Technical Details: Topcon PN-A5 GNSS Antenna, Part 2
Professional Surveyor Magazine - April 2012
by Dmitry Tatarnikov
Part one of this series introduces the technicalities of antennas for GNSS reference stations in the context of GNSS spectrum expansion, and it provides details on Topcon
’s PN-A5, including the basics of its unique design. Part two compares the antenna’s performance to a CR4 antenna, Topcon’s version of the original JPL choke ring design. Figure numbers continue from part one.
The PN-A5 antenna comprises a newly designed, full-spectrum GNSS antenna element. Figure 7
shows phase center offset in vertical versus frequency of the radio signal. (The results represented in the figure were obtained from anechoic chamber measurements. An offset for GPS L1 (1575MHz) is used as reference.) As demonstrated by this figure, the choke groove structure of the CR4 has resonance at approximately 1150MHz with rapid phase center variation near resonance. This is opposed to a phase center offset for the PN-A5 that is smooth versus frequency, with variations not exceeding 1cm over the entire GNSS band. This provides with consistent response for the different GNSS signals.
Figures 8 and 9
show normalized antenna gain patterns. (GPS L1 and L2 frequencies are examples, and the data plotted is from anechoic chamber measurements.) Right-hand circular
polarization (RHCP), which coincides with those transmitted by GNSS satellites, is shown as solid lines. Left-hand circular (LHCP) is shown as dotted lines. This figure demonstrates that the antenna gain pattern roll-off from zenith to horizon for the PN-A5 antenna is 10-12dB, which is approximately 5dB less than the CR4.
The 5dB of antenna gain improvement is extremely important for low-elevation satellites being tracked by a receiver. With the receiver signal processing algorithms, a 5dB gain provides up to 10dB improvement in signal-to-noise ratio (SNR) for the P code of GPS. This is illustrated by Figures 10a and b
with the plots representing SNR versus elevation. (The receiver used for data collection is the Topcon GB500.) This improvement allows a receiver to reliably track satellites to the horizon.
Note that the antenna gain for zenith for the PN-A5 is 2dB less compared to the CR4. This is in agreement with the main antenna directivity theorems based on energy conservation law: an antenna with a wider pattern has less maximal gain. Demonstrated in Figures 10a and b, this maximal gain lessening of 2dB in SNR decreases for directions close to zenith when compared to the CR4. This does not lead to signal tracking difficulties due to the already high SNR values for these directions.
Multipath Rejection Capabilities
As is well known, multipath error is proportional to the ratio between the reflected signal and the direct satellite signal magnitudes. When reflected from the ground, the original satellite signal changes its polarization properties. For most soil types, the reflected signal is generally left-hand circular polarized rather than RHCP. If the terrain underneath the antenna is
homogeneous, then the ground surface acts as a mirror, thus providing a reflected signal coming from below the horizon at an angle equal to a direct signal from above the horizon. This is schematically shown with Figure 11
. This is why, when characterizing the multipath reflection capabilities of the antenna, it is common to use the down-up ratio (DU) as a proportion between antenna gain patterns for LHCP signals for the same certain angle from below the horizon as for the RHCP signals from above the horizon at the same angle.
The DU ratio is plotted in Figures 12 and 13
, which demonstrate the DU versus elevation angle for GPS L1 and L2 frequencies. Figure 14
shows the DU for the zenith direction versus frequency of the radio signal (the data represented is from anechoic chamber measurements). As seen by these plots, the multipath-rejection capabilities of the PN-A5 antenna are competitive to those of CR4 antenna. Figure 14 illustrates the slight advantage of the PN-A5 antenna multipath rejection for zenith direction over the entire GNSS frequency band.
The undesired resonance at the lowest GNSS band demonstrated by this plot has been discussed previously regarding phase center offset in vertical. For the PN-A5 antenna, this resonance is shifted far below GNSS band.
Finally, note that the PN-A5 antenna is equipped with a state-of-the-art low noise amplifier (LNA). The LNA provides a 1.0dB noise figure, 48dB gain, and 50dB or better of out-of-band signals rejection starting from 100MHz offset from the GNSS bands. The antenna has a robust and environmentally protected design. It is housed within the exisiting Topcon and SCIGN radomes (as previously mentioned) with a total weight of 19.8lbs (9kg).
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.
- A. Leick, GPS Satellite Surveying. Second ed. John Wiley & Sons, Inc, New York, 1995
- 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
- 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
- D. Tatarnikov, A. Stepanenko, A. Astakhov, V. Filippov, Compact circular-polarized antenna with expanded frequency bandwidth, Patent of Russian Federation, №2380799, 2010
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