Constellation Mission Operation

4.2.1 Launch and Orbit Injection

After successful launch the F3 constellation has the following orbit characteristics [57], [61]:

– SMA: 6893 km;

– Eccentricity (E): 0.00323;

– Inclination (I): 71.992^o;

– Right Ascension Ascending Node (RAAN, Ω): 301.158^o.

The six identical satellites are deployed into six mission orbits with the following orbit characteristics for i=1~6:

– SMAi: 7178 km;

– Eccentricity (Ei): < 0.014;

– Inclination (Ii): 71.992^o;

– RAAN (Ωi): Ω5, (Ω5 - 30°), (Ω5 - 60°), (Ω5 - 90°), (Ω5 - 120°), (Ω5 - 150°) ±5^o; – (AOL, L_i): L₅, (L₅ - 52.5°), (L₅ - 105°), (L₅ - 157.5°), (L₅ - 210°), (L₅ - 262.5°) ± 8^o

4.2.2 Collision Avoidance

The separations of F3 spacecraft from the final stage of the launch vehicle relied on the separation mechanism built into the structure of each spacecraft. All the six satellites were injected heading along the velocity direction. The separation of each spacecraft from the spacecraft stack and the final stage of the launch vehicle obey the conservation laws of momentum and energy. As a result of calculation, the velocity after separation should be

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We conclude that the spacecraft will not collide with each other because the velocity of spacecraft N is always faster than the velocity of spacecraft N+1. When taking into account the variance and the accuracy of measurement, there may be approximately 12.5% variance in the energy of the spring in F3’s case. To avoid collisions, the compressions of the sets of springs for each spacecraft are different: x_FM1 > x_FM2 > x_FM3 > x_FM4 >x_FM5 > x_FM6. The resulting separation simulation results are illustrated in Figure 4-2. The separation intervals are set at 60 seconds. The higher dashed line represents +12.5% of specified spring energy, and the lower dashed line represents -12.5%. In Figure 4-2 Distance = 0 represents an imaginary object which is the non-separated final stage and spacecraft suite. And the different slopes correspond to different velocities. If the lines do not intersect each other, no collision is expected to happen.

4.2.3 Separation Sequence

the final stage of the launch vehicle as the first separation is triggered. Additional simulation analyses were performed and the results indicated that the relative positions with respect to the six satellites and final stage are adequate to avoid collision. However, the effect of residual thrust did result in changes to the spacecraft sequence. The expected spacecraft sequence should be FM6->FM5->FM4->FM3->FM2->FM1 based on the designed installation of a separation spring without the 4^th stage residual thrust. The satellite cluster sequence with the anticipated 4^th stage residual thrust after launch became FM6->FM1->FM5->FM4->FM3->FM2. FM1 has lagged behind as expected in the cluster sequence since it has the least effect due to the 4^th stage residual thrust. This sequence change has no practical impact on flight operations or mission operations [57], [61].

4.2.4 Beacon Mode Exit

Each of the satellites flew in a cluster after launch and all beacon modes of the satellites worked well for the first and second orbit. However, problems were encountered when not receiving telemetry from spacecraft at the third and the fourth orbit after launch. The exit-beacon-mode-flag uplink command was sent to all six satellites and verified the downlink signals of all satellites at the fifth orbit. It was later determined that the reason for the erroneous telemetry reception on orbits three and four was that the onboard bus GPSRs aboard FM3, FM4, and FM6 were unable to lock onto the GPS signals for proper time synchronization for the beacon mode [32].

4.2.5 Spacecraft and Payload Checkout

The spacecraft checkout starts when the satellite exits the beacon mode after the initial spacecraft acquisition. The flight software configurations were checked and confirmed as normal on all six satellites, initially, and later the navigation anomalies that were attributed to the erroneous GPSR behaviors appeared at Launch plus three (L+3) days. It was not possible to isolate the root cause of these erroneous GPS behaviors. However, an alternative

resolution of feeding the known state vector to each spacecraft via uplink commands regularly was able to stop the GPS-related navigation anomalies. All six satellites were ready to be powered on the payload at L+6 days. The GOX payload of each spacecraft was powered on first at L+6 days, the TIP payload on at L+8 days, and TBB payload on at L+13 days respectively, according to the operation in-orbit checkout plan [32].

4.2.6 Constellation Deployment

During the L&EO phase the satellites were separated one by one into the same injection orbit with the same RAAN and RAAN drift rate. The strategy to differentiate the RAANs among the six orbits is to maneuver the six satellites into the mission orbit altitude of 800 km at different “maneuvering windows” (typically 45 days) in the year in order to get into the designated separate orbital planes through nodal precession. All satellites will reach their final orbits with each designed RAAN and AOL at this phase [32], [57].

4.2.7 Final Constellation and Extended Mission

The final constellation of F3 has six orbit planes as shown in Figure 4-3. Each orbit is at an altitude of 800 km with an inclination angle of 72^o. The separation angle among orbit planes is 30^o and the AOL separation between satellites in adjacent orbit planes is of 52.5^o. The final constellation allows the six satellites to collect 1,800 to 2,200 atmospheric sounding data on an average per day worldwide.