Topcon PN-A5 GNSS Antenna, Part 1

by Dmitry Tatarnikov

This article introduces the technicalities of antennas for GNSS reference stations in the context of GNSS spectrum expansion. It also provides details on Topcon’s PN-A5, including the basics of its unique design. Part two discusses characterizations of the antenna’s performance.

Accuracy in positioning at centimeter level or better allows for the broad use of GNSS signals for geodesy, land surveying, construction, and agriculture. The highest accuracy of positioning is achieved with differential modes when the rover receiver uses high-quality corrections generated by the reference station. Support of differential corrections over large territories requires the installation of a reference antenna for each station within the network. Presently, there are thousands of reference stations across the globe.

An ideal antenna for a reference station receives signals from all satellites in view while fully rejecting signals that come from underneath or indirectly. These indirect signals are the result of what we call “multipath.” They are original satellite signals reflected from the terrain under the antenna. These signals mix with the direct signals from the satellites, thus providing what is known as multipath error to the positioning. With today’s technology, multipath is the largest error source within high-precision applications.

Typical antennas used to date for reference networks are of the choke ring style (Figure 1). This antenna type was originally designed by the Jet Propulsion Laboratory (JPL). This antenna design comprises a special ground plane made of concentric grooves with a total diameter of approximately 40cm. The antenna has typically used a Dorne & Margolin (DM) antenna element mounted in the center. The DM element is a cross-dipole type. The purpose of the choke groove structures used by the antenna is to decrease the antenna gain for the directions below the horizon. This reduces multipath error significantly.

JPL-designed choke ring antennas have been serving satellite positioning applications for more than 20 years with many antennas based on this design currently in operation. However, new antenna developments address two considerations.

The first consideration is the ongoing GNSS constellation expansion for new satellite systems and new signals. The JPL choke ring with the DM antenna element was designed at a time when only the United States’ GPS constellation was being used, for all practical purposes. Later, Russia’s GLONASS constellation was supported by the antenna functionality. Both the GPS and GLONASS systems have been radiating two signals known as L1 and L2. In addition, currently under deployment are the GALILEO system from Europe, the QZSS systems of Japan, and the COMPASS system of China. Several other systems from different countries are being explored. This is in addition to the modernization for the new L5 signal of GPS and the L3 signal of GLONASS. The GPS L5 signal is already being transmitted by new-generation satellites while GLONASS L3 is planned and pending.

Electrical engineers refer to this as GNSS spectrum expansion. One example is the L2 signals of GPS and GLONASS occupying 1217MHz to 1252MHz radio frequencies. Considering that all new signals and systems will occupy the range within the spectrum from 1160 MHz to 1300 MHz with a bandwidth of 140MHz, this results in a factor of four versus the bandwidth of 35MHz for the L2 band. This is in addition to the L1 segment from 1560 MHz to 1615MHz. A reference station antenna needs to serve for many years without replacement, and the natural wish of a network administrator is to have the antenna fully compatible with these existing and new signals.

The second consideration comes from the fact the choke groove structure of the original design contributes to narrowing of the antenna pattern, if compared to more portable antennas of non-choke ring design that are typically used as GNSS rovers, for instance. The signals from lower elevated satellites will be suppressed by the choke ring antenna more than those of a rover antenna. The result is more difficulty with signals from low elevation satellites being tracked by the reference receiver. However, the low elevation satellites are of prime importance for satellite positioning considering the Dilution of Precision factor (see online reference #1) directly affecting the precision. Antenna pattern narrowing is an unavoidable feature of the plain choke groove structure.

The new antenna development goals of Topcon are to address both of these aforementioned considerations, specifically:

  1. to obtain robust antenna tracking performance over the expanded GNSS frequency band covering all the existing signals as well the new signals expected over the next 10-to-15-year span, and
  2. to increase the antenna gain for low elevation satellites, making the gain comparable to a typical rover antenna.

These goals are to be achieved without decreasing the proven multipath rejection capabilities of the choke ring antenna.

It is worth noting that any improved antenna performance can be theoretically attained if the antenna design size and weight are unlimited; this is a fact of electromagnetic technology. One criterion of Topcon’s new PN-A5 antenna development was to keep size and weight consistent with the original choke ring antenna. The Topcon PN-A5 antenna is designed to fit the existing Topcon radome and the SCIGN radome that are well known to the GNSS community.
 

PN-A5 Design Basics

Design basics of Topcon’s PN-A5 antenna are discussed in detail in the online references (#2-4) and are briefly summarized below. 

