<p>The Project for On-Board Autonomy – 3 (PROBA-3) of the European Space Agency (ESA) is a technological demonstration mission for precise formation flying of two spacecrafts in a Highly Elliptical Earth Orbit (HEO). The mission’s main objective is to verify and validate high precision formation flying techniques, performing formation control, collision avoidance, and rendezvous operations; this goal can be achieved by operating a suite of different metrology systems. In addition to the technological goals, PROBA-3 accommodates the Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun (ASPIICS) scientific payload that takes advantage of the high precision formation flying capabilities to create a 144-meter externally occulted Coronagraph Instrument (CI), with the occulter on one satellite and the telescope on the other one. The metrology systems play the critical role for the success of the mission, and among these, the Shadow Position Sensors (SPS) has been designed and engineered to return the formation position with the highest accuracy. The SPS system consists of a set of 8 silicon photo-multipliers (SiPM) mounted in front of the ASPIICS telescope monitoring the symmetry of the penumbra projected by the external occulter on the coronagraph’s entrance pupil plane. A dedicated metrology algorithm converts the SiPM readout currents in relative and absolute lateral and longitudinal position coordinates that are provided to the GNC for fine formation maintenance manoeuvring. The flight algorithm has been developed by the team of the Astrophysical Observatory of Turin (OATo), part of the Italian National Institute for Astrophysics (INAF), and is based on a pseudo-paraboloidal description of the expected penumbra profile. The algorithm has been successfully tested during the ASPIICS calibration campaign held in 2021 in Turin, Italy. The same algorithm has been implemented in the data reduction pipeline of the Science Operation Center (SOC) together with an alternative algorithm, proposed by ESA, in order to have a cross-validation and, when possible, a refinement of the SPS metrology performances. In this work, we followed the approach proposed by ESA to implement and validate this alternative algorithm. The suggested approach uses the Cardano’s method to determine the radial position of each sensor with respect to the centre of the projected umbra and to calculate the coordinates of the umbra centre in the reference system centred on the telescope’s entrance pupil. This procedure is computationally costly with respect to the flight algorithm; however, it can achieve higher accuracy in those regions where the pseudo-paraboloid returns greater positioning errors, while guaranteeing an axis-symmetric modeling of the penumbra profile. In this paper, the implementation of this alternative algorithm is described and the performance of the two algorithms are compared and discussed in order to determine their advantages and disadvantages when executed in the SOC pipeline.</p>