Biomass is ESA’s seventh Earth Explorer mission, dedicated to study the Earth’s forests from space. It is the first satellite to carry a P-band Synthetic Aperture Radar (SAR), which is being used to measure the global forest biomass. It was successfully launched on 29 April 2025 from Kourou aboard a Vega-C rocket. The LEOP and ensuing operations have been conducted from the European Space Operations Centre (ESOC) in Darmstadt.

The satellite is operated in a dawn-dusk, sun-synchronous orbit, with a local time of ascending node at 06:00, at an altitude of approximately 670 km. During the first 6 months, in its Commissioning phase (COM), the satellite and operational concept were validated. The operational tomographic phase (TOM) started in November 2025, which will last until April 2027, followed by the interferometric phase (INT). The latter will conclude in September 2031. The spacecraft follows a ground track repeat pattern of 44 orbits in 3 days, with a westward drift in longitude specific to each of the two phases.

The selected repeat cycle yields a large distance between nodes, of about 910 km, which the swath of the instrument cannot cover with the selected westward drift. Two novel processes were defined to achieve global coverage with the given conditions. Firstly, the spacecraft can perform observations in three distinct roll attitudes, denominated swath A, B, and C, thus covering more ground. Secondly, after finishing a loop of observations in the three attitudes, the satellite performs a relocation to a different longitude by the means of a Satellite Repositioning Manoeuvre (SRM). Both techniques are combined differently in COM, TOM, and INT. As a result, the planned mission consists not of following a repeat orbit, as is typical for Earth Observation spacecraft, but instead a non-repeatable reference defined from beginning to the end of the planned mission.

The reference ground-track of the mission corresponds to a trajectory composed of periods of scientific observation spanning multiples of three days with the aforementioned westward drift, intercalated by repositioning phases. The repositioning phases can last 9 days (TOM) or 12 days (INT), comprising a pair of drift start plus two pairs of drift stop SRMs. A reference orbit is generated by FD following the ground-track of the scientific phases, plus the one corresponding to an ideal execution of the defined SRMs. The generation of this reference orbit for a span of 7 years is computationally demanding. The failure of the scientific objectives of an observation phase could require its entire repetition, which entails recomputing and rescheduling the entirety of the reference orbit to insert the repeated phase, potentially on short notice. The partial parallelization of reference orbit generation allows for an acceleration of the process.

The operational orbit follows the reference ground-track within a maximum deviation of 2 km during scientific observation. In addition, it is required that the ground-track repeats every three days, within 100 m (TOM) or 200 m (INT) of accuracy, to fly over the same areas of interest. This drove the decision to define a manoeuvre slot every three days, not interfering with scientific activities. Finally, the altitude of the operational orbit must be within 500 m of the reference orbit.

The concept developed by ESOC Flight Dynamics (FD) for orbit control consists of, on one hand, delegating the definition of the manoeuvre slots, and overall complete scheduling to the process of reference orbit generation. This allows for a knowledge of the mission schedule years in advance. On the other hand, the orbit control is reduced to the control of ground-track and eccentricity against the reference orbit, at the slots defined by the reference. The ground-track during science phases is optimized to repeat every three days, while during repositioning phases the ground-track deviation accumulated during science is corrected.

This paper will describe the reference orbit definition plus the orbit control algorithm. Using the knowledge of the spacecraft dynamics collected during operations, the predicted performance of the orbit control for the remainder of the mission will be presented. Finally, it will include an update of the altitude control analysis, already presented in the 29th ISSFD, as well as its current implementation.