Tips for Choosing Precise, Power-Efficient GNSS Receivers for Location-Based Applications
April 23, 2019
| By: Christine Young
Blogger, Maxim Integrated
Personal navigation is just one of many applications that rely on the U.S. Global Positioning System (GPS), which provides geolocation and time information to GPS receivers. From mapping forests and the ocean floor to helping farmers harvest their fields, these technologies are benefiting a wide range of industries. Given that location-based products can be as small as jewelry or as a large as, say, an aerospace navigation system, choosing a global navigation satellite system (GNSS) RF receiver IC with the right characteristics calls for careful consideration.
GPS consists of three segments:
- Space, consisting of a nominal constellation of 24 operating satellites that transmit one-way signals providing current GPS satellite position and time
- Control, where worldwide monitor and control stations ensure that the satellites remain in their proper orbits
- User, which consists of the GPS receiver equipment1
In a GPS system, radio signals are transmitted from satellites to receivers on or near Earth. At any given moment, the satellites’ positions are known. The distance or “range” from the receiver to each satellite can be calculated from the propagation delay of the radio signal from that satellite. The receivers’ spatial coordinates can be calculated once the distance to each of several reference points (i.e., satellites) is known.
Figure 1. Whether you want to travel across state lines or simply to the next neighborhood, navigating from point A to point B is much easier thanks to GNSS and GNSS RF receiver ICs.
GNSS receivers process the signals broadcast by satellites and then determine user position, velocity, and precise time (PVT). A typical GNSS receiver consists of:
- An antenna
- An optional external low-noise amplifier (LNA) to provide low-noise amplification near the antenna
- An optional SAW filter to reject jammers
- Temperature-controlled crystal oscillator (TCXO)
- RF front-end IC, which amplifies, down-converts, filters, and samples the GNSS signal
- Baseband digital signal processor (DSP). Typically implemented in an FPGA for real-time receivers, this DSP outputs navigation message (NAV) bits and information such as carrier phase and code phase.
- Baseband processor subsystem to perform all of the mathematical calculations to compute the navigation solution, interpret NAV messages, and apply corrections2
Of course, for a GNSS to deliver accuracy, the receiver has to be able to get a “good read” on position even if the satellite signal has been compromised by some sort of interference or other direct line-of-sight error. Contributing sources of errors include everything from the satellite clocks to ionospheric and receiver noise3. The target application—and how it will be impacted by performance, accuracy, and power consumption—should determine the selection of the GNSS receiver. An application that calls for highly precise positioning, for instance, will benefit from a receiver that supports multiple frequencies and multiple constellations. By accessing signals from several constellations, a multi-constellation receiver can reduce acquisition time, improve position and time accuracy, and minimize multipath errors from reflected signals. For wearables like smart jewelry that send alerts, with location data, to the wearer’s emergency contacts, important characteristics for the underlying ICs in the GPS receiver front-end include low noise, low power, and small size. Reliability is also critical because any type of misread in the GPS coordinate or an abrupt power loss can lead to potentially serious consequences4. Smart jewelry is typically powered by a tiny battery whose voltage can be as high as 1.5V (silver oxide and alkaline cells). The underlying electronics should operate within these battery voltage limitations while dissipating very little power. Let’s consider the low-power LNA in the receiving electronics for these tiny devices. The LNA attains a very low-level signal, doing so without any substantial degradation to the signal-to-noise ratio (SNR) and also without introducing distortion. Its power dissipation equals the power supply voltage times the quiescent supply current. With no output load from the GPS tuner, the LNA’s power dissipation remains consistent as signals travel from the antenna to the GPS tuner. The lower the power dissipation, the better for battery life of the smart jewelry5.
GNSS Receiver ICs for Many Needs
One multi-constellation, multi-band GNSS receiver to consider for applications requiring highly precise positioning is the MAX2771. In a single chip, this receiver covers the E5/L5, L2, E6, and E1/L1 bands and the GPS, GLONASS, Galileo, QZSS, IRNSS, and BeiDou navigation satellite systems. The chip integrates a complete receiver chain (including a dual-input LNA and mixer), a filter, programmable gain amplifier (PGA), multi-bit analog-to-digital converter (ADC), a fractional-N frequency synthesizer, and a crystal oscillator. Its total cascaded noise figure is as low as 1.4dB. Since it features on-chip monolithic filters, there’s no need for external IF filters. This receiver is ideal for portable electronics such as laptops, digital still cameras, telematics, and vehicle tracking and fleet management systems.
For smart jewelry, wearables and other small, portable electronics, there’s the MAX2679 and MAX2679B GPS/GNSS ultra-low-current LNAs. With support for GPS L1, Galileo, and GLONASS, this IC achieves high gain and low noise figure while maximizing the input-referred 1dB compression point and the 3rd-order intercept point. The MAX2679 consumes only 1mA supply current while providing 0.95dB noise figure, while the MAX2679B consumes only 650µA while providing 1.03dB noise figure.
Who would have expected that something as small as a piece of jewelry could be used to notify designated contacts of an emergency situation—with the wearer’s location? Navigation, mapping, and location tracking are benefiting us on a personal level and are also a boon to a variety of industries. With the right GNSS receiver technology, your GPS application can go a long way.
- In-depth article, “Navigating the GNSS Landscape for More Precise, Power-Efficient Receivers”
- Application note, “Concerned About GNSS Inaccuracy? Expand Your Constellations”
- Design solution, “Empower Wearable Electronics by Turning Down the Current”
- 1 https://www.gps.gov/systems/gps
- 2 https://www.maximintegrated.com/content/dam/files/design/technical-documents/white-papers/navigating-the-GNSS-landscape-for-more-precise-power-efficient-receivers.pdf
- 3 https://www.maximintegrated.com/en/app-notes/index.mvp/id/6883
- 4,5 https://www.maximintegrated.com/content/dam/files/design/technical-documents/design-solutions/DS74-Empower-Wearable-Electronics-by-Turning-Down-the-Current.pdf