Get distance from an RFID Tag [solved]

dear arduino community,
hi there,

i've got ir+rfid guided robot. have target many objects in room, 1 after other. in random position. each 1 rfid passive-tagged.
i proceed that: target object ir, reach it, , check if target right 1 rfid validation.

now, validate objective before to totally reach  

a way roughly compare ir measured distance , rfid measured distance, , conclude if target going right one. think compare evolution of ir-distance , evolution of rfid distance should sufficient.

i understood methods distance rfid more or less @ "research" level. (is still true ?)
i know 2 of these methods: "rssi method", measure signal strength, , "radar" method, measure time between signal emission , tag answer reception.

the rssi sounds not easy use, i'm looking signal transit time measuring.

now, questions

programming feasibility :
> in opinion, how can recover time between signal emission , tag answer reception?
> there conditions fulfil (particular material, tags, etc) ?

physical feasibility:
i know 2 way trip should depend of tag response time.
the field clear, no obstacles between reader , tags.
>  i assume response time should more or less constant in same condition of field / same reader/ same tag. stupid ?
> consequently, believe calibration should sufficient reduce error. stupid ?

global feasibilty
> know tips make method more reliable?
> think should reasonably easy combine rssi , radar method improve reliability ?


thanks lot rfid experts of community!
kind regards,
meric.

[edit]
material consider project :
- cottonwood long range uhf rfid reader
- linksprite pcb uhf rfid antenna
computed on arduino mega2560.
... pertinent choice ?

radio signals travel @ speed of light, not possible measure transit time typical consumer grade electronics, arduino.

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