Figure 1. The International Space Station
In the framework of the Agreement for the Early Utilisation of the International Space Station (ISS), ESA has negotiated with NASA to have early flight opportunities for European payload instruments prior to the launch of the Columbus Orbital Facility, in exchange for the delivery to NASA of a set of items that constitute the early European contributions for the utilisation of the International Space Station.
One of these early European contributions is the Hexapod positioning/pointing system, which is being developed under the responsibility of ESA's Directorate of Manned Spaceflight and Microgravity to provide a pointing platform for the NASA Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument. Hexapod and SAGE III are designed to be accommodated on an ISS Express Pallet and have a five-year design lifetime. They will be launched with Space Shuttle flight UF-4 at the beginning 2002.
The ISS-VIEW software tool has been developed to support the mission analysis and to define the observational windows for Hexapod/SAGE III, but it can also be used for other instruments. For example, it also allows one to predict astronomical or Earth-target observing opportunities for Express Pallet payload instruments from their actual mounting locations on the Space Station Truss. In addition to the orbital and attitude parameters of the Station, ISS-VIEW is also able to take into account the field-of-view restrictions imposed by both the fixed and movable elements of the overall Space Station assembly.
The International Space Station (ISS) will be a large permanently manned space platform capable of supporting long-duration scientific and technology research projects in space. Traditional microgravity disciplines such as the science of materials and fluids, human physiology, biology, chemistry and bio-technology will benefit considerably from the long-term availability of such a facility with a gravity-free environment. Earth remote sensing and space science too will be able to take advantage of the availability of such a large platform in low Earth orbit with frequent access opportunities.
The ISS (Fig. 1) is the biggest international space enterprise ever and is the result of the combined efforts of the United States, Russia, Japan, Canada and Europe. The European contribution is provided through the ESA Programme for European Participation in the International Space Station, funded by 10 ESA Member States*. It is the biggest manned space programme ever undertaken by ESA. The Station will provide the space infrastructure needed to carry out experiments in both pressurised and unpressurised environments (further information can be found in the ESA Guide for European Users, ESA SP-1202, available from ESA Publications Division). The first assembly elements are scheduled to be launched in 1998, starting an assembly sequence that will take about half a decade. The permanent habitation of the Station by a crew will, however, already start some six months after the launch of the first element.
Figure 1. The International Space Station
* Belgium, Denmark, France, Germany, Italy, Netherlands, Norway, Spain, Sweden, and Switzerland.
Dedicated platforms called Express Pallets will accommodate external payload instruments at four locations on the starboard side of the Space Station Integrated Truss Assembly, inboard of the solar arrays. Two Express Pallet mounting locations will offer zenith-oriented viewing, and the other two locations will be nadir-oriented. Each Express Pallet will provide the interface infrastructure (six adapters) and the resources needed to accommodate several smaller payload instruments.
Figure 2 shows the segment of the Station's Truss (known as segment S3) that will host the Express Pallets, and two Pallets are shown accommodated on top of it. Figure 3 shows the Express Pallet architectural concept, with square boxes representing the payload instruments that can be accommodated on the six Express Pallet Adapters.
Figure 2. The zenith- and nadir-facing Express Pallets, attached to the Station's truss structure
Figures 3. Express Pallet architectural concept
On the basis of the ESA/NASA Agreement on Early Utilisation, in exchange for the Hexapod's delivery to NASA European users will benefit from early flight opportunities for their instruments on these Express Pallets.
The NASA Stratospheric Aerosol and Gas Experiment III (SAGE III) is the fifth in a series of spaceborne Earth remote-sensing instruments developed at NASA's Langley Research Center. The industrial contractor for its development is Ball Aerospace (Boulder, Colorado). SAGE III will use both solar and lunar occultation techniques to perform the spectrometric measurements needed to monitor the global distribution of aerosol and gas constituents in the Earth's atmosphere. Three flight units will be produced, each one adapted to fly on a different spacecraft carrier. One of these units will be flown on the ISS, mounted on the Hexapod pointing system developed by ESA.
