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September 1995

HALOGEN OCCULTATION EXPERIMENT (HALOE)

Introduction

HALOE logoNASA'S Halogen Occultation Experiment (HALOE) is designed to monitor the vertical distributions of ozone and key upper atmosphere trace gases that affect the global ozone distribution by measuring the attenuation (reduction in intensity) of the Sun's energy in selected spectral bands as it passes through the Earth's atmosphere.

One of the 10 highly-instrumented experiments on NASA's Upper Atmosphere Research Satellite (UARS), HALOE is scheduled to be launched in September aboard the Space Shuttle Discovery. The instrument has a planned lifetime of at least 18 months.

UARS is the first comprehensive space experiment ever mounted to study the chemistry, dynamics and energetics of the Earth's upper atmosphere. It will provide valuable scientific input to critical policy decisions aimed at protecting the thin, fragile ozone layer which blocks harmful ultraviolet radiation from reaching the Earth's surface.

The UARS spacecraft will be delivered into a 348-mile (560 km) circular Earth orbit. After deployment by the Shuttle remote manipulator system, UARS will be boosted by a propulsion system on the spacecraft to a 363-mile (600 km) circular orbit, inclined 57 degrees to the equator.

Goddard Space Flight Center manages and directs the UARS mission for the Office of Space Sciences and Applications, NASA Headquarters, Washington, D.C. The spacecraft was designed, built, integrated and tested by GE Astrospace, Valley Forge, Pennsylvania, and East Windsor, New Jersey.

HALOE Science Objectives

Scientific objectives of the HALOE mission are to:

  • Improve understanding of stratospheric ozone depletion by collecting and analyzing global vertical profiles of ozone and gases important in its destruction: hydrogen chloride, methane, water vapor, nitric oxide and nitrogen dioxide.

  • Study the chlorofluoromethane impact on ozone by measuring hydrogen chloride and hydrogen fluoride in addition to the other key chemical species, and using data on Freon 11, Freon 12 and chlorine nitrate obtained from other UARS experiments.

  • The science investigations will include studies of trace gas sources and depositories, transport mechanisms, dynamics, and validation of atmospheric and photochemical dynamics models.

  • The experiment uses gas filter correlation radiometry to measure hydrogen chloride, hydrogen fluoride, methane and nitric oxide, and broadband filter radiometry to measure water vapor, nitrogen dioxide, ozone and carbon dioxide. The carbon dioxide data will be used to obtain atmospheric temperature versus pressure profiles.

HALOE Instrument Description

The HALOE instrument hardware is contained in two separate packages, the Platform Electronics Assembly (PEA) and the Sensor Assembly. The PEA links the instrument with the spacecraft. The Sensor Assembly makes the science measurements and consists of eight science detectors and radiometers, a Cassegrain telescope, a two-axis gimbal assembly, a Sun sensor, the spacecraft adapter and supporting electronics.

The operating principles of gas filter correlation radiometry and conventional broadband filter radiometry are described below:

Gas Filter Correlation Radiometry Principle - Solar energy enters the gas correlation section of the instrument optics and is divided for each channel (hydrogen chloride, hydrogen fluoride, nitric oxide and methane) into two paths. Each channel has its own broadband optical filter and detector. The first path contains a cell filled with the gas to be measured; the second path is a vacuum path without gas. By electronically comparing the outputs of the gas and vacuum path detectors, scientists can derive chemical measurements.

Broadband Filter Radiometry Principle - The water vapor, nitrogen dioxide, ozone and carbon dioxide channels are conventional broadband filter radiometers. In this case, energy from the Sun comes in through only one path for each channel. After passing through a broadband optical filter, the energy is focused on a detector. By tracking the Sun, a signal is recorded outside the atmosphere and during occultation after absorption by the atmosphere. The ratio of the attenuated signal to the signal outside the atmosphere can be used to measure the gas concentration .

The Cassegrain telescope reflects solar energy through a series of beamsplitters and spectral filters to the photovoltaic detectors for the gas filter correlation channels and to the broadband filter radiometer channels. The atmospheric target gases are detected at specific wavelengths between 2.5 microns and 11 microns.

The instrument size is approximately 36 in. (spacecraft adapter to frame radiometer) by 24 in. (elevation gimbal to telescope) by 32 in. (telescope to Gimbal Electronics Assembly)or (92 x 62 x 81 cm). The Platform Electronics Assembly is approximately 9 x 10 x 6.6 inches (23.5 x 24.3 x 22.1 cm). The total mass of the instrument (Sensor Assembly and PEA) is 222 pounds (101 kg).

The initial design phase for HALOE was conducted by TRW Defense and Space Systems Group in Redondo Beach, California. The final design, fabrication, assembly and testing was completed in-house at Langley Research Center.

Instrument Operation

The HALOE instrument operates autonomously once powered and initialized. Commands are sent to the spacecraft computer to operate the instrument for one day to perform the sunrise and sunset data observations. When the spacecraft sends a command to perform a sunrise or sunset sequence, the instrument automatically performs a solar acquisition a balance of all gas filter correlation radiometer channels, limb to limb scans of the solar disk, a calibration activity, the science data measurement during occultation and then slews back to the stow position .

