GlobalTop Technology, maker of positioning modules with embedded antennas, has launched the Titan X1, a compact multi-GNSS patch module for applications where small footprint, ease of integration and flexible interface options are essential in addition to robust positioning performance.
At 12.5 x 12.5 mm, Titan X1 is one of the smallest embedded patch antenna GNSS modules based on Mediatek’s MT3333 chipset. It features a specially tuned (12 x 12 mm) GPS+GLONASS patch antenna that offers excellent performance for a module so small.
Titan X1 offers a fully integrated design as standard, with a complete set of components including TCXO, RTC Crystal, SMPS, SAW filter and an additional LNA, all of which are considered vital for optimum performance.
Titan X1’s introduces multi-interface support (UART, I2C and SPI), and includes external antenna detection circuit with interface so users don’t have to choose between compact size and advanced features.
“We understand the growing need for ultra-small positioning modules, especially those with embedded antennas,” said Sam Khan, vice CEO of GlobalTop Technology. “But we don’t believe in compromising essential components and functions for the sake of a smaller size. With Titan X1 we show our commitment to making ultra-compact modules that lack nothing in terms of interfaces and functions compared to their larger counterparts. IOT devices are getting smaller but more complex with most devices featuring a wide array of sensors and connectivity modules. Being just a receiver radio, we believe that integrating a positioning module to an IOT device should be the easiest part of the development and Titan X1 is perfect for that.”
Samples of Titan X1 are available now, with mass production starting March 30.
A diversely polarized antenna array combines signal processing in the spatial and polarization domains for significant improvement in receiver robustness against interference.The C/N0 of line-of-sight components is improved since the receiver can use the power present in the left-hand circularly polarized channels, and also interference mitigation improves.
By Matteo Sgammini, Stefano Caizzone, Achim Hornbostel and Michael Meurer
INNOVATION INSIGHTS with Richard Langley
POLARIZATION. We use the word in everyday speech to mean a division into two groups with sharply contrasting opinions or beliefs.
But the word has another use in physics and electrical engineering to describe a characteristic of electromagnetic waves. Electromagnetic waves, whether they be light waves or radio waves, have electric and magnetic fields vibrating perpendicularly to each other and to the direction of propagation. If the electric field (and, correspondingly, the magnetic field) vibrates in a specific non-changing plane, we say that the wave is linearly polarized.
In terrestrial radio communications, signals are typically transmitted as linearly polarized waves with the electrical field oscillating in the vertical plane or the horizontal plane. Receiving antennas are designed and oriented to preferentially respond to the particular polarization of the signals. Before the widespread use of cable and satellite distribution platforms, VHF and UHF TV signals were received using rooftop antennas consisting of multiple parallel metal rods (similar antennas are used now for terrestrial digital TV).
In North America, the rods were in the horizontal plane since the transmitted signals were horizontally polarized. In Europe, on the other hand, the rods were sometimes in the vertical direction since there, some TV signals were transmitted with vertical polarization.
If the plane of vibration of the electric and magnetic fields rotates uniformly as the signal propagates, we have the case of circular polarization. Since the sense of rotation can be clockwise or anti-clockwise, we have right-hand circularly polarized (RHCP) and left-hand circularly polarized (LHCP) signals following the direction of curl of the fingers of the right and left hands. Circular polarization is typically used for signals from satellites in low and medium Earth orbit, such as GNSS satellites, where the relative orientation of the transmitting and receiving antennas is not fixed. For maximum signal reception, the polarization of the receiving antenna should match the polarization of the signal. All GNSS satellites transmit RHCP signals and therefore most GNSS receiving antennas are designed for such signals.
However, a funny thing can happen to a satellite signal on the way to a receiving antenna. If the signal bounces off a nearby structure or the ground or the sea surface, its polarization is modified and it will become LHCP or a combination of the two polarizations. While this multipath phenomenon can be a pest, as discussed in last month’s column, it can be used to advantage in measuring sea-surface roughness, for example, by monitoring reflected GNSS signals from a low Earth orbiting satellite or an aircraft using a LHCP antenna.
But GNSS receiving antennas are not perfect—especially for direct line-of-sight low-elevation-angle signals. A primarily LHCP antenna can capture a significant portion of the energy in such a RHCP signal and could provide a strong response to a reflected signal when the line-of-sight signal is missing or very weak. So, there could be a benefit in having a dual-polarized antenna to improve positioning capability in marginal situations. Furthermore, jamming signals can be of arbitrary polarization and a dual-polarized antenna array with beamforming capability could better separate and mitigate such interference. In this month’s column, we examine the principles of operation of such an antenna array and how one performed in real-world jamming and non-jamming scenarios.
The rapid growth of the wireless telecommunication sector and, consequently, the high demand of spectrum assigned to the new services make the frequency spectrum very crowded and quite saturated. With the weak received signal power of GNSS signals, spurious harmonics from other systems can cause unintentional interference and, therefore, a serious problem to the reliable estimation of user position, velocity and time (PVT). Besides unintentional interference, more virulent intentionally radiated signals, called jammers, may knock out the GNSS receiver; this is especially the case when a jammer with high time-frequency dynamics (such as a chirp-like jammer) affects the GNSS signal spectrum.
Whether unintentional or intentional, interference represents a serious threat to GNSS in applications ranging from safety-of-life to critical sectors like law enforcement, transportation, communication and finance. In such critical applications, it is important that the GNSS receiver provides a minimum level of reliability and robustness, even at the cost of increased price and complexity.
To meet this need, some manufacturers and research institutions have been developing GNSS receivers equipped with anti-jamming capabilities.
In this article, we propose a novel approach to interference mitigation. We equipped a GNSS receiver with a diversely polarized antenna array to combine signal processing in the spatial and polarization domains in a novel way. By doing this, we demonstrated achievable improvement in interference mitigation. For this purpose, we extended an existing two-step blind adaptive beamforming algorithm to a new algorithm that includes the polarization domain. We evaluated the new algorithm through measurement data gathered during a campaign carried out at the Automotive Testing Center in Aldenhoven, near Jülich, Germany. We used different interference sources, including low-cost jammers, euphemistically called personal privacy devices (PPDs), in real-life situations such as in a moving car approaching a GNSS receiver.
The receiving antenna used in our work is a four-element rectangular dual-polarized (DP) array in a two-by-two configuration. Each element has two feeds available, one ideally receiving the right-hand circularly polarized (RHCP) field and the other the left-hand circularly polarized (LHCP) field of the polarized incoming signals. Due to antenna imperfections and coupling effects, part of the LHCP field impinging on the antenna will be received by the RHCP port and vice versa. Generally, the antenna axial ratio is fine-tuned at boresight so that the energy of a RHCP satellite signal impinging on the array at high-elevation angles will be mostly captured by the RHCP port, while the energy flowing through the LHCP channel can be ignored. This statement does not hold for satellite signals coming from lower elevation angles or in general for signals with polarization other than RHCP, this being generally the case for multipath and interference. In particular, the response of a planar antenna array for angles-of-arrival (AoAs) close to the horizon is almost linearly polarized. It follows that a significant portion of the RHCP energy in a signal is likely to be captured by the LHCP channels and can be used either to strengthen the line-of-sight (LOS) component, or to better separate and mitigate both multipath and interfering (jamming) signals.
Adding the polarization domain makes it possible to better discriminate spatially and temporally correlated signals. In some environments, such as urban canyons, the LOS signal might not be available or might be strongly attenuated. In this case, the reflected non-LOS signals can be used to perform positioning and would benefit from a DP antenna approach. As a matter of fact, the reflected signals will be no longer RHCP, thus the LHCP channel can be used to strengthen the echoes and improve positioning. Diversely polarized antenna arrays also have the advantage of increasing the total number of available degrees of freedom. The number of degrees of freedom of an antenna array corresponds to the number of nulls that can be placed in the direction of arrival of interfering signals. For a single-polarization (SP) array with M elements, M-1 nulls can be placed in the spatial domain. In the case of a DP array, 2M-2 nulls can be placed in the space and/or polarization domains. This is a key factor in counteracting high-power and highly-dynamic jammers, such as PPDs. Furthermore, the use of a diversely polarized array improves signal detection, as well as direction of arrival and polarization-estimation performance. This is particularly true for closely spaced signals with sufficiently separated polarizations. On the other hand, the introduction of the second polarization increases the computational complexity of signal processing, since the number of elements is doubled.
