Fighter aircraft are one thing, but the challenge of knowing where everything is becomes vastly more complex when scaled up to myriad items being transported across vast distances. For example, some 68 million containers are used to move cargo around the world (according to statista) and any one could contain millions of dollars of manufactured goods or materials. Keeping track of these assets is a logistical nightmare for companies. Wireless tech is providing the answers.
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 kilometers 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.
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 -130 dBm (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 modeling, 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.
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.
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.