  Line intensity mapping (LIM) is a new technique for tracing the global
properties of galaxies over cosmic time. Detection of the very faint signals
from redshifted carbon monoxide (CO), a tracer of star formation, pushes the
limits of what is feasible with a total-power instrument. The CO Mapping
Project (COMAP) Pathfinder is a first-generation instrument aiming to prove the
concept and develop the technology for future experiments, as well as
delivering early science products. With 19 receiver channels in a hexagonal
focal plane arrangement on a 10.4 m antenna, and an instantaneous 26-34 GHz
frequency range with 2 MHz resolution, it is ideally suited to measuring
CO($J$=1-0) from $z\sim3$. In this paper we discuss strategies for designing
and building the Pathfinder and the challenges that were encountered. The
design of the instrument prioritized LIM requirements over those of ancillary
science. After a couple of years of operation, the instrument is well
understood, and the first year of data is already yielding useful science
results. Experience with this Pathfinder will drive the design of the next
generations of experiments.