The results of our measurement campaign show a significant improvement in receiver robustness against interference when the DP approach is used compared to the general SP case.
SIGNAL MODEL
In this section, we will briefly describe the theory of signal and antenna polarization with a minimal number of equations. A more complete discussion is included in the paper on which this article is based (see Further Reading).
Polarization of a Plane Wave. A received electromagnetic signal is assumed to be narrow band, and the source of radiation is assumed to be located in the far field. The plane wave propagating in free-space has the property that the direction of propagation is orthogonal to the electric and magnetic field vectors. This allows the electric field e of a polarized wave to be completely described in terms of the two unit vectors, and , orthogonal to the direction of propagation
(1)
where ∈xand ∈y are the real-valued, non-negative, amplitudes of the components of the electric field, Φxand Φyare the phase components of the field, ω is the angular frequency of the carrier and k is the wave number.
Only the real part of Equation (1) is physically relevant, with the complex exponential containing information about the phase of the oscillating field.
Switching from the linear to the circular basis vector set:
(2)
where and are the unit vectors of the RHCP and LHCP components, respectively and omitting the explicit time and spatial dependence, we can write the normalized electric field as
(3)
The polarization state of an electromagnetic signal is fully described by ∈R and ∈L.
More generally, the electric field of any plane wave impinging at the antenna can be expressed in the form
(4)
Dual-Polarized Antenna Array. A circular DP antenna features two orthogonal circular polarization output ports, meaning each element ideally receives the voltage induced by the RHCP and LHCP field components separately on the two different antenna ports. Due to antenna imperfections and the coupling effect, part of the received RHCP field is received by the LHCP port and vice versa. These undesired voltages are responsible for the emergence of the cross-polar components.
In view of this, we characterize the antenna in terms of its response to circularly polarized plane waves and express the electric field using the Jones vector notation in the circular basis as
(5)
where is the complex total electric field received by the RHCP port, is the complex electric field induced at the RHCP port by a purely RHCP electromagnetic wave, indicated as a co-polar component, is the complex cross-polar component of the electric field obtained by exciting the antenna with a purely LHCP electromagnetic wave, φ is the azimuth angle and θ is the elevation angle of the impinging signal assuming the antenna to be at the origin of the spherical coordinate system. Similar statements apply for the total electric field received by the LHCP port, and for the co-polar ( ) and cross-polar () components.
If vR and vL are the complex voltages induced at the RHCP and LHCP antenna outputs by the signal in Equation (4), respectively, the antenna outputs are given by
(6)
where ψ = [θ, φ] is the vector parameter carrying the information about the direction of arrival (DoA) of the incident signal and τ is the time delay of the incident signal.
With an M-element array of DP sensors, we can vR and vL to represent the complex array responses of the DP antenna array:
(7)
where and define the steering vector of the DP antenna array given a signal incident at angle ψ and polarization defined by the Jones vector .
Problem Formulation. The analog signals collected by the antenna array are then passed through the receiver front end where they are amplified, filtered and shifted to baseband. The resulting complex baseband signal with bandwidth B that is received by an antenna array with M DP sensor elements at polarization port P is
(8)
where sp(t) defines the superimposed satellite signal replicas with l = 1 identifying the LOS signal and l = 2, …, L the non-LOS (multipath) signals and zp(t) denotes the superimposed radio frequency interference (RFI) signals with i ranging from 1 to I.
Additionally, we assume temporally and spatially uncorrelated complex white Gaussian noise np(t). can be expressed in terms of the steering vectors given the lth signal’s incident angle, the polarization vector and a complex scalar term involving the signal complex amplitude, Doppler frequency, carrier-phase offset and the particular pseudorandom noise sequence and associated
The baseband signals are then digitized at sampling frequency 1/Ts ≥ 2B. The observations are collected at K periods of the pseudorandom sequence at N time instances and the polarizations of the satellite signals and interferers as well as their DoAs are assumed to be constant over each single observation. We finally combine the two outputs of the DP antenna to benefit from both polarizations with a resulting unified signal output X. This increases the number of available degrees of freedom; furthermore, it allows us to carry out filtering in the polarization domain. On the other hand, the overall system complexity is increased; in particular, the computational complexity of the matrix inversion needed for pre-whitening (to be discussed next) is increased by a factor of about 23.
PRE-WHITENING AND EIGEN-BEAMFORMING
Interference mitigation and beamforming uses a two-step blind beamforming approach based on orthogonal projection. It is similar to an approach we developed for the single-polarization case, with the only difference here being the introduction of the orthogonal LHCP channel, which doubles the number of sensors. Doubling the number of sensors does not necessarily mean that the number of degrees of freedom is also doubled. It has been shown that when using diversely polarized antennas, to discriminate signals unambiguously it is required that the maximum number of signals D = L + I satisfies the relationship D ≥ 2M–2.
This means that one additional degree of freedom is required to discriminate in the polarization domain in comparison to the case of an antenna array of uniformly polarized sensors, where it is required that D ≥ M–1.
Pre-Whitening. We establish a sample spatial-polarimetric covariance matrix, where we assume that the satellite signals, the interfering signals and the noise are uncorrelated among each other. Furthermore, we ignore the influence of the signal replicas, because their power is usually much smaller than the power of the noise and interference. We then obtain the approximate pre-whitening matrix to be applied to X. The pre-whitening matrix is applied before signal despreading.
Eigen-Beamforming. In the next stage, the complex pre-whitened signal passes through the tracking loops for despreading and code and carrier wipe-off. We collect the post-correlation signal at K integration intervals to obtain the data matrix and the post-correlation spatial-polarimetric sample covariance matrix. The post-correlation eigen-beamformer is obtained following the same optimization problem that we solved for the single-polarization case. We apply the optimum weight vector, maximizing the ratio between the power of the desired signal and the power of the undesired signals plus noise, using the eigenvector with respect to the dominant non-zero eigenvalues of the post-correlation covariance matrix.
MEASUREMENT CAMPAIGN
The receiving antenna used in our work is a planar four-element rectangular DP array in a two-by-two configuration, similar to one we have used previously, apart from the additional hybrid couplers needed to provide the LHCP channel outputs. Each element has a double feed, one ideally receiving the RHCP field and the other the LHCP field of the polarized incoming signals, resulting in a total of eight output channels. The single antenna elements are designed for the reception of the GPS L1 and L5 and Galileo E1 and E5 bands, but in this work we focus only on the reception of GPS L1 signals.
The eight signals are passed through a front end, where they are amplified, filtered and down-converted to the intermediate frequency of 2.5 MHz. The analog signals are then digitized with a sampling rate of 8 megasamples per second. The resulting 8-bit digital data are collected and stored on a solid-state drive for data analysis in post-processing. Data analysis is then performed by using a GNSS software-based receiver.
Description of Test Scenarios. The DP system has been tested using measurement data to assess its dual capability of improving the quality of LOS signal reception and robustness against both unintentional RFI and jamming. As mentioned previously, the measurement campaign was conducted at the Aldenhoven Automotive Testing Center. The location provides seven tracks of different lengths, inclinations and shapes. The test track used for this measurement campaign was the so-called autobahn, providing two lanes in each direction of travel and a total length of 1,000 meters (see FIGURE 1).
FIGURE 1. Test track layout.
In this article, we report and analyze the results of three different test scenarios. In the first test, we collected GPS L1 data over 60 seconds in an interference-free environment. The aim of this baseline scenario was to verify if the additional LHCP channels improved signal reception in terms of carrier-to-noise-density ratio (C/N0) and PVT errors.
The second test scenario involved a horn antenna mounted on a mast, transmitting a continuous wave (CW) interference signal in the GPS L1 band and steered in the direction of the receiving antenna, as shown in FIGURE 2. Both the horn antenna and the receiving antenna were kept static during the measurement interval.
FIGURE 2. CW interference scenario.
