The use of position-based functions using multi-constellation global navigation satellite system (GNSS) receivers for Europe’s Galileo, the USA’s Global Positioning System (GPS), Russia’s GLONASS, China’s BeiDou navigation satellite system, and Japan’s QZSS is growing across a range of applications including robotics, autonomous vehicles, industrial automation, logistics and asset tracking, drones, and agricultural and heavy construction equipment. The benefits of using multi-constellation GNSS receivers include; better availability of the position, navigation, timing (PNT) signals, increased accuracy and integrity, and improved application robustness.
But developing multi-constellation receivers is a complex and time-consuming activity that includes: optimization of the L-band antenna; designing the radio frequency (RF) front end; integration of the baseband signal processing algorithms to acquire, track, and apply corrections to the various PNT signals; coding of the application’s processing software to extract PNT data from each channel of the baseband and use the information to implement system functions. Designers must also choose an appropriate antenna and place it correctly.
As an alternative, designers can turn to pre-engineered GNSS modules and development environments to quickly and efficiently integrate positioning capability into a system. These GNSS modules include the RF front-end, the baseband processing, and the embedded firmware to speed applications processing software development. Some GNSS modules also include the antennas.
This article reviews the basics of GNSS, PNT, and the operation of multi-constellation GNSS receivers. It then looks at the pros and cons of integrating antennas into GNSS modules before introducing several GNSS modules—with and without integrated antennas—as well as associated evaluation boards from STMicroelectronics, Septentrio, and Würth Elektronik that designers can apply toward the efficient and cost-effective development of accurate and robust position-based applications.
What are GNSS and PNT?
GNSS and PNT are closely related concepts. GNSS satellites are the most common source of PNT signals. GNSS satellites are essentially highly accurate synchronized clocks that constantly broadcast their PNT information. A GNSS module receives PNT signals from a given satellite and calculates its distance from that satellite. When the receiver knows the distance to at least four satellites, it can estimate its own position. However, the accuracy of the position estimation is affected by a variety of error sources, including:
- Clock drift of the timekeeping circuitry in GNSS satellites
- Inaccuracies in the prediction of the exact orbital position of GNSS satellites
- General performance drift in the overall satellite equipment relative to other satellites, otherwise known as satellite biases
- Distortions and delays in signal transmission as it passes through the ionosphere and troposphere.
- Multipath reflection and variable performance and drift in the receiver
There are various techniques available to designers to correct for satellite-based and atmospheric GNSS errors.
Improving GNSS performance
The best way to minimize the impact of errors originating in the GNSS receiver is to use the highest performance receiver that fits the cost and size constraints of a given application. But even high-performance receivers are not perfect; their performance can very likely be improved. It is important to understand these correction methods since they offer varying performance, and some GNSS modules are not capable of implementing all of them.
Ground-based reference stations are used by several GNSS correction methods (Figure 1). The most established methods for using ground-based reference stations to provide GNSS corrections to receivers are Real-Time Kinematic (RTK) and Precise Point Positioning (PPP). More recently, hybrid RTK-PPP methods have become available.
Figure 1: A GNSS user receiver can get information about atmosphere, clock, and orbit errors from a reference network to improve positioning accuracy. (Image source: Septentrio)
RTK relies on a single base station or a local reference network for correction data that can eliminate most of the GNSS errors. RTK assumes that the base station and the receiver are closely located—a maximum of 40 kilometers (km) or 25 miles apart—so they experience the same errors. Post-Processed Kinematic or PPK is a variation on RTK and is widely used in surveying and mapping to obtain high-precision positioning data or centimeter-level accuracy.
Only orbit and the satellite clock errors are used to make PPP corrections. These satellite-specific errors are independent of the user’s location, which limits the number of reference stations that are needed. However, PPP does not account for atmosphere-related errors and therefore has lower accuracy relative to RTK. In addition, PPP corrections can have initialization times of about 20 minutes. The longer initialization time and lower accuracy makes PPP impractical for many applications.
