High Resolution Doppler Imager
HRDI at a glance
The High Resolution Doppler Imager (HRDI) was one of ten instruments on board the Upper Atmospheric Research Satellite (UARS). UARS was the first satellite in NASA's Mission to Planet Earth which was implemented to study the Earth's stratosphere and mesosphere. HRDI observed the emission and absorption lines of molecular oxygen (and other atmospheric constituents) in small volumes (~6 km in height by 50 km in width) above the limb of the Earth.
UARS was launched on 12 September 1991 into a 585 km circular orbit inclined 57 deg to the equator. Launch site was Cape Kennedy on the Space Shuttle Discovery (STS-48).
The HRDI instrument conducted scientific measurements from November 1991 until April 2005. The UARS spacecraft was deactivated on 14 December 2005.
The UARS spacecraft completed 78,084 orbits around the Earth over a time period of 5,208 mission days. The UARS spacecraft is expected to re-enter the atmosphere sometime in 2009 or 2010.
HRDI measured winds by determining the Doppler shift of absorption and emission lines in the O2 Atmospheric band. From the Doppler shift of the lines, the horizontal winds could be determined, while the line shapes and strengths yielded information about the temperature and atmospheric species make-up. The goal of HRDI was to measure the vector winds in the stratosphere (10-40km), mesosphere and lower thermosphere (50-120km) during the day, and the lower thermosphere at night (~95km) to an accuracy of approximately 5 m/s. The horizontal wind vector was determined by observing the Doppler shift of rotational lines of molecular oxygen along two lines of sight. In addition to winds, temperatures and volume emission rates were obtained in the mesosphere and lower thermosphere. From the volume emission rate the density of minor species, such as ozone during the day and atomic oxygen at night were determined.
The HRDI instrument consisted of a two-axis, gimbaled telescope, a triple-etalon-Perot interferometer, interferometer electronics, support electronics, and a dedicated instrument processor.
The telescope was a well-baffled off-axis parabola telescope mounted on a two-axis gimbal structure. This provided the ability to point anywhere within a hemisphere, to measure wind vectors at various altitudes.
The interferometer assembly consisted of an optical bench, interferometer optics, and support electronics. The bench was supported by kinematic mounts.
- Relay optics provide input from the telescope subassembly.
- Two eight-position filter wheels select spectral regions of interest.
- The multiple-etalon Fabry-Perot design provides white light rejection and high throughput, allowing measurement of absorption features. The low-resolution etalon and medium-resolution etalon use piezoelectric spacers to tune gap spacing between the etalons.
- The piezoelectric spacers are controlled by feedback circuits commanded by the dedicated instrument processor. The etalon gap was controlled to better than 0.1 nm. Etalons for the interferometer were 132 mm in diameter with 90 mm clear field of view.
- The folding mirrors reduced the overall length of the instrument thereby reducing weight.
- The interferometer's ruggedized Questar telescope focused interference patterns received from the high-resolution etalon onto the image plane detector.
- The image plane detector is a modification of the design flown on the Dynamics Explorer Satellite, but had a larger anode array consisting of 32 concentric ring elements. The anode array converts the ring pattern of photons (produced by the etalons) into sets of discrete electron pulses representing spectral elements.
Paul Hays, Principal Investigator (until 2000)
Wilbert Skinner, Instrument Scientist (1984-1993), Science manager (1993-2000), Principal Investigator (2000-2005)
Brian Kennedy, Project manager (until 1984)
Michael Dobbs, Project manager (1984-1989)
Heinz Grassl, Project manager (1989-1993)
Vince Abreu, Science manager (1984-1993)
Current Points of Contact
Wilbert Skinner, Principal Investigator
Alan Marshall, Data Manager
|Type of measurement:||Doppler shift and line broadening of scattered sunlight and atmospheric emission in the visible wavelengths.|
|Type of instrument:||Triple-etalon Fabry-Perot interferometer.|
|Geophysical Parameters determined:||Horizontal-vector wind, atmospheric temperature, volume emission rate.|
|Wavelength coverage:||550 to 770 nanometers.|
|Viewing geometry:||45 degrees, 135 degrees, 225 degrees, and 315 degrees +/- 5 degrees to spacecraft velocity vector.|
|Maximum latitude sampled:||74 degrees.|
|Comments:||Gimbaled telescope provides potential for azimuth direction; orthogonal measurements for same atmospheric volume separated by approximately 8 minutes.|
|Spectral Resolution:||0.001 nanometers.|
|Vertical Resolution:||6 km at limb (0.12 degree field of view).|
|Horizontal Resolution:||80 km at limb (1.7 degree field of view).|
|Time required for one vertical scan:||Approximately 30 seconds.|
|Instrument weight:||348 lb.|
|Average power:||109 watts.|
|Data rate:||4.750 kbps.|
Some of the more difficult technical challenges encountered by the HRDI team included:
- Development of the piezoelectric posts for the etalons. The gap of the low and medium resolution etalons had to be controlled to better than 0.1 nm over a changing thermal environment
- Development of integration and calibration techniques for a complicated optical instrument.
- Development of instrument control software capable of reliable execution of extensive scripted commands. The flexibility of this software allowed the mechanical components, which were initially operated in an open loop configuration at the beginning of the mission to, be operated in a closed loop mode in the latter part of the mission to compensate for aging of the components.
- Development of a relatively large aperture telescope with a pointing control of better than 0.1 degrees and pointing knowledge of better than 0.01 degrees.
Engineering Feats of Note
HRDI performed wavelength analysis on the light detected from atmospheric emission or absorption features by spatially scanning the interference fringe plane with a multichannel array detector. A sequential altitude scan performed by the commandable telescope provides global coverage of state of the atmosphere from cloud top through the thermosphere.
The instrument operated from November 1991 until April 2005. The quality of the measurements at the end of the mission were as good as those at the beginning. The instrument suffered two partial electronic failures that were overcome using the flexibility provided by the flight software with no loss of scientific capability.
Approximate Cost and Time to Build
Work on the HRDI instrument started in the early 1980s. The instrument was delivered in November 1989. It was brought back to SPRL in the spring of 1990 for final calibration and was returned for integration on the spacecraft in the fall of 1990. Launch was in September 1991 and scientific operations started in November 1991.
Cost of the HRDI project including design, construction, calibration, flight operations, and scientific processing and analysis exceeded $35M in 1990 dollars.
HRDI data resides in the public domain and is available for scientific analysis. Visit http://hrdi.engin.umich.edu for more information.