The objective of the third test scenario was to replicate a real-life situation involving jamming, similar to the so-called “Newark scenario,” where a GPS jammer in a truck driving past Newark Liberty International Airport caused ground-based and satellite-based augmentation systems receivers to malfunction. To carry out this test, we installed a type K-320 PPD jammer transmitting in the GPS L1 band (see FIGURE 3) in the 12-volt auxiliary power outlet (cigarette lighter receptacle) of a moving car approaching the receiver and driving by.
FIGURE 3. The K-320 in-car PPD jammer.
The car started its run at a distance of about 260 meters from the receiver. During the first 20 seconds, the car holds its position. After this time, it was driven in the direction of the receiver with a constant speed of 30 kilometers per hour, finishing its route on the other side of the autobahn track, as depicted in Figure 1.
Baseline Scenario. The benefits that come to light using a DP array are of a dual nature. First, the C/N0 of LOS signals is improved since the receiver can make use of the power present on the LHCP channels due to polarization mismatch, in particular for satellites with low AoA, resulting in better receiver-computed PVT solutions. This effect appears evident if we analyze the behavior of C/N0 values over time collected during the non-interference experimental test in the GPS L1 band.
With reference to the sky plot in FIGURE 4 indicating the positions of the satellites at the time of observation, we analyzed the subgroup composed of those satellites having an elevation AoA lower than 30°. There was a sensible improvement of C/N0 using both polarizations from the DP antenna compared to just using the RHCP output (see FIGURE 5(a)). On the contrary, satellites with an elevation AoA higher than 60° do not benefit from the DP antenna and experienced almost the same C/N0 whether the LHCP channel was used or not, as can be seen in FIGURE 5(b).
FIGURE 4. Receiver sky plot for GPS L1 on October 22, 2015, at 13:10:00 UTC.FIGURE 5. C/N0 improvement using the dual-polarized antenna: (a) low-elevation-angle satellites (elevation angle <30°), (b) high-elevation-angle satellites (elevation angle >60°).
While the advantage of using the DP array is evident when observing the C/N0 behavior, this achievement does not translate with the same clear evidence when assessing the 2-D horizontal position error. Nevertheless, an improvement of about 6 centimeters in terms of the standard deviation of the 2-D position solution error in the horizontal plane has been obtained using the DP antenna (see TABLE 1). It is reasonable to expect that in a scenario where the availability of satellites is not as high as in our test case, the use of low-elevation angle satellites becomes more important for the accuracy of the PVT solution. In this case, the use of a DP antenna could play a key role in improving positioning accuracy.
Table 1. Interference-free RMS positioning error, in meters, in the horizontal plane over 60 seconds. Note that the data for the single-element result was obtained using just one sensor element of the 2 × 2 array in the same test run from which the array DP and array SP results were obtained.
CW Interference Scenario. The use of a DP array provides the ability to filter signals in the polarization domain, and at the same time we benefit from the additional degrees of freedom available. Thus, interference mitigation becomes more effective than using a SP array, increasing the receiver robustness and enabling tracking and successful PVT solutions in a severe interference scenario. This outcome appears evident analyzing the results of our test, where the linearly polarized CW interference described in TABLE 2 impinged on the array.
Table 2. Direction and calculated interference-to-signal ratio (ISR) for 25 dBm transmit power CW interference signal.
We show the 2-D horizontal position errors from this test in FIGURE 6. The figure highlights the improvement in position accuracy when both RHCP and LHCP channels are jointly used, limiting the root mean square (RMS) error to 2.65 meters, while it increases to 3.88 meters when only the RHCP channels have been used.
FIGURE 6. Horizontal position errors over 60 seconds in the presence of CW interference.
The advantages of using the DP array as assessed above are well summarized in FIGURE 7. The figure shows the history of the C/N0 split into two clusters. The upper cluster is from measurements during the interference-free period while the lower cluster is from measurements during the period the receiver is affected by the interference. In the figure, the improvement in terms of C/N0 is notable when using the DP array, in particular for low-elevation angle satellites, and for those satellites having a DoA close to the DoA of the interference. The latter case, when satellite signals and the interfering signal almost overlap in space, has been fully analyzed in a technical note (see Further Reading).
FIGURE 7. C/N0 history for all tracked GPS L1 satellites placed in order of their elevation AoA collected over 120 seconds: (a) using the single-polarized antenna, (b) using the dual-polarized antenna.
PPD Jammer Scenario. The goal of this test was to compare the overall performance of the DP array to the SP array, as well as to the case when only a single-element antenna was used and with no pre-whitening applied. The K-320 PPD employed in this test scenario poses a serious threat to any commercial receiver in obtaining a valid PVT solution. The spectrogram of the K-320 is shown in FIGURE 8(a), which illustrates that the chirp signal sweeps very rapidly (with a sweep interval of about 40 microseconds) across a frequency range of 15 MHz centered at the L1 carrier frequency, as can be seen in the plot of power spectral density in FIGURE 8(b). The frequency range is much larger than the receiver bandwidth of about 8 MHz (dual-sided). This means that the RFI is seen as pulsed RFI by the receiver.
FIGURE 8. Chirp-like signal generated by the K-320 PPD jammer: (a) spectrogram, (b) spectral density.
An estimate of the jamming behavior during the test in terms of interference-to-signal ratio (ISR) is shown in FIGURE 9. The estimated ISR counts only for the portion of jamming power falling within the receiver bandwidth in baseband after down conversion; it is not an estimate of the ISR at the antenna array. The closer the jammer in the passing car is to the receiver, the higher the PPD power affecting it. The minimum distance between the jammer and the receiver is about 14 meters and is reached at 13:13:45 UTC as indicated in the figure.
FIGURE 9. Estimated interference-to-signal ratio (ISR) of the K-320 PPD jammer.
In FIGURE 10, we can observe the impact of the RFI when the car drives past the receiver by means of the number of tracked satellites, or rather by the number of valid pseudoranges available for PVT computation. When the jammer is close to the receiver, the DP antenna is always better than the SP one. When the RFI is at the minimum distance (about 14 meters) from the receiver, the SP antenna is no longer able to deliver a valid position, while the DP antenna still can.
FIGURE 10. Number of available pseudoranges.
The higher number of valid pseudoranges when using the DP antenna is translated into a better position accuracy. This result can be seen in TABLE 3, which lists the RMS horizontal position error computed during the time the estimated ISR is greater than 25 dB. In the computations, only valid PVT solutions and 2-D positioning errors below 20 meters have been considered.
Table 3. RMS positioning error, in meters, in the horizontal plane computed when ISR > 25 dB.
CONCLUSION
The results of the measurement campaign have shown a significant improvement in positioning accuracy and robustness against interference when the dual-polarization approach is used compared to the general single-polarization case. Position accuracy takes advantage of the better C/N0 for those satellites with an AoA below 30°, which experienced up to 2 dB C/N0 improvement. Although the benefit in PVT accuracy was not remarkable in our testing, this should become more notable in scenarios where a lower number of satellites are visible or when the LOS signals are obstructed, such as in urban environments. Receiver robustness takes advantage of the possibility of filtering in the polarization domain and the additional number of available degrees of freedom, enabling tracking and PVT solution availability in severe interference scenarios. In particular, a valid PVT solution was still available for an ISR of 53 dB using the dual-polarization array, while the single-polarization array was unable to deliver a valid position. While these improvements are noteworthy, they do come with added cost and complexity of the receiving system, since the number of channels to be processed is doubled.
ACKNOWLEDGMENTS
This article is based on the paper “Interference Mitigation Using a Dual-Polarized Antenna in a Real Environment,” presented at ION GNSS+ 2016, the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation, held Sept. 12–16, 2016, in Portland, Oregon.
MATTEO SGAMMINI received an M.Eng. degree in electrical engineering in 2005 from the University of Perugia, Italy. He joined the Institute of Communications and Navigation of the German Aerospace Center (DLR), Wessling, Germany, in 2008. He is pursuing a Ph.D. in electrical engineering with research interests in interference mitigation techniques for GNSS.
STEFANO CAIZZONE received an M.Sc. degree in telecommunications engineering and a Ph.D. degree in geoinformation from the University of Rome “Tor Vergata,” Italy, in 2009 and 2015, respectively. Since 2010, he has been with the antenna group of DLR’s Institute of Communications and Navigation, where he is responsible for the development of innovative miniaturized antennas.
