  We report on the behaviour of Resistive Plate Chambers (RPC) developed for
muon detection in ultra-high energy cosmic ray (UHECR) experiments. The RPCs
were developed for the MARTA project and were tested on field conditions. These
RPCs cover an area of $1.5 \times 1.2\,{m^2}$ and are instrumented with 64
pickup electrodes providing a segmentation better than $20\,$cm. By shielding
the detector units with enough slant mass to absorb the electromagnetic
component in the air showers, a clean measurement of the muon content is
allowed, a concept to be implemented in a next generation of UHECR experiments.
The operation of a ground array detector poses challenging demands, as the RPC
must operate remotely under extreme environmental conditions, with limited
budgets for power and minimal maintenance. The RPC, DAQ, High Voltage and
monitoring systems are enclosed in an aluminium-sealed case, providing a
compact and robust unit suited for outdoor environments, which can be easily
deployed and connected. The RPCs developed at LIP-Coimbra are able to operate
using a very low gas flux, which allows running them for few years with a small
gas reservoir. Several prototypes have already been built and tested both in
the laboratory and outdoors. We report on the most recent tests done in the
field that show that the developed RPCs have operated in a stable way for more
than 2 years in field conditions.