Applications needing near-RTK accuracy and quick initialization times often employ the newest GNSS correction service, RTK-PPP (sometimes referred to as state-space representation (SSR)). It uses a reference network with stations spaced about 100 km (65 miles) apart that collects GNSS data and calculates a combination of satellite and atmospheric corrections. The reference network uses Internet, satellite, or mobile phone networks to send the correction data to subscribers. GNSS receivers using RTK-PPP can have sub-decimeter accuracies. The choice to use RTK, PPP, and RTK-PPP correction methods involves a series of design tradeoffs that developers need to review to select the optimal solution for the specific application profile. (Figure 2).
Figure 2: Strengths and weaknesses of three common GNSS correction methods. (Image source: Septentrio)
Satellite-based augmentation systems (SBAS) are starting to become available on a regional basis to replace the RTK, PPP, and RTK-PPP ground station-based correction methods. SBAS still uses ground stations to measure GNSS errors, but the stations are spread across entire continents. The measured errors are processed at a central location where the corrections are calculated and transmitted to geosynchronous satellites over the covered area. The correction data is broadcast from the satellites as an overlay or augmentation to the original GNSS data.
GNSS accuracy is dependent on the availability and the accuracy of satellite measurements and associated corrections. High-performance GNSS receivers track GNSS signals at multiple frequencies and use multiple GNSS constellations and various correction methods to deliver the needed accuracy and resilience. The resulting redundancy enables stable performance even if some of the satellite measurements and data experience interference. Designers can select from a variety of GNSS accuracy and redundancy capabilities (Figure 3).
Figure 3: GNSS accuracy grades with corresponding correction methods and selected applications. (Image source: Septentrio)
GNSS modules: integrated vs external antennas
Due to the complexity of multi-constellation positioning, modules are available from various suppliers that help accelerate time to market, lower cost, and ensure performance. That said, designers need to consider whether to use an internal antenna or instead opt for one that resides external to the GNSS module. For applications where time to market and cost are a priority, an integrated antenna may be preferable as significantly less engineering is involved. For applications that need FCC or CE certification, the use of a module with an integrated antenna can also speed the approval process. However, solution size can increase, and flexibility may be limited with integrated antenna solutions.
External antennas provide designers with a wider range of performance and layout options. A large high-performance antenna or a smaller and lower-performance antenna can be selected. In addition, antenna placement is more flexible relative to the location of the GNSS module, further enhancing design flexibility. Placement flexibility also allows external antennas to provide more reliable GNSS operation. However, antenna placement and connection routing can be a complex and time-consuming process and requires specific expertise, potentially increasing costs and slowing time to market.
Tiny GNSS module for space-constrained designs
Design teams with the required expertise in antenna placement and routing can use STMicroelectronics’ Teseo-LIV3F, a multi-constellation (GPS/Galileo/GLONASS/BeiDou/QZSS) GNSS module that uses an external antenna (Figure 4). The module comes in an LCC-18 package measuring 9.7 mm x 10.1 mm, and features 1.5 meter (m) circular error probable (CEP) position accuracy, with a time to first fix (TTFF) for cold and hot start as low as under 32 seconds (s) and under 1.5 s, respectively (GPS, GLONASS). It has a standby power consumption of 17 microwatts (µW) and a tracking power consumption of 75 milliwatts (mW).
Figure 4: The Tesco-LIV3F GNSS module includes the GNSS core & subsystems, plus all required connectivity and power management, in a package measuring 9.7 x 10.1 mm. It requires an external antenna. (Image source, STMicroelectronics)
The Tesco-LIV3F’s on-board 26-megahertz (MHz) temperature compensated crystal oscillator (TCXO) helps ensure high accuracy, and the dedicated 32 kilohertz (kHz) real-time clock (RTC) oscillator enables a reduced time to first fix (TTFF). Features such as data logging, seven-day autonomous assisted GNSS, firmware (FW) reconfigurability, as well as FW upgrades, are enabled by the 16 megabit (Mbit) embedded flash memory.
Applications suitable for the Tesco-LIV3F include insurance, logistics, drones, tolling, anti-theft systems, people and pet location, vehicle tracking, and emergency calls.
As a pre-certified solution, use of the Teseo-LIV3F module can result in a reduced time to market of the final application. It has an operating temperature range of -40°C to +85°C.
