Argos Mission to Yield New Data on Earth's Ionosphere and Space Radiation

Satellite Launches Successfully on February 23, 1999

Teams of scientists at the Naval Research Laboratory (NRL) have developed a suite of ultraviolet (UV) and X-ray remote sensing instruments, a dual frequency radio beacon for computerized ionospheric tomography, and a superconductivity experiment to fly on board the Air Force Space Test Program's (STP's) Advanced Research and Global Observation Satellite (ARGOS). The instruments were integrated onto a Delta II rocket for the satellite's launch, from Vandenberg AFB, CA, into a polar orbit on February 23, 1999.

Observational data from the UV and X-ray instruments, together with the radio measurements of total electron content, will be used to study "space weather," the ever-changing effects of the sun's radiation on the upper atmosphere and ionosphere. Scientists will be able to create the first dynamic, global maps of the ionosphere, showing chemical, density, and temperature changes. A number of other objectives will also be addressed, including mapping of ultraviolet stars, timing of X-ray pulsars and the application of these measurements to satellite navigation and timing, space tests of new superconducting electronic devices, and space tests of radiation-resistant and fault-tolerant computers.

ARGOS, which has a 3-year planned operational life, will carry nine primary experiments that contain 31 different sensors and subexperiments. Of the primary experiments, three were developed by NRL's Space Science Division (SSD), one was developed by the Laboratory's Naval Center for Space Technology (NCST), and one was developed by the NRL Plasma Physics Division. Two others were developed with NRL collaboration.

Mr. Gil Fritz, head of SSD's X-ray Astronomy Branch and of the ARGOS SSD experiment coordination office, reports that ARGOS contains a unique combination of new imaging, timing, and spectroscopic sensors which will obtain unprecedented, simultaneous information on the upper atmosphere. Since the earth rotates beneath the fixed polar orbit of ARGOS, truly global measurements will be made and three-dimensional maps can be produced. The simultaneous coverage in wavelength, spanning almost four decades from X-rays to ultraviolet, is also a powerful asset in understanding the space radiation in the near-earth environment. NRL scientists anticipate a variety of spin-offs and follow-on missions, including operational sensors to monitor space weather, and imaging sensors in deep orbits that can pinpoint ionospheric conditions anywhere on the earth.

The three NRL SSD experiments are the High Resolution Airglow/Aurora Spectroscopy (HIRAAS), the Global Imaging Monitor of the Ionosphere (GIMI), and the Unconventional Stellar Aspect (USA) experiments.

HIRAAS is a multi-instrument experiment that will scan the edge of the Earth's atmosphere (called the limb) about every 90 seconds to measure naturally-occurring airglow emissions in the 50 to 340 nanometer (nm) wavelength range over a wide array of geophysical conditions and at varying local times. The instruments will perform continuous observations over several spectral bands with resolution up to ten times better than with previous experiments. These measurements will be used to infer the composition (O+, N2, O, and O2) and temperature.

Data from the HIRAAS experiment will be used to explore new concepts in monitoring space weather from satellites, and to improve high frequency communications and over-the-horizon radar, which rely on propagation through the atmosphere. The measurements will also help researchers assess the long-term effects of the increases of atmospheric greenhouse gases on the upper atmosphere and ionosphere.

HIRAAS is made up of three ultraviolet (UV) spectrographs:

· the High-resolution Ionospheric and Thermospheric Spectrograph (HITS), a very high resolution extreme UV/far UV (EUV/FUV) spectrograph;

· the Low-resolution Airglow and Auroral Spectrograph (LORAAS), a moderate resolution EUV/FUV spectrograph; and

· the Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) experiment, a high resolution middle ultraviolet spectrograph.

GIMI will obtain wide-field FUV/EUV images of ionospheric and upper atmospheric emissions simultaneously, covering large areas of the earth from a low-earth orbit. These images will be used to determine chemical densities [O+, nighttime O2, NO and N2] on a global basis and
to detect disturbances in the ionosphere that are caused by auroral activity, gravity waves and foreign materials from meteors, suspected "ice comets," rocket exhausts and chemical releases. In between the atmospheric observations, GIMI will also perform an all-sky survey of stars and celestial diffuse sources at far-ultraviolet wavelengths.

The GIMI instrument has two coaligned cameras for simultaneous observations of selected targets. Camera 1, which is sensitive in the 75-110 nm ranges will primarily be used for observations of the dayside ionosphere, auroras, and stellar occultations, and for star field surveys. Camera 2 is sensitive in the 131-160 and 131-200 nm far-UV wavelength ranges and will be used for observations of the nightside ionosphere, airglow, stellar occultations, star field surveys, and also gas releases and rocket plumes at night.

