Satellite positioning, with its global availability and high accuracy, is a key enabler for the growing number of consumer, industrial, and automotive tracking applications. At the same time, users still have higher expectations for satellite positioning technology in terms of performance, size, power consumption and cost. To optimally balance these features to meet the needs of a particular application or use case, a deep understanding of the technologies, hardware, software, and services used is required.
Best performance with lowest power consumption
GNSS technology is able to provide highly accurate location, speed and time data around the world, so it has a place in the emerging smart connected solutions. At the same time, users are increasingly expecting GNSS technology, and to stand out in today's competitive market, GNSS devices need to outperform their competitors in terms of accuracy, responsiveness, and power consumption, which often puts a lot of pressure in terms of cost reduction and size.
Particularly in consumer, industrial, and vehicle tracking use cases, Global Navigation Satellite System (GNSS) receivers have long been considered power consumers. Back in 2010, single-band receivers consumed slightly more power than 120 mW in continuous tracking mode. By 2015, power consumption had dropped to around 70 mW. Today, thanks to technological improvements, tracking applications consume only 25 mW of power.
Over the past five years, the power consumption of GNSS technology has climbed to new heights. Today's low-power GNSS receivers can track more satellite constellations, each supporting multiple frequency bands, providing higher positioning accuracy at faster speeds and lower power consumption. In some use cases, the power consumption of a GNSS receiver can be reduced to less than ten percent of the end device's power budget.
However, achieving this low-power target is challenging. Today, advanced GNSS receivers typically offer a series of settings that allow users to configure themselves to optimize power consumption while meeting the performance requirements of specific use cases.
In this article, we outline some basic design considerations that enable GNSS receivers to consume only a fraction of the power consumption budget of a standard GNSS solution. The design factors that need to be considered for a particular use case will depend on how the user chooses between precision, dynamic performance, size, cost, and more.
Use cloud-based positioning technology to track the distribution of power between cellular communications and GNSS.
Use cases determine power saving options
Which power-saving options product developers can use to balance the accuracy, performance, size, and cost of GNSS receivers largely depends on their application scenario. For example, sports watches are small in size and typically only have small antennas and batteries, so they require relatively high 1 Hz update rates and have demanding requirements for GNSS performance. Most sports watches only offer limited internet connection (via smartphones).
Logistics Cargo Trackers are less demanding in terms of update rates and GNSS receiver performance, but require months of operation on small batteries. In between are handheld devices and pet and child trackers.
Car trackers are a class of their own, with features including low size restrictions, the ability to power them from a power source, and the use of cellular data for continuous communication.
The following table outlines the common limitations of the five classes of end devices, and the specific limitations of a given end device will depend on the specific requirements specified by its use case and may differ from the examples provided below.
Typical limitations of five common types of end devices.
Performance and power consumption
As we have seen, the power consumption of GNSS receivers is determined by a variety of factors. Most of the power is consumed during start-up (satellite signal capture), which typically consumes about 20% more than continuous satellite tracking, so the main drivers of power consumption are related to satellite signal capture and continuous satellite tracking.
In addition to the size and cost of the end device, the performance requirements of the application for GNSS also limit the options available to reduce power consumption. As we've seen, each of these options has its own set of trade-off rules that developers need to carefully consider to fully meet the needs of the target application.
Usability and precision
While some use cases always require highly accurate GNSS positioning, others are less demanding. Today's state-of-the-art GNSS receivers offer multiple ways to balance the need to position availability and accuracy against power consumption.
The first method involves selecting the number of GNSS constellations to track. Tracking multiple GNSS constellations at the same time increases the number of visible satellites at any given time, thereby increasing the robustness of positioning readings. In poor signal environments, such as deep urban canyons or under forest canopy, this approach can significantly improve the availability, accuracy, and reliability of location services.
In terms of power consumption, tracking more GNSS constellations forces the receiver to spend more time acquiring satellite signals, which in turn increases power consumption. In addition, receivers need additional RF paths to capture signals when tracking GNSS signals in different frequency bands, which also increases power consumption.
