Giri Krishnamurthy, Mini-Circuits Engineering

Boris Benger, Mini-Circuits Applications

Introduction: The History & Modern Uses of GNSS Applications

GNSS (Global Navigation Satellite Systems) has become an increasingly prevalent RF application for both military and consumer 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.

Due to the geostationary nature of GNSS, the various constellations each serve specific geographic regions. The United States constellation known as GPS (Global Positioning System) was the first of many to follow. Because the GPS system hovers above North America, and the signal will only reach those in America, Canada and Mexico, additional GNSS systems were launched subsequently to serve other regions of the globe. Some of these additional systems include Galileo over Europe, GLONASS (Global Navigational Satellite System) over Russia, and BeiDou (Compass) over China.

The satellites in these systems each transmit signals over L-band frequencies (L1, L2 and L5) that are picked up by the receiver 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 the programmed destination.

Design Requirements

The L1, L2 and L5 bands function at low frequencies in the 1-2 GHz range, which makes them accessible and relatively cheap to operate within. As a result, 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 an immediate data error that will present itself either as an absence of information or as 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 graph. Narrow bandwidths are ideal for L-band applications where spectrum allocations are crowded, and low insertion loss is important for receiving the relatively weak signals transmitted from satellites to receivers on Earth.

GPS, Galileo and GLONASS often share center frequencies in the L1, L2 and L5 bands allowing interoperability for LBS-enable devices between regions, although the bandwidths within each system vary. GPS generally has the widest bandwidth (approximately 15 MHz) compared to Galileo and GLONASS. Because each constellation is geographically separated from the others, there is no risk of interference between the different systems while occupying the same center frequency.

This interoperability is convenient both for filter designers and end users. A GPS filter meant to capture a frequency range in the L1 band, for example, will be able to encompass the narrower bandwidths of Galileo’s E1 band and GLONASS’s II-L1 band. This overlap allows one bandpass filter to be used in multiple GNSS systems and gives the user greater value and mobility with a single device.