USA will observe bright X-ray sources, mostly binary X-ray sources in our Galaxy, to test new approaches to satellite navigation. The sensors, built in collaboration with the Stanford Linear Accelerator Center, which are sensitive to X-ray wavelengths of about 1 to 10 Angstroms, will measure celestial sources and characterize those sources for potential use as reference points for autonomous military space systems. It will make measurements aimed at furthering scientific understanding of the physical nature of the sources. USA will also perform the first X-ray tomographic survey of the earth's atmosphere.

To accomplish its mission, USA will observe a small number of targets, selected prior to launch, and re-measure the X-rays from these targets repeatedly. The scientists anticipate that each of approximately 30 bright sources will be observed several times during the first month of operation. Two to four observations are planned for each orbit, depending on particle background and source location.

USA consists of two X-ray sensors and a gimbaled mounting. With a field-of-view of about 1.5 degrees, the detectors will measure the time-varying X-ray output of celestial sources, which can be processed to provide an autonomous timekeeping system, observe horizon crossings for autonomous position determination, observe modulations of steady X-ray sources for aspect determination and observe terrestrial X-ray emission including some associated with very low frequency transmissions.

Secondary goals of the USA Experiment will further the understanding of cosmic X-ray sources and test new concepts for fault-tolerant computing in space. The X-ray sources which USA will observe contain white dwarfs, neutron stars, or black holes. Studying them allows astronomers to glimpse matter in its most extreme states where densities are as high as in an atomic nucleus, and extraordinarily strong gravitational forces and magnetic fields are present. USA will observe time variable phenomena in these sources with timescales of less than 1 millisecond. USA also carries a two-computer testbed that consists of a military radiation-hardened processor side-by-side with a commercial off-the-shelf (COTS) processor. This testbed will allow scientists to demonstrate the capability of using advanced fault tolerant software algorithms to enable the COTSprocessor to be used for high-performance space-based computing tasks.

The NRL SSD ARGOS research and development team involves participants from numerous government, industry and university communities. The NRL principal investigators (PI) and project scientists (PS) for the UV and X-ray remote sensing instruments are: USA: Dr. Kent S. Wood (PI), Dr. Michael N. Lovellette (PS); GIMI: Dr. George Carruthers (PI); HIRAAS: Dr. Robert P. McCoy, Office of Naval Research (PI), Dr. Kenneth F. Dymond (PS).

The Coherent Electromagnetic Radio Tomography (CERTO) instrumentation, developed by NRL's Plasma Physics Division, consists of a stable radio beacon transmitter on the satellite and a chain of receivers on the ground. Radio transmissions from the CERTO beacon are processed by the ground receivers to produce two-dimensional maps of the electron densities in the ionosphere. The CERTO measurement technique provides images of the ionosphere with 10 km vertical and horizontal resolution. In addition, ionospheric irregularities of 1 km or less in size can be determined by fluctuations in the CERTO radio waves.

CERTO can also be used to calibrate the ionospheric densities obtained using the EUV instruments such as HIRAAS, GIMI, and EUVIP on ARGOS. The CERTO radio-based technique has the advantage of higher spatial resolution than provided by the EUV-based techniques, but requires ground-based receivers aligned under the satellite orbit. The two techniques together on the same satellite provide substantial improvements over each technique separately. CERTO principal investigator, Dr. Paul Bernhardt notes that the NRL instruments on ARGOS will be the first demonstration combining EUV and radio sensors for enhanced imaging of the ionosphere.

NRL NCST is responsible for the High Temperature Superconducting in Space Experiment II (HTSSE II), which will demonstrate the operational space capability of superconducting components and technology that are light weight, faster, and use much less power than the silicon or gallium arsenide (GaAs) based electronics used today. [See related NRL Press Release 2-99r].

NRL collaborative experiments include the Space Dust Experiment (SPADUS) with the University of Chicago, which is funded by the Office of Naval Research, and the Extreme Ultraviolet Imaging Photometer (EUVIP) with the Army Space Command. The remaining two experiments, the Electric Propulsion Space Experiment (ESEX) and the Critical Ionization Velocity (CIV) experiment, were developed by the Air Force Research Laboratory. These two experiments will be turned on and operated during the first 1-2 months after launch of the ARGOS satellite. Subsequently, the remaining seven experiments will be turned on and will operate continuously for the duration of the mission.

For website information on ARGOS and its payloads, please refer to http://bcoe.nrl.navy.mil/SSD/Argos/html/index.html and http://xweb.nrl.navy.mil.

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