ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 371 of 422
From the Figure 4-106 it can be mentioned that the long rendezvous technique does not require
any optimal correction manoeuvres in between. This strategy is targeting several intermediate
orbits until reaching the final one. It has also two possible sub-scenarios: to place the MAV in a
low orbit at 500x500 or to place the MAV in a high orbit at 17000x17000 (the Mars geostationary
orbit).
Supposing that the final orbit is 500x500, the long rendezvous needs to:
• adjust the nodes of the two orbits of our vehicles (the so called phasing part of the
rendezvous): the Capsule Module (CM) of the MAV with three astronauts on-board at
450 km altitude with the Orbital Module (orbiter) at 500 km altitude
• establish a plane correction (PC) burn: this burn will align the orbital planes of the
capsule and the orbital module. Most likely that this PC will be small, but is any case it
will be needed due to the MAV ascent dispersions.
• travel the altitude difference of 50 km between the two orbits
• prepare and allow terminal rendezvous
4.5.2.4 Phases of RvD for the long rendezvous strategy
This section is devoted to explain the long rendezvous strategy as selected by the system
enginner.
Let us select the LHLV coordinate system, which has the origin in the target, one axis pointing in
the direction of the flight and the other one perpendicular pointing towards the planet. The
selected approach for the RvD mission arc is based on the “above and ahead manoeuvre” type:
the active chaser moves towards the target from a lower orbit and behind the target in the relative
position.
There are three phases during this RvD in circular near co-planar orbits:
• 1st Phase. Preliminary RvD: Find the target (wherever the target is) and determine its
orbit
• 2nd Phase. Intermediate RvD: Find the target (wherever the target is) and determine its
orbit.
• 3rd Phase. Terminal RvD: Final approach and structural latching
• 4th Phase. Transfer of astronauts to the orbiter and de-docking of the MAV.
A general overview of the activities involved in the different phases is sown in Figure 3. The
GNC units needed in each mode are listed at the end of each subsection.
4.5.2.4.1 Preliminary rendezvous
First thing to do is to achieve a precise orbit determination of the orbiter. This is done by means
of Deep Space Network Doppler from Earth ground station. One week is needed to achieve an
accuracy of meters and meters per second in position and velocity.
At the same time the orbit of the MAV will be computed, ranging from the orbiter to the MAV
first, and via Deep Space Network after. The radio finder (RF) system will be used to locate
initially the MAV.
Finally, the phase angle between the orbiter and MAV will be progressively reduced (lower orbit
has shorter orbital period). This phase ends with a relative small true anomaly between chaser
and target, and a small difference in orbital planes.