In order for the data to be of use to scientists, industry, and the public, rapid, efficient, and accurate operational algorithms must be developed to convert the radiance observations into environmental parameters. Data from the GUVI instrument is processed on the ground to generate data products at the different levels.

GUVI Data Level Definitions
Data Level Description
Raw Telemetry Unprocessed digital telemetry from the TIMED spacecraft
Level 0 Unprocessed instrument data at full resolution that has been separated by instrument or subsystem
Level 1A Unprocessed instrument data at full resolution, time referenced and annotated with ancillary information including radiometric and geometric parameters
Level 1B Level 1A data processed to sensor units (e. g. Rayleighs/color). This is a virtual product, no data files will be produced.
Level 1C Level 1B radiance data mapped on a uniform 25x25 km2 earth-referenced grid
Level 2B Derived geophysical variables mapped on a uniform 100 x 100 km2 earth-referenced grid
Level 3 Derived geophysical variables over multiple orbits mapped on a uniform earth-referenced space-time grid shared with other TIMED instruments

Level 1 consists of radiance data, which is converted into Rayleighs by a calibration routine and mapped on to the Earth by geolocation software. The "disk" portion shows the pixels centered at the location of the pierce point of the GUVI viewing line of sight at 150 km altitude. The "limb" portion shows the pixels centered at the appropriate tangent altitude for each limb viewing angle.

At Level 2 the data is averaged into 100x100 km2 resolution superpixels, and the geophysical parameters are calculated from the radiance of the different colors. Both the calibration and the calculation of environmental parameters are based on table look-ups, so that the procedures may be easily modified to improve accuracy as better results become available. A different algorithm is used for disk and limb retrievals. The auroral region requires a separate algorithm, so the software must interpret the data to identify the auroral boundaries and choose the proper algorithm.

Level 3 then maps the retrieved environmental parameters on a uniform earth-referenced space-time grid shared with the other TIMED instruments, for easy comparison and combination of results. This global map of environmental parameters, along with additional data, will be made available at the GUVI web site.


Environmental Parameters and Requirements Flowdown

The LBH1 and LBH2 colors refer to parts of the Lyman-Birge-Hopfield bands of molecular nitrogen. The LBH1 region is defined as between 140 nm and 150 nm. The LBH2 region is defined as 165-180 nm.

Environmental parameters measured by different colors
  HI (121.6 nm) OI (130.4 nm) OI (135.6 nm) N2 (LBH 1) N2 (LBH 2)
Dayside Limb     O altitude profile Solar EUV N2, O2, Temperature
Dayside Disk     O/N2 N2, O2  
Auroral Zone Proton auroral boundaries Mixed auroral boundaries Effective energy flux Q, Mixed auroral boundaries Effective average energy <E>, Mixed auroral boundaries Effective average energy


Requirements flowdown for the GUVI instrument
Science Objectives Measurement Goals Capability Requirements Functional Requirements Implementation Requirements
Determine seasonal and local solar time variation of the major species composition in the MLTI region in accordance with the TIMED science requirements for accuracy, temporal and spatial scales, and coverage.

Provide a global determination of O, N2, O2 and temperature profiles through measurement of the spectral radiance of principal atomic and molecular ultraviolet dayglow emissions.

Global coverage of HI (121.6), OI (130.4, 135.6), and N2 (LBH)   Imaging spectrograph Scan mechanism repeatability better than º step
Spatial and spectral FUV airglow and aurora 115-180 nm
15-50 nm resolution Integration period: limb 0.034 s, disk 0.064 s
Maximize local solar time and geographic coverage Disk dayglow global coverage 0.18° -0.74° x0.85° pixels
Handle max radiance at solar max at noon on equator Low background count Scan range -60° to +80° ; steps 0.4° on limb
Scan mechanism
Limb profiles 110-300 km Limb coverage tangent altitude 60-500 km Nadir spatial resolution < 10 km Mechanical alignment knowledge: < ± 0.1° all axes
Know S/C altitude £ 1 km; lat&long £ 3 km 3s pointing accuracy 0.3° (<6 km 1s on limb)
Composition to &plusmin; 15% during solar max Day/night dynamic range 1000:1 140° field of regard Spectrograph internal stray light < 0.1% /spectral bin
Low instrument and analysis errors Data system
Spatial resolution: ý scale height (vertical),

100 km (horizontal)

Limb altitude accuracy £ 6 km Downlink binned sensor and pointing data Off-axis rejection: FUV BRDF < 0.5 at 1°
100% duty cycle
Disk dayglow resolution 100 km 5 color definitions Brightness error budget
Measure energy inputs in the auroral region to understand the global MLTI energy balance in accordance with TIMED requirements.

Provide the precipitating auroral electron and proton fluxes and average energy. Map auroral boundaries.

Auroral HI (121.6), OI (135.6), N2 (2 LBH)   Detector electronics
200 kHz max count rate Counting statistics: 4%
Auroral boundary determination for 1 erg/cm2/s electron aurora Orbit 600-900 km Low nonlinearity
Orbit inclination > 70° Control gain over 2 years Dark count: 1%
Accuracy < &plusmin; 20%   Error budget
Map boundary to 10 km Brightness measurement: 8% Stray light: 2%
Pointing: 6% Nonlinearities: 3%
20 km spatial resolution for energetic particles Auroral resolution 20 km post-processing Calibration: 8% Data compression: 2%
Inversion/theory: 7%