Hexapods are positioning/pointing devices that are able to control the six degrees of freedom (positional and rotational) of a rigid body in three-dimensional space. They are frequently used in several types of ground applications, ranging from high-accuracy applications in optics and manufacturing machinery, to dynamic applications in car/flight-dynamics simulators and active anti-vibration mounts.
Hexapods are typically composed of six linear actuators (legs) arranged in the shape of three trapezia and interconnected by means of 12 universal joints to a lower and an upper mounting plate. The payload is normally accommodated on the upper mounting plate and the lower mounting plate is fixed to the hosting carrier. Computerised coordinated control of the lengths of the six legs allows the relative displacements and attitudes of the upper and lower mounting plates to be precisely controlled.
The pointing device for SAGE III will be the first European space mechanism based on the Hexapod principle. Following preliminary feasibility assessments, the Hexapod Project underwent a Pre-Phase-B pre-development activity (contractor ADS Italia) lasting from December 1994 until October 1995, which included the prototyping and testing of several linear actuator options, and a Phase-B design study, completed in March 1997 (prime contractor Alenia Spazio, with ADS Italia and Carlo Gavazzi Space as subcontractors), which included the breadboarding of a development model with full functionality (Fig. 4), and extensive testing activities.
Figure 4. The fully functional Hexapod development model
Hexapod and SAGE III will be mounted on a nadir-oriented ISS Express Pallet.
Nominally, the International Space Station will operate in a 90 min orbit with very low eccentricity and 51.6 deg inclination, in a local vertical - local horizontal attitude fixed with respect to the local nadir and the direction of motion. Tracking a target from the Station requires relatively high pointing rates for relatively short periods. The rates for tracking the Sun or a star are typically in the order of 0.07 deg/sec, and the viewing time for a 60 deg viewing range for a celestial source lying in the orbital plane is 15 min/orbit. This time increases for sources located away from the orbital plane.
The actual field of view will be very much limited by various obstructions associated with the Station's structure, which introduces constraints on the visibility of the payload targets. The fixed parts of the Station assembly will intrude into the payload fields of view in such a way that their profile may change as the Station's configuration evolves over time during the assembly sequence. Obstructions from the solar arrays and radiators will impose important limitations on the fields of view from the Express-Pallet mounting locations (Fig. 5). The field-of-view obstruction caused by the solar arrays and by the radiators will also be time-dependent. In order to maximise energy generation, the solar arrays will normally be rotated during the day both around their axis and around the Truss to maintain perpendicularity to the Sun; at night they will be aligned with the Station's flight direction to reduce drag. The radiators will also turn to optimise heat rejection.
Figure 5. Zenith-looking field-of-view from the Express Pallet
Temporary field-of-view limitations will be caused by re-supply vehicle docking, by the movements of robotic manipulators, etc. Other obstructions may be mission-dependent, such as the presence of large payload facilities. These complications, combined with the short-duration visibility of many of the targets - which will call for relatively high tracking rates, particularly if they are located on the Earth's surface - mean that accurate mission analysis and observation planning for the remote-sensing and space-science instruments that will be operating from the Station are of critical importance.
All in all, therefore, the Station's complex configuration, combined with the regularly changing geometry of its moving parts, imposes the use of sophisticated software tools for accurately predicting the visibilities of specific targets of interest.
Observations from Hexapod/SAGE III will require unobstructed viewing of the Sun and the Moon during sunrise (moonrise) and sunset (moonset), i.e. when the Sun's (Moon's) light passes through the terrestrial atmosphere prior to entering the SAGE III spectrometer. It is precisely to identify these opportunities that ISS-VIEW has been developed (by CARSO of Italy, under the supervision of the Hexapod prime contractor).
ISS-VIEW has been developed as a software tool primarily to support the engineering analysis and the mission planning for Hexapod/SAGE III. However, it has been conceived in such a way that its use can be generalised to other Earth-observation, solar and astronomy payload instruments mounted on either the upper or lower Express Pallets of the ISS.
