Leica AR20 GNSS Antenna

The Leica AR20 is a new innovative design antenna that brings new 

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levels of performance for reference station and monitoring applications. 

The AR20 contains a new 3D design choke ring for improved tracking 

performance, excellent phase centre symmetry and unmatched multipath suppression across all GNSS frequency bands.

More Robust for Longer Durability

The newly designed AR20 choke rings are treated with a newer and more robust double treatment for longer lasting durability in all environments. An optional radome cover is also available for added protection.

Additional Power Surge Protection 

The innovative integrated multi-stage power surge arrestor for the AR20 enhances protection against power surges in the electrical system, typically caused by lightning or any other source of high voltage. 

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Superior Multipath Performance

The AR20 has excellent Axial Ratio at low elevations which results in superior multipath rejection across all GNSS frequency bands including L5, due to the new 3D (inside) design. Traditional 2D choke ring designs are typically optimised for a single frequency band, while the AR20 has been optimised for superior multipath performance across all GNSS frequency bands.

1. Introduction

One of the key features of all measurement systems, including GNSS (Global Navigation Satellite Systems), is the accuracy of their operation, which can be assessed based on the analysis of measurement errors. The mentioned errors can be systematic, i.e., repeated with each measurement, or can be random for individual measurements. Manufacturers try to minimize systematic errors at an early stage of design. Systematic errors include, for example, receiver clock errors, which are effectively eliminated through the use of compensation methods. On the other hand, most of the observed errors in GNSS result from the operation of random factors, the description of which requires the use of mathematical statistics methods [ 1 2 ].

The key aspect in the analysis of the operation of satellite systems is the propagation conditions of the radio signal. They are related to, among others, the following phenomena affecting the damping of radio waves: covering the wave by buildings and trees, reflections of radio waves from various types of surfaces, and multipath wave propagation [ 3 ].

The factors obscuring the horizon include, among other buildings, that are an obstacle to the propagating wave, reducing its energy level. In turn, possible reflections and multipath of the radio signal are a direct cause of radio wave fading. It is a common phenomenon, the negative impact of which can be minimized by, inter alia, the use of several transmission frequencies in the receiver, the use of the special design of antennas, or the development of data processing algorithms in the receiver [ 3 ].

Most modern GNSS receivers already use at least two transmission frequencies and have an extensive signal processing layer. They also have external antennas, which makes the development of the antenna design the most available method of minimizing the reflection and multipath phenomena among the other mentioned options [ 4 ].

Several different antennas are used in GNSS: from small patch antennas (car navigation) to complex geodetic antennas [ 5 6 ]. There are also antennas for special applications, which have mostly larger dimensions, allowing them to obtain a sealed structure and the highest possible resistance to adverse environmental phenomena [ 7 9 ]. These types of antennas include choke ring antennas, which enable high damping of signals arriving at low or negative elevation angles [ 10 12 ]. For this purpose, the discussed antennas use coaxial rings (quarter-wave chokes) with a specially selected depth (usually ¼ wavelength of the antenna operating frequency) [ 13 ]. One of the advantages of such a structure is that it prevents the transverse flow of the induced electric current, which also allows the active element of the antenna to be electrically isolated from the side walls. In addition, this type of antenna significantly increases the protection of the receiver against jamming and interference from other radio devices nearby. Their main disadvantage is usually heavy weight resulting from the need to use metal (often steel) as the material from which the ring cover is made [ 5 ].

Localized effects such as multipath result from the environment around the receiving antenna [ 14 ]. Position fix errors are mainly caused by multipath propagation of GNSS signals. Providing a clear and accurate signal to a GNSS receiver is the primary purpose of the antenna. Integrated choke ring antennas are commercially available and very expensive [ 15 16 ]. Leica, among others, publishes the results of localization scatter studies for the AR25 choke ring antenna [ 17 ], which indicate a slight standard deviation of the obtained results (Easting 1.1 [mm], Northing 1.3 [mm], Height 2.5 [mm]) and confirms the high repeatability of positioning. Research work on choke ring antennas also focuses on the design of miniaturized [ 10 ] and increasingly lighter versions of antennas [ 18 ]. Most often, choke ring antennas are composed of deep concentric rings on a flat circular metal ground plane [ 11 ]. The subject of research is also antennas of a different shape, such as trapezoidal [ 19 ]. The time and frequency-domain responses of various choke ring shielding structures are also investigated [ 20 22 ]. Choke ring antennas can also be used for low-cost GNSS receivers in static measurements [ 23 ]. Positioning accuracy tests were also the subject of research in the context of using smartphones as GNSS antennas placed in a choke ring [ 24 ]. In the discussed research, it was shown that the use of a choke ring type cover improves the accuracy of localization in the X, Y, and Z coordinate systems. A choke ring antenna can also be an effective tool for establishing a baseline of possible multipath error as compared to a standard beacon antenna configuration under forest canopies [ 25 ]. However, other studies have shown that the precision of GNSS signals in static measurements within typical forest stands for Central Europe with the use of a choke ring antenna is greater than the sub-meter level [ 26 ]. The influence of GNSS antenna height on pseudorange multipath was analyzed in the article [ 27 ]. Three antennas in total were analyzed: a choke ring antenna, a geodetic microstrip patch, and an antenna used for Real Time Kinematic (RTK) measurements. Multipath signals reflected from the ground were mitigated most effectively by the choke ring antenna [ 27 ]. In conclusion, no works related to the performance tests of a dedicated GNSS antenna in a version without/with a low-cost choke ring cover have been found.

One of the possible uses of choke ring antennas is the positioning of Unmanned Ground Vehicles (UGVs). Localization problems, including the application of GNSS in UGVs, are the subject of many scientific articles [ 28 30 ]. GNSS is used mostly as a system for determining a reference position with high accuracy. In most cases, it is important to ensure the long-term and stable operation of the GNSS signals. Mentioned high accuracy of positioning is also required, which is most often provided by the RTK mode. It is especially critical when GNSS is the only UGV location system. This technology is often used in the field, but it has several limitations related to the need for an open horizon or satellite signal interference. In a lot of cases, the use of GNSS is simply impossible, e.g., due to environmental factors (independent of humans), while the goal is to protect GNSS antennas against the phenomenon of signal multipath or simply against external interference under good visibility. The solution in such cases could be choke ring antennas; however, their use is associated with reduced visibility of the horizon, which may affect the accuracy of the system. The positioning accuracy of GNSS is the subject of scientific articles [ 31 35 ]. In the case of stationary tests, long-term measurements are most often used to eliminate the occurrence of random errors. In conclusion, no works related to the positioning accuracy of the GNSS with the choke ring antennas working in RTK mode on UGVs have been found.

Accordingly, the purpose of the article is to determine the influence of the low-cost choke ring antenna on the GNSS receiver positioning accuracy working in RTK mode in the environment with possible multipath effects. For comparison, the article specifies positioning errors for two types of antennas: the aforementioned choke ring antenna and the dedicated GNSS antenna supplied by the manufacturer. In this way, the possibility of using the choke ring antenna in RTK measurements was also analyzed. The positive results of the experiment will determine the possibility of using the designed antenna on UGVs.

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