Asset tracking is big business. Knowing where valuable items are at any time with a high degree of accuracy increases productivity, enhances security, and lowers cost. However, asset tracking has been limited to high value items because high-quality Global Navigation Satellite System (GNSS) receivers can be difficult to design and implement, expensive, and power hungry.
The introduction of modular GNSS solutions that are specifically designed to extend asset tracking to a much wider range of applications by offering compact, relatively inexpensive, and low power consumption alternatives are much easier to implement. Still, there are many module options to pick from, and they are all sensitive to power supply quality. In addition, battery-operated end products require particular attention to a module’s low-power modes.
This article introduces some GNSS modules and explains how to use them as the basis for real time location systems (RTLS). The article shows how the modules are set up for maximum sensitivity, communication with the host microcontroller, rapid satellite acquisition, and optimal power efficiency.
The basics of GNSS
GNSS RF receivers take advantage of satellite constellations such as the United States’ GPS, Russia’s GLONASS, or Europe’s Galileo, to allow outdoor, mobile assets to be located with an accuracy of a few meters or better.
GNSS works by picking up the unique navigation signal from at least three satellites in the orbiting constellation. Synchronization between satellites and receivers enables the respective signal propagation delays (and hence distances) from the satellites to be determined. The intersection of the satellites’ signal spheres (with radii equal to the calculated distance) and the Earth’s globe determines the receiver’s precise location.
Each satellite transmits low-power RF signals comprising the satellite’s identification, ephemeris data (detailing the satellite’s current and future orbital position), and its status.
The receiver accuracy is determined by the accuracy of its synchronization with the satellite clocks; an error of 1 nanosecond (ns) can cause a positional error of 30 centimeters (cm).
A key operational parameter of the receiver is time-to-first-fix (TTFF). A cold start TTFF, whereby the receiver starts up with no signal or previous positional information, can take up to a minute. If the receiver has not been moved since it was shut down, and still has the previous positional information in the onboard memory, it can perform a warm start which results in a TTFF of around 25 to 30 seconds (s). A hot start occurs when the receiver has very recent positional information in its memory and can therefore accurately predict satellite positions and results in a TTFF of just 1 s.
Because GNSS satellite transmissions are weak, high receiver sensitivity is important. Signal acquisition time improves once the receiver has located the satellites because if the signal is obstructed, the receiver can anticipate the satellite’s next position to regain the signal without having to scan wide swathes of the sky.
New modules ease design
Designing a GNSS system from scratch is a complex business. If the developer has the expertise, then such a strategy could possibly lead to a more differentiated end product in terms of cost, size, and performance, but for the less experienced a module is often a wiser alternative. Modules are assembled, packaged, tested, and (typically) verified units that can be dropped into the end product. Design complexity is eased because the modules eliminate the need for the engineer to develop complex RF circuitry from scratch.
Better yet, a new generation of GNSS modules has been designed to target the kind of applications previously impractical because of the drawbacks of traditional GNSS solutions. Examples include wearable devices, smart watches, and asset tracking of relatively low value items.
Key factors the designer should consider when selecting a module are:
- Price: This varies according to volume and other factors.
- Positional accuracy: Better positional accuracy relies on a superior clock. The designer should match the accuracy to the requirement of the application.
- TTFF: The time taken to acquire satellite signals and calculate position when the module is first activated. Some modules boast algorithms that predict the orbital position of satellites for up to a month in the future, greatly shortening TTFF from a cold start.
- Size: Compact dimensions allow the module to be incorporated into small form factors.
- Power consumption: Solutions for applications such as asset tracking will typically run from small batteries. Low power consumption will extend battery life.
- RF sensitivity: High initial sensitivity is demanded to pick up relatively weak GNSS signals during a cold start. Sensitivity increases once a satellite has been acquired and is being tracked by the receiver.
- RF interference immunity: Depending on the host system, GNSS operates in the 1176.45 to 1602.0 MHz range which sits adjacent to GSM frequency allocations. Good GSM band rejection is required to ensure good reception of the GNSS signals.
Antenova’s Radionova M20050 module is a good example of this new generation of GNSS modules (Figure 1). The module is a drop-in GNSS receiver, operates at 1.575 GHz, measures 13.8 x 9.5 x 1.8 mm, and can run on three different GNSS systems simultaneously to enhance TTFF and location accuracy. The module operates off a 2.8 to 4.2 volt power supply and features several low-power modes to extend battery life. The TTFF is less than 35 seconds from a cold start, and cold start sensitivity is -148 dBm (Figure 1).
