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THE INSTRUMENTS . . .
GROUNDWINDS NEW HAMPSHIRE
The
GroundWinds New Hampshire LIDAR is located in the town of Bartlett in the
White Mountains region of the state. The location of this facility was chosen
in order to make wind and atmospheric observations in a mountainous region with
relatively high aerosol levels. Since this facility must be operated by staff
onsite, the location is also convenient to two GroundWinds team institutions:
the University of New Hampshire and the Mount Washington Observatory.
This photo shows the facility building, which is 541 ft ASL. The building houses the LIDAR control room, a laser/interferometer room, and a utility room. The dome on the roof allows a 300 degree azimuth sweep of the laser beam through the sky. The beam elevation is fixed at 45 deg.
The container in the upper center in this photo holds the 532-nm pulsed laser. The orange plexiglass box contains the post-laser optics that steer the beam through the vertical pipe up to the exit telescope in the dome. A portion of the beam is diverted directly to the interferometer through a fiber optic. This serves as a reference (un-Doppler-shifted) signal.
This telescope assembly holds both the outgoing beam optics and the main (receiving) telescope. The outgoing beam passes through a beam expander, which matches the beam divergence to the field of view of the telescope. The beam, telescope, and dome opening track together as the LIDAR sweeps through the sky.
The computers which control the LIDAR system and monitor, store and analyze the data are in a separate room for reasons of eye safety and to better isolate the laser and interferometer from vibrations and temperature changes.
The laser beam is not intense enough to be visible during daytime operation, but can be seen easily at night in the vicinity of the facility. The scattered light is strong enough to be seen up to altitudes that include the so-called “boundary layer” where aerosols produce a significant return signal. Above that, the signal is not visible to the eye, but can be detected by the LIDAR system.
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THE INSTRUMENTS . . .
GROUNDWINDS HAWAII
The GWHI facility was established at the NOAA Mauna Loa Observatory (MLO) on the “big island” of Hawaii in February 2002. This location at 11,141 ft ASL in the middle of the Pacific Ocean was chosen to demonstrate the technology’s ability to make wind measurements in a clean, aerosol-free atmosphere. The tall, isolated Hawaiian Islands make for interesting mountain flows and diurnal effects. The facility is across the valley from the Mauna Kea Observatories, which use the GWHI data for improved weather forecasts.
The GWHI instrument uses a UV (355-nm) light source, which provides an increased molecular return compared to the green (532-nm) light source. UV light is also heavily attenuated by cockpit windows, making this instrument eye safe to pilots. The instrument was designed for remote operations and with a lightweight interferometer to take an additional step toward BalloonWinds and a space-borne instrument.
Because of its isolation from population centers, GWHI is operated remotely from the University of Hawaii (UH), the University of New Hampshire (UNH) and the Michigan Aerospace Corporation (MAC.) The facility is serviced on an as-needed basis by MLO personnel who are stationed on-site.
The GW Hawaii interferometer is a second-generation version, with improved accessibility compared to the original GWNH design. The two black boxes protruding from the side of the interferometer container hold the CCD cameras for the aerosol and molecular channels.
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THE INSTRUMENTS . . .
BALLOONWINDS BUILD AND LAUNCH LOCATIONS
The GroundWinds facilities in New
Hampshire and Hawaii have fulfilled their roles of demonstrating the
successful measurement of tropospheric winds using Direct Detection Doppler
LIDAR. They continue to acquire useful synoptic data sets of local winds
and other information about atmospheric conditions. From the perspective
of demonstrating technology for flight on a satellite, there is, however,
an inherent mismatch between a ground-based wind measurement LIDAR system
and a space-based system. The ground-based system is “up-looking” and probes
first the lower, thick atmosphere and then the higher, thin atmosphere.
It is also approximately fixed in relation to atmosphere. A space-based
system is “down-looking” and sounds first the thin atmosphere
and then the thick atmosphere. It is also on a moving platform and
sees returning light scattered from the Earth’s surface.
The BalloonWinds phase of the GroundWinds program is designed to more closely match the situation of a space-based LIDAR. BalloonWinds will represent a satellite mission in that it will be down-looking, flying above 99% of the atmosphere. The LIDAR instrument is being designed and built by teams from UNH and MAC (directing Raytheon as a partner and Fibertek as a subcontractor). In addition, the Air Force Research Laboratory (AFRL) at the Kirtland Air Force Base in Albuquerque, New Mexico will provide technical and hardware support for the payload gondola as well as facilities for payload integration and launch. The following table lists the payload teams and their major responsibilities.