It is possible with ISS-VIEW to simulate the payload observations by using a polar display showing the hemisphere of observational interest. The payload instrument is assumed to be in the centre of the polar display; the entire spatial sphere is represented, with the hemisphere behind the instrument displayed at a lower resolution. ISS-VIEW simulations include modelling of the profiles of the ISS fixed and moving obstructions as a function of the Express Pallet payload position and of time. The key data of the analysis run, such as time, attitude, obstruction/observation periods and target positions, can be recorded as outputs for the user.
Background
Precursors of ISS-VIEW are the UVS-VIEW and the GLO-VIEW software tools, developed for two Space Shuttle Hitchhiker pointing instruments UVSTAR and GLO, respectively. The latter were part of the International EUV Hitchhiker (IEH) experiment flown in 1995 on Shuttle flight STS-69 and are planned to fly a second time (five flights in total) in August 1997 on STS-85. GLO is a University of Arizona (Tucson) programme, while UVSTAR is a collaborative effort by the Universities of Trieste and Arizona. GLO-VIEW and UVS-VIEW are used to help scientists predict available observing windows, thereby assisting in the effective planning of both observation and mission strategies.
With UVS-VIEW and GLO-VIEW, the centre of the display is the Space Shuttle's -Z axis (out of the bay). The viewing horizon is the Space Shuttle envelope as seen from the Hitchhiker bridge. The entire spatial sphere is shown, with the +Z hemisphere (the one behind the Space Shuttle bay) represented at lower resolution.
When UVS-VIEW or GLO-VIEW are started, after the initialisation phase (in which they read the input data files), they switch into a graphical mode to display the spherical map, and the Mission Elapsed Time clock starts running. Current Mission Elapsed Time and Space Shuttle attitude are shown to the user. The Earth or space targets selected are displayed; Earth, Sun, Moon, Jupiter, and the spacecraft velocity vector direction are always shown by default. The Earth's night and sunlit limbs and the terminator are also indicated.
The ISS-VIEW package
ISS-VIEW, developed under an ESA contract, retains many features of its UVS-VIEW precursor. The major modifications introduced consist of several improvements to the package's man/machine interface and adaptations to take into account the particular configuration and attitude characteristics of the International Space Station. It allows observers to simulate observing windows, to plan target sequences, and to select instrument pointing criteria. The software simulation results can be recorded, and the user also has a playback facility.
Running ISS-VIEW requires an IBM-compatible 386, 486, or Pentium computer running DOS 3.1 or higher. Both standard VGA and Super VGA cards are supported. The software can be made available to users upon request.
The modelling of the Space Station consists of the envelope (mask) of the Station 'horizon', represented in a polar geometry, as seen from one of the lower (or upper) Express Pallets. The mask model takes into account both the fixed parts of the Station and the main moving parts: the US and Russian solar arrays and the radiators. The simulation also takes into account the fact that during the day (i.e. during the part of orbit when the Station is exposed to the Sun) the US solar arrays rotate both around their axis and around the Truss in order to remain perpendicular to the Sun's direction. The modelling also considers the movement of the Russian solar panels to track the Sun, and the rotations of the radiators to optimise heat rejection.
ISS-VIEW can be easily modified to account for the presence of additional mission-dependent Space Station obstruction sources (e.g. large payloads like the Materials Exposure Facility), and if necessary the time-dependent profiles of robotics arms and docking vehicles can also be introduced. Changes in the Station's geometry can be programmed into a specific file modelling the Station mask (ISS.MSK). Different masks are provided for instruments mounted on the upper and lower Express Pallets.