Straight pins structure versus choke grooves.  Choke grooves of the initial design in Figure 1 form what we call an impedance structure. The term “impedance” means that an imaginary surface exists where the relationship between the electric and magnetic fields is a different type than regular conductors or isolators. This impedance surface passes through the choke groove openings. Figure 2 schematically shows a cross-sectional view of the choke grooves structure with the impedance surface shown by a dashed line. Properties of regular conductors or isolators normally do not vary much with changes of the radio frequency of an applied signal. Conversely, the surface impedance of the choke groove structure does exhibit variations over the GNSS frequency band.

Another design to create an impedance surface is a straight pins structure shown schematically with Figure 3. The impedance surface is located at the pins’ ends as shown by the dashed line. Within the design process, the desired 
property of the impedance surface formed by the pins structure demonstrates 30% less frequency derivative compared to a choke groove structure. This allows for more consistent antenna functionality over the expanded GNSS frequency band.
 
Convex impedance ground plane versus flat ground plane.  Antennae used with satellite positioning are essentially the receiving type. However, as with most cases related to antenna technology, it is easier to consider the transmitting mode of the antenna rather than receiving. Equality of antenna properties for both modes of operation is established by basic theorems of the antenna area.

With the transmitting mode, the DM antenna element placed in the center of the structure of figure 1 would be the exit source. If the impedance surface of the choke groove openings is properly tuned, then it generally forces the wave travelling from the source to leave the surface faster than it would if the surface were made of a plain conductor such as metal (Figure 4).

This is why the field along the surface decays faster, resulting in a small portion of the radiated power reaching the ends of the impedance surface. Thus only a small portion of the power will diffract over the structure’s ends and propagate in directions underneath the antenna.

This also explains why an antenna gain for the directions underneath the antenna is small. Considering the receiving mode, one can say the impedance structure does provide suppression of multipath signals coming from underneath.  However, “forcing” the wave travelling from the source to leave the impedance surface also results in antenna gain degradation for directions close to grazing. These grazing directions coincide with low elevation angles.

If the surface is made convex rather than flat, then the same scenario just discussed holds true, only the grazing directions are now below the horizon (Figure 5). This improves sensitivity to low elevation satellites. An important consideration is that the radius of curvature of the surface be properly chosen so as not to increase the antenna sensitivity for signals coming from underneath. These signals from underneath are multipath (online reference #3).
 
Broadband antenna element design. The PN-A5 antenna comprises a newly designed, full-spectrum GNSS antenna element. This antenna element utilizes an array of vertical convex dipoles. Figure 6 shows the main components of the antenna element. It has an antenna radome (1), a cup with dipoles (2), and a power summarizing unit (3). The latter is coupled regarding capacity with the dipoles.

Such an array of dipoles possesses a property that could be called an inverse reactance behavior (see online reference #4). With such a property, an input reactance of the array structure has a null of the derivative with respect to frequency inside the desired frequency band. The result is a very smooth behavior of the reactance versus frequency and, in turn, broadband functionality. This antenna element possesses relative bandwidth of more than 40%, which is larger than the entire GNSS band from 1160 up to 1615 MHz.

Part two discusses the performance characteristics of this 
antenna.

References 
  1. A. Leick, GPS Satellite Surveying. Second ed. John Wiley & Sons, Inc, New York, 1995 
  2. D. Tatarnikov, A. Astakhov, A. Stepanenko, Broadband Convex Impedance Ground Planes for Multi-System GNSS Reference Station Antennas, GPS Solutions, v15, N2, 2011, pp. 101-108
  3. D. Tatarnikov, A. Astakhov, A. Stepanenko, GNSS Reference Station Antenna with Convex Impedance Ground Plane: Basics of Design and Performance Characterization, Institute of Navigation, International Technical Meeting (ION ITM), 2011, San Diego, CA, USA, January 24-26
  4. D. Tatarnikov, A. Stepanenko, A. Astakhov, V. Filippov, Compact circular-polarized antenna with expanded frequency bandwidth, Patent of Russian Federation,  №2380799, 2010

Dmitry Tatarnikov holds a Master EE, PhD, and a Doctor of Science degree, all in antenna theory and technique from Moscow Aviation Institute, Moscow, Russia. He began his GNSS antenna developments in 1994 with Ashtech Moscow. Since 2000 he has been antenna design chief for the Topcon Technology Center in Moscow, Russia. 
 

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