Problem building a smart shopping cart I'm building a smart shopping cart with Arduino for my final year project. 

The cart is supposed to be able to follow its user while shopping. I don't know of many feasible indoor positioning systems to use.

Proposed solution 1

I have recently seen some RF/ultrasonic positioning systems that make use of:


one ultrasound transmitter and one RF receiver, carried by the person, 
and two ultrasound receivers and one RF transmitter carried by the robot. 


The robots RF transmitter sends out a signal, which when received by the person's RF receiver, triggers the person's ultrasound transmitter to transmit a signal. This signal is received by the robot's two ultrasound receivers (with a distance, d between them), and the distance and angle to the target person can be calculated.

That sounds very feasible, but my question is, if the robot is in a crowded place, where the other people are also carrying ultrasound transmitters and RF receivers:


Will the RF signal meant for the owner of the robot be accidentally received by the RF receiver on another person, or is there some kind of unique ID shared between RF receiver/transmitter pairs that could prevent this problem, or maybe some kind of frequency hopping system, so there would be no mistaken identity.
How will the ultrasound receivers on the robot be sure that the received signal is from its matching ultrasound transmitter. Again, is there some kind of special ID systems employed by some ultrasonic transmitter/receivers.


Proposed solution 2

I have also read about this new device called Trackr, it makes use of bluetooth low energy to track objects based on their proximity to bluetooth enabled devices like smart phones.

Considering a scenario where various bluetooth devices (beacons) are placed at specific points in a room, and the target to be tracked is equipped with a bluetooth emitter/bluetooth chip/dongle. 

When the target passes each of those beacons, the signal strength intensity is received and distance from the target is calculated.

I understand this concept, but have some questions:


When distance is calculated between beacon and target, what about the angle, how would the robot follow the target based on distance alone?
If this method is not accurate enough to be used in a crowded space, what other method can be used, I have checked out some indoor GPS devices and they are way to expensive (above $200). 


Would a camera-based tracking system be better and how easy would it be for a non expert like me to implement.
I have a couple of ideas for you but know these are only suggestions. No hate please.

Have you tought of IR following?

IR is a almost unique signal that only you have. You can either use multiple IR sensors to determine where the emitter is or you can use the servo motor to do it as a radar. Note that you have to isolate each sensor so it knows where you are and it doesn't captures the wave from a different angle by accident.

Extra safety?

You can also incorporate an untrasonic distance sensor so that the cart doesn't bump into you or any one. Like you can say, if the distance is smaller then 12 inchs / 30 cm, stop the cart. It is practically an extra safety feature.