Figure 3.1.: Overview of the International Space Station showing the Local Vertical (LV)/Local Horizontal (LH) Reference Frame.
The ISS flies in a 51° inclination orbit. The nominal maximum altitude of the quasi-circular orbit is 450 km. The perturbation forces cause a natural orbit decay of the ISS that has to be compensated by periodic reboosts. The altitude of the station is therefore slowly decreasing from maximum to minimum, and is then periodically boosted up back to the maximum. The resulting short term altitude variations are shown in Figure 3.2.
Figure 3.2: ISS Short Term Altitude Variations.
The Space Station altitude is determined mainly by logistics and safety considerations. It will be kept as low as possible to maximize the upioad/resupply capability and as high as necessary to minimise drag effects even if a reboost manoeuvre - normally scheduled every 90 days - is missed. As a consequence of this compromise, there will be considerable long term variations in the orbital altitude of Space Station dictated by the solar activity variation (11 year) cycle. At times of low solar activity the atmospheric density at any given altitude decreases and the minimum safe altitude can be as low as 350 km. Around solar maximum the atmosphere expands and the minimum safe altitude will be around 450 km. The resulting long term altitude profile will be as shown in Figure 3.3. Superimposed over these long term variations will be the variations of Figure 3.2.
Figure 3.3.: ISS Long Term Altitude Variations. (A long term strategy for altitude maintenance is shown. The sawtooth curve is an altitude profile which guarantees that the ISS stays above the nominal drag threshold of 278 km, should one 90 day reboost be missed).
ISS shown in Figure 3.1 will fly in an attitude where the
velocity vector is roughly
perpendicular to the ITA and aligned with the long axis of the
forward pressurized module
group. The ITA will thus always be oriented perpendicular to
the orbital plane and the third
axis of the space station will be aligned with the
zenith/nadir direction. The attitude dehned in
this way is the Local Horizontal/Local Vertisal (LH/LV)
attitude. More precisely the station flies
in a Torque Equilibrium Attitude (TEA) where the average
environmental torques sum to
zero. The combined forces of atmospheric drag and gravity
gradient acceleration will cause
deviations from the ideal attitude. The attitude control
system will limit these deviations to less
than 5°axis maximum. The inertia of the overall
configuration will ensure slow variations
in attitude limited to 0.02 degrees per second for each axis.
In summary the payload sites attitude oscillates around the LV/LH by up to 3.5° to peak over one orbit, corresponding to a worst case attitude variation of typically ±1.5° over an observation period of 15 minutes.
External payloads can be accommodated at several locations on the ISS (See Figure 3.1 .):
The handling and installation of individual EXPRESS Pallets and single EPAs with payload will be achieved with the Space Station Remote Manipulator System (SSRMS) and the Special Purpose Dextrous Manipulator (SPDM). These operations are described in more detail in Section Ground Operations for the Flight Segment
The salient features of the SSRMS are as follows:
The salient features of the SPDM are as follows:
The SPDM standard envelope is what ultimately limits the allowable height (1.25m) of instruments on the EPA (see Section 4)
The SSRMS and the SPDM are shown as part of the Mobile Servicing System (MSS) in Figure 3.6. whilst Figure 3.7 shows the SPDM with two EXPRESS Pallet Adapters attached.
Figure 3.6.: The Space Station Remote Manipulator System (SSRMS) and the Special Purpose Dextrous Manipulator (SPDM) shown as parts of the Mobile Servicing System (MSS).
Figure 3.7.: The SPDM with two EXPRESS Pallets (arrowed).
The salient features of the EXPRESS Pallets are as follows:
Dimensions; 3.937 m x 2.286 m; Payload Mass Capability; 1350 kg (at launch); Payload Power; 120Vdc and 28Vdc - total upper limit is 2.5kW; upper limit for 28Vdc supply only is 1 kW; Payload Command,Control, Data & Telemetry; RS232/422,MIL-STD-1553B,HRDL,nalog,Discrete.
The EXPRESS Pallet concept includes the EXPRESS Pallet Adapter, which allows the mounting of up to six individual payload complements on one Pallet.
The salient features of the EXPRESS Pallet Adapters are as follows:
Dimensions; 1.213 m x 1.054m x 0.053m; Payload Mass Payload Mass Capability; 225 kg per adapter; Payload Power; 120Vdc and 28Vdc-total upper limit is 2.5kW; the actual power to the payloads on a single EXPRESS Pallet Adapter depends on manifesting & resource allosation; upper limit for 28Vdc supply only is 500W; two non-redundant) power outlets per EPA, each either 120VDC or 28VDC; each outlet separately switched and fused inside the EP power subsystem. Payload Command7 Control, Data & Telemetry; RS232/422, MIL-STD-1553B, HRDL, Analog, Discrete; two non-redundant data lines per EPA. HRDL single link is switched between EPAs according to demand, with usage timelined.
Attached payloads will be able to face the zenith (upwards), the nadir (towards Earth), the Earth limb, the ram (forward of the Space Station) or the wake (behind the Space Station) orientation depending on which EXPRESS Pallets and Adapters they are mounted. Six single EXPRESS Pallet Adapters are shown on the EXPRESS Pallet in Figure 3.8.
Figure 3.8: EXPRESS Pallet with Six Single Adapters.
The sharing of resources between the various instruments mounted on the EXPRESS Pallet Adapter will result in specific interface requirements which are dealt with in Sections 4 to 8.
For the early opportunity mission ESA will provide a pointing capability. The Phase B of the development of a Coarse Pointing Device (CPD) has started. The CPD is foreseen to be provided in two versions. The first is intended for space science instruments and the second is intended for space biology exposure payloads. Both will be equipped with a solar sensor to enable more accurate tracking. The pointing system will guarantee a required performance for a given specified ISS environment at the interface level with the instruments. The performance achievable in terms of pointing accuracy and stability will be directly dependent upon the following factors:
The performance characteristics common to both CPD versions are as follows:
Location: Upper side of the ISS ITA Pointing Accuracy: ±1° Pointing Stability: tbd arcsec/sec Number of rotational axes: 2 Sun exposure time per year: 600 hours minimum (approximately 15 minutes per orbit on average) CPD height (inclusive of payload): 1.25m
The performance characteristics specific to CPD version 1 are as follows:
CPD load capability: 100 kg (payload / instruments mass)
The performance characteristics specific to CPD version 2 are as follows:
CPD load capability: 30 kg (±10%) Payload mounting area: 650 x 450 mm (±10%)
A conceptual block diagram of the CPD and its power and data interfaces to the EPA, accommodating some instruments with a solar pointing requirement. is shown in Figure 3.9.
Instruments may be mounted on a support/mounting plate provided by the mission integrator. This option is described further in Section 5.
A custom-built PDU will be developed for each grouping of instruments on an EPA once the instruments have been selected and their overall integration scheme decided. The PDU funstions are described in more detail in Section 7. Figure 3.9. shows a PDU.
SPLCs will manage some data requirements of instrument groupings. Other data requirements may be handled by direct connection to the EPA. Section 8 describes the SPLC and the data services provided in more detail. SPLCs are standard items which may be customised for individual instruments or groupings of instruments. Figure 3.9 shows an SPLC.
Figure 3.9 Conceptual Design of the Coarse Pointing Device (CPD) with Solar Pointing Instruments. The PDU and SPLC are also shown.