Hi, 

I would like to track ants in real time. I need to track a large
number (>250) of ants in a lab setting and I thought to use RFID
technology for tht purpose.

I found out that RTLS is a RFID based technology uses actve RFID tags.
My question is: If there a passive-tag basesd solution, if so are the
passive tags
necessary are small enough to stick to a big ant (1/2 inch) and
finally what is the percision of the RTLS positioning.

Thanks
Roy

You can get rfid tags small enough to fit into a needle. It's what
they use for cat and dog identification chips. These are passive
chips. The problem you have is active chips require their own power
source so are instantly to big therefore passive is the only solution.
However passive chips because they have no power source and being this
small and a very limited range I would imagine that it is not possible
to use an rfid tag for an ant. That said if you were able to get the
scanners within about half a metre maybe a metre it might be able to
detect the chip.

Real Time Locating Systems
RTLS or Real time Locating System is RFID upgraded to produce instant
location information.

Normal RFID tags are read when a reader is brought near the tag or
vice versa. RTLS allows a reader unit to "see" the actual location of
a tagged item, without the tagged item being near the reader. Using
special readers placed around a property, tags are located using a
triangulation system.

An RTLS solution typically utilizes battery-operated radio tags and a
cellular locating system to detect the presence and location of the
tags. The locating system is usually deployed as a matrix of locating
devices that are installed at a spacing of anywhere from 50 to 1000
feet. These locating devices determine the locations of the radio
tags. The systems continually update the database with current tag
locations as frequently as every several seconds or as infrequently as
every few hours for items that seldom move. The frequency of tag
location updates may have implications for the number of tags that can
be deployed and the battery life of the tag. In typical applications
systems can track thousands of tags simultaneously and the average
battery tag life van be 5 years or more. average tag battery life can
be five or more years.

Smallest Passive RFID tag:

  Lack of an onboard power supply means that the device can be quite
small, commercially available products exist that can be embedded
under the skin. As of 2006, the smallest such devices measured 0.15 mm
× 0.15 mm, and are thinner than a sheet of paper (7.5 micrometers).
And you can use Passive RFID tags for RTLS.
For more Information 

Hitachi advances paper-thin RFID chip
http://www.eetimes.com/news/design/showArticle.jhtml?articleID=179100286
http://en.wikipedia.org/wiki/RFID