jtd wrote:
You do not rely on signal strength cause that is dependent on
everything and the cat. You use time stamps either as actual time or
a periodic signal. In either case all the devices in the computing
chain need to sync and be extremely accurate. In the cae of gps a few
microseconds drift will result in several meters of inaccuracy. The
shorter the distance to be measured the higher the accuracy,
including relative accuracy. for our situation we require centimeter
accuracy over tens of meters. which would translate to nano sec
resolutions. The costs would be impossible. Precision guidance
systems use inertial navigation (gyro compass) and verrrryyy high
accuracy initial location.
Lets do it from the bottom first. We are getting into very high
precision navigation apparatus, something which may not be necessary for
local use in a locality. Lets start with simple directional TX RX
systems. They will give the direction and area information on the basis
of where the user is standing. The need for the RF system would be
limited to helping the user make a choice of direction and its
destination. The guide railings and audio beeps at traffic signals will
provide the `last meter' connectivity.
I was also wondering about the use of optical Tx and Rx devices to
eliminate the problems of rf mixing, interference and direction tuning.
An optical device works in line of sight and that will give the
necessary directional focus. This can also be used for the bus number
suggestion put up by Krishnakant. As the bus passes by the user, the IR
device on the bus door will send out an optical signal, that will be
picked up by the hand held device. This makes it highly directional and
eliminates signals from adjacent buses. To eliminate the problems of
public blocking the optical path, these devices can be mounted at the
top of the bus and the receptor mounted at the same level on the bus
stop. A public address loudspeaker will announce every bus that passes
the post.
Regards,
Rony.
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