GEOScan is a grassroots effort, proposed as globally networked orbiting observation facility utilizing the main Iridium
NEXT 66-satellite constellation. This will create a revolutionary new capability of massively dense, global geoscience
observations and targets elusive questions that scientists have not previously been able to answer, and will not answer,
until simultaneous global measurements are made. This effort is enabled by Iridium as part of its Hosted Payload
Program. By developing a common sensor suite the logistical and cost barriers for transmitting massive amounts of data
from 66 satellites configured in 6 orbital planes with 11 evenly spaced slots per plane is removed. Each sensor suite of
GEOScan's networked orbital observation facility consists of 6 system sensors: a Radiometer to measure Earth's total
outgoing radiation; a GPS Compact Total Electron Content Sensor to image Earth's plasma environment and gravity
field; a MicroCam Multispectral Imager to measure global cloud cover, vegetation, land use, and bright aurora, and
also take the first uniform instantaneous image of the Earth; a Radiation Belt Mapping System (dosimeters) to
measure energetic electron and proton distributions; a Compact Earth Observing Spectrometer to measure aerosol-atmospheric
composition and vegetation; and MEMS Accelerometers to deduce non-conservative forces aiding gravity
and neutral drag studies. Our analysis shows that the instrument suites evaluated in a constellation configuration onboard
the Iridium NEXT satellites are poised to provide major breakthroughs in Earth and geospace science. GEOScan
commercial-of-the-shelf instruments provide low-cost space situational awareness and intelligence, surveillance, and
reconnaissance opportunities.
The Iridium NEXT satellite constellation has designed space to accommodate hosted payloads that
provided not only access to space but also allow the user to leverage Iridium's real time communication
capability. This is ideal for small sensor payloads and mission areas that require real-time data. The
detection of volcanic ash is one such application, meeting a critical need of warning aircraft on the location
of volcanic ash. To this end, we have described a system concept that uses small lightweight sensors the fit
within the Iridium NEXT hosted payload allocation and provide critical data needed to predict the location
and movement of volcanic ash in the atmosphere.
Many contamination lessons have been learned since the Midcourse Space Experiment satellite was launched on April 24, 1996. FOremost is the inclusion of spacecraft design and thermal engineers with the contamination team early in the program, which resulted in the placement of vents away from the optical sensors, the separation into clean and 'dirty' sections, the exclusion of thrusters, the use of non- perforated silver/Teflon on the optical sensor axis, and the outgassing procedures instituted for all subsystem suppliers. The use of contamination instruments as monitors during integration and testing enabled correct technical decision to be made during several contamination incidents. In space, the contamination monitoring instruments supported programmatic decisions during the early mission planning. During the warm-up of the cryogenic telescope, it was learned that a small gap between the multilayer insulation and the internal baffles contributed to mass redistribution of water vapor. Consequently, it was also learned that a careful warming procedure may potentially be used to clean space-based cryogenic mirrors of condensed water vapor. Particles brought from the ground can be released by mechanical motions such as door openings as well as by thermal shock induced by the Sun during umbra exit. Solar- induced water evaporation from multilayer insulation can dominate the gaseous environment over the spacecraft's lifetime of several years.
The MSX Contamination Experiment team was responsible for establishing design and operational guidelines and the contamination control plan for the Midcourse Space Experiment (MSX), as well as for tracking hardware cleanliness prior to launch. The approaches taken and the results are described.
The Midcourse Space Experiment (MSX) is a Ballistic Missile Defense Organization (BMDO) demonstration and validation
satellite program that has both defense and civilian applications. MSX has UV, visible, and infrared instruments including
the SPIRIT 3 cryogenic telescope. It also has several contamination measuring instruments for measuring pressure, gas
species, water and particulate concentrations and condensable gas species. A cryogenic quartz crystal microbalance (CQCM)
and four temperature controlled microbalances (TQCMs) are part of this suite of contamination measuring instruments. This
paper describes some of the flight QCM data obtained and analyzed to date. The CQCM is located internal to the SPIRIT 3
cryogenic telescope and is mounted adjacent to the primary mirror. Real-time monitoring of contaminant mass deposition on
the primary mirror is provided by the CQCM which is cooled to the same temperature as the mirror -20 K. The four
TQCMs are mounted on the outside of the spacecraft and monitor contaminant deposition on the external surfaces. The
TQCMs operate at -50°C and are positioned strategically to monitor the silicone and organic contaminant flux arriving at
the UV and visible instruments, or coming from specific contaminant sources such as the solar panels. During the first week
of flight operation, all QCMs recorded deposition in the 10-20 ng/cm2-day (1-2 A/day) range. These TQCM deposition rates
have continuously decreased, and after 270 days mission elapsed time (MET), the rates have fallen to values between 0 and
0. 15 A/day depending on TQCM location. Thermogravimetric analyses (TGAs) on the CQCM and TQCMs have provided
valuable insight into the amount and species of contaminants condensed.