ACHIM HORNBOSTEL holds a diploma degree in electrical engineering and a Ph.D. degree from the University of Hannover, Germany. He joined DLR in 1989 and heads a working group on algorithms and user terminals at the Institute of Communications and Navigation.
MICHAEL MEURER received a diploma in electrical engineering and a Ph.D. degree from the University of Kaiserslautern, Germany. Since 2006, he has been with DLR’s Institute of Communications and Navigation, where he is the director of the Department of Navigation and of the Center of Excellence for Satellite Navigation. Since 2013, he has also been a professor of electrical engineering and director of the Institute of Navigation at Rheinisch-Westfälischen Technischen Hochschule (RWTH) Aachen.
FURTHER READING
• Authors’ Conference Paper
“Interference Mitigation using a Dual-Polarized Antenna in a Real Environment” by M. Sgammini, S. Caizzone, A. Iliopoulos, A. Hornbostel and M. Meurer in Proceedings of ION GNSS+ 2016, the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, Sept. 12–16, 2016, pp. 275–285.
“Detection and Suppression of PPD-Jammers and Spoofers with a GNSS Multi-Antenna Receiver: Experimental Analysis” by A. Hornbostel, M. Cuntz, A. Konovaltsev, G.C. Kappen, C. Hättich, C.A. Mendes da Costa and M. Meurer in Proceedings of ENC 2013, the European Navigation Conference, Vienna, Austria, April 23–25, 2013.
“Blind Adaptive Beamformer Based on Orthogonal Projections for GNSS” by M. Sgammini, F. Antreich, L. Kurz, M. Meurer and T.G. Nollin in Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, Sept. 17–21, 2012, pp. 926–935.
“Field Test: Jamming the DLR Adaptive Antenna Receiver” by M. Cuntz, A. Konovaltsev, M. Sgammini, C. Hattich, G. Kappen, M. Meurer, A. Hornbostel and A. Dreher in Proceedings of ION GNSS 2011, the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, Oregon, Sept. 19–23, 2011, pp. 384–392.
“Suppression of Multipath and Jamming Signals by Digital Beamforming for GPS/Galileo Applications” by Z. Fu, A. Hornbostel, J. Hammesfahr and A. Konovaltsev in GPS Solutions, Vol. 6, No. 4, March 2003, pp. 257–264, doi: 10.1007/s10291-002-0042-2.
• Diversely Polarized Antenna Arrays
“Subspace Fitting with Diversely Polarized Antenna Arrays” by A.L. Swindlehurst and M. Viberg in IEEE Transactions on Antennas and Propagation, Vol. 41, No.12, Dec. 1993, pp.1687–1694, doi: 10.1109/8.273313.
“Direction Finding with an Array of Antennas Having Diverse Polarizations” in IEEE Transactions on Antennas and Propagation, Vol. 31, No.2, March 1983, pp. 231–236, doi: 10.1109/TAP.1983.1143038.
• Antenna Array Signal Processing
“Two Decades of Array Signal Processing Research: The Parametric Approach” by H. Krim and M. Viberg in IEEE Signal Processing Magazine, Vol. 13, No. 4, July 1996, pp. 67–94, doi: 10.1109/79.526899.
“Multilinear Array Manifold Interpolation” by R.O. Schmidt in IEEE Transactions on Signal Processing, Vol.40, No.4, April 1992, pp. 857–866, doi: 10.1109/78.127958.
A controlled radiation-pattern antenna can preserve GNSS positioning while providing at least an azimuth angle towards an interference source. If integrated with an attitude and heading reference system (AHRS), only a few lines of position pointing towards the RFI source could provide a fast indication of the probable ground location.
By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
GNSS is an essential enabler for many aviation applications that rely on either accurate position or time synchronization. While the idea of “sole means” GNSS is disappearing, it remains challenging to match the performance and coverage of GNSS with terrestrial systems. This is why aviation is working on Alternate Positioning, Navigation and Time (A-PNT) to cope with the potential for a wide-area GNSS outage. Current navigation aids are clearly part of this approach in the short term. We will continue to need a terrestrial capability for some time, but we don’t expect that it will support the same level of performance as GNSS. Even if we have back-up, we must be able to resolve GNSS outages efficiently.
Among principal GNSS vulnerabilities — constellation performance issues, space/solar weather and radio-frequency interference (RFI) — RFI is the one where observability on the ground is often limited. While the protection of radio services from interference is a state responsibility typically assigned to a telecommunications or other government agency, it is in the interest of an air navigation service provider (ANSP) to be able to request help and enforcement action from the telecommunications regulator in an efficient manner.
As a part of its contribution to Single European Sky ATM Research (SESAR, a collaborative project to improve European airspace and its air traffic management), Eurocontrol has developed an RFI Mitigation Plan as a guidance framework with the objective to maintain risks to GNSS and the associated operations at tolerable levels. The document will be published by ICAO in its GNSS Manual in the new 2017 edition.
MITIGATION PLAN
RFI can be a security issue. Consequently, a commonly used philosophy in the security domain was used in the mitigation plan: there are many potential threats, but not necessarily all of them translate into operationally relevant risks. Threats are thus sort of dormant risks, which, if left to develop unmitigated, could develop into risks to aviation. The mitigation process monitors threats, assesses risks, and then implements suitable mitigation to stop threats from developing into risks. Three successive stages have been identified where such barriers can be applied:
Prevent transmission of RFI, mostly through radio regulatory actions and coordination;
Prevent interruption of positioning and navigation capabilities in the presence of RFI. This is achieved at the avionics level by making sure receivers can tolerate some RFI as well as redundant capabilities;
If interruption cannot be avoided, ensure that other communication, navigation and surveillance capabilities provide continued safety while being able to detect, locate and eliminate an RFI source efficiently.
This third barrier is where flight inspection or other aerial work platforms can play a significant role. However, this role is not limited to risk mitigation. Aerial measurement capabilities can also play a role in threat monitoring by getting data on RFI emissions that are too weak to pose operational risks, and facilitate risk assessment by providing a reliable reference of the impact of such signals on an aircraft in flight.
FLIGHT INSPECTION
Similar to the subject of flight validation, airborne GNSS signal-in-space testing must not necessarily rely on traditional flight inspection capabilities. Other aerial work capabilities can be used, and it is hoped that, over time, data from regular aircraft operations and event recording systems can be used at least for threat-monitoring purposes. However, as soon as a significant RFI occurs, purpose-built aerial detection and localization capabilities are hard to beat. Given that aviation is carrying the risks related to RFI, and telecom regulators are unlikely to have such capabilities, this naturally points to the experience and resources of flight inspection aircraft and their crews.
Even if a significant amount of ground-based RFI sensors are available, local building shadowing can make it difficult to impossible to detect and locate an RFI emitter. Aircraft-provided data can be superior to ground data, and a rough aircraft-based localization can greatly increase efficiency of ground-based localization and source elimination efforts. Aerial RFI localization capabilities offer unique strengths in an overall cooperative process.
EVOLVING SIGNALS
GNSS manifests the transition from analog signals of conventional navigation aids to digital ones. A common characteristic of digital signals is their better use of a frequency channel by spreading the carrier energy such that distinct carrier or subcarrier tones become difficult to observe. Unfortunately, RFI sources have kept up with this, and now most commonly employ swept CW signals, easy to produce but still looking essentially like broadband signals. Many unintentional RFI sources also look like broadband.
Because GNSS is a multi-modal system not uniquely used by aviation, a new type of RFI threat is becoming more common: intentional RFI, which is not directed at aviation, but may nonetheless have an impact. Because there is no direct intent to harm aviation, the nature of these signals and RFI scenarios can become diverse and unpredictable. Furthermore, given the prevalent and ubiquitous nature of GNSS, the number of potential RFI threats is more significant and will evolve more dynamically than aviation capabilities.
A recent effort collecting GPS outage data reported by pilots revealed that a small but surprising number of outages that could potentially be linked to RFI occur on a regular basis, even during en-route operations in some limited regions of the world. For flight inspection, this implies it would be useful to increase the sensitivity of RFI source detection commensurate with the digital nature of GNSS and consistent with the power levels that can impact receivers.
