ION GNSS 2012
Session B2: GNSS Simulation, Testing & Timing Applications 2

Title: A Calibration Method for a Controlled Reception Pattern Antenna and Software Defined Radio Configuration
Author(s): Z. Bauer, C. Bartone, S. Gunawardena, Ohio University
Room: 103/104 (NCC)

This paper presents a demonstrated method for the performance of a calibration method for controlled reception pattern antenna (CRPA) using a Software Defined Radio (SDR) configuration. The combination CRPA and SDR system consists of a low-cost 7-element configuration where the antenna RF inputs are feed directly into the multi-channel SDR system. This combination CRPA and SDR system was characterized in an anechoic chamber environment to closely replicate the fielded antenna/receiver system. This combined CRPA and SDR system configuration will provide multi-antenna element characterization and calibration measurements that can be used to remove carrier and code phase biases caused by the antenna elements and receiver front-end components for down-stream adaptive signal processing algorithms. A CRPA is an adaptive antenna array that can implement beam forming and null steering techniques to improve the overall performance of the antenna, especially in the presence of multiple interfering signals such as jammers [1]. Each antenna element of the array can inherently produce code and carrier phase biasing within the received satellite signals. To correct for the induced biases caused be the receiver signals, each antenna element must be calibrated [2]. Additional consideration must be taken into account regarding the calibration requirements for a CRPA system implementing adaptive processing techniques; however, adaptive algorithms cannot remove all biases produced by the antenna hardware without predetermined calibration data for the antennas in the array [3].

The objective of the paper is to present the demonstrated method for the performance characterization of a calibration method for a CRPA and SDR configuration that closely represents the eventual fielded systems. This will provide a method of calibration for an array using a real world CRPA/SDR system. For verification, the calibration data produced by the CRPA/SDR system configuration will be compared with calibration data produced using a traditional CRPA test approach where each element of the array is characterized one at a time with a traditional antenna test approach.

The SDR part of the configuration was implemented utilizing the multi-channel RF-front end and processing capability of the Ohio University SDR. The multi-channel data acquisition system presented in this paper uses two SDR RF front-ends to perform data sampling. Each software receiver has four input channels and a local oscillator, allowing up to seven signals to be sampled simultaneously. Each software receiver front end can detect both the GPS L1 frequency (1575.42 MHz) and the GPS L2 frequency (1227.6 MHz) and has a sample rate of 56.32 MHz. The SDR RF front-ends sample the signal received by the antenna elements. Next, the RF front-ends perform the amplification, filtering, down conversion, and analog-to-digital conversion for the receiver system. Finally, the digital information is stored to disk for post processing. Through post-processing the carrier and code tracking loops can be implemented, and the magnitude and phase of each antennal element can be determined. The antenna-under-test (AUT) for this investigation was a CRPA consisting of 7-active antenna elements, each mounted on a 14.12in circular ground plane. One reference element was placed in the center of the ground plane, with six outer elements mounted approximately 3.748in (?/2) from the reference element. Each antenna is a commercially available dual-frequency L1/L2 dual-layer patch antenna. The test environment for this investigation was in the Ohio University Antenna Anechoic Chamber located at Ohio University, Athens, OH. The shielded test chamber also has a hybrid near-field antenna test system developed by Antcom. The test system consists of a scanner, a motion control system, a vector network analyzer, and a personal computer [4]. The test system allows the AUT to be mounted on to a dielectric tower which is attached the motion control system. The motion control system allows for full 360 degree rotation in the azimuth and elevation plane. Calibration data was collected in two primary modes: 1) Traditional calibration test methods using a traditional antenna test technique, and 2) the CRPA/SDR combination configuration where the motion base of the antenna system was used but not signal generation or vector network analyzer measurements were used.

A traditional calibration test method was first performed using the CRPA to determine "truth measurements" for the magnitude and phase of each element in the CRPA. Each antenna element was tested separately by connecting one individual element to the vector network analyzer and placing 50 ? impedance loads on the remaining elements. The vector network analyzer determined the magnitude and phase measurements of each element over the spatial. During testing, each AUT element was rotated 360 degrees in elevation and 180 in the azimuth. Each test was also performed using co- and cross-polarization.

The second method using the same 7-element CRPA and the SDR multi-channel data acquisition system was then tested in the anechoic chamber. Each antenna element was connected to the SDR RF front-ends by coaxial cable. For these demonstration tests, a CW signal was generated by a simple RF signal generator, that was phase locked to the SDR receiver system. This setup allowed for simultaneous data collection for each of the seven antenna elements of the CRPA. Multiple tests were performed by rotating the AUT across the azimuth and elevation plane. The digital data was stored, and the magnitude and phase measurements where determined in post-processing.

The calibration data results determined by the SDR multi-channel data acquisition system were then compared with the calibration data results obtained with the traditional antenna test measurement methods. These comparisons show that the SDR multi-channel data acquisition calibration data results compare very well with the traditional antenna single-element calibration data results; however, the SDR multi-channel data acquisition system provides several advantages. These advantages include higher resolution amplitude and phase response measurements and a more representative CRPA/SDR configuration that would be fielded. Thus, not only is the RF characteristics of the CRPA antenna characterized, but also the RF front-end of the SDR RF front-end all the way up to the point where the signals are digitized. This paper also highlights that the calibration corrections as well as the antenna steering algorithm can be implemented in one step with in the SDR and eliminates the requirement for a traditional antenna electronics unit after the CRPA. The paper concludes by summarizing the benefits of using a SDR multi-channel data acquisition system over a traditional testing method and how this method can improve overall CRPA/SDR implementation. The work in this paper is significant because it provides a calibration method that implements the actual CRPA/SDR system that would be used in real world applications. Using a CRPA/SDR configuration is advantageous because it removes the necessity for additional associated antenna electronics that are used to perform adaptive processing techniques and error mitigation, whereby these functions could be implemented with the SDR. With the CRPA/SDR system, the antenna calibration and adaptive processing can be done with embedded signal software. This allows the receiver system to be application specific and easily upgradable. It also allows for multiple processing techniques to be implemented on the same set of data within the embedded software of the receiver system.

References:

1. Jonathan A. Ulrey and Inder J. Gupta. "Optimum Element Distribution for Circular Adaptive Antenna Systems," In Proc. ION NTM, 2006, pp. 76-81.

2. C. M. Church and I. J. Gupta, "Calibration of GNSS adaptive antennas," in Proc. 22nd International Meeting of the Satellite Division of the Institute of Navigation, 2009, pp. 344 - 350.

3. Ung Suok Kim. "Mitigation of Signal Biases Introduced by Controlled Reception Pattern Antennas in a High Integrity Carrier Phase Differential GPS System." Doctor of Philosophy, Stanford University, 2007.

4. Ian Matthew Barton. "Antenna Performance Analysis for the Nationwide Differential Global Positioning System." Master of Science, Ohio University, Athens OH, 2005.

5. David S. De Lorenzo. "Navigation Accuracy and Interference Rejection for GPS Adaptive Antenna Arrays." Doctor of Philosophy, Stanford University, 2007.

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