This paper discusses the pressure spike phenomenon measured by the Total Pressure Sensor (TPS) on the Midcourse
Space Experiment (MSX) spacecraft as it passes through aurora! regions. The TPS is an inverted magnetron, cold cathode
gauge that has a range of 10'° to i0 Ton and is sampled at a 1 Hertz rate. The operation of the instrument depends uponthe
conversion of neutral molecules to ions and the measurement of the resultant current. The TPS has monitored large (>10 fold
increase) pressure transients in the auroral regions of the Earth, which have very short temporal width (< 3 seconds). It is
shown experimentally that the injection of electrons into the TPS orifice results in a higher apparent pressure measurement,
presumably due to an increase in the ion formation rate. An equivalent positive ion flux, however, does not noticeably effect
the TPS pressure measurement. It is therefore believed that the on-orbit transients seen by the MSX TPS are consistent with
an increased flux of electrons from the auroral regions into the entrance aperture of the gauge. This paper describes ground
experiments to quantify the phenomenon with a flight spare TPS and correlates these results to the measurements made onboard
the MSX spacecraft.
Initial results from the midcourse space experiment (MSX) ion mass spectrometer (IMS) is presented. The IMS is designed to measure the concentration of contaminant and ambient ionospheric ions from 1 - 56 amu. In addition, the IMS is used to infer the floating potential of the satellite. Ambient ions observed in the IMS data include H+, He+, N+, and O+. The instrument was operated in a 'contaminant ion' mode during the early-operation phase (first seven days) in which the instrument was placed in its most sensitive state. This was done to search for contaminant ions, produced through charge exchange reactions between neutral water vapor and ionospheric O+ ions moving at 7.5 km/s relative to the satellite. Mass peaks near those expected for H2O+ were observed. Finally, the initial results on the satellite charge inferred from changes in the ion current collected as a function of the stopping potential are presented. It is found that the satellite potential floats between minus 6 to minus 10 V with respect to the plasma potential.
The electrodynamics of particulate contamination in an ionospheric plasma near spacecraft surfaces is being investigated by a new 2D particle-in-cell (PIC) simulation. Particulate charging in the PIC simulation is treated as a collisional event and is implemented in a Monte Carlo fashion. The simulation has been applied to dusty plasmas both in an unbounded plasma and near a bounded plasma (spacecraft surface). Results from the simulation include estimates of the charge and potential structure of finite-size dust clouds. Low-density dust clouds do not perturb the electric potential of the plasma, similar to the case for isolated particulates. In high-density dust clouds, the potential forms a sheath-like structure while the charge forms a double layer on the outer boundary of the cloud. Particulates can have negative or positive charges depending on the applied surface potential, thus raising the possibility of controlling particulate contamination by controlling the spacecraft potential.
The Midcourse Space Experiment (MSX) ion mass spectrometer (IMS) is a Bennett radio- frequency mass spectrometer designed to measure the concentration of contaminant and ambient ionospheric ions from 1 - 60 amu. The instrument is one of a suite of six onboard contamination instruments, which also includes a neutral mass spectrometer, pressure sensor, quartz crystal microbalance, krypton water vapor monitor, and particulate flashlamp experiment. The instrument is sensitive to an incoming ion flux on the order of 1 X 107 ions/cm2s, which corresponds to an ion concentration of approximately 10 ions/cm3 for ionospheric ions that move at a speed of approximately equals 8 km/s with respect to the satellite. The IMS instrument and calibration are described in the paper. The purpose of the IMS calibration is to determine the detection efficiency for ionospheric and contaminant species that the MSX satellite is likely to encounter.
A particle monitor based on the near forward scattering of light from an AlGaAs laser diode was modified for space flight and proved to be robust and reliable during an actual space launch. Near-field particles could result in large extraneous signals from the IR, visible and UV telescopes on board a spacecraft because of their proximity to the sensors. It is therefore desirable to build a particle monitor to go with optical sensors in order to correlate various particulate events with spacecraft operations, so that their effects on the sensors can be corrected. This device, along with the power supply, associated analog and digital electronics, and mechanical mounting will be described. Particulate measurements during ground testing will be presented.
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