The second approach focuses on antenna selection, that is, selecting antennas that meet the performance requirements of the final application. Active antennas use low-noise amplifiers (LNAs) to increase the gain of the input RF signal, especially in weak signal scenarios such as improper antenna placement or indoors, where active antennas are critical to capturing weak satellite signals in noise. However, active antenna LNA consumes considerable power to achieve the desired antenna gain, and such increased sensitivity is usually at the expense of increased power consumption. Therefore, for low-power use cases that require excellent positioning availability and accuracy, large passive antennas can be used (the size of the end device needs to be compromised).
Trace mode
While some use cases require continuous tracking, others only need to read positional data no more than once per minute, hour, or even per day. The position update rate determines the time interval between position calculations and can significantly affect the power consumption of the GNSS receiver.
On this plot, the continuous tracking mode consumes the most power. In applications that combine GNSS positioning with LTE cloud connectivity, continuous tracking mode also presents the challenge of rfetching between the transmitted LTE signal and the GNSS signal received in a nearby frequency band.
By effectively mitigating RF interference, the power consumption of GNSS receivers can be reduced by preventing signal loss from forcing the receiver back into the high-power signal capture phase. Mitigating RF interference requires an upfront investment in a precision board design and may require additional filters, which increase signal attenuation in the RF path. When using a passive antenna setup, you may need to add a low noise amplifier (LNA).
For applications that do not require continuous tracking, many GNSS receivers offer a power-down mode (PSM) that significantly reduces power consumption by limiting the GNSS receiver's tracking capabilities between position calculations.
There are many types of PSMs, each with different advantages and disadvantages. To strike the best balance between receiver performance and power consumption, developers need to choose wisely the PSM that best meets the needs of their use case.
Hardware-based power optimization strategy
Designers of tracking solutions can use a full set of design strategies to optimize the power consumption of devices. Because each decision affects not only power consumption, but also the performance, size, and cost of the tracking solution, solution designers must carefully weigh the pros and cons of each strategy to find the lowest power configuration that provides the required tracking performance.
Optimal component selection
Selecting low-power components (LNA, crystal oscillator, real-time clock) will slightly improve the total power consumption during GNSS receiver operation.
power supply
Although GNSS devices operate over a narrow voltage range, they have an optimal voltage at which power dissipation is minimized. The components chosen to provide the correct voltage also affect the overall power consumption of the GNSS receiver: a low-dropout regulator (LDO) - a solution that is less costly but consumes power in the form of heat, thus not conducive to optimizing power consumption. Switch-mode power supplies (SMPS) are more efficient. However, this power supply uses coils, which can cause unnecessary RF interference at the front of the antenna.
Spare battery
A power interruption causes the GNSS receiver to lose its positioning as well as all downloaded time and GNSS track data, so the GNSS receiver is forced to perform a full cold boot when power is restored. By saving this data in spare RAM, GNSS receivers with battery backup can recover from power interruptions more quickly, saving power. When the position update period is longer than two hours (roughly corresponding to the effective time of the ephemeris data), the backup battery becomes redundant, so the backup battery can be removed, thereby reducing power consumption.
Real-time clock (RTC)
In the event of a power outage, the built-in real-time clock allows the GNSS receiver to start up faster after the primary power is restored. Therefore, the power consumption can be reduced by using this device. However, RTC requires a battery as a backup power source, so it increases the size and cost of the device.
RF path
In the case of sufficient signal strength, the power consumption of passive antennas is less than that of active antennas. In use cases where higher sensitivity is required, the use of active antennas may not increase the gain of the RF path. In such cases, an active antenna with external LNA control can turn off the LNA (when not using GNSS) instead of always remaining on. By adjusting the RF path gain based on environmental conditions, an LNA with internal power settings can reduce the power consumption required to provide the necessary GNSS sensitivity and accuracy.
Crystal oscillator
Although the crystal oscillator outputs a stable frequency signal with lower power consumption, temperature fluctuations affect its frequency, which in turn affects the sensitivity and first positioning time of the GNSS receiver, and therefore increases power consumption. Temperature-controlled crystal oscillators (TCXO) can solve the problem of temperature sensitivity and reduce positioning power consumption, but consume slightly more power in continuous operation. The choice of crystal oscillator depends on the desired positioning performance and the specific combination of components used. Use cases that rely on small antenna designs or are expected to operate in weak signal environments can improve the sensitivity of GNSS receivers by selecting ACXO.
memory
In some trackers, GNSS receivers are equipped with flash memory for firmware upgrades or data storage on the device. However, by storing data in the host's storage, a device can shorten its bill of materials and reduce power consumption.
