Advanced Ultra-Mini Hidden Antenna for Successful GNSS Navigation
By Desmond C. Wong, Parsec Technologies
The market acclaimed PTA1.5M and PTA1.5x2M GNSS active antenna modules – which only use 8% volume to outperform the top 25x25x2 mm traditional patch antenna for ultra-compact GNSS design. So does this information imply that it is possible to hide the antenna in a hostile environment? This article delves into the vast world of antenna miniaturization and creates a GNSS antenna solution that will work -100% - well inside a car compartment or in a wearable compact product.
Low-cost mass-produced Global Navigation Satellite System (GNSS) receiver modules or ICs are a compelling solution for small form factor design for location tracking devices. It provides reasonable expectation in Time-To-First-Fix (TTFF) and position accuracy. However, it is vulnerable to variation in systems design practices that include antenna, band pass filter, LNA, frequency reference, thermal management, and background noise level inherent in electronic equipment. To analyze each of the design practices, the qualities of four major functional blocks in a typical GNSS receiver are partitioned (Figure 1). Block 1 and Block 2 govern the quality of the antenna and RF front-end, respectively. Block 3 is the baseband and Block 4 is the source of frequency reference. The structure of Block 1 determines to a larger extent the final noise contribution of receiver to the signal. LNAs with noise contribution of 1dB are easy to realize, while low noise narrow band filters on the other hand are much more difficult to realize. However, if a high out-of-band (OOB) Input referred 3rd Order Intercept Point (IIP3) LNA is used, the narrow band filter can be placed behind the LNA where its contribution to the final noise figure is less.
In a system without jamming or radio cohabitation, Block 1 could be simplified to be a passive antenna by eliminating the narrow band filter and discrete LNA. However, if there is radio cohabitation in a system design, unintentional jamming or high level of EMI interference from other electronic devices, the inclusion of narrow band filter and LNA with good linearity  are required after the antenna but before the second stage of RF front-end (Block 2) to prevent the RF front-end from operate under compression. The on-chip LNA inside Block 2 (i.e. the RF front-end) typically does not provide enough linearity and dynamic range. Gain compression and noise figure degradation of the on-chip LNA can result from strong OOB signals, resulting in C/No degradation. OOB IIP3 is important for co-existence with cellular and wireless transmitters. Including a high linearity discrete LNA (Parsec PT1233D) or high linearity active antenna (Parsec PTA.15M-9/-16) can improve the sensitivity performance of the receiver when responding to external interference. The combination of discrete LNA and narrow band filter in Block 1 insure OOB IIP3 is high enough not to cause a blocking problem in the GNSS band.
To reduce the impact of satellite signal multipath reflections and ground-based jamming and reflections, common practices imposed on the standard GNSS antennas are right hand circular polarized (RHCP) and insensitive below 5 degrees of elevation (Figure 2). The installation is on the top to provide line-of-sight signal reception from the sky. It also helps to avoid negative effects of in-vehicle jamming and unintentional interference caused by in-vehicle electronic devices. These standard practices though can mitigate negative effects described above; however, with the increasing need for compact product design, tactical tracking that hides the tracking device is required (Figure 2B). The receiver is hidden and the installation direction of the antenna is arbitrary. This requires an omnidirectional antenna and possibly a discrete LNA to compensate for the additional path loss. So the standard practices of GNSS antenna installation are no longer applicable.
Optimal Miniature GNSS Antenna
Loss in antenna performance cannot be compensated if it is viewed as an individual component. The demand for miniature GNSS antennas is growing so quickly that miniature antennas are commonly done with linear polarization and omnidirectional properties. With the recent advancement in 3D heterogeneous integration technology, the impairment can be compensated for with enhanced system noise, frequency reference, filter and thermal management . These new techniques impose new competing requirements on each of the four blocks to design smaller form factor, higher accuracy, lower cost, and higher reliability GNSS trackers based on miniature GNSS antenna. Therefore, the focus of antenna design shifts to the matters of size, efficiency and position accuracy.
Parsec PTA1.5-16 miniature passive antenna in the 9x16x0.8 mm size was selected to compare with a standard 25x25x2mm ceramic patch antenna; the reduction in antenna volume is over 90%. Both were referenced to the same ground plane of 45x45mm. An ultra-small, low-cost mass-produced GPS SiP  was used as the baseline for system integration. Horizontal position accuracy under static and dynamic conditions was compared in both open sky and indoor environments.
A 24 hour static test with a surveyed roof-mounted antenna is the standard practice to examine the performance of the GNSS receiver. The antenna used in the test was a standard 25x25x2mm ceramic patch antenna mounted on a 45x45 mm ground plane. Circular error probability (CEP50) was then calculated based on four samples measured at the same time.
In particular, since the selected GPS SiP is an ultra-small and mass-produced commercial receiver with a circuit footprint of less than 40mm2, it is suitable to test the concept of miniaturization with the target antennas. The goal is to provide indicative results toward miniature tracking devices in which the antenna is small, omnidirectional and hidden in an arbitrary position where no line-of-sight satellite signals are possible.
