provides background on the design methodology of an adaptive GPS antenna system and the testing methodology used to evaluate such a system. The challenge in evaluating multi-element active antenna arrays is that conventional antenna characteristics such as element phase, gain, or side lobe levels do not easily translate into system level performance measures such as dynamic response time, pattern null width and null depth. The GPS Antenna System (GAS-1N) Foreign Comparative Testing (FCT) program evaluated a four-element, four channel adaptive antenna system for U.S. Navy airborne applications by using various measures of performance based on the stage of testing. Testing methodologies included anechoic chamber, F/A-18 full-scale model on an outdoor range, modeling and simulation techniques, and dynamic flight testing on a C-12J and F/A-18C. Additionally, side-by-side comparisons were made during flight testing using Fixed Radiation Pattern Antenna (FRPA), GAS-1N (4-element), and GAS-1 (7-element) antenna systems.

A Novel GPS Survey Antenna

Abstract

A novel GPS antenna for surveying applications is proposed. It is a fixed beam phased array of aperture coupled slots optimized to receive a right hand polarized signal. The proposed antenna is made out of a single PCB board. Another PCB board is placed underneath the antenna to act as a reflector in order to reinforce the antenna directivity and reduce the back-lobe radiation. The radiation pattern roll-off of this antenna is sharper than the conventional GPS patch antennas mounted in the “choke ring” configuration. The sharp pattern roll-off allows reducing the antenna’s susceptibility to multipath generated replicas of the GPS signal. The antenna is much smaller and lighter than a corresponding typical “choke ring” antenna. There is no phase center offset between the L1 and L2 GPS frequencies and the antenna does not require any alignment with respect to a given direction (such as North) due to its natural symmetry. Due to its planar structure it can be easily buried in the vehicle or aircraft skin.

GPS C/A Code Tracking with Adaptive Beamforming and Jammer Nulling

Abstract

A space-time adaptive processing (STAP) algorithm for delay tracking and acquisition of the GPS signature sequence with interference rejection capability is developed. The interference can consist of both broadband and narrowband jammers. The narrowband jammers are modeled as vector autoregressive (VAR) processes and rejected by temporal whitening. The spatial nulling is implicitly achieved by estimating a sample covariance matrix and feeding its inverse into the extended  Kalman Filter (EKF). Computer simulations demonstrate robust performance of the algorithm in severe jamming, and also show that the algorithm outperforms the conventional delay-locked loop (DLL).

Advanced Pinwheel Compact Controlled Reception Pattern Antenna (AP-CRPA) designed for Interference and Multipath Mitigation

Abstract

This paper describes the design of a compact, dual GPS frequency L1/L2, multi element adaptive antenna array, where each array element is comprised of an aperture coupled spiral slot array. In short, it is an array of arrays. The design of a seven element array is described. The design is quite different from existing CRPA antennas which use a stacked patch approach. The aperture coupled spiral slot array methodology allows a reduction of mutual coupling between the adjacent elements of the array, and hence a reduction of the overall size of the antenna. In addition, its wide band performance will allow it to meet future GPS M-code requirements. The array has a low profile with a maximum height of 0.8”.

Miniaturized  GPS Antenna Array Technology and Prediction Anti-Jam Performance

Abstract

NAVSYS has developed a miniaturized GPS antenna array technology that reduces the size of the antenna elements and the array dimensions. This technology enables GPS controlled reception pattern antenna arrays (CRPAs) with anti-jamming capability to be installed on vehicles where their size has previously prohibited their use. This includes aircraft where size and weight constraints resulted in fixed reception pattern antenna (FRPA) installations instead of CRPAs and munitions where space and surface area are at a premium. BOEING has developed AGHAST™ (Another GPS High Anti-jam Simulation Tool), which allows the anti-jam performance of CRPAs and their null-steering electronics to be predicted. Using detailed antenna pattern and  coupling data collected in the instrumented anechoic chamber at Boeing, AGHAST™ can accurately predict the expected installed performance of anti-jam hardware in a jamming environment. In this paper, the miniaturized antenna array technology is described, along with test results and an AGHAST™ evaluation of the mini-array anti-jam performance.

High Performance GPS Pinwheel Antenna

Abstract

There is a need for improved antenna performance in view of a recent decision by the DOD to disable SA at L1 GPS channel. For single frequency receivers SA had been the main cause of the largest errors of code and carrier phase measurements. With SA turned off, the multipath, ionosphere and antenna phase center variations are now the biggest contributors to code and carrier phase measurements. Ideally the antenna must be smaller and lighter than a typical “choke ring” antenna for surveying and man pack applications. In this paper, a new high performance GPS antenna is proposed using “Pinwheel” (patent pending) technology [16], characterized by a stable phase center and very good immunity to multipath generated replicas of GPS signal. The antenna is a fixedbeam twelve-element phased array antenna comprised of aperture-coupled spiral slots optimized to receive L1 GPS right-hand polarized signal. In addition there is an array of eleven concentric slot rings located outside the perimeter of spiral slots in order to suppress the diffraction and reflection from the edges of the antenna, and to achieve much improved right-hand circular polarization at very low elevation angles. This allows further reduction of the predominantly left-hand polarized multipath signals generated at low elevation angles. The antenna has a very simple mechanism of establishing a proper phase gradient or all spiral slots in order to achieve right-hand polarization for the whole upper hemisphere. A spiral slot antenna element has a wide-band performance, therefore preventing unwanted phase (group delay) and amplitude modulation of the spread spectrum GPS signal typically found in narrow band antennas. In order to limit the antenna to out-of-band interference, the GPS signal is band limited with RF filters in the LNA section of the antenna. The proposed antenna is made out of a single PCB board. Another PCB board is placed underneath the antenna to act as a reflector in order to reinforce the antenna directivity and reduce the back-lobe radiation. The radiation pattern roll-off of this antenna is much harder than the conventional GPS patch antenna. The sharp pattern roll-off allows further to reduce the antenna’s susceptibility to multipath-generated replicas of the GPS signal. The antenna does not require any alignment with respect to a given direction (such as North) due to its natural symmetry.

Small Affordable Anti-Jam GPS Antenna (SAAGA) Development

Abstract

The U.S. Army CECOM is developing a low-cost GPS anti-jam Solution refined to as the Small Afllordable Anti-Jam GPS Antenna (SAAGA), for ground vehicle and helicopter applications. The SAAGA product will provide 30 dB of additional anti-jam protection for a large mix of wideband and narrowband interferers. The SAAGA program goals are to mitigate up to 10 interferers (up to 3 wideband and up to 7 narrowband) at an affordable cost of $4,000 per unit in production quantity. The cost/performance goal of SAAGA will be achieved by employing a novel all-digital antenna spatial-temporal processing technology and a proven low-cost antenna manufacturing technology. A simpler and lower cost version of SAGA to mitigate narrowband interferer threats, will utilize a conventional Fixed Radiation Pattern Antenna (FRPA) and can be produced at a cost of $1,000 in production quantity. Both versions of the SAAGA product will be field tested and evaluated by the Army on a UH-60A helicopter platform, This paper describes the novel features of the SAAGA antenna array design and anti-jam processing, presents laboratory test results, analysis and simulation performance data and presents the Army field test and evaluation plan. The significance of the SAAGA research and development program is to ovemome the cost barrier of adaptive antennas for interference mitigation to make GPS anti-jam technology affordable to both military and commercial GPS users.