  Due to the advancement of nano-satellite technology, CubeSats and fleets of
CubeSats can form an alternative to high-cost large-size satellite missions
with the advantage of extended spatial coverage. One of these initiatives is
the Cubesats Applied for MEasuring and LOcalising Transients (CAMELOT) mission
concept, aimed at detecting and localizing gamma-ray bursts with an efficiency
and accuracy comparable to large gamma-ray space observatories. While precise
attitude control is not necessary for such a mission, attitude determination is
an important issue in the interpretation of scintillator detector data as well
as optimizing downlink telemetry. The employment of star trackers is not always
a viable option for such small satellites, hence another alternative is
necessary.
  A new method is proposed in this series of papers, utilizing thermal imaging
sensors to provide simultaneous measurement of the attitude of the Sun and the
horizon by employing a homogeneous array of such detectors. The combination
with Sun and horizon detection w.r.t. the spacecraft would allow the full 3-DoF
recovery of its attitude. In this paper we determine the spherical projection
function of the MLX90640 infrasensors planned to be used for this purpose. We
apply a polynomial transformation with radial corrections to map the spatial
coordinates to the sensor plane. With the determined projection function the
location of an infrared point source can be determined with an accuracy of
~40', well below the design goals of a nano-satellite designed for gamma-ray
detection.