Another particular challenge comes from the specification of an interference mask for GNSS. Other navigation systems do not have such a mask, or any kind of minimum signal-to-noise ratio standard. The mask represents a realistically achievable interference environment. It has been adopted as a global benchmark where receivers experiencing signals above the mask may not produce misleading information, but may stop operating.
However, in practice, little is known about by how much typical receivers exceed the minimum masks. Some tests have reported a margin as significant as 23 dB to CW and 10 dB to broadband signals. This means that an RFI which may not bother one type of receiver at all could be a significant problem for another, limiting the possibility to rely on observed receiver performance. It also implies that signal-in-space effects should be detectable at the low levels of the ICAO receiver RFI mask.
CRPA LOCALIZATION
For civil aviation as opposed to military operations, a CRPA could make sense provided that it outperforms current RFI localization methods at a reasonable price. In military applications, the exact location of the RFI source may be of a secondary nature, as long as desired signal tracking can be maintained.
However, by steering a null (negative gain) towards the angle of arrival of an undesired signal source, a line or sector of possible source positions can be obtained. In this case, the main objective would not be to null a deliberate interferer or jammer, but to obtain a bearing on the type of the interferer. The main scenario we worry about that leads to low-power events are those where aviation is not the desired target, such as a PPD. Unintentional cases can be a mix of high- or low-power cases. The use of a GNSS-specific antenna is expected to provide the required sensitivity, while being able to profit from the military off-the-shelf development. When further integrated with standard flight-inspection sensors such as an attitude and heading reference system (AHRS) and additional geolocation software, this approach has the potential to increase the reliability, accuracy and speed of geolocation while reducing operator effort and flying time. An additional potential benefit is the preservation of ownship position when flying into an area of significant RFI.
The suggested use of military technology brings with it the question on how such use could be authorized. CRPA antennas and associated antenna electronics manufactured in the United States fall under the International Traffic in Arms Regulation (ITAR). While this is a solvable but, nonetheless, cumbersome issue, the approach taken by this project was first to evaluate possible benefits from using a CRPA before worrying about the ITAR issue.
This study was conducted by Eurocontrol in the frame of a SESAR Project on GNSS, including a contract with Rockwell Collins for a feasibility study of the CRPA RFI localization concept. The French (DSNA/DTI) and U.S. FAA Flight Inspection service supported the project with expertise and in-kind contributions. The FAA conducted an overflight with a direction-finding-equipped aircraft for direct comparison between the CRPA approach and other, non-GNSS specific, commercial solutions.
TECHNOLOGY OPTIONS
Current, common GNSS CRPAs come in either 4- or 7-element variants. CRPAs always require antenna electronics for further processing of the RF inputs, and perform either nulling (steering negative gain towards RFI sources) or beamforming (steering positive gain towards GNSS satellites), or both. The most performant system is a 7-element CRPA in combination with digital beam-former antenna electronics. The 7-element CRPA has a diameter of 36 cm (14 inches), which is of some concern for installation on a typical flight-inspection aircraft such as the Beech King Air. But for a feasibility study, it makes sense to first evaluate the most-performing option. If there is unnecessary margin, the solution can be simplified afterwards.
A top-mounted solution on the airplane fuselage was retained due to experience with military anti-jam performance suggesting that RFI localization performance would be sufficient while retaining the benefit of stable ownship position. A key element of the assessment focused on how to best use aircraft banking to facilitate geo-localization.
As shown in Figure 1, the CRPA is connected to the Digital Integrated GPS Anti-Jam Receiver (DIGAR). As there is one RF cable per CRPA element, it is useful to install the DIGAR as close as possible to the CRPA. The standard military-production DIGAR contains not only the antenna electronics but also the receiver including baseband processing. For civil purposes, either a civil receiver would need to be integrated into the DIGAR or, alternatively, a single RF output is available to connect a standard civil GPS receiver. The DIGAR will also feed angle-of-arrival information into a direction-finder software.
Figure 1. System configuration. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
The software provides angle-of-arrival information with respect to the antenna/aircraft reference frame. To provide a geolocation capability, this must be combined with ownship position and aircraft attitude. As most flight inspection aircraft are equipped with an AHRS, this is not expected to be a problem. Project resources did not permit full integration, so testing was done using the direction-finder display only. The AHRS would need to provide 10–50 Hz updates with an error of not more than ±2 degrees.
Figure 2 shows an example of the direction-finder output. Lighter areas show where the antenna electronics produce negative gain, while darker areas represent stronger positive gain. The red dot indicates a potential interferer has been identified. Source location is at about 280 degrees of azimuth with respect to aircraft nose.
Figure 2. Excerpt from direction finder polar display of RFI signal angle of arrival. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
Correct detection probability will depend on the sensitivity threshold and associated false detection probability being considered acceptable. A visual localization may still be possible at carrier-to-noise density ratios (C/N0) below those needed to produce the red dot here, especially if the visible ambiguity can be removed through some aircraft maneuvering. It can be inferred from the system description that once the full integration is accomplished, the provision of a direct output using only a few lines of position to find a probable RFI source location in terms of approximate lat/long coordinates should be straightforward.
SIMULATOR TESTING
A well-calibrated simulator capable of feeding the seven RF inputs was used to assess detection performance for different flight patterns near an RFI source. The tested patterns include a rectangular, a circular and an oscillating, S-shaped trigger-and-hunt trajectory. A variety of different encounter scenarios in terms of power levels and free space path loss were tested. Power levels were adjusted to produce a 1-dB reduction in the C/N0. Both a continuous wave (CW) interferer at the L1 center frequency and a broadband (BB) interferer were simulated (using a 20-MHz-wide PSK signal). Figure 3 shows an example of achieved detection accuracies in both azimuth and elevation angle.
Figure 3. Example result of angular detection performance. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
While there is a strong peak within ±10 degrees of azimuth, there are also significant outliers. For the elevation (note the normalized scale), however, the main peak is thinner with even stronger sidelobes. Due to the installation of the antenna on top of the aircraft fuselage, the simulation results indicate that the elevation angle output is not very useful for detection. The time series result for the azimuth is given in Figure 4, where it can be seen that there are many good detection matches but also some “sympathetic nulls” that move in the opposite direction of the ground track truth reference (circled in grey). It is expected that with additional software processing, these sympathetic nulls can be filtered out.
Figure 4. Azimuth Time Series Result Corresponding to Figure 3. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
For all tested scenarios (assuming additional filtering), azimuth detection capability was better than ±10 degrees (one standard deviation), and in some cases as accurate as ±2 degrees. There was no significant difference between CW and BB results. As could be expected, simulated aircraft banking significantly improved detection capability. Consequently, the use of orbits seems to be the best search strategy. The simulator testing used a figure-eight pattern with one of the orbits passing over the interference source.
LIVE-SKY VAN TESTING
Rockwell Collins has an authorization to broadcast RFI test signals at the GNSS L2 frequency. Previous work showed that the results at L2 can be applied equally to L1. Figure 5 shows the test area, including a –100-dBm signal level boundary. The interferer was installed on a tripod and fed by a signal generator using a normal GPS fixed radiation pattern antenna (FRPA).
Figure 5. Live-sky test area. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
Locations B and C were used to both calibrate the RFI level and as check points for the van trajectory. The test van included a fixture that allowed a tilting of the CRPA by 30 degrees from zenith to either side. Figure 6 shows a schematic of the tilt fixture. It can be seen that this set up creates a realistic RFI path that arrives with an elevation slightly below the horizon at the unit under test. Two sets of tests were performed: one where the van drove straight into or out of the area of interference to determine overall equipment sensitivity, and varied paths to quantify angular detection performance. Again, both CW and BB RFI signals were evaluated.
Figure 6. CRPA with tilt fixture. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
Not surprisingly, elevation angle results turned out not to be very reliable given the below horizon signal path. But azimuth errors were slightly greater than obtained during the wavefront simulator testing (±12 degrees, one sigma). This can be attributed to both multipath and a less accurate heading truth reference. Taking these additional factors into account, the results are very consistent. Tilting the antenna by 30 degrees towards the RFI source significantly improves azimuth resolution (to about ±8 degrees) while also reducing sympathetic nulls. When the tilted antenna points away from the RFI source path, azimuth accuracy will decrease, which is considered helpful in avoiding false detections.
