How to optimise asset tracking with Wi-Fi locationing

By Finn Boetius, Product Marketing Engineer - Wi-Fi & Matter, Nordic Semiconductor

This article originally appeared in the April'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

One tech solution is the Global Positioning System (GPS) – the principal component of Global Navigation Satellite System (GNSS). Billions of people already rely on GNSS daily to help them find  their way around, and it’s now also providing a foundation for many IoT applications to help keep  track of valuable assets that might otherwise go missing.

Made up of 24 satellites positioned around 20,000 kilometres above the Earth, the GPS constellation  is arranged to ensure four satellites can be observed at any point on the planet. These satellites  transmit their locations, status, and the precise time from onboard atomic clocks to an earthbound GPS receiver. The receiver notes the arrival time of the signal and then calculates the distance to  each satellite (using the time difference between signal transmission and reception, and then  multiplying by the speed of light). Information from four satellites can then be used to pinpoint the receiver’s position on the surface of the planet. Attach the receiver to the valuable cargo and you’ll  always know where it is.

The challenges of GNSS

Well, perhaps not quite always. Despite its sophisticated design, GNSS is still subject to several  issues. For instance, inaccuracies with the onboard clocks can cause timing errors. To combat these  issues, GNSS systems compare multiple satellites and use algorithms to determine which clocks are  in error and then reset them.

Other problems occur because the relatively weak signal between satellites and earthbound  receivers can easily be disrupted. With an average GPS signal strength at Earth’s surface of just - 130dBm (as reported by gps-repeaters.com), obstructions such as rows of tall buildings – known as  ‘urban canyons’ – can block the transmission.

And even in instances when the data is successfully transmitted, so-called multipath errors can  occur. These involve the signal reflecting off buildings before reaching the receiver, causing timing errors and leading to incorrect positional information. Other errors can occur because of density  variations in the Earth’s atmosphere, which can delay or distort the GNSS signal.

Electromagnetic interference (EMI) from other radio sources can also cause timing errors. To  mitigate these problems, techniques such as filtering, correlation, and signal power measurement,  as well as ionosphere and troposphere modelling, are employed. But despite all this technology there are times when GNSS just doesn’t work, and logistics companies need alternatives to keep track of everything.

Maximising battery life with Wi-Fi SSID scanning

Another downside of GNSS is that it can take several minutes to determine the initial location of a  tracked item – taking a heavy toll on batteries. Some relief for the power source can come from using technologies such as Assisted- and Predicted-GPS (A-GPS and P-GPS). These methods, supported by Nordic’s nRF Cloud Location Services in conjunction with the company’s nRF9160 SiP, a  multimode cellular IoT solution, use satellite assistance data stored in a database. This information is transmitted to the nRF9160 via the LTE-M or NB-IoT network, speeding up the first satellite fix and saving power.

But even with A-GPS and P-GPS, the battery requirements for GNSS remain significant. This becomes  important consideration for certain applications, such asset trackers that are typically equipped with  smaller batteries while still requiring long lifetimes. If you need precision down to few meters, only GNSS will do, but if accuracy of tens of meters is acceptable, there are alternatives that can lead to reduced power requirements.

One such locationing technique, which provides a reasonably precise alternative to GNSS while also consuming less power, is Wi-Fi Service Set Identifier (SSID) scanning. Every Wi-Fi access point (AP) network is identified with an SSID – a technical reference for the AP’s name. With knowledge of the network’s SSID, it’s possible to cross reference against databases that will detail its location.

Wi-Fi chips such as Nordic’s nRF70 Series of companion ICs devices can use the technique to  determine location by scanning any nearby Wi-Fi AP for its SSID. In the Nordic case, a partner  nRF9160 SiP can then forward the SSID (and other useful information) to nRF Cloud using cellular connectivity. nRF Cloud then checks one or more Wi-Fi SSID databases and returns the tracked item’s location based on the position of nearby APs, plus a degree of uncertainty for that location, either to the nRF9160 or elsewhere as directed.

Striking the right balance between precision and battery life with Wi-Fi locationing

While it’s hard to beat the accuracy of GNSS, when a slightly lower precision is acceptable and battery life is critical, or in asset tracking applications where the GNSS signal is likely to be interrupted, Wi-Fi SSID locationing is an excellent alternative.

With Nordic’s nRF91 and nRF70 Series, and nRF Cloud, it’s simple to switch seamlessly between GNSS and Wi-Fi locationing methods in an asset tracker to trade-off location precision against battery life to best suit the situation. There is now no excuse for the loss of expensive aircraft parts or any other valuable assets.