  PODIUM is a compact spacecraft navigation unit, currently being designed to
provide interplanetary missions with autonomous position and velocity
estimations. The unit will make use of Pulsar X-ray observations to measure the
distance and distance rate from the host spacecraft to the Solar System
Barycenter. Such measurements will then be used by the onboard orbit
determination function to estimate the complete orbital elements of the
spacecraft. The design aims at 6 kg of mass and 20 W of power, in a volume of
150 mm by 240 mm by 600 mm. PODIUM is designed to minimize the impact on the
mission operational and accommodation constraints. The architecture is based on
a grazing incidence X-ray telescope with focal distance limited to 50 cm. The
effective area shall be in the range 25 to 50 cm2 for photon energies in the
range 0.2-10 keV, requiring nesting of several mirrors in the Wolter-1
geometry. Grazing incidence angles will be very small, below 2 deg. The current
target FOV is 0.25 deg. The pulsars photon arrivals are detected with a single
pixel Silicon Drift Detector (SDD) sensor with timing accuracy below 1usec. The
unit has no gimbaling to meet the applicable power, size and mass requirements.
Instead, the host spacecraft shall slew and point to allow pulsar observation.
The avionics architecture is based on a radiation hardened LEON4 processor, to
allow a synchronous propagation task and measurement generation and orbit
determination step in an asynchronous task. PODIUM will enable higher autonomy
and lower cost for interplanetary missions. L2 space observatories and
planetary flybys are the current reference use cases. Onboard autonomous state
estimation can reduce the ground support effort required for navigation and
orbit correction/maintenance computation, and reduce the turnaround time, thus
enabling more accurate maneuvers, reducing the orbit maintenance mass budget.