Summary. Even if a good bit of integration work remains necessary to produce a production-ready system for flight inspection or other similar aircraft, the approach shows promise. Further testing, especially using an actual aircraft installation, is recommended. Installation of a 7-element CRPA will be challenging on a typical Beech King Air, but possible. Antenna calibration requirements are expected to be manageable with a standard network analyzer. To avoid further complications with export regulations, the use of a separate civil GNSS receiver is recommended. The overall system is, at this stage, still on the costly side.
While a 4-element CRPA could be used, this was estimated to double or triple angular azimuth detection errors and reduce the detection distance, and consequently not likely to be worth the additional cost. While smaller 7-element CRPAs than the one used are available, their performance would need to be assessed.
For a top-mounted CRPA, aircraft banking is essential to ensure good performance. This could increase the amount of airspace required for detection and lead to operational complications. Furthermore, since the aim is to increase detection sensitivity to geo-locate weak power sources such as personal privacy devices, maintaining ownship position is not that critical, as it can be managed by maintaining an appropriate distance from the RFI source if needed. Consequently, both DSNA and FAA recommend using a bottom-mounted CRPA. In addition to adding 10 dB of detection sensitivity on average and reducing the need for maneuvering, it may restore the utility of the elevation output, thereby potentially further reducing search time. Either way, it will be useful for equipped aircraft to have alternate positioning capabilities to GNSS both for aircraft guidance and truth reference systems.
The system required a 15-dB stronger signal to transition from detection to localization. However, this is dependent on the accepted false-alarm rate. A tunable procedure can be envisaged where the software accepts a higher false-alarm rate at first to maximize search capability and moving to a lower alarm rate to confirm suspected RFI source locations later. Both the potential of the additional filtering software and any human-machine interface aspects would need to be further evaluated.
GENERIC CAPABILITIES
The two common options for in-flight detection of RFI sources in any relevant frequency band are the use of either a spectrum analyzer or, if available, a direction finder. The spectrum analyzer approach depends on connection to a suitable antenna, preferably with some directionality. In this way, the aircraft can be maneuvered to point the antenna either towards or away from the RFI source. Normally there is very little directivity, making this a challenging search. A direction finder is a significant improvement. Figure 7 shows the L-band antenna array used by a DF-4400 as installed on the bottom of the aircraft.
Figure 7. CRPA with tilt fixture. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
Newer generation spectrum analyzers with a good GNSS-specific pre-amplifier, using digital sampling with a fast A/D converter, could provide useful capability. However, the subject is beyond the scope of this discussion, and we focus here on comparing the CRPA approach with a standard direction finder.
The FAA Flight Inspection service conducted complementary flights during the Rockwell Collins live-sky van testing. The flights included orbits and a direct overflight of the RFI source. This was complemented by additional laboratory calibration to ensure that results could be compared. The sensitivity results of the CRPA approach are more meaningful in comparison with a generic direction-finder capability. Since test data is only available for a top-mounted CRPA, the comparisons here are made for the preferred bottom-mounted CRPA using engineering estimation.
The key finding was that while direction-finding capability was quite comparable between the CRPA- system and the DF-4400 for CW, the CRPA-system outperforms the DF-4400 by a significant margin when encountering broadband signals. This is considered to be a significant improvement given the expected nature of RFI sources. During the FAA overflight, the aircraft did not manage to detect the broadband signal. Consequently, the values given here are reconstructed from laboratory analysis. Table 1 compares the estimated achievable sensitivities.
Table 1. Comparison of direction-finding sensitivity. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
In view of the limitations of the data analysis performed, these values must be interpreted with caution. In general, we can conclude that the direction-finding sensitivity of the CRPA system is relatively insensitive to the encountered modulation of the RFI signal, and that the bottom-mounted CRPA system outperforms the DF-4400 system by a small margin in the CW case and by a large margin in the broadband case. How many additional dBs can be gained by both approaches through further optimizations is for future analysis. The performance improvement of the CRPA system does come at a cost, as could be expected.
DETECTION
Before the search for an RFI source can begin, it must be detected. Normally it should be easier to detect an RFI source than to locate it, since direction-finding requires a certain signal strength to obtain bearing information. However, given the directionality of DF arrays, this may not necessarily be true. Another potential factor is the reliance on a spectrum analyzer to detect RFI, which may not achieve the corresponding noise floor, especially when using a broad scan across a wide frequency range. The direction-finder system needs about a 15-dB difference between detection and localization ability.
Figure 8 shows the detection ranges for the top-mounted CRPA system for a given ground-based emitter while the aircraft altitude is assumed at 2000-ft AGL. The bottom mounted system would improve the minimum detection threshold further. Given that 15 dB can translate into a significant difference in free space path loss distance, concepts for efficient direction finding once an RFI source is detected deserve further attention.
Figure 8. Detection ranges for top-mounted CRPA system. Source By Gerhard E. Berz, Pascal Barret, Brent Disselkoen, Michael Richard, Vincent Rocchia, Florence Jacolot, Todd Bigham and Okko F. Bleeker
HUMAN FACTORS
During the FAA overflight, the broadband RFI couldn’t be detected by either the spectrum analyzer in use or the DF-4400. Part of the challenge was using the right equipment settings. For the DF-4400, it was found that best performance could be obtained for detecting broadband RFI when using the FM wide mode of demodulation. Similar findings were obtained for the use of the spectrum analyzer, where specific skills are necessary to use the equipment to its fullest capability. Similar issues are expected when having to interpret the display of a CRPA-based system. This means that regardless of the RFI source geo-location approach used, specific training should ensure that aircraft operators have the greatest chance of success in finding RFI sources.
CONCLUSIONS
An approach using a CRPA antenna, electronics and processing software proved superior to current, generic direction-finding capabilities, especially with respect to broadband signals. Maintaining ownship position in the presence of RFI is a secondary objective when looking for the expected weak signal sources, and the use of a bottom-mounted CRPA system is preferred. Additional filtering to eliminate sympathetic nulls and other issues require further investigation.
Significant benefit derives from employing aerial work aircraft in cooperation with ground-based capabilities. We recommend that equipment manufacturers further study all aspects of GNSS RFI geo-location and improve their capabilities. Such capabilities are expected to limit the exposure time to RFI cases and allow a more efficient deployment of ground-based spectrum enforcement resources. These studies should include the improvement of detection and localization equipment, and the development of corresponding operational procedures for flight crews.
ACKNOWLEDGMENTS
The Eurocontrol-funded contract with Rockwell Collins is part of the Eurocontrol contribution to SESAR Project 15.3.4, GNSS Baseline and the GNSS RFI Vulnerability Mitigation Task.
Rockwell Collins provided the DIGAR and Direction Finder Software.
This article is based on a paper presented at ION-GNSS+ 2016.
Disclaimer. This article does not contain any official Eurocontrol, SESAR, FAA or DSNA position or policy. It does not constitute any endorsement of a particular product, or a statement of any kind relating to any future procurement activity.
GERHARD BERZ and PASCAL BARRET work at Eurocontrol, Belgium; VINCENT ROCCHIA and FLORENCE JACOLOT with Direction des Services de la Navigation Aerienne, France; BRENT DISSELKOEN and MICHAEL RICHARD at Rockwell Colins, U.S; Okko F. Bleeker with OFBConsult System Engineering, the Netherlands; and TODD BINGHAM with the U.S. Federal Aviation Administration.
Tallysman, a manufacturer of high-performance GNSS antennas and related products, has introduced a magnetic-mount triple-band (plus L-band) GNSS antenna, TW7972, and a dual-band antenna, TW7872.
They are designed for precision agriculture, autonomous vehicles, navigation, real-time kinematic (RTK), precise point positioning (PPP), and other applications where precision matters. The ability of the TW7972 to access L-Band correction services extends its utility to a wider range of applications.
The introduction of these antennas is a continuation of Tallysman’s expansion into broader band GNSS antennas. These antennas are the first releases in a line of new enclosures that will be used for additional broadband GNSS solutions.
Photo: Tallysman
The antennas employ Tallysman’s Accutenna technology.