The Sun pointer/tracker subsystem consists of two coarse and one fine Sun sensors, a two-axis gimbal assembly, a microprocessor and drive electronics for gimbal motor control. Its function is to acquire the Sun, scan the solar disk and track a specified location on the solar disk during balance, calibration, and science data measurement activities for orbital sunrise or sunset events. Acquisition and tracking control signals for the gimbals are derived from the Sun sensors.

During a typical event, measurements will begin at a tangent height of 93 miles (150 km), where there is no atmospheric interference, down to the Earth's surface or until the Sun is obscured by clouds. HALOE will view approximately 15 sunrises and sunsets each day collecting data on vertical trace gas concentrations. Each event will occur at a different latitude and longitude, and global coverage is repeated every 3 to 4 weeks.

Data Processing

Data from the HALOE instrument will be stored in one of two tape recorders aboard the UARS observatory. The data will be transmitted to the White Sands receiving system through the Tracking Data Relay Satellite System (TDRSS). Routine processing of the data will occur at the Central Data Handling Facility (CDHF) at the Goddard Space Flight Center with software provided by the Langley science team. Interaction with the CDHF will be through a direct data link between the HALOE Remote Analysis Computer at Langley Research Center and the CDHF.

Processed data will contain species concentration profiles as a function of global location and time. The profiles will be mapped out on a global and seasonal basis as the data accumulates during the mission.

All UARS data will be archived at the Goddard Space Flight Center.

Scientific Value

Atmospheric problem & chemistry

Ozone in the Earth's atmosphere reaches concentrations of only about 12 parts ozone to one million air molecules, yet it has profound effects on Earth life. If the ozone level is changed, the solar ultraviolet level at the Earth's surface is altered. Serious biological and economical impacts can occur in areas such as human health, crop and plant growth, perturbations to micro organisms in the soil and oceans, weathering of materials, and possible climate and weather alterations. A 1 percent decline in ozone levels, for example, can lead to a 2 percent rise in human skin cancer. Also, if ozone is depleted, the stratospheric temperature rise will be altered leading to changes in atmospheric stability due to the weakened temperature inversion.

A growing body of evidence has led to a consensus in the scientific community that manmade activities are perturbing the ozone layer. The recent Antarctic ozone "hole" finding, for example, can only be explained by considering reactions involving aerosol particles and chlorine compounds formed after dissociation of the man-made chlorofluoromethanes (CFM's). These CFM's are used as refrigerants and in various industrial applications. The extent to which such effects occur outside the Antarctic region is unknown. Consequently, it is very important that the ozone layer be monitored globally and over a long time period.

The overall goal of HALOE is to provide global-scale data on temperature, ozone and other key trace gases needed to study and to better understand the chemistry, dynamics and radiative processes of the middle atmosphere (6-74 miles or 10-120 km) and to study the impact of CFM's on ozone using hydrogen fluoride observations, in combination with other HALOE data. The figure shows the major chlorine source species entering the middle atmosphere.

They interact with ozone, the nitrogen and hydrogen oxides, and with solar radiation to form the reservoir molecules hydrogen chloride and hydrogen fluoride. For every chlorine atom formed in the middle atmosphere by dissociation of the CFM molecules, approximately 1,000 ozone molecules are destroyed. HALOE studies will be aimed at evaluating the relative importance of man-made and natural chlorine sources in ozone destruction. Since the primary man-made chlorine sources (i.e., the CFM's) contain both chlorine and fluorine in the molecule, while natural sources (e.g. methyl chloride and carbon tetrachloride) contain only chlorine, hydrogen fluoride becomes an indicator of man-made chlorine input to the middle atmosphere. Hydrogen chloride is an indicator of the total chlorine input. The relative importance of these two sources can be inferred by studying changes in hydrogen chloride and hydrogen fluoride with time.

HALOE Implementation

The Langley Research Center is responsible for providing the scientific instrument for the HALOE investigation, the instrument flight operations, the science data products through HALOE data processing and data management systems, and managing the Science Team and its investigations.

HALOE Science Team

Dr. James M. Russell III, HALOE Principal Investigator, Langley Research Center
Dr. Ralph J. Cicerone, University of California/lrvine
Prof. S. Roland Drayson, University of Michigan
Prof. John E. Frederick, University of Chicago
Dr. Adrian F. Tuck, Aeronomy Lab/NOAA/ERL, Boulder, Colorado
Prof. Dr. Paul J. Crutzen, Max Planck Institute for Chemistry, Federal Republic of Germany
Dr. John E. Harries, Rutherford Appleton Laboratory, United Kingdom
Dr. Jae H. Park, NASA Langley Research Center
Larry L. Gordley, Gats Inc., Hampton,Va.
W. Donald Hesketh, SpaceTec Ventures Inc., Hampton, Va.

HALOE Project Management

Dewey M. Smith, Project Manager
Thomas C. Jones, Deputy Project Manager
Dr. James M. Russell lil, Principal Investigator
John G. Wells, Flight Operations and Science Manager
Kenneth V. Haggard, Science Software and Data Processing Manager

For more information, check out the HALOE Homepage.