Implementation of efficient geolocation through the use of swarm radio
Communication and location awareness together are enabling a whole new category of geolocation applications. Collision avoidance (CAS) – to mention just one of them – will be introduced in this paper illustrating the benefits of the extended swarm approach.
The swarm platform technology
Low power swarm radios – autonomous 2.4 GHz chirp spread spectrum wireless nodes – are the basic swarm building blocks – see figure 1. They are able to broadcast and exchange messages while monitoring distances to other individuals in the swarm which are the key capabilities that allow for coordinated swarm behavior.
Each individual in a wireless swarm consists of a swarm radio that is controlled by a host through its application interface (API). There are several categories of API commands – see figure 2. The RangeTo <node ID> command for instance returns the distance to another node.
The quality of location-awareness depends on two basic criteria: accuracy and latency. Accuracy is the difference between measured and true distance. Usually it could be characterized by a fixed off-set and the spread of results as shown in figure 3. Latency specifies the time required to obtain a ranging result. It has a strong impact on the real-time character of the application. Short messages and quick responses help to minimize latency thus maximizing throughput. A typical swarm radio requires 1.8 milliseconds of air time for executing a SDS-TWR cycle, nanotron’s patented Symmetrical Double-Sided Two Way Ranging. To broadcast its ID it only requires 350 microseconds.
The maximum obtainable range of the swarm radios determines how far apart individuals in the swarm are still able to interact. Maximum range is highly dependent on the application environment. Under ideal line-of-sight conditions range might exceed 500 meters; however, in reality it often will be much shorter due to obstacles, reflections, interference from other radio signals, antenna miss-alignment etc.
Figure 4 shows a real world example with one swarm radio inside a car and the other carried by a person. Range could be extended by placing the antenna on the outside of a car or by having the antenna installed on a hard-hat instead on a belt.
Collision avoidance solution (CAS)
There is a need for automatic collision avoidance in mining. In order to prevent accidents a reliable alarm is required whenever vehicles come too close to people, assets or other vehicles. The swarm geolocation technology is well-suited for implementing such collision avoidance solutions (CAS).
A simplified set-up with vehicles, assets and people – a total of three node types – is used to illustrate the essential outline of the application. In the worst case scenario two objects move towards each other at maximum speed – see table 1. The system needs to react faster than the time necessary for the objects to traverse the respective safety zone for the shortest path collision course. In our example the shortest time is 2.2 seconds; therefore latency of the CAS system must be kept short and the whole group of nodes needs to complete the full location awareness cycle faster than in 2.2 seconds. For reliable operation one might decide to accelerate the sequence in order to execute it several times within this interval.
Figure 5 shows the steps of the location awareness cycle and how they are supported by the swarm radio:
- Get IDs (4): As a first step the swarm radio makes itself visible by broadcasting its own ID. SetBroadcastIntervall=01 for example sets the blink interval to 1 second. After activating the broadcast by SetBroadcastNodeID=1 the swarm radio broadcasts its ID every second. Node IDs of other participants are automatically stored in the NodeID list when received. The host application can read the NodeID list by using the GetNodeIDList command. This way neighbors are identified to the CAS application.
- Range to IDs (5): As a second step the swarm radio measures the distance to all neighbors. This is accomplished by subsequently executing the RangeTo <node ID> command. Resulting distance values are communicated back to the host application.
- Evaluate distances (6): In a third step the CAS application needs to decide whether any of the measured distances violates a safety zone requirement and needs to take action if it does. It may involve a simple audio alarm on approach or exercising the brakes of a truck to prevent an imminent collision.
As part of designing the CAS application it is now possible to estimate the time required to execute one location awareness cycle and trigger an alarm if required. The sequence in our example takes less than 30 milliseconds; hence the time constraint mentioned above can be easily met.
All swarm radios share the same air interface. The CAS application works in an entirely asynchronous fashion and packet collisions may occur. Several location awareness cycles instead of just one increase the probability of a successful sequence. At the same time traffic through the air interface must not exceed channel capacity. Broadcasting the node ID together with a full ranging cycle takes about 2.2 milliseconds of the air time. This is just 0.1% of the 2.2 second cycle time for the CAS application. As a rule of thumb no more than 17% of the available airtime should be used as a good trade-off between success rate and throughput. This is important when scaling the application by adding more swarm radios.
In real swarm applications safety zones could be designed to be dynamically adjusted to the actual speed of the moving object and the last measured distance on a potential collision course. This way the total number of alarms can be minimized and the number of swam radios that can be used in the system before channel saturation occurs, can be maximized.
Nanotron’s swarm platform is well-suited to build geolocation applications quickly. Swarm radios are location aware since they are able to measure distances amongst themselves and exchange the results. Range, ranging accuracy, latency and throughput are important design criteria for geolocation applications based on the swarm platform.
About the Authors
Dr. Gunter Fischer is Field Application Engineering Manager responsible for application support for nanotron’s swarm business. He can be reached at email@example.com.
Dr. Thomas Förste is Vice President of Sales and Marketing at nanotron Technologies. His email address is firstname.lastname@example.org.
Dr. Frank Schlichting is Director of Product Management for the swarm product line at nanotron. He can be reached at email@example.com.