The TW7972 is capable of receiving GPS L1/L2/L5, GLONASS G1/G2/G5, BeiDou B1/B2, Galileo E1/E5a+b and L-band correction services (1164 MHz to 1254 MHz + 1525 MHz to 1606 MHz).
The TW7872 is capable of receiving GPS L1/L2, GLONASS G1/G2, BeiDou B1 and Galileo E1.
The precisely tuned antennas have a tight pre-filter to protect against intermodulation and saturation caused by high-level cellular 700 MHz and other signals.
The antennas provide superior multi-path signal rejection, a linear phase response, and a tight phase-center variation (PCV) at a new economical price point, Tallysman said. The antennas provide comparable or superior performance to higher priced triple- and dual-band GNSS antennas on the market.
The TW7972 and TW7872 are housed in a magnetic-mount, IP67 weather-proof enclosure with pre-tapped screw holes. The antennas can also be ordered without the magnet.
The TW3967 (28-dB gain) and the TW3972E (35-dB gain) are the embedded versions of the TW7972. The TW3867 and TW3872E are the embedded versions of the TW7872. They are available with a wide selection of connectors and custom cable lengths, and can be custom tuned by Tallysman to ensure optimum performance within the customer’s enclosure.
Professional GNSS users now expect lightweight, easy-to-use receivers optimized for their particular workflows. Meanwhile, a streamlined manufacturing process means design and production of sophisticated instruments now takes months rather than years, and relies on global teams of networked specialists.
Carlson Software approached Hemisphere GNSS in early 2015 with the goal of bringing a new GNSS receiver to market, one optimized for land surveyors with high precision, convenience, and small form factor. “We work closely with land surveyors, and we definitely saw a need,” said Carlson’s director of special projects Karl Nicholas. “Our clients were asking for smaller, lighter receivers. We also felt that a new receiver could be better optimized to work with the multiple satellite constellations now available, and with the array of RTK solutions that surveyors use routinely.”
Hemisphere recognized that a new lightweight receiver would also serve its own marine clients well, especially if it was optimized to work with the company’s Atlas GNSS Global Correction Service as both rover and base station.
The S321 by Hemisphere GNSS. Photo: Hemisphere
Carlson focuses on computer-assisted design (CAD) software, field data collection, and machine control products for land surveying, civil engineering, construction, and mining. Through the partnership, Hemisphere gained access to a deep knowledge base of how surveyors work with GNSS in real-world conditions, and how to optimize a new receiver for fieldwork of all kinds.
This aided decisions about interface, form factor, and features. Project dialog between the two companies identified specifications for particular functions and features, as prototypes became available for testing and feedback.
Specifications included:
Compact and Durable. A form factor for a smaller receiver had already been developed. “Our hardware design and manufacturing division in China presented a hardware design that we really liked, so we didn’t have to redesign from scratch in that area,” explained Hemisphere senior product manager Lyle Geck. “We were able to move ahead with only minor modifications.”
Carlson tested rigorously before signing off on the hardware design. “I put mine on top of a two-meter pole and dropped it onto concrete and dirt, and I also tried it out in wet weather — worked fine!” recalled Nicholas.
Multiple Constellations. “We now have a receiver that works seamlessly right now with GPS, GLONASS, and the Chinese BeiDou system,” added Nicholas. “And when Europe’s Galileo system becomes available, we’ll be ready for it too.”
RTK, Correction Sources. Hemisphere’s Athena RTK engine, is designed to process the new signals with high-accuracy performance. In addition to traditional RTK correction methods using NTRIP and UHF/900 MHz radios, Hemisphere also provides Atlas, its own L-band correction service: subscription-based, flexible, available over the Earth’s landmass, from approximately 200 reference stations, providing up to sub-decimeter accuracies via L-band satellites or over the Internet.
The new receiver was also designed with a built-in UHF radio, and multiple wireless communication ports to enable corrections via radio, cellular modem, Wi-Fi, Bluetooth, or serial connections.
Base Station Capacity. The receiver can serve as both rover and base station. “For our marine clients, this receiver is actually more likely to be used as a base station,” said Geck, typically set up in a port for construction or other maritime operations. Not a closed system, it works with Atlas, other protocols like TrimTalk, and with external radios that can be connected as needed.
Productivity. For surveyors, Carlson specified a compass and a tilt sensor so the receiver knows if the pole is vertical, how it’s oriented horizontally, and how to correct for those factors. It works for stakeouts and recovering points; the unit directs the user to the next point graphically, saving time.
For surveyors in obstructed areas, position reliability will often degrade. “Surveyors are aware of this, but it’s hard to compensate when they don’t have information about just what’s happening with accuracy.” SureFix uses proprietary algorithms and various inputs to give a quality indicator for particular points, for confidence when shooting in difficult multipath conditions, or telling a surveyor to slow down to get the required precision. This improves fieldwork and can eliminate trips back to the field to correct errors.
Carlson Software leveraged its 30+ years in land surveying, while Hemisphere GNSS added manufacturing experience and GNSS and RTK expertise. The result is a compact receiver, BRx6 from the former and S321 from the latter, tuned for the requirements and workflows of customers’ daily projects.
Harxon has released its next-generation triple-frequency Helix Antenna HX-CH7603A, which has excellent performance and high efficiency in a compact form factor, the company said.
Harxon’s new-design helical antenna HX-CH7603A is capable of GPS L1/L2, GLONASS L1/L2 and BDS B1/B2/B3. Though compact, it provides high peak gain (more than 3.5 dBi) and wide beam width to ensure the signal receiving performance of satellite at low elevation angles.
HX-CH7603A is equipped with an O-ring and gold-plated SMA (sub-miniature version A) connector that makes the antenna waterproof-grade, reaching IP67 once installed on a mating surface. The antenna is designed for applications requiring minimal integration effort or for retrofitting existing products.
Features
High gain (3.5dBi) with superior tracking performance
Very low noise figure
High stability and high repeatability at phase center
An antenna upgrade for U.S. Navy submarines is being provided to improve GPS anti-jamming capabilities.
Mayflower Communications Company, subcontractor to Lockheed Martin Sippican, is applying its Submarine Anti-Jam GPS Enhancement (SAGE) capability to the U.S. Navy Multifunction Mast Antenna System (OE-538B) upgrade to improve submarine communications and meet Navigation Warfare (NAVWAR) requirements.
The SAGE (NavGuard 501) GPS anti-jam unit.
The Mayflower SAGE — a variant of Small Antenna System (SAS) — was developed specifically for inclusion on Submarine Platforms to support U.S. Navy requirements for GPS anti-jam.
The SAGE’s small size and feature set make it capable for ease of integration by Lockheed Martin Sippican into the OE-538B antenna mast.
The SAGE is a high performance and low size, weight and power (SWaP) cost-effective antenna system that will enable the U.S. Navy submarine fleet to operate in GPS contested or denied (NAVWAR) environments.
The SAGE (NavGuard 501) can supply clean GPS Signals to multiple GPS receivers from a single antenna and is compatible with C/A, SAASM P(Y), and M-code receivers. The SAGE fits he small SWaP requirements of the OE-538B antenna mast.
The SAGE is Mayflower’s latest federated, affordable anti-jam solution that leverages proven small antenna system (SAS) technology and provides Iridium capability in an integrated antenna. The SAS solution has been extensively tested by the federal government on multiple platforms.
The SAGE is the highest performance and smallest GPS anti-jam federated solution with Iridium capability in the market. The SAGE AJ solution offers an affordable SWaP-C alternative over larger and more expensive existing anti-jam systems.
The Space and Naval Warfare Systems Command (SPAWAR HQ) awarded the sole source contract for the development of an OE-538B antenna upgrade and procurement to Lockheed Martin Sippican/Granite State Manufacturing Submarine Antenna Joint Venture. The contract is in support of the Program Executive Office for Command, Control, Communications, Computers, and Intelligence (PEO C4I), Undersea Integration Program Office (PMW/A 770).
Mayflower was selected by the U.S. Navy and Lockheed Martin Sippican to design, develop, and integrate the Submarine Anti-Jam GPS Enhancement (SAGE) (NavGuard 501) product.