To experiment with the module and accelerate application development, designers can use the AEK-COM-GNSST31 evaluation board. When used in conjunction with the X-CUBE-GNSS1 firmware, the evaluation package can support acquisition, tracking, navigation, and data output functionality without external memory. This EVB is also designed for use with an SPC5 microcontroller for automotive application development.
GNSS module with interference mitigation
Septentrio’s 410322 mosaic-X5 multi-constellation GNSS receiver is a low-power, surface-mount module measuring 31 mm x 31 mm x 4 mm that provides designers with an array of interfaces, including four UARTs, Ethernet, USB, SDIO, and two user-programmable GPIOs.
Designed for use in robotics, autonomous systems, and other mass-market applications, the mosaic-X5 features an update rate of 100 Hertz (Hz), a latency of under 10 milliseconds (ms), and a vertical and horizontal RTK positioning accuracy of 0.6 cm and 1 cm, respectively. It can track all GNSS constellations, supporting current and future signals, and is compatible with PPP, SSR, RTK, and SBAS corrections. The module’s TTFF is under 45 s cold start and under 20 s warm start.
The mosaic-X5 features several Septentrio patented technologies, including AIM+, an onboard interference mitigation technology that suppresses a variety of interferers, from simple continuous narrowband signals to complex wideband and pulsed jammers.
The modules’ interfaces, commands, and data messages are fully documented. The included RxTools software allows receiver configuration and monitoring, as well as data logging and analysis.
Septentrio’s 410331P3161 mosaic-X5 development kit enables designers to explore, evaluate, and develop prototypes that take full advantage of the mosaic-X5’s capabilities (Figure 5).
Figure 5: Designers can create a prototype using the 410331P3161 mosaic-X5 development kit using a variety of connections, including Ethernet, COM ports, or USB 2.0, or by using an SD memory card. (Image source: Septentrio)
The kit uses the mosaic-X5’s intuitive web user interface for easy operation and monitoring, allowing designers to control the receiver module from any mobile device or computer. The web interface uses easy-to-read quality indicators to monitor the receiver operation.
Designers can create a prototype by integrating the mosaic dev-kit using any of the of following connections: Ethernet, COM ports, USB 2.0, SD memory card.
GNSS module with integrated antenna
For designers of applications that can benefit from the use of a GNSS module with an integrated antenna, Würth Elektronik offers the 2614011037000 Erinome-I module with a high-performance system-on-chip (SoC) (Figure 6). The module supports the GPS, GLONASS, Galileo, and BeiDou GNSS constellations, and comes with an integrated antenna on top that simplifies hardware integration and shortens time to market. The module, including the integrated antenna, measures 18 mm x 18 mm.
Figure 6: The 2614011037000 Erinome-I is a complete GNSS module with a high-performance GNSS SoC plus an integrated antenna. (Image source: Würth Elektronik)
Also included on the module are the TCXO, RF filter, low-noise amplifier (LNA), and serial flash memory.
Würth also offers the 2614019037001 evaluation board (EVB) for the Erinome-I (Figure 7). The EVB can also serve as a reference design for the integration of the GNSS module in an application. A USB port can be used to connect the EVB to a PC. A multi-pin connector gives designers access to all the pins of the GNSS module.
Figure 7: The 2614019037001 evaluation board for the Erinome-I (near the center of the board, with the integrated antenna visible in the center of the module) also acts as a reference design. (Image source: Würth Elektronik)
Würth Elektronik Navigation and Satellite Software (WENSS) is a simple PC tool to interact with the Erinome-I GNSS module using the UART interface. It supports:
- Control of EVB operation
- Bidirectional communication with the Erinome-I module
- Evaluation of Erinome-I features and capabilities
- Familiarization with Erinome-I protocols, sentences, and commands
- Configuration of the Erinome-I without knowledge of the protocols
- Parsing of sentences and commands used by the Erinome-I
WENSS enables easy evaluation of positioning applications without advanced knowledge. Experienced developers can also use WENSS for more advanced configurations.
Accurate and reliable positioning capability is best achieved using multiple constellations with associated correction technology support. These are complex systems, but designers can turn to pre-engineered GNSS modules, associated development kits, and environments to quickly and efficiently compare options and implement position-based features and services.
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