A Study of Geometry and Commercial Off-The-Shelf (COTS) Antennas for Controlled Reception Pattern Antenna (CRPA) Arrays

Controlled Reception Pattern Antenna array (CRPA) is an effective approach for rejecting interference and enhancing the received GNSS signal power. Conventionally, the antenna array used for CRPA is designed for reducing imbalance between elements. Additionally, full calibration should be made for calculating the spacing between elements and the cabling latency before performing CRPA. Hence, the high cost and complex calibration of antenna arrays decrease the attractiveness of CRPA to many user communities. We develop an approach which can perform CRPA without any prior information. Specifically, inexpensive and Commercial Off-The-Shelf (COTS) antennas can be used as elements of an antenna array. This allows the elements to be easily arranged to any desired array layout. With this new freedom in array geometry, the important issue of its effect on performance is raised. As the characteristics of antenna and array layout will affect the anti-interference performance, it is important to study the effect of antenna array geometry and select the best units/layouts combination. In this study, we examined both COTS antenna use and the effect of array geometry by assessing three models of commercial antennas and arranging these in two selected layouts. The antenna patterns of two physical layouts are analyzed in terms of beamwidth and illustrated. Signal collection hardware is built to record the data sets of antenna array. A positioning procedure of calculating electrical layout is developed using differential carrier phase measurements. We collected data sets for each antenna/layout combination. And, these data sets are processed by our software receiver to log the carrier phase measurements. The electrical layouts determined from the positioning procedure are compared with physical ones for examining the mutual coupling effects on the antenna array. From these results, we can assess the array layouts and the COTS antennas INTRODUCTION Global Navigation Satellite System (GNSS) signals are relatively weak and thus vulnerable to deliberate or unintentional interference. An electronically-steered antenna array system provides an effective approach to mitigate interference by controlling the reception pattern and steering beams/nulls. As a result, so-called Controlled Reception Pattern Antenna (CRPA) array have been deployed by organizations such as the US Department of Defense which seeks high levels of interference rejection. Our efforts have focused on developing a commercially viable CRPA system using Commercial Off-The-Shelf (COTS) components to support the needs of Federal Aviation Administration (FAA) alternative position navigation and timing (APNT) efforts. In 2010, we implemented a 7-element, 2-bit-resolution, single-beam and real-time CRPA software receiver [1]. In 2011, the receiver was upgraded to support all-in-view, 16-bitresolution and 4-element. Even though we can implement all these CRPA software receivers in real time, the performance of anti-interference is highly dependent on the antenna array layout and characteristic of antenna element. Our beamforming approach allows us to utilize several COTS antennas as array rather than custom designed and fully calibrated antenna [2]. The use of COTS antenna is important as the goal of the effort is to develop CRPA for commercial endeavors specifically robust timing for the national airspace. Hence, it is important to study the geometry layout of the individual antennas of the array to assess the layouts and how antenna performance affect the results. We adopted three models of COTS antenna and two possible layouts for a 4-element array. Then, signal collection hardware consisting of 4 Universal Software Radio Peripheral (USRP) [3] and one host Personal Computer (PC) is built to collect array data sets for each layout/antenna combination. Our developed CRPA software receiver is used to process all data sets and output carrier phase measurements. We address a procedure to calculate the electrical layouts of antenna array by differential carrier phase positioning. When compared to physical layout, the results of electrical layouts can be used to determine the mutual coupling effect of each combination. Using the electrical layout, the resultant gain patterns can be calculated and used to see the beamwidth and side lobe issue. This is important as these factors have significant effect on anti-interference performance. This study focuses on understanding the performance effects of geometry and developing a method for describing the best geometry. This paper is organized as follows. First, pattern analysis for two selected layouts is provided. Then, the specifications of COTS antennas are listed. The signal collection hardware and experimental setups are described in detail. The approach for logging carrier phase measurement by our developed CRPA receiver is explained. For calculating the electrical layout, a procedure to solve the spacing between elements is provided. The calculated results of electrical spacing for all layouts are compared to physical layout. And, the resulting patterns are shown for discussing the mutual coupling effect of antenna. Finally, some concluding remarks are made. PATTERN ANALYSIS OF ANTENNA ARRAY Pattern is defined as the directional strength of radio frequency signal from the antenna. The pattern of antenna array is the product of the isotropic array factor and the isolated element pattern. We assume that the pattern of each element is identical and only consider the isotropic array factor. Figure 1 shows the coordination of an antenna array. First element is set as reference position. The x-axis is the east direction, y axis is the north direction and z axis is up direction. 1 i p  is the baseline vector of the ith antenna. r̂ is the unit vector to satellite.