The first step is to find out how much the performance margin of the GPS SiP can provide toward the minimization of effort on the antenna. The tests are to be coherent with the concepts as explained in Figure 1. The more the performance margin could be shifted to the system as represented by Block 2 to Block 4, the higher the probability that the navigation device worked well in the hostile environment with a miniature antenna.
Getting the Job Done Regardless of Standard Antenna Practices
Figure 4 is the test results with the standard reference patch antenna, where the tests were conducted at a GNSS test lab. The lab has been used for many benchmarking tests for GNSS receivers and the people are well experienced with GNSS testing. After all the results were computed from the four units under test, it showed a very tight result that the average value of CEP50 was 1.66m, where the data ranged from 1.59m to 1.79m. The improvement is more than 30% over the standard 2.5m CEP50. A reference was also done at an arbitrary indoor position for about 2 hours that showed the navigation error was well within the 25m circle.
In order to replace the standard patch antenna with a miniature omnidirectional passive antenna, three parameters are important — the antenna gain, radiation efficiency and phase error. The Parsec miniature passive antenna under test provides an average gain of -0.54 dBi with the peak gain of 0.9 dBi at zenith. Radiation efficiency is more than 60%. Since direct phase error measurement is complex to conduct, it was observed indirectly by looking at the navigation accuracy under severe multipath reflections. In theory, the smaller the antenna, the smaller the phase error. So in principle, if the miniature antenna introduced additional phase error under multipath reflections, the results would be aggregated and amplified. Conversely, if the phase error was indeed smaller, the navigation accuracy would be beneficial with a miniature antenna.
Both indoor static and drive tests were conducted with the miniature antenna (Figure 5). The indoor static test was done at an arbitrary office location for a period of 4 hours, which was double the duration of the previous reference done with the standard patch antenna. It showed a satisfactory result that the navigation accuracy was also tight within the 25m circle.
Drive tests were conducted at a location where both open sky and indoor covered conditions were available. The tests were done in parallel with both antennas at the same time. Under the open sky locations as shown on the left side of Figure 5, both antennas showed back to back overlapping on the navigation results, although the miniature antenna is more susceptible to multipath reflections since it is 4dB lower in gain and 25% lower in efficiency than the patch antenna.
The final navigation test in the two story covered garage is the most hostile condition. The building has two layers of concrete structure. Although there are some openings on the side walls, it is very deep in both x and y dimensions. In particular, there are no openings on one side of the wall, as shown in the side view of the garage building. It corresponds to the upper trajectories as shown by the blue and red traces in the picture. The drive test was on the ground floor and the speed was less than 4 miles per hour, with some stops in between. So the advantage of increased position accuracy under high dynamic didn’t apply to this test. It solely relied on the qualities of the antennas, the system noise and the accuracy of frequency reference.
The trajectory in blue represented the test unit with the miniature passive antenna. In comparison to the standard patch antenna in red, there was no significant deviation of navigation results between the two antennas. Specifically, there was one 180 degree turn and two 90 degree turns on the course. If there were significant impairment with the miniature antenna, these turns would result in large trajectory deviations after those turns. But in this test, the miniature antenna exhibited very good accuracy and it also indirectly confirmed that the Parsec miniature antenna didn’t suffer from phase error under the severe multipath reflections in the indoor covered garage.
In any GNSS navigation devices, the antenna is still a system’s key component. Additional narrow band filter and high OOB IIP3 LNA can be selectively combined with a miniature passive antenna according to the specific applications and conditions. However, with the need for miniaturization and tactical needs for hiding the navigation devices at any arbitrary locations, it is not practical to follow the standard practice of GNSS antennas. A growing awareness of this fact is evident among systems designers and end users. Indeed, the combination of a well-engineered miniature omnidirectional antenna and low cost mass-produced GNSS receiver could work very well together and get the job done in hostile environments. This combination is a much stronger method than ones existing years before.
The miniature antenna makes it easier for system developers to maintain healthier performance and design margins. And in the end, integrators have more freedom to make the navigation devices smaller and to innovate as the miniature antenna provides an extra degree of freedom to many unconventional applications.
 Desmond Wong, Yasuhiko Katsuhara and Masaya Tanaka, “Novel GPS SiP Module with Miniaturized Substrate by Using RF Sub-circuit Embedding Technology,” International Conference on Electronics Packaging Proceedings, April 2013, TC1-2.
 Jupiter SE880 GPS receiver module, “EDN Hot 100 Products of 2012, RF/Microwave”, November 2012.
 Low Noise Amplifier Linearity Impacts to Close Proximity Co-located GPS L1 Receivers, Parsec Technologies White Paper, July 2013.
Desmond Wong is a pioneer in the 3D heterogeneous integration technology for RF-SiP design and has 20 years of experience in disruptive innovation for the Wireless, RF and Microwave industry. He is the director of GNSS Technology at Parsec Technologies in Irvine, California. Desmond held various posts in advanced technology leadership and strategic programs at Fortune 500 companies including Brunswick, Conexant, and Rockwell. He is an inventor and has multiple patent applications pending. He received a patent grant for an optical memory design from the Innovation and Technology Commission of Hong Kong in 2004. He can be reached through LinkedIn or email@example.com.
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