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Definitive Ephemeris If available, the Landsat 7 definitive ephemeris is used for geometrically correcting ETM+ data. Definitive ephemeris substantially improves the positional accuracy of the Level 1G product over predicted ephemeris. An ephemeris is a set of data that provides the assigned places of a celestial body (including a manmade satellite) for regular intervals. In the context of Landsat 7, ephemeris data shows the position and velocity of the spacecraft at the time imagery is collected. The position and velocity information are used during product generation. The Landsat 7 Mission Operations Center receives tracking data on a daily basis that shows the position and velocity of the Landsat 7 spacecraft. This information comes from the three US operated ground-receiving stations and is augmented by similar data from NASA's Tracking and Data Relay Satellites. The Flight Operations Team processes this information to produce a refined or "definitive" ephemeris that shows the position and velocity of Landsat 7 in one minute intervals. Tracking data are used to compute the actual spacecraft position and velocity for the last 61 hours and to predict these values for the next 72 hours. The predicted ephemeris data are uploaded to the spacecraft daily. On-board software interpolates from this data to generate the positional information contained in the Payload Correction Data (PCD). Engineers with the Landsat Program have completed a predicted versus definitive ephemeris analysis. Comparisons to ground control points demonstrate the definitive ephemeris is, in fact, reliably more accurate than the predicted ephemeris. Geometric accuracy on the order of 30-50 (1 sigma) meters, excluding terrain effects, can be achieved when the definitive ephemeris is used to process the data. Level 1G products produced after March 29, 2000 use definitive ephemeris if available. The .MTL field "ephemeris_type" in the product metadata files identifies whether a product was created with definitive or predicted ephemeris. Daily definitive ephemeris profiles have been archived since June 29, 1999 and are available for downloading. |
