How BLE tag tracking works
Learn the technical fundamentals in Bluetooth Low Energy tag tracking
As tag trackers do not communicate directly to the internet, they work differently from connected trackers.
Tags and scanning devices
A BLE tag is attached to an asset. It is a small piece of tracking hardware. The tag broadcasts its own unique identification every few seconds (for example, every 4 seconds), using a wireless Bluetooth signal. More specifically, it uses the Bluetooth Low Energy standard – which, compared to classic Bluetooth, focuses more on energy efficiency (meaning, longer battery life) than high data transfer speeds. Bluetooth Low Energy is commonly abbreviated as “BLE”; and hence we call the this tag a “BLE tag”.
It's important to stress, that a BLE tag is a pretty basic, dumb device. It does not contain a GPS, a modem, or a processor, and it’s not capable of knowing its location. All it does, is periodically broadcast its own ID. So, tags on their own are an incomplete solution.
For any Bluetooth tag tracking solution to function properly, there should also be several scanning devices active. These scanning devices will periodically listen for the BLE signals emitted by the tags. They keep a list of detected tags in memory, and periodically – for example every 30 minutes – send the list of detected BLE tag IDs to the cloud platform. Since the platform knows the location of scanning devices ; it can update the location of tagged assets to the location of the scanning device.
BLE tags using iBeacon format
Bluetooth Low Energy is a communication protocol that does not specify the actual data format. The Sensolus solutions currently support BLE tags that broadcast their unique ID in the iBeacon format, which is the most commonly used industry format.
In the iBeacon format, a tag is programmed emits three data fields to identify itself.
- UUID value: A high level identifier, used to identify an organization or manufacturer. Sensolus scanning devices are configured to filter on a specific UUID, to avoid picking up iBeacon signals from other tags and devices not known to the platform.
- Major value: A numeric field (8 bit).
- Minor value: A numeric field (8 bit). The combination major-minor should be unique within the UUID space.
Sensolus offers BLE tags and ensures that the UUID-major-minor values are unique and correctly registered in the tags and in the platform.
BLE tag location behavior
It is important to understand that the platform will always set the location of tags to be exactly at the exact location of the scanning device. For simplicity reasons, this can be described the scanning device 'claiming' a nearby BLE tag. There are some slight nuances, depending on the type of scanning devices:
- Zone anchor scanning devices: the location of the scanned BLE tags is set equal to the configured infrastructure device in the platform (see Set the location of a BLE anchor or geobeacon). The location of tags will be represented as moving from one zone anchor to the another anchor.
- Externally powered trackers as scanning device: The location of the scanned BLE tags is set equal to the last GPS scan performed by the vehicle tracker. Note that the a single BLE scan might take 30 seconds; and their might be a timing difference between the actual BLE scan and the GPS localization.
- Mobile app as scanning device: The location of the scanned BLE tags is set equal to the current location according to the smartphone operating system. Note that modern smartphones automatically use a combination of GPS and Wi-Fi localization.
Contrary to what some new users might expect, with regular BLE zone scanning there is no estimation or interpolation being done of the distance between the tag and the scanning device. The reason is that the received signal strength cannot be translated to a distance because it is highly variable, depending on the type of tag, transmission power setting of the tag, reflections, objects blocking line of sight, and atmospheric variation, … There are solutions on the market that perform high-precision location tracking based on Bluetooth signals, but these solutions are outside the scope of zone scanning (and also have more expensive price points and more specific infrastructure requirements).
When multiple scanning devices observe the same tag simultaneously, the general rule is that the tag will be set to the scanning device having the strongest signal strength. For more details, refer to What happens when multiple devices scan the same BLE tag.
BLE scanning accuracy limitations
BLE tag scanning should not be seen as a high-accuracy (indoor) localization solution that would allow easy finding of asset precise location. For such solutions, more advanced (and expensive) equipment is required.
It is very important to be aware of the accuracy limitations of BLE tag scanning.
- When not obstructed, BLE signals can easily be detected up to a 100m distance.
- But, large objects placed between the tag and scanning device will bounce (especially in metal), reducing the distance. When used indoors, BLE signals can bounce on walls and ceiling and again other objects, but still the distance is reduced.
- Hence, in general, we estimate a range of '20m' per scanning device.
- There is no triangulation, or distance estimation (see above).
Instead, BLE tag scanning should be seen as way to known which assets are present in a specific zone.
Proper testing in an environment that closely mimics the real use case(s) is strongly recommended.
BLE emitting, scanning and sending intervals
Understanding the different timings and intervals helps to understand the responsiveness and accuracy of a solution.
First, BLE tags have a periodic advertisement interval (also called the emitting or sending interval or frequency). For example, a BLE tag announces its ID every 3 seconds. This interval is a compromise between easier detectability and battery lifetime. It is very uncommon and time consuming to modifying advertisement frequency after purchase of tags.
Secondly, the scanning devices themselves often operate using a scanning interval. They will listen for a period of time that covers multiple tag advertisement intervals (for example 10 seconds or more) to ensure that occasional misses of a single advertisement are covered. Battery powered scanning devices do not scan continuously, but scan at intervals. Externally powered scanning devices, on the other hand, can afford to scan almost continuously.
Finally, scanning devices use a sending interval at which they periodically announce the list of detected BLE tags to the platform. Having short sending intervals means a higher temporal responsiveness (knowing when an asset arrived in the zone, or left the zone) but comes at the cost of higher energy and communication volume usage - which is an important consideration for battery powered zone anchors.
In short, if it is really important to know the time of arrival or departure as accurately and fast as possible (for example detect arrival within 5 minutes), fixed zone anchors are probably the best choice (externally powered, using the local area network). In other cases, battery powered anchors are recommended due to the higher flexibility and strongly reduced infrastructure impact.
Scanning devices overview
| BLE zone anchor ZA 3500 | BLE zone anchor ZA 3505 | Vehicle trackers (TRACK 12xx range) | Sensolus mobile app on mobile device | |
|---|---|---|---|---|
| Ideal environment | Indoor | Indoor - outdoor | Outdoor in-transit | Indoor - outdoor |
| Network communication protocol | LAN, Wi-Fi or 4G | NB-IoT | 4G (2G legacy) | Network communication used by the mobile device |
| Power source | USB power or PoE | Internal battery pack (replaceable) | Vehicle battery | Battery of mobile phone |
| Wires needed for installation | Yes, wire for power & wire if communicating over LAN | No | Yes, wire for power | No |
| Positioning accuracy range | ~20m up to 200m | ~20m up to 200m | ~20m up to 200m | ~20m up to 200m |
| Tags getting location from | Location of zone anchor as appointed on the platform | Location of zone anchor as appointed on the platform | GPS in vehicle tracker | Location of the mobile device |
| Location capture behavior (simplified) | Snap-to-strongest BLE zone anchor | Snap-to-strongest BLE zone anchor | Carry detection & coarse relocation | Check-in on location of the mobile phone |
| Responsiveness | Continuous scanning Upload every 5 min |
Sweep scan & upload Configurable max. every 30 min |
Continuous scanning & upload | On demand |
| Tags scanned at the same time | 2000 (recommended) | Strongest 100 | First seen 100 | 1000 |