Figure 1: Antenova’s Radionova M20050 GNSS module features an internal antenna and operates at 1.575 GHz. (Image source: Antenova)
The device uses the NMEA 0183 data output protocol, a proprietary protocol controlled by the U.S.-based National Marine Electronics Association. The protocol uses simple ASCII serial communications, is widely used in the industry for serial connectivity with GPS receivers, and has been adopted by navigation software such as Google Maps and Microsoft MapPoint. The M20050 exports position data via a UART interface at a default 9600 baud rate.
Linx Technologies offers its TM Series of GNSS receiver modules. The devices operate at 1.575 and 1.602 GHz from a 3.0 volt to 4.3 volt power supply. TTFF is less than 33 seconds from a cold start, with cold start sensitivity of -147 dBm. The device uses the NMEA 0183 protocol through a UART interface that also has a default rate of 9600 baud.
The device’s core handles all the necessary initialization, tracking, and calculations autonomously, so no programming is required. The RF section of the TM Series is optimized for low level RF signals and requires no production tuning.
Telit also offers a GNSS module solution in the form of its Jupiter JF2. While the company lists the device as a GNSS product, it only supports the U.S.-based GPS system, although a GLONASS unit is also available. The Telit module is based on the SiRFstarIV™ core, is powered from a 1.8 volt supply, and comes in an 11 x 11 x 2.6 mm package. The TTFF is less than 35 seconds from a cold start, and has a cold start sensitivity of -147 dBm. A key advantage of the Telit product is that it is interoperable with the company’s cellular modules making it easier to design a combined GPS/cellular solution (Figure 2).
Figure 2: Telit’s Jupiter JF2 module is one of the smallest integrated GNSS receivers even though it also combines cellular capability. (Image Source: Telit)
While the modules from Antenova, Linx Technologies, and Telit simplify the design of location tracking systems, it is not a case of soldering the module onto a printed circuit board, adding power and waiting for the first fix. The main design steps required for a working system are antenna selection (or antenna tuning if the module has an internal antenna), a power supply, pairing with an appropriate microprocessor, and programming.
Some modules are supplied with a built-in antenna, but others leave the choice to the developer. A built-in antenna removes another design step but will inevitably be a “one-size-fits-all” solution. Leaving the choice of antenna to the designer allows better matching of antennas to the applications.
For example, if the module is fitted to a handheld device, the antenna will be presented to the sky in various orientations, so an antenna with a wide and uniform pattern may yield better overall performance than one with higher gain but a narrower beam. Employing a module without a built-in antenna, such as Linx Technologies’ TM Series, allows for experimentation to match the application.
For GNSS applications, the antenna requires good right hand circular polarization characteristics to match the polarization of the satellite signals. Ceramic patches are the most commonly used style of antenna, but many other shapes, sizes and styles are available.
GNSS applications can employ passive or active antenna. The active type operates with a low-noise amplifier (LNA) to improve sensitivity. When using an active antenna, it’s good practice to add a 300 ohm (Ω) ferrite bead to connect the VOUT line to the RFIN line. This bead blocks RF interference from the power supply, while allowing a DC voltage onto the RF trace to feed the antenna. A series capacitor inside the module prevents the DC voltage from affecting the bias on the module’s internal LNA.
The key parameter that determines a good RF circuit from a poor one is its impedance (Z). Care must be taken such that the pc board layout of the end product maintains a 50 Ω impedance path between the module and antenna. Manufacturers typically provide layout guidelines in their module datasheets to assist in matching impedance and antenna board clearance to maximize sensitivity (Figure 3).
Figure 3: Antenna clearance, facilitated by a pc board area free of traces, is important to maximize sensitivity and improve TTFF. (Image source: Antenova)
Modules with an internal antenna, such as Antenova’s Radionova M20050 device, may require some external tuning to maximize sensitivity and accelerate TTFF. This is not overly complex, typically requiring the addition of a few passive components to compensate for slight antenna detuning caused by board components adjacent to the module. In the case of the Radionova M20050 module, Antenova has simplified the process by adding “AT1” and “AT2” inputs to the module to which the appropriate resistors and inductors can be connected to tune the internal antenna (Figure 3, again).
Each of the Antenova, Linx Technologies and Telit modules must be connected to a suitable microprocessor for control and configuration. The microprocessor requirements are typically modest and a mid-range, 16-bit device is up to the job. Most GNSS modules communicate via a serial GPIO or UART, so ensure the selected microprocessor has one, or both.
The microprocessor typically uses the NMEA protocol mentioned earlier to communicate with the GNSS module. The protocol defines three types of inputs/outputs: Commands, Write, and Read messages. The modules output a response for each input/output. Commands are used to change the operating state of the module. Write messages change the module’s configuration, and Read messages detail the current configuration.
