Using LoRa for Geolocation
LoRa geolocation is a GPS-free solution for low-power wide area networks. LoRa-enabled sensors support tracking applications by using time-difference-of-arrival (TDOA) techniques to determine the approximate location of a device, with an accuracy down to the nearest city block. Let’s watch the following video to learn more.
(Time: 5:03 minutes)
As noted in the video, all gateways and devices on a LoRaWAN network must have highly-accurate clocks that are synchronized with each other–down to the nanosecond. When a device transmits a signal, all of the gateways in range receive the signal and pass it on to the network server. The network server then calculates the location of the device based on the TDOA, RSSI, and SNR. For this to be successful, however, data packets must be received by at least three gateways. The more gateways that receive and forward the signal, the more accurate the results.Improving Geolocation with Fine Timestamping
To understand how fine timestamping works, let’s look at an example. Say you have a device with a LoRa chip, and gateways with a processor that supports this feature. The device sends a data packet. When each gateway in range receives the packet, it stores the precise time of arrival (TOA) down to the nanosecond. The gateway closest to the device receives the packet first; and the ones farther away receive the packet later. When the packet is received by at least three gateways, the device can be located based on the packet’s Time Difference of Arrival (TDOA) at each gateway.
You can also look at it this way: when you throw a stone into a lake, it creates a ripple effect. Presume that we have set up gateways at different locations in the lake. When the stone hits the water, the water breaks into ripples that spread out at the same rate in all directions. Because the gateways are at different distances from the stone, the ripples hit each gateway at different times. Based on those time differences, we can estimate the position of the stone.
In much the same way, a LoRa-enabled device (the stone) sends a data packet (the ripple) which is then time stamped at each gateway in order to locate the device. After the data packet is received by the gateways, each timestamp is sent to the cloud, where the timestamps from all of the receiving gateways are used to calculate the location of the device. The location accuracy is generally within 75–150 meters, depending on radio channel, line-of-sight, or multipath conditions, as well as the number of receiving gateways.
By enabling TDOA-based geolocation, which is a network-enabled function, fine timestamps make locating assets easier, more affordable, and more efficient; there is no additional impact on the end device and the device does not need a GPS chip.
Practical Implementations of Fine Time Stamping
There are practical advantages when it comes to implementing a gateway that supports fine timestamping. For example, if you already have sensors with LoRa chips monitoring the temperature in your delivery trucks and you also need to track the location of the trucks, setting up gateways that use such processors allows you to track the trucks and monitor their temperatures, all with a single LoRa-enabled device. As the truck travels, the device sends packets to the gateways in range. With the Fine Timestamp feature, every time a gateway receives a packet from a device, it will attach a timestamp (in nanoseconds) and send the data to the network and application servers. The servers then send the timestamp data to the cloud, where a TDOA location solver calculates the location of the device based on the different timestamps received from the various gateways.