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How to trade-off location accuracy against battery life in cellular IoT asset trackers

Truck driver usin asset tracker

A global supply chain that was already under some strain has been placed under far greater pressure by the pandemic as locked-down consumers turned to online shopping in their hundreds of millions.

One knock-on effect is a shipping container bottleneck; there are 170 million of these standard modular boxes globally, yet it's almost impossible to find a spare one right now.

But the challenge has motivated logistics companies to up their game by turning to technology. The IoT is a key solution for supply chain problems because it forms a global network of connected devices that can monitor and track any cargo. Consequently, logistics providers can find out exactly where everything is, when it's due to move, and how it's being handled, making supply chains far more robust and reliable.

The cellular IoT option

Cellular IoT asset-tracking devices are finding favor with logistics companies because they are reliable, compact, and relatively inexpensive. Logistics firms can use a cellular IoT device to monitor the location and condition of valuable cargo. The asset tracker periodically reports its location to a cloud server along with other helpful information such as the temperature of a refrigerated container or whether a fragile load has been subject to a high-G impact.

Even when the cellular IoT device is out of range of cellular coverage, for example on a ship far from land, it can continuously record and store the cargo location and whether it has been subjected to damage like water ingress. Once back on dry land, the device can upload its data to the cloud server.

Read more: Building better asset trackers for monitoring fragile shipments

The importance of battery life

Another key advantage of cellular IoT is its power efficiency. For example, Nordic's nRF9160 SiP has been designed to take full advantage of the low-energy possibilities—such as PSM and eDRX power-saving modes—associated with the cellular LTE-M and NB-IoT (cellular IoT) standards. Because Nordic designs all the hardware and software, the SiP offers a highly efficient and optimized low-power cellular IoT solution.

This high efficiency allows asset-tracker manufacturers to build lightweight, compact devices with powerful processing capabilities that can run for months or even years from a single battery charge. Such devices lower logistics companies' maintenance requirements because the firms don't have to recharge or change device batteries frequently. This also benefits the environment as battery production and disposal are reduced.

Read more: Towards a battery-free IoT-world

However, even though cellular IoT devices like the nRF9160 SiP are highly efficient, the application ultimately determines battery life. The key to long battery life is to maximize the time the device spends in a very low power "sleep" mode. Activating cellular IoT and GNSS radio devices and intensive processor use quickly push up the average power consumption with a proportional decrease in battery life.

Read more: The Importance of Average Power Consumption to Battery Life

Location finding using the cellular network

There are several things an application developer can do to increase battery life. For example, he or she could reduce the frequency of cellular IoT device communication with the cloud. The precision with which the position of the asset tracker needs to be fixed has an even greater impact on battery life. Excellent accuracy cost more energy. Sometimes that accuracy is required, but there's little point in expending energy to locate the device down to a few meters if all the application needs to know is whether the goods have arrived in the destination city.

To assist developers in the quest for maximum battery life, Nordic's nRF Cloud Location Services solution offers several techniques for nRF9160 SiP location determination, each trading off more battery power for greater position precision.

The most battery-friendly technique uses the SiP's LTE-M/NB-IoT modem to find out the nearest cellular base station's ID and then reference that against a known base station locations database. This technique offers precision down to kilometer level with minimal battery drain. Multi-cell location builds on the single-cell technique by referencing the position of several nearby base stations to trilaterate position down to a few hundred meters while still keeping power consumption low.

Location finding using GPS

In addition to LTE-M/NB-IoT, the nRF9160 SiP incorporates GNSS. GNSS includes the U.S.'s GPS as well as Russia's GLONASS, China's BeiDou, and Europe's Galileo. GPS can be used, for example, to ascertain location with a precision of a few meters. The downside is that a cold-start 'time-to-first-fix' (TTFF) on a group of satellites can take several minutes and use significant battery capacity.

But to shorten TTFF, satellite assistance data can be sent down to a ground-based GPS receiver and stored in a GPS database. This Assisted-GPS (A-GPS) data can then be relayed to the cellular IoT device via the LTE-M network at a rate up to 3000 times faster than it's delivered from the satellite itself. The rapid transmission means the device can quickly revert to a sleep mode, saving significant power. The IoT device can then find the satellites in seconds when required, saving even more power.

Predictive-GPS builds on A-GPS by providing over two weeks' assistance data to the IoT device. The result is a short TTFF and accurate location determination but with even greater power savings, resulting from lower assistance data requests to the cellular network compared to A-GPS.

Using P-GPS does increase the computational load on the IoT device's processor. Still, in the case of the nRF9160 SiP, the processor is highly efficient, so the additional operation time has only a small impact on battery life. nRF Cloud Location Services offers customers a combination of either A-GPS or P-GPS or a combination of both.

Cellular IoT asset tracking is on the rise. Analyst Research and Markets, for example, believes the asset tracking sector will be worth $35 billion by 2026, while another analyst, ABI Research, estimates 80 percent of the industry will use cellular IoT connectivity. That's a lot of asset trackers and a lot of batteries, making it doubly important that engineers maximize the efficiency of their solutions.

Read more: Cellular IoT asset tracking applications gain momentum


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