University of New Hampshire (UNH) – System/Integration |
CCD Camera |
Thermal Management, Power Distribution, and Telemetry System |
Gondola Design and Systems Engineering |
Control Electronics Chamber Design |
Michigan Aerospace Corporation (MAC) - Instrument |
Instrument Systems Engineering |
Interferometer and Environmental Packaging |
Laser/Telescope and Environmental Packaging |
Instrument Control System |
Control Electronics Packaging |
Raytheon - Santa Barbara Remote Sensing |
Telescope, Laser Development Oversight |
Fibertek |
Diode-pumped Laser |
The flight plan calls for three balloon launches beginning in the spring of 2006. All launches will be done at Holloman Air Force Base at White Sands, New Mexico. The nominal float altitude is 100,000 feet (30km).
The first two flights are intended to be concept demonstrations. The first flight will demonstrate the electrical, thermal, mechanical, and optical performance of the integrated instrument for nighttime flight conditions. The second flight will demonstrate the ability to operate during the daytime given the additional thermal load and the increased optical background. Instrument modifications based on experience from the initial flights will be made in the 6 months leading up to the final flight, which will experience both day and night conditions. A typical flight timeline going from launch to landing, including about 10 hours at float altitude would last about 14 hours.
THE INSTRUMENTS . . .
BALLOONWINDS INSTRUMENT SPECIFICATIONS
The BalloonWinds LIDAR must meet or exceed
the total optical transmission of the GroundWinds Hawaii (GWHI) system
in order to achieve the photometric efficiencies required of a space-based
instrument. The remotely operated instrument must be designed to function
on a high-altitude, down-looking platform for multiple daytime and
nighttime missions with flight durations of up to 14 hours. The following
table lists some of the main features of the instrument.
Parameter |
Baseline Value |
Laser Output Power |
3 W |
Laser Pulse Frequency |
100 Hz |
Laser Beam Divergence |
0.11 mrad |
Wavelength |
355 nm |
Gondola Altitude |
30 km |
Telescope Diameter |
0.5 m |
Telescope Field of View |
0.19 mrad |
Vertical Resolution |
0.25 km for 0 – 2 km 1.0 km for > 2 km |
A major challenge of the balloon mission is to adapt the LIDAR technology to a balloon-based platform and to support the power and thermal requirements of the instrument in a high-altitude environment. Specifically, the low atmospheric pressure (< 10 mbar) means that critical subsystems must be hermetically sealed and held at a constant (near 1 bar) pressure. The low efficiency of the laser requires substantial battery power to operate over the duration of the mission. Furthermore, the high input power of the laser and, to a lesser extent, the control electronics requires a cooling system, while the need for temperature stability of the interferometer chamber requires heaters. The cooling system uses an ice/water reservoir as a heat sink. Onboard system data acquisition and communication is handled by single-board computers linked via Ethernet. Data is radioed to the ground station in Internet Protocol format.
Summary of BalloonWinds System |
Gondola Mass: 4500 lbs (~900 lbs for cooling; <400 lbs for batteries) |
Size: 8’ H x 8’ W x 12’ L |
Power Requirements: 1300 W |
24-26 Lithium Ion Battery Stacks |
Thermal Management: Ice Phase Change; 0 C coolant temperature |
Technologies Implemented:
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THE INSTRUMENTS . . .
BALLOONWINDS PERFORMANCE PREDICTIONS
The BalloonWinds flights
are expected to produce a well-defined set of data products that will
demonstrate the potential of a DDD LIDAR to make useful wind measurements
from space. These products will include:
- Measurement of a spectrum and a photometric return to confirm the atmospheric response and the instrument performance model for a set of atmospheric conditions that are representative of those encountered by a space-borne Doppler LIDAR.
- A performance model for a space-borne tropospheric Doppler LIDAR sounder with appropriate subsystem scaling and a comparison of that projected performance to the Global Tropospheric Wind Sounder (GTWS) data product requirements.
- Assessment of the scalability of key subsystems to a space-borne Doppler LIDAR instrument.
- Measurement of wind velocity profiles within an accuracy and precision determined by our knowledge of the instrument and its limitations for a given photometric return including the effects of spatial and temporal atmospheric variability and other uncontrolled parameters.