The centre of the graphical display is the instrument Z-axis (zenith-nadir direction for Earth-oriented instruments; the opposite direction for sky-oriented instruments); the hemisphere behind the instrument is shown at a lower resolution. The current Observation Elapsed Time and Station attitude are displayed on the screen. The default display includes the Sun, the Moon, Jupiter, the Earth's centre and the Station velocity vector. The Earth's limb is represented, with dark blue used to indicate the night limb and bright blue the sunlit limb. When visible, the terminator is shown as a bright yellow line. Earth latitude and longitude lines (if selected for display) are shown at 5 or 10 deg intervals, facilitating correlations with specific points on the Earth's surface. The auroral oval is also displayed, as a green line or as an oval, when it is visible; it is calculated for a height of 100 km and so it appears slightly above the Earth's limb. Other command-line options include the possibility to select the stars displayed, based for instance on the intensity of their ultraviolet emission. The program can be run in either interactive or automatic mode.
To facilitate its use, ISS-VIEW has an online Help feature.
Mounted on one of the lower Express Pallets of the ISS, and suitably aligned by Hexapod, SAGE III will use the solar/lunar occultation technique to perform spectrometric measurements to monitor the global distribution of aerosol and gas constituents in the Earth's atmosphere. Sun- or moonlight will be collected via an aperture near the nadir end of the instrument. A scan mirror directs the collected light to a telescope, which focuses it at the entrance slit of a spectrometer. Light entering the spectrometer is reflected by a folded mirror to a spherical holographic grating, which disperses first-order light onto a Charge-Coupled Device (CCD) detector. The CCD will record eight different spectral channels of science data. A ninth data channel is provided by the zero-order light, appropriately filtered; in this case the detector is a photodiode.
Observations from Hexapod/SAGE III require unobstructed viewing of the Sun or the Moon during sunrise (moonrise) and sunset (moonset). Once the target has been detected and acquired, the SAGE III azimuth pointing system will keep the instrument locked onto the target's vertical centroid whilst the elevation mirror starts scanning at constant angular velocity until it scans off the source disk, and then reverses scan direction. In the sunrise (moonrise) case, SAGE III will then continue to scan the light source disk through the atmosphere from the moment when the disk is in an apparent position tangential to the Earth's surface, until the moment it reaches an apparent altitude of 300 km (the opposite sequence will apply for sunset/moonset). Observations made when the source disk has a tangent height of between 150 and 300 km (i.e. when the incoming light does not pass through the Earth's atmosphere) will be used to obtain the unattenuated source intensity for instrument self-calibration.
To illustrate the application of ISS-VIEW for Hexapod/SAGE III operations, Figures 6a-d present a sequence of ISS-VIEW displays for different viewing situations. Figure 6a shows the unobstructed sky as seen by an observer looking at the Earth from the same orbit as the International Space Station. In Figures 6b-d, the Space Station masks are as seen from one of the lower Express Pallets, where Hexapod/SAGE III is to be accommodated. The mask used in Figure 6b refers only to the fixed parts of the Station - basically the pressurised elements and the Truss assembly. In Figures 6c-d, the Space Station 'horizon' includes the obstructions generated by the rotating panels (solar arrays and radiators) of the station and also the Materials Exposure Facility, as an example of a mission-dependent temporary obstruction, assumed to be accommodated on an Express Pallet adjacent to Hexapod/SAGE III. Figures 6c and d refer to sunrise and sunset, respectively.
Figure 6a is intended to illustrate the unobstructed-sky view as seen from a free-flying spacecraft that looks towards nadir (centre of the graphical display) from the same orbit as the International Space Station. The heavy white circle is the observer's horizon. The hemisphere behind the observer, lying outside the horizon circle and represented at lower resolution, contains the Moon (green crescent) and Jupiter (magenta circle). The Sun (yellow circle) is on the horizon and illuminates part of the Earth (marked in bright blue). The terminator is visible as a bright yellow line. The Earth's night limb is represented in dark blue. The latitude and longitude lines are displayed on the Earth's surface in red and magenta, respectively, at 5 deg intervals. The auroral oval is visible near the South Pole as a projected green oval. The direction of the Space Station's velocity vector is indicated by a red cross surrounded by a circle; it fixes the X-axis direction and has (0, 0) azimuth and elevation coordinates.
