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During the launch phase, the solar panels are deployed and Sun-pointed, communications with the ground are established, ranging is activated, and the contents of the Solid-State Recorder (SSR) partitions 4 and 6 are sent to the ground. Of particular interest are the housekeeping data recorded prior to signal acquisition by the Madrid Deep Space Network (DSN) ground station. This initial SSR playback will require on the order of a few hours. That data is sent to the Mission Operations Centre (MOC) for assessment. If this assessment reveals no anomalous behaviour or unexpected Spacecraft condition, then the Observatory is put into Sun-point State and the mission phase transitions to EOP. During the first week in flight, all spacecraft subsystems perform their initial functional checkouts. This first phase of basic subsystem checkouts will occupy approximately the first week of the flight mission. During the first days, Mission Design (MD) updates the reference trajectory to reflect the known launch accuracy and launch time.

On mission day 10, MD provides the updated trajectory to the Navigation team to allow them to begin designing TCMs 1 through 3. At this time, ground communication is via the coupled low gain antennas (LGA), with contacts providing continuous coverage. The data downlink rate is commanded to 10 or 20 kbps for use in the coming days. The use of coupled antennas at separation provides the greatest solid angle coverage to maximize the probability of successful communication with the Observatory in the event of an attitude anomaly after separation. However, data rates via the coupled LGA will diminish rapidly in the coming days due to the Earth moving into the null between the two antenna patterns. Spacecraft will use Inertial-point State to roll 20 degrees about the Y axis in order to move Earth out of the null between the coupled LGAs. (On day 6, the spacecraft will transition to the single forward LGA and the 20 degree off-point can be removed.) The successful completion of the launch phase demonstrates that a number of spacecraft subsystems are functioning correctly. The first priority in EOP is to ensure the proper functioning of the spacecraft subsystems needed to remain power-positive and in contact with the ground. To this end, the ADCS performs a health assessment of two-point states (Earth and Inertial) that were not exercised during Launch.

Next, a checkout of the telecommunication subsystem is carried out. Data rates between 101.6 bps and 50 kbps are tested at varying mod indices. The delta-V thrusters are initialized, followed by EPS subsystem calibration of the battery charge monitor and shunt monitor. On day 6, the Sun-Probe-Earth (SPE) angle will be approximately 90 degrees and steadily diminishing. This makes the geometry favourable for the use of the single forward LGA. The spacecraft will transition from the coupled LGAs 2/3 and begin using LGA1 as the default. At this time, the 20-degree off-point will be removed, and nominal Sun-point state will be used.

Late in the first week, the propulsion subsystem will test the catbed heaters for both the RCS and delta-V systems. At some point during this first week, it is expected that the reaction wheels will be desaturated. This will be triggered by the wheels reaching a pre-set threshold and will be scheduled opportunistically between other activities. The first instrument activity on day 14 consists of EMUS performing initial power on and checkout, then opening the detector door to allow outgassing and detector cavity clean-up to begin. To prepare for the subsequent detector scrub process, EMUS performs more extensive tests of its spectrograph and electronic systems. EXI then performs its electronic box checkout, followed by the EMIRS instrument initial power on and aliveness test.