Design considerations when selecting crystal oscillators.
Firmware-based power optimization strategy
In addition to the hardware-based power optimization strategies proposed in the previous section, product developers can use a range of firmware-based power optimization strategies.
Although power optimization strategies vary from vendor to vendor, they generally fall into the four types shown below.
Update rate
The update rate is the first factor to consider when seeking to reduce the power consumption of the GNSS receiver. In continuous tracking mode, most GNSS receivers support an update rate of 10 Hz or higher. Some common use cases only require locations to be updated every minute or daily. Therefore, GNSS receivers can significantly reduce power consumption by reducing the update position rate to meet the practical requirements of the use case and allowing it to enter power-saving mode (PSM) during the update interval.
Multi-constellation multi-band GNSS
Tracking multiple GNSS satellite signals simultaneously can significantly impact power consumption, especially when receiving signals from different frequency bands, which is also rewarding: especially when sky views are limited or small antennas are used, tracking more GNSS constellations improves positioning availability. In such poor signal environments, positioning accuracy can be improved by mitigating multipath effects and receiving signals from multiple frequency bands (L1, L2, L5). In addition, carefully select the constellation to track based on the geographic location of the constellation, and devices with geographical limitations can reduce power consumption without compromising performance.
When high positioning accuracy and short capture time (TTFF) are critical to the application, GNSS receivers need to download the ephemeris data of the tracked satellite every 30 minutes, and the more constellations the receiver tracks, the more frequent the download operation. Since the GNSS receiver needs to remain on to download ephemeris data, this operation is only possible in continuous tracking mode, in which case the receiver cannot save power.
Tracking more GNSS constellations will improve location availability
Power-saving mode
Advanced GNSS receivers offer one or more power-down modes (PSMs) to maintain high performance while reducing power consumption (compared to continuous tracking mode). Each PSM balances power consumption and GNSS receiver performance differently, so it is critical to use the correct mode based on the constraints of your application.
Continuous tracking mode
In continuous mode, the GNSS receiver first acquires its position, then determines the position correction and downloads the almanac and ephemeris data. Once this is done, the receiver switches to tracking mode to reduce power consumption, and the receiver remains in tracking mode unless it loses its position. For applications that require several updates per second, continuous tracking mode provides the perfect balance between performance and power consumption.
Loop tracking mode
Some GNSS receivers have a power-down mode that reduces calculations in short intervals of less than ten seconds to save power. Cyclic tracking mode is ideal for use cases where the signal is strong enough or the antenna is large enough: if the signal is too weak, the receiver will revert to normal tracking mode with higher power consumption.
Super-E mode
Super-E mode is a proprietary power saving mode on u-blox GNSS receivers that is essentially an optimized cyclic tracking mode. By minimizing the resources required during tracing, Super-E mode reduces overall power consumption with little to no impact on performance. When the signal is weak or only a few satellites are visible, full power mode is activated to maintain positioning performance. When there is a sufficient and sufficiently strong satellite signal, the power-saving Super-E mode is activated. Tests have shown that super-E mode saves power up to three times more than standard u-blox 1 Hz full-power mode in open environments with minimal impact on positioning and speed accuracy.
On/off operation
Some GNSS receivers can switch between the capture/track phase and the sleep phase, a feature called an on/off operation. During the sleep phase (off), the receiver consumes very little backup battery power. For situations where sleep is long, this mode is a wise choice. As is the case with cyclic tracking mode, on/off operation requires a strong satellite signal at the RF input to minimize the time (and power) required for the first positioning after each "off" cycle.
Cloud positioning
Another popular way to save power with GNSS receivers is to leave the high-energy calculations involved in calculating location output to the cloud. In cloud positioning, GNSS receivers perform the reception and signal processing of GNSS signals, while handing over positioning estimation processing to cloud services. Although this operation requires an Internet connection, it can reduce the power consumption on the GNSS receiver to one-tenth.
Cloud positioning is an ideal solution for use cases where GNSS receivers can sleep for long periods of time (for example, only a few location updates per day) and where the device itself does not need to use location information. With an update cycle of one hour or less, the power required to establish cellular communications and transmit data to the cloud outweighs the benefits of putting the receiver to sleep.
