Conception, Realization, Evaluation of a Seven-Element GNSS CRPA
By Frederic Leveau, Solene Boucher, Erwan Goron, and Herve Lattard
A controlled radiated pattern antenna can be an effective way to protect GPS receivers against jamming. A new CRPA, composed of seven elements, works on the E5a, E5b, E6, L2, and L1 bandwidths. This article reports on radiation pattern measurements of the array in a test facility.
Controlled radiation pattern antenna (CRPA) technique is considered to be the best GPS pre-correlation protection technique against interference. It consists of an antenna array and a processing unit that performs a phase-destructive sum of the incoming interference signals, this process being equivalent to making nulls towards interferers in the array radiation pattern.
Considering the growing Galileo system and the possible interest of the French Ministry of Defense in the Public Regulated Service (PRS) , a prospective study was undertaken to develop an array compatible with GPS M-code, Galileo PRS, and aeronautical radionavigation signals in the E5 bandwidth. The French Expertise & Procurement Defence Agency (DGA) awarded the French company SATIMO a feasibility contract to design, conceive, realize, and evaluate a circular array composed of seven elementary patch antennas (see Figure 1).
SATIMO, a company specializing in R&D for antennas and in innovative antenna test ranges, has since developed this GPS-Galileo CRPA antenna, shown below.
The CRPA consists of seven elementary patches covering E5a, E5b, L2, E6, L2, and L1 frequency bandwidths, using microstrip multilayer technology. Each element is housed in a 9-centimeter (diameter) by 2-centimeter (height) radome, connector excluded. In that volume, a space provision has been reserved to include a low-noise amplifier (LNA) and two filters for a sharp out-of-band rejection. As a consequence, it is possible to configure three types of arrays: passive without filters, passive with two passband filters, and finally active (including a LNA, with a gain > 26dB, NF<0.9dB) with two passband filters. The maximum gain levels in these configurations are from 3.6 dBi to 29.8 dBi. For radiation patterns, see Figure 2.
The design of the single element has been optimized to control the deviations of each patch antenna when included in a seven-element array.
To limit mutual coupling with respect to the array dimensions, the distance between the elements’ phase centers has been chosen close to 0.7 λ at L1 frequency. This value results in a 36.5-centimeter (diameter) array. The standalone antenna and the CRPA antenna have been validated through an environmental testing campaign.
The usual iterative tuning and the optimization process for prototyping have been performed on SATIMO’s arch test range. This test facility indeed significantly reduces the time required to characterize the antenna-under-test (AUT) radiation pattern, in comparison with classical anechoic chamber test facilities.
More precisely, the arch test range instantaneously scans the field in one whole site angle cross-section plane, whereas the legacy systems mechanically scan the same cross-section plane by rotating the AUT for each incremental angle value. The spatial sampling of the near-field radiated by the AUT, thanks to a large number of probes along the arch surrounding it, enables a significant savings in time. The near-field results in the current plane can be displayed in real-time on a computer screen. Then, the rotation of AUT around its axis is automatically controlled by the measurement system, and a new acquisition is performed for each new cross-section plane. A Fourier transform computation is eventually applied to the 3D near-field to get the far-field radiation pattern.
The radiating characterization of the CRPA has been performed with a SATIMO SG24 system. With such a system, we have measured the complete 3D radiation patterns of each single element in less than 40 minutes per antenna.
The evaluation of the CRPA array was performed with this test bed in SATIMO’s facility (see photos below). The process begain with measuring an element alone on a ground plane, in order to extract the gain, the axial ratio, the aperture angle, the matching values, and every feature that defines a fixed-radiation pattern antenna. The evaluation secondly consisted of characterizing the array, that is, extracting the gain and the phase of each element in the array, with respect to a reference element. To implement such a reference anytime during the near-field acquisition process, the arch test range (Figure 3) is very powerful, because all the probes constantly point at the center of the array, despite AUT’s motions. On the contrary, the need for such a reference makes measurements difficult in anechoic chambers, which often require canceling out misalignments, thanks to specific motions that must be taken into account in the computations.
Functional tests are another important part of the CRPA unit evaluation. Usually, two kind of tests can be conducted: outdoors or in anechoic chamber.
Classical Tests. DGA plans to perform outdoor test campaigns by utilizing an array placed on the roof of an all-terrain vehicle (see photo). The array will be connected to a CRPA GPS processing unit and to a receiver in the vehicle. Some interferers will be located along the trajectory of the vehicle, according to various scenarios defining their waveforms and their power levels. The CRPA capability to reject those interferers can then be assessed. These kinds of outdoor tests naturally suit CRPA’s processing unit and array characterization, as they involve radiated GPS and interfering signals. However, these kinds of tests are not reproducible and are quite complicated to set up.
Some tests in anechoic chambers could be an alternative in order to obtain reproducible test results, but in that case, transmitting GPS constellation signals indoor becomes a challenge. An option could be the use of a GPS signal simulator, but this means a unique direction of arrival of GPS signals. Moreover, no dynamic trajectory could be done.
New Test Bed. DGA recently acquired a test bed, developed by INEO Defense, that enables evaluating CRPA units in conducted mode, for example. There is no longer a need to radiate either GPS signals or interfering signals. The purpose of this test bed, called BAnc de Caractérisation des Antennes Réseaux Antibrouillage (BACARA), or test bed to characterize anti-jamming antenna arrays (Figure 4 and Figure 5), is to replace the array and simulate its GPS and jamming environment. This means that it is able to create elementary antenna phase delays and gains resulting from the array geometry, by using finite impulse response (FIR) filters (Figure 6). This is the reason why this test bed must be fed with the array phase and gain measurement results obtained with the arch test range.
Alternatively, these results can be obtained with traditional anechoic chamber measurements. 10 channels of a multi-channel GPS simulator, each one matched with a satellite, are used by the test bed. Thus, BACARA coherently sums GPS constellation simulator output channels and interfering signals, so as to accurately simulate the array’s behavior in the laboratory. As a result, for any CRPA processing unit, it is possible to compare the array’s impact on a processing unit with an ideal array being composed of perfect elementary antennas.
Unfortunately, BACARA currently operates on L1 or L2, but not on the E6 and E5 bandwidths. On the other hand, this test bed is able to simulate dynamic trajectories, with the mobile positions and attitudes. Up to 10 internal jammers with various waveforms can be set up, and their power levels over time are computed by software like Warfare or Matlab. A numerical calibration allows some transparency of the test bed for CRPA units under test.
SATIMO, a company specializing in electromagnetic field measurements in the microwave frequency range and part of the Microwave Vision Group, has developed an array for the reception of M-code, PRS, and aeronautical radionavigation signals. This antenna array has been fully evaluated and qualified through electrical and environmental tests. The measurement methods have enabled the company to demonstrate the feasibility of the performances expected. Functional evaluations restricted to GPS are still under way. To do so, DGA will utilize its complementary outdoor and indoor test means, especially its laboratory test bed BACARA, as a tool to precisely evaluate GPS CRPA units.
Frederic Leveau works at the French MoD (DGA Information Superiority) as a radionavigation expert. His main interests are Galileo PRS prospective studies and developments and the integration of CRPA systems within French platforms.
Solene Boucher works at the French MoD (DGA Information Superiority) as a radionavigation expert. Her main interests are Galileo PRS prospective studies and developments. She is also responsible for the test bed BACARA.
Erwan Goron is an engineer at SATIMO Industries (Microwave Vision Group). His main activity is antenna conception.
Herve Lattard is an engineer at SATIMO Industries (Microwave Vision Group). His main activity is antenna conception.