Next Generation Ultrasonic Positioning
This page outlines a few research ideas/goals for the next generation of
ultrasonic positioning, as discussed by Paul, Henk, and Mike at the end of
We would like our indoor positioning system to have the following
Cliff's current system has achieved the first two, cheap and
simple, but reliability and scalability are still an issue.
- Cheap - this is in terms of dollars and resources like power
- Simple - easy roll out and configuration
- Reliable - performs well in all locations, regardless of noise
- Scalable - roll out is possible for installations of any size
Attaining reliability means that we must overcome two main obstacles,
interference and reflections.
The current ultrasonic system that we employ uses narrow-band transmitters
and receivers operating in the 40kHz band. The receivers use a simple level
sensitive, one bit protocol for recognising ultrasonic pulses from the
transmitters. As a result, any noise in the 40kHz band with large enough
amplitude easily confuses the system. This noise is produced by various
things in the environment, like rustling crisp packets or jingling keys.
A reliable positioning system must be able to differentiate noise and
interference from authentic ultrasonic pulses. Some ideas for tackling this
problem are discussed in the Proposals section below.
Echoes and Reflections
Each transmitter is allocated a time slice in which it must transmit an
ultrasonic pulse. The receiver listens for the first pulse within this
slice and assumes that it is from the designated transmitter. However, if
there are reflections in the environment from the previous time slice, then
the system will record an invalid distance reading.
The current solution to this problem is to make the time slices large
enough to let all reflections dissipate before the next transmitter is
activated. The draw back to this is that large time slices result in slower
response times and large latencies for the system. Latency between distance
readings also causes errors in position calculations since they assume that
a set of distances correlate to the same point in time and space. Of
course, this is not the case for objects in motion where the position
changes as time passes. It is also important to note that this solution
degrades the scalability of the system. Larger rooms, for example, require
more transmitters - each increasing the latency further.
Alternative solutions to this problem is discussed in the next
It may be possible to reduce the effects of noise by changing the ultrasonic
pulse from its one bit protocol to a several bit protocol. Giving bit
patterns to each transmitter may also allow us to increase the frequency of
the system since the receiver could differentiate between pulses from
different transmitters. This may be a cheap way to improve reliability.
Broadband ultrasonic systems get around noise and reflection problems by
utilising a larger portion of the available ultrasonic spectrum. See Mike
Haza's paper in the
proceedings of UbiCom 2002. This is heavy duty stuff and is expensive in
terms of computing power and component cost.
We have talked about using multiple microphones on a receiver to measure the
angle of incidence of incoming pulses. With this information we may be able
to eliminate reflected signals or noise that originate from invalid
positions (we know where the transmitters are supposed to be). This can be
done in two ways:
Of course, we have to be certain of the orientation of the receiver.
- by placing the mics very close together (within one wavelength ~ 10mm)
and measuring the pulse phase difference between them. Three microphones
will give the angle at which the pulse is incident.
- by placing the mics slightly further apart and measuring the time
difference between receipt of the same pulse. Instead of phase we measure
time to achieve the same result.
It is also possible to eliminate erroneous readings by using the spatial
relationship between the distances given by the system. The relationship
gives a set of valid distances for each transmitter - given the distance to
other transmitters. See Henk's 3D hull diagram.
Kalman filters may provide a nice way to model the state of the system and
allow us handle data in a stochastic manner - taking care of those nasty
outliers. Henk has started on this one.
Is there a way to use signal strength to gain information?
Is is possible to get rid of the synchronising RF pinger?
Cliff and Amoss have begun work on examining probability densities of
pre-recorded location plots. I think that they plan to use this data to
build a probability model for locating mobile users. Can you guys add more
I believe that Cliff plans to do some reading over the next couple of weeks
on sensor fusion. Anything to add here Cliff?
It will probably be possible to combine a number of these techniques to
improve on the reliability of the ultrasonic system.
Existing Systems and Reading
Below is a list of some of the current papers and projects relating to
- MIT Cricket - an
ultrasonic positioning system that locates an object to the nearest
transmitter (very similar to our stuff).
- The Bat - an old
AT&T (Cambridge) project that does tracking.
- Andy Ward's PhD
Thesis - a nice introduction to positioning and ultrasonics.
- Mike Haza's paper
in the proceedings of UbiCom 2002 - broadband ultrasonics.
This page last updated November 4, 2002
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