Joseph Thomas, Mayflower’s Director of Government Programs, said, “The SAGE product has given Mayflower the opportunity to support a U.S. Navy National Strategic Level Platform and to expand into the next generation of small SWaP NAVWAR GPS Anti-Jam systems. The SAGE ensures we can continue to offer the warfighters the very latest and most efficient technology to support operations in an A2AD Environment”.
Mayflower is working closely with Lockheed Martin Sippican to complete integration and environmental qualification of the SAGE to support the OE-538B program requirements.
The United States Naval Observatory (USNO) has selected NovAtel’s GPS Anti-Jam Technology (GAJT) to satisfy a requirement for a controlled reception pattern antenna capability at sites throughout the Department of Defense Information Network (DoDIN).
The DoDIN is the core global enterprise network of the United States military and is depended upon for secure and sensitive voice, data, video and bandwidth services. This latest order brings the number of NovAtel GAJT antennas ordered by the U.S. Navy to more than 600.
GAJT protects GPS-based navigation and precise timing receivers from intentional jamming and accidental interference. It is a null-forming antenna system that ensures satellite signals necessary to compute position and time are always available.
The commercial off-the-shelf product comes in versions suitable for land, sea, fixed installations and smaller platforms such as UAVs. Military vehicles and platforms, networks and timing infrastructure also benefit from the protection that GAJT provides. There is no need to replace GPS receivers already installed, as GAJT works with civil and military receivers, and is ready for M-code, according to NovAtel.
NovAtel’s manufacturing techniques and quality processes mean that that the company can ramp up quickly to meet volume requirements, the company said.
“This order underlines our ability to deliver GAJT in volume and on time,” said Michael Ritter, president and CEO of the Canada-based NovAtel. “GAJT has now been shipped and is in use operationally by 12 allied nations around the globe. We are grateful for the rigorous technology selection process conducted by USNO which led to this latest order.”
The U.S. Naval Observatory is located in Washington, D.C.
Located in Washington, D.C., the USNO is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation and Timing for the United States Navy and the United States Department of Defense.
Antenova Ltd., manufacturer of antennas and RF antenna modules for machine-to-machine (M2M) and the Internet of Things, is now offering Beltii (P/N SR4G013), an embedded antenna that measures 15.6 x 3.3 x 4.4 millimeters and operates with all satellite navigation constellations.
The antenna has been designed to work over a very small ground plane on a small printed circuit board (PCB), where it can be placed in a corner position, and does not need any ground clearance.
Beltii works with GPS, GLONASS, BeiDou and Galileo, and can add a positioning capability to any small, lightweight device at 1559-1609 MHz. It is suitable for wearable electronics, trackers, drones, navigation devices, and sports applications, the company said.
It is the latest in Antenova’s lamiiANT range of antennas, which are manufactured from FR4 materials and use the latest in dielectric constant laminate substrates to construct smaller, more efficient antennas.
“Beltii is a brand-new design using new materials. It beats the typical larger, ceramic patch antennas on all counts: it is smaller, more efficient, and performs better,” said Antenova CEO Colin Newman. “It is a great addition to our range of positioning antennas.”
Beltii radiation pattern at 1557.42 MHz.
Because the performance of a wireless device depends on the performance of its antenna, and the performance of the antenna depends on how it is integrated into the PCB, Antenova’s antenna designers give the integration requirement top priority and design each antenna so that it can be integrated easily, and to perform well in situ within the device. Antenova calls this concept DFI, or Design For Integration.
Antenova also provides detailed datasheets with advice on integrating the antenna, and offers the services of its engineering team to help customers and OEMs with antenna integration if required.
Evaluation boards (P/N SR4G013-U1) are available with full details of all of Antenova’s embedded antennas.
NovAtel has introduced its new VEXXIS series of GNSS antennas. NovAtel made the announcement at ION GNSS+, which is being held this week in Portland, Oregon.
The VEXXIS series includes two lines of antennas, the new GNSS-800 series and the GNSS-500 series introduced earlier this year. The series offers the latest advancements in GNSS antenna technology for multi-constellation and multifrequency GNSS applications.
The VEXXIS GNSS-800 series of antennas provide exceptional tracking performance previously unachievable in such a small form factor. Patented multi-point feeding network and radiation pattern optimization technology provides stable phase center and enhanced multipath rejection as well as exceptional low elevation satellite tracking while achieving high peak zenith gain.
The new technology enables the antenna to track the maximum number of satellites in any environment for an enhanced positioning solution. The GNSS-800 family of antennas are the toughest high precision antennas NovAtel has designed to date, ensuring their survivability in even the harshest operating environments.
The VEXXIS GNSS-500 series of antennas were designed with a low profile, aerodynamic enclosure, useful for ground vehicles in applications such as agriculture, machine control and mobile mapping.
Featuring the same multi-point feeding network as the GNSS-800 family, GNSS-500 antennas offer excellent multipath rejection and stable phase center. Signal reception is unaffected by the rotation of the antenna or satellite elevation, simplifying placement and installation. Vehicle mounting is easy with the antennas’ magnetic or screw mounting options.
VEXXIS GNSS-500 antennas are available for immediate ordering. GNSS-800 antennas will be available in the fourth quarter of 2016.
Taoglas, a provider of IoT (Internet of Things) and M2M (Machine to Machine) antenna solutions, has launched a new series of high-performance LTE + GNSS or Wi-Fi antennas. The announcement was made at CTIA Super Mobility, held Sept. 7-9 in Las Vegas.
The Taoglas Guardian X 11-in-1 antenna.
The Guardian series includes 4, 5, 6 and even 11-in-1 antenna options for 4G LTE cellular applications that also require GNSS or Wi-Fi or satellite options.
“Drilling holes in assets and doing long coaxial cable runs is a thing of the past for many IoT applications, particularly in the transportation industry,” said Dermot O’Shea, joint CEO at Taoglas. “Most vehicles and assets are no longer made from metal, but of a carbon fiber or composite material. This means the antenna does not need to be outside the asset but can be mounted internally.”
One example is in the trucking industry, where antennas are mounted under the roof and above the headliner, eliminating the need for holes to be drilled. “This saves huge amounts of time and cost for the installation as well as increasing device performance due to the cable runs being shorter,” O’Shea said. “It also decreases the likelihood of antenna damage due to impact or vandalism.”
The Guardian series antennas are delivered in a gloss-finished, compact square-shaped enclosure (146 x 134 x 20 mm). In the series are these options:
MA931 – 6 in 1 (2 x Cellular, 3 x Wi-Fi, 1 x GNSS)
MA930 – 6 in 1 (2 x Cellular, 2 x Wi-Fi, 1 x GNSS, 1 x Satellite)
MA950 – 5 in 1 (2 x Cellular, 2 x Wi-Fi, 1 x GNSS)
MA961 – 4 in 1 (2 x Cellular, 2 x Wi-Fi)
Also, an extension to the line is the Guardian X series, with the first product being MA4000, an 11-in-1 antenna (six cellular, four Wi-Fi, one GNSS).
The Guardian X dimensions are 540 x 183.1 x 35.4 millimeters. Despite its small size, the MA4000 antenna eliminates the requirement for multiple holes to be drilled in a valuable asset. The enclosure material is flame retardant, as is the CFD-200-FR low-loss cable. This means the antenna is compliant for airline, bus and rail passenger applications and complies with UNECE regulation R 118.
Harxon has released a utility beacon antenna — HX-CS7615A — to professionally solve marine satellite positioning challenges.
The HX-CS7615A supports GPS L1/L2, GLONASS L1/L2 BDS B1/B2/B3 and beacon frequencies (282.6 to 326 KHz), which greatly overcomes the defects of long-distance transmission limits, the company said. In addition, combining wide frequencies in one antenna makes the new device more cost effective.
Inside, a multipath rejection board significantly eliminates measurement error, Harxon said. The phase center of the antenna remains constant as the azimuth and elevation angle of the satellites change.
The HX-CS7615A Harxon marine antenna.
The HX-CS7615A is designed with high gain and wide beam width. It is test approved — even in some severe blocking situations, its reception remains stable.
A specialized antenna made for rugged environments, the HX-CS7615A beacon antenna is sealed against water and dust, is salt and fog resistant, and operates in extreme weather conditions.