For setups that use LTE-M to connect to the Internet, the GNSS receiver consumes approximately 10 percent of the power, with the remaining 90 percent consumed by the cellular modem.
The following overview provides rough guidance for determining the most appropriate power saving mode based on the update rate.
For update rates between once per minute and once per hour, you may need to calculate the energy consumption to determine the most appropriate power saving mode. As we'll explore below, using auxiliary GNSS to save power during cold start is also an effective way to do so.
Rough guidance for determining the appropriate power-saving mode based on the update rate.
Auxiliary GNSS
The most obvious way to save power is to cut off the power to the device. However, shutting down the device completely forces the receiver to perform a cold boot the next time it is turned on, which involves a capture phase of about 30 seconds required to acquire the first location — the first positioning time (TTFF). If the RF signal is weakened due to challenging environments, antenna size, or antenna placement, this phase can take several minutes. GNSS receivers often try to complete the first position as quickly as possible, which affects the accuracy of the first position.
A good way to significantly reduce TTFF is to use a secondary GNSS service that provides the satellite system's ephemeris, almanac, and accurate time and satellite status correction data. Secondary GNSSs come in many forms, including ancillary data that is downloaded in real time over the Internet or downloaded in bulk for a few days at a time. u-blox also offers an autonomous mode in which GNSS orbit prediction is calculated directly by the GNSS receiver itself, without the need for external auxiliary data or communication.
Benefits of secondary GNSS
The data in the table are obtained under good signal conditions (-130 dBm). If the RF signal level is low, the capture time is longer. Good antennas and careful antenna placement are important for optimal results.
The shorter the capture phase, the less power the GNSS receiver consumes. To benefit from continuous communication, online assistance is the best option, followed by offline and autonomous assistance, keeping in mind that autonomous assistance requires occasionally turning on the GNSS receiver for a few seconds to a minute in order to download ephemeris data.
Three common key features of auxiliary GNSS
Data batching
The number of messages transmitted between the GNSS receiver and the host MCU increases the processor load on both sides, so transmitting only basic messages helps reduce power consumption. GNSS receivers typically offer two configuration options: the user can choose which messages should be sent and set the update interval for continuous message delivery.
Another way to reduce host-side power consumption is to have the GNSS receiver collect a certain amount of data before it starts transmitting it to the host, which is called data batching. Because the host consumes significantly more power than the GNSS receiver, bulk data transfer from the GNSS receiver to the host MCU allows the host to go into hibernation as much as possible, thereby reducing power consumption (see figure below).
Compares the power consumption of continuous messaging (left) and data batching (right).
The best strategy for every type of application
The following table provides guidance on the hardware and firmware options that apply to each of these set of use cases. The recommendations made apply to the general implementation of each use case and outline the factors that product developers must consider when optimizing their tracking device power consumption. Rather than providing explicit recommendations, the guidance is intended to provide a starting point for discussions with component suppliers. The best settings can only be finalized after considering the precise specifications of the target use case.
Example of a sports watch
Cyclic tracking mode and data batching can save a lot of power. Offline auxiliary GNSS data can be downloaded after communication is established, which will reduce startup time, but require battery backup and a real-time clock. For most sports watches, using TCXO and receiving signals from 2 GNSS constellations simultaneously is the best trade-off for both power consumption and performance.
Logistics cargo tracker
Logistics cargo trackers prioritize long battery life over positioning accuracy and reduce the bill of materials to a minimum for small size and low cost.
Car Tracker
Car trackers are usually connected to the car battery via the OBD interface. Trackers that rely on small embedded antennas often struggle to obtain RF signals. LNA and multi-GNSS in combination with auxiliary GNSS can help cope with this situation.
summary
As more consumer, industrial, and automotive tracking applications begin to leverage satellite positioning, customer expectations for the performance, size, cost, and power consumption of their solutions are higher than ever. Since these four factors are closely related, any measure to reduce power consumption will affect the performance of the product.
Today's most advanced GNSS receivers typically offer multiple ways to optimize power consumption while meeting use case-specific performance requirements. Exactly which design considerations and equipment configurations apply to a particular use case will depend on how the developer balances these four conflicting factors.
By Bernd Heidtmann, Product Manager, Product Strategy, U-blox AG Standard Precision GNSS