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By Finn Boetius, Product Marketing Engineer, Nordic Semiconductor
Thursday, 09 November, 2023
Global Positioning System (GPS), the principal component of Global Navigation Satellite System (GNSS) which also includes Galileo, BeiDou and GLONASS among others, is a technological marvel.
The GPS constellation is made up of 24 satellites positioned around 20,000 kilometres above the Earth. The arrangement of the constellation ensures at least four satellites can be observed at any point on the planet. A GPS receiver picks up the signals from the satellites which provide their locations, status and the precise time from onboard atomic clocks. The receiver notes the arrival time of the signal and then determines the distance to each satellite from the difference in time between signal transmission and reception, and then multiplying by the speed of light. Information from four satellites fixes the receiver’s position to a unique point.
Billions of people rely on GNSS daily to help them determine where they are on the Earth’s surface. GNSS is also now providing a foundation for many IoT applications in the logistics and transportation sectors helping keep track of valuable assets that might otherwise go missing. This is why for asset tracking and other applications, Nordic’s cellular IoT solution, the nRF9160 SiP, incorporates GNSS capability.
Satellite signal lost
Despite its impressive technical foundation, GNSS is not flawless. Some problems do occur with the satellites, such as inaccuracies with the onboard clocks resulting in timing errors. To mitigate such drift errors, GNSS systems compare multiple satellites and use algorithms to determine which clocks are in error and then reset them compared with an earthbound reference.
Other problems occur because the relatively weak signal between satellites and earthbound receivers can easily be disrupted. For example, ‘urban canyons’ — formed by rows of tall buildings — can obstruct the signal. And there’s little chance of GNSS signals penetrating buildings.
But even if the signal does get through, so-called multipath errors can occur when it reflects off buildings before reaching the receiver. That can result in timing errors which in turn lead to incorrect positional information. Other errors can occur because of anomalies in the Earth’s atmosphere that can delay or distort the GNSS signal. Electromagnetic interference (EMI) from other radio sources can also cause timing errors. To mitigate these problems, receivers use techniques such as filtering, correlation and signal power measurement, and for the atmospheric challenges, methods such as ionosphere and troposphere modelling are employed.
Another challenge with a GNSS modem is that it can take several minutes to fix the location of a group of satellites from a cold start. That uses significant battery capacity. One solution, used by Nordic’s nRF9160 together with the company’s nRF Cloud Location Services, is Assisted- and Predicted-GPS (A-GPS and P-GPS). These methods use satellite assistance data stored in a database which is relayed to the nRF9160 via the LTE-M or NB-IoT network — saving significant power compared to an extended first fix. When required, the IoT device can then find the satellites in seconds, saving further energy. The P-GPS technique builds on A-GPS by providing over two weeks of assistance data to the IoT device resulting in even greater power savings.
Even with power saving techniques, GNSS can still extract a heavy toll from batteries; that’s an important consideration for things like wearables or asset trackers which are typically equipped with modest batteries yet are expected to deliver long battery life.
If high accuracy is needed then the battery trade-off of GNSS is worth it, but if less accurate locationing is acceptable there are ways to save power. One option to overcome the power consumption of GNSS — and which is also supported by the nRF9160 SiP and nRF Cloud Location Services — is to use the known location of cellular base stations to narrow down the position of the receiver. The single-cell location method relies on identifying in which cell the tracked device is situated and then referencing the cell identification against a database of known base station locations. The technique offers accuracy down to kilometre level while only modestly impacting the receiver’s battery life.
Multi-cell location builds on the single-cell technique by referencing the position of several nearby base stations instead of just one, to offer accuracy down to a few hundred metres while keeping power consumption low.
An interesting locationing technique which complements GNSS — and which can also be used to trade-off location precision against battery life — 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.
SSID locationing is supported by Nordic’s nRF70 Series of companion ICs. When used for Wi-Fi locationing, the nRF70 Series devices scan any nearby Wi-Fi AP for its SSID; a partner nRF9160 SiP then forwards 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 SSID’s location, plus a degree of uncertainty for that location, to the nRF9160, or elsewhere as directed.
It’s hard to beat the precision of GNSS. But when precision of tens of metres is acceptable and battery life is critical, or when the GNSS signal is interrupted, Wi-Fi SSID locationing is an excellent alternative as it consumes significantly less power than GNSS. If it’s only necessary to determine the location of an asset to within a kilometre and battery life is critical, cell-based locationing is the answer. With Nordic’s nRF91, nRF70 Series and nRF Cloud Location Services it’s simple to switch seamlessly between all three methods to optimally trade-off location precision against battery life. With this locationing tech there is now no reason for valuable assets to ever be lost again.
Need to know
SSID information is found in the packet header of each communication transmitted over a Wi-Fi network and is distinct from the payload of the packet. The data is publicly broadcasted by every Wi-Fi enabled device and is accessible by any other Wi-Fi device within range, regardless of whether the Wi-Fi network uses encryption.
Never to be seen again
You might think that owners of valuable assets would make very sure they look after them. But no, just like your keys or pocketbook, items worth thousands or even millions of dollars have a habit of just disappearing.
Some, like the 1816 containers lost from the container ship ONE Apus, are down to pure bad luck. As reported in Slash Gear, the unfortunate vessel met with disaster due to extreme weather conditions in the Pacific Ocean, about 3000 kilometres from Hawaii. $90 million worth of goods sank into the abyss. Others, such as the lost Nazi train of Walbrzych, said to have been loaded with 270 tonnes of gold, weapons, jewels and art, and which allegedly disappeared late in WWII between Breslau and Walbrzych, might just be the stuff of myth. An extensive search for the train revealed naught, leading many to believe it never set off in the first place. If the carriages ever turn out to be real, the gold alone would be worth a cool $19 billion at today’s prices.
But then there are the foolhardy. Reported in UK newspaper Metro, in 2009 IT worker James Howells got his hands on 7500 bitcoin which he stored on his PC’s hard drive. When Howells ditched the computer, the hard drive went with it to landfill, only for him to later realise the bitcoin therein were worth nearly $5 million. Weeks of grubbing among the trash left him empty-handed. And then there was British journalist Nigel Reynolds. He was one of the first journalists to interview author JK Rowling and received a first edition copy of Harry Potter and the Philosopher’s Stone for his trouble. Reynolds assumed the book would fall flat and threw it in the bin — yet today, similar copies sell for over $60,000.
And finally, there’s the downright incompetent. Popular Mechanics magazine reports over one million spare parts needed to keep F-35 fighter aircraft in the air have gone missing. The parts are believed to have a total value of at least $85 million, but no one really knows because of some questionable bookkeeping. One can only hope the government keeps the actual weaponry on a tighter leash.
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