Inputs are sent to the receiver on the RX line and outputs are sent from the receiver on the TX line. By default, output messages are sent once every second. The protocol allows for both standard and proprietary inputs/outputs (Figure 4).
Figure 4: The microprocessor communicates with the GNSS module via a GPIO or UART, TX and RX connections. Note the use of a 300 Ω ferrite bead to limit RF interference from the power supply to the active antenna.) (Image Source: Linx Technologies)
The message structure of the NMEA protocol is straightforward, for example:
“command-ID[,parameter1,parameter2,...,parameterN]<cr><lf>” for commands;
“message-ID,<data1,data2,...,dataN>*<checksum><cr><lf>” for messages.
The message ID for standard messages begins with an NMEA ‘talker’ ID: “$GP” indicates GPS, “$GL” indicates GLONASS, and “$GN” indicates global navigation. The GNSS module typically echoes the command back out to the host processor after the command has been executed.
“$GPZDA,183746.000,22,08,2017*56<cr><lf>”, for example, is a GPS message detailing the universal time and date (18:37:46.0 on 22 August 2017).
Many module makers, such as Telit, opt for proprietary schemes for commands (Figure 5). (“$P…” indicates this is a proprietary scheme, and “…STM…” is a manufacturer’s ID, in this case the STMicroelectronics chip used in Telit’s Jupiter JF2 GNSS module.)
Figure 5: NMEA protocol proprietary commands for Telit’s Jupiter JF2 GNSS module. (Image source: Telit)
“$PSTMCOLD,0x02<cr><lf>” performs a cold start and (optionally) clears ephemeris data;
“$PSTMINITTIME,23,02,2018,09,44,12<cr><lf>” initializes the current GPS universal time to 9:44:12 on 23 February 2018.
The module manufacturers have eased the development process by offering evaluation kits that incorporate the GNSS module and include an antenna, or antenna tuning if the module features an internal antenna. Antenova, for example, offers the M20048 RF module evaluation board, and Linx Technologies offers the MDEV-GNSS-TM development kit for the TM Series. In neither instance is a microprocessor required for evaluating the module as the evaluation kit connects directly to the USB port of a PC, with the PC then supplying the input messages and supervising operation (Figure 6).
Figure 6: Antenova’s M20048 evaluation kit includes a micro USB interface to allow control and configuration from a PC. (Image source: Antenova)
Powering GNSS modules properly is critical
Because they are required to detect weak RF signals, GNSS modules demand a clean, stable power supply to ensure a high signal-to-noise ratio (SNR). The specification varies depending on the selected GNSS module, but generally speaking, peak noise should be kept to less than 20 millivolts (mV) to avoid problems.
While some GNSS modules include an onboard voltage regulator, it is advisable to employ an external primary regulator to supply the module. If a switch-mode voltage regulator is selected to increase efficiency and extend battery life, the designer should consider teaming the device with a low dropout (LDO) linear regulator to limit noise at the GNSS module’s voltage input.
If efficiency is less of a challenge, an LDO regulator alone is a good choice as the bill of materials is lowered and the regulated output is cleaner than that of a switching regulator. It is also good design practice to employ filter circuits to clean up the input voltage regardless of voltage regulator choice.
GNSS modules (for example, the M20050) often include two voltage inputs, one to power the main digital and processing circuitry and a second to provide backup to the RAM and clock.
In addition to controlling and configuring the GNSS module, the host microprocessor manages power supply modes to help extend battery life. For example, the M20050 module supports three such modes: standby, backup, and periodic. Standby shuts down the RF section of the module and puts the processor on standby. The clock and RAM remain powered to maintain module configuration.
Backup mode is entered when the main voltage input is switched off. This mode is designed to aid rapid TTFF when the module awakens from a power saving (sleep) mode because historical ephemeris data is retained in the module’s RAM.
Periodic mode reduces current consumption by waking the module for short durations to reestablish a satellite fix and then return to a sleep mode. This mode is useful for accelerating TTFF because ephemeris data is periodically refreshed rather than just retained (Figure 7).
Figure 7: Periodic mode enables frequent refresh of ephemeris data to ensure rapid TTFF when the GNSS module is activated. (Image source: Antenova)
The proliferation of international satellite-based navigation systems has increased the accessibility and accuracy of GNSS for precise location tracking. While the design of GNSS enabled systems is not easy, GNSS modules greatly simplify matters by providing an assembled, tested and verified solution for asset tracking.