Efficient geolocation using swarm radio
In wireless sens or networks the concept of a swarm is used to illustrate how
individuals in a group interact. Individuals in a swarm need to know their position relative to each other. Nanotron has added location-awareness to wireless sensor networks so that the swarm members can measure the distance between each-others, and are able to make decisions using this information. Communication and location awareness together are enabling a whole new category of geolocation applications.
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 mis-alignment etc.
Fig. 4: Range measured between a pedestrian with a swarm radio mini and another swarm radio mounted onto the dashboard of a passenger car.
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 of 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.
Table 1: Travel time through various safety zones on a straight 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.
Fig. 5: Collision avoidance application flow chart.
Example: RangeTo command.
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.
Dr. Gunter Fischer is Field Application Engineering Manager responsible for
application support for nanotron’s swarm business. He can be reached at g.fischer@nanotron.com.
Dr. Thomas Förste is Vice President of Sales and Marketing at nanotron Technologies. His email address is t.foerste@nanotron.com.
Dr. Frank Schlichting is Director of Product Management for the swarm product line at nanotron. He can be reached at f.schlichting@nanotron.com.
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