Figures 6a. Sequence of ISS-VIEW displays for different viewing situations
In the simulation of Figure 6a the observer's viewfinder is assumed to be 5 deg wide, represented by the 5 deg field-of-view box pointed according to the azimuth and elevation coordinates reported on the right side of the display (Az = 90 deg, El = 5 deg). The corresponding inertial coordinates of right ascension and declination (RA and Dec) of the centre of the field of view are also reported on the display. For the sake of greater clarity, no stars have been represented. The Log-Off sign in the top left corner indicates that the data are not being recorded. Other self-explanatory information is given on the right side of the display.
In Figure 6b, the fixed elements of the Station (as foreseen at assembly completion) are shown as seen from Hexapod/SAGE III (which is at the centre of the display). Under the proposed orbital conditions, the full Earth is now dark and the Sun is illuminating its other side.
Figures 6b. Sequence of ISS-VIEW displays for different viewing situations
Figures 6c,d represent the sky obstruction with the full profile of the Station, including solar arrays, radiators and with the Materials Exposure Facility present. The simulations have been run for sunrise and sunset, respectively, when SAGE III performs its spectrometric measurements. Here a 25 deg-wide field-of-view box has been used. The attitude file used for the simulations shown in the sequence of Figure 6 has a local vertical - local horizontal hold with constant zero roll, pitch and yaw angles. Like its predecessor UVS-VIEW, ISS-VIEW allows one to select several different attitudes (although in reality the Station will not use all of them). However, as a further improvement introduced in ISS-VIEW, it is possible to simulate the Station torque equilibrium attitude also, adding amplitude and frequency periodic perturbations around the nominal local vertical - local horizontal attitude.
Figures 6c,d. Sequence of ISS-VIEW displays for different viewing situations
These simulations have been performed with a given set of orbital parameters for the International Space Station. It should be noted, however, that given updated knowledge of detailed orbital parameters, different sets of inputs can be selected for ISS-VIEW runs.
Figures 7a,b show the effect of precession by changing the ascending node of the orbit from 109.6 deg to 169.6 deg.
Figures 7a,b. Effect of precession caused by changing ISS orbit's ascending node from 109.6 to 169.6 deg
The zenith-oriented Express Pallets on the Space Station will mostly be used by payload instruments that need to view large parts of the sky, such as solar and astronomy instruments. Figure 8 shows an example of the field-of-view capabilities (zenith direction) from the upper Express Pallets. In the top/left part of the hemispherical display, we can identify the mask shapes of the Russian elements, with their solar arrays. The Sun and other celestial bodies are visible from the Express Pallet mounting location when they are above the line of the Space Station mask (which is a sort of local horizon).
Figure 8. Typical field-of-view for solar/astronomy payloads from the upper Express Pallet
Figure 9 gives the visibility of the Sun at three different selected Observation Elapsed Times. The parameter that measures the Sun's visibility is the solar zenith angle, i.e. the angular distance, measured on the geocentric celestial sphere, between the right ascension and the declination coordinates of the station and the Sun. Three solar-zenith-angle curves are plotted in Figure 9. Curves (a) and (b) have been calculated starting at Observation Elapsed Times of 02/04:24.:00 and 10/03:44:00, respectively. They both show the effect of obscuration by the Russian solar arrays and their supporting structures. Curve (c) has been calculated at an Observation Elapsed Time of 11/02:53:00 with a change in the value of the ascending node angle to simulate the precession.
Figure 9. Visibility of the Sun at three different Observation Elapsed
ISS-VIEW is a valuable new tool for supporting mission analysis and planning activities for Earth-observation and solar/astronomy instruments to be operated from the International Space Station. The simulation software includes knowledge and display of Earth latitude and longitude, and hence it can also be used to make visibility predictions for targets located on the Earth's surface.
ESA Bulletin Nr. 91