Introduction: The History & Modern Uses of GNSS Applications
GNSS (Global Navigation Satellite Systems) has become an increasingly prevalent RF application for both military and commercial use. Early GNSS technology, or location-based services (LBS), was developed exclusively for military use and operated with a margin of error of about 10 yards (9.14m), which was sufficient at the time, but limited its suitability for end uses requiring a higher degree of precision. In the time since the first launch of GPS more than 40 years ago, the evolution of the technology has improved precision to the order of 2-3 yards (1.83-2.74m). This advancement of the technology combined with significant miniaturization and cost reduction of LBS-enabled devices has opened up a large and growing commercial market for GNSS services. For example, GNSS is now used in the agricultural market to calculate statistics for weather, soil conditions, and crop health to help farmers maximize their yields and profits. Such innovations have stimulated demand for components that support military, industrial and consumer applications.
There are four GNSS systems that provide global coverage. GPS (Global Positioning System) is owned and operated by the United States government. GLONASS (Global Navigational Satellite System) is owned by Russia, Galileo by the European Union, and BeiDou (Compass) by China. These systems orbit the earth in Medium Earth Orbit (MEO) and complete one revolution around the earth in 12 hours and one minute on average, roughly half a sidereal day. In addition to the global systems, there are two regional systems that belong to Japan and India. The Japanese system, Quasi-Zenith Satellite System (QZSS), covers the Asia-Oceania region, and the Indian system, Navigation with Indian Constellation (NavIC), covers India and nearby regions.
The satellites in these systems each transmit signals over L-band frequencies that are picked up by system-specific receivers in LBS-capable devices on Earth. The receiver accepts the incoming carrier frequencies from four satellites, and takes the coordinates from each in order to determine its current position in relation to a given programmed destination.
L-band frequencies occupy the 1-2 GHz range, which makes them accessible and relatively cheap to operate within. However, this region of the spectrum is also occupied by multiple application bands for other communication systems. The dense development of L-band frequencies creates an increased risk of interference between different system bands. In GNSS systems, that interference can result in errors that present either as missing information or incorrect information.
For these reasons, when selecting filters for GNSS applications, it is important to choose a filter that is capable of high rejection with high selectivity or “sharp skirts” on an insertion loss vs. frequency plot. Narrow bandwidths are ideal for L-band applications where spectrum allocations are crowded, and low insertion loss is important for processing the relatively weak signals transmitted from satellites to receivers on Earth.
GPS, Galileo and GLONASS sometimes share center frequencies, particularly in the GPS L1 and L5 bands (see Figure 1), which allows one filter to be usable in multiple systems, depending on the specific application. GPS generally has the widest bandwidth (approximately 15 MHz) compared to Galileo (12 MHz) and GLONASS (6 to 10 MHz).
Choosing the Right Filter Technology
Several filter technologies are potentially capable of meeting the requirements for GNSS applications. Ideally, the filter needs to have a high Q value to prevent interference from adjacent signals and low insertion loss to preserve the integrity of relatively weak incoming signals. Cavity filters offer the highest Q value of practically any filter technology as well as low insertion loss. However, cavity filters are typically large and expensive. These practical limitations preclude their suitability for most real-world GNSS applications in which both size and cost are important factors.
L-C lumped element filters are generally smaller and more cost effective. In theory, they are capable of achieving deep rejection with very high selectivity, but this might require designing nine or ten poles into the filter response. Although more poles will create high rejection and sharp skirts, this approach comes with a compromise on insertion loss on the order of roughly 5-7 dB which degrades the sensitivity of the receiver.
Ceramic resonator filters, by contrast, can achieve high rejection and selectivity with much lower insertion loss (typically 1.5 dB or better) than L-C filters and comparable rejection performance. Although they are slightly larger than L-C filters, ceramic resonator filters are still an acceptable size and cost for GNSS application requirements. This combination of performance, size and cost makes ceramic resonator filters an ideal solution for use in GNSS applications.
Ceramic Resonator Filters and Diplexers for GNSS Applications
Mini-Circuits offers a broad range of ceramic resonator filters specifically designed for GNSS applications, including both off-the-shelf models and custom designs. Our ceramic resonator filters are designed with high Q resonators that provide narrow passbands with low insertion loss ranging from 0.9 to 3.0 dB. They boast excellent rejection and selectivity as well as low profile packaging for dense system layouts. They also offer excellent temperature stability and rugged construction, making them suitable for critical applications in harsh operating conditions.
For systems with special requirements, Mini-Circuits’ applications engineers can advise on solutions to accommodate additional screening, qualification, and custom designs. For example, stopband rejection of standard models can be further extended by cascading LTCC filters in series. Mini-Circuits also has the in-house design and production capability to make passband modifications of existing filters or any other customization on demand.