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.
___________________________________________________________ Inbox full of spam? Get leading spam protection and 1GB storage with All New Yahoo! Mail. http://uk.docs.yahoo.com/nowyoucan.html