  CHICOS (California HIgh school Cosmic ray ObServatory) is presently an array
of more than 140 detectors distributed over a large area (~400 km^2) of
southern California, and will consist of 180 detectors at 90 locations in the
near future. These sites, located at area schools, are equipped with
computerized data acquisition and automatic nightly data transfer (via
internet) to our Caltech lab. The installed sites make up the largest currently
operating ground array for ultra-high energy cosmic ray research in the
northern hemisphere. The goal of CHICOS is to provide data related to the flux
and distribution of arrival directions for ultra-high energy cosmic rays.
  We have performed detailed Monte-Carlo calculations to determine the density
and arrival-time distribution of charged particles in extensive air showers for
the CHICOS array. Calculations were performed for proton primaries with
energies 10^18 to 10^21 eV and zenith angles out to 50 degrees. We have
developed novel parameterizations for both distributions as functions of
distance from the shower axis, primary energy, and incident zenith angle. These
parameterizations are used in aperture calculations and reconstruction of
shower data, enabling preliminary analysis of ultra-high energy shower data
from CHICOS.
