In a general sense, validation is the process of assessing by independent means the uncertainties of the data products derived from the systems outputs. Verification verifies the instrument performance and calibration corrections may result from the verification process.
To assist the verification/validation (V/V) process after launch, an orbit that produces a repeat track after 8 days has been adopted. Since the Verification Phase lasts for about 90 days after spacecraft checkout, an 8 day repeat orbit will allow about 8 opportunities for overflight of ground V/V sites. The 8 day repeat allows overflights, or near overflights, of V/V sites located at White Sands Space Harbor (NM), Bonneville Salt Flats (UT) and Madison (WI). Other candidate sites are being investigated, such as Africa, S. America and Australia. The 8 day repeat track in the Western U.S. is illustrated in the figure (21K) (illustrated in Adobe PDF format).
Several concepts for V/V are being investigated. These include use of flat areas (e.g., White Sands) that can be appropriately characterized with ground measurements. These V/V sites are suitable for one aspect of V/V for altimetry. With ground instrumentation at the V/V sites, the spot on the ground illuminated by GLAS from an altitude of 600 km can be positioned independently from the technique used to produce the surface profile data product. Comparison of the inferred surface profile with the directly measured profile provides the basis for validating the data product as well as verifying the instrument performance. In addition, the influence of surface slope can be further evaluated by the use of a well characterized undulating surface, such as sand dunes. Temporal change in the dunes will be mitigated by performing the V/V experiment with independent measurements during the GLAS overflight. A third surface planned for use is the ocean. In these experiments, the spacecraft will be commanded to perform a slow roll up to 5 degrees off the nadir direction. Analysis of the returned laser pulse arrival time and pulse shape will enable an independent estimate of pointing biases.
V/V of atmospheric lidar will utilize various approaches. However, direct comparison of the GLAS lidar data products with measurements from a well characterized ground-based lidar enables one approach. As shown in the Western U.S. ground track, both ascending and descending passes occur near a candidate site at Madison, WI. Since the GLAS atmospheric products are obtained with some averaging, a direct overflight of the ground lidar is not required. Nevertheless, the ICESat has an off-nadir pointing capability which may be invoked for either altimetry or lidar V/V.
More information can be found in the GLAS Validation Plan (Version 1.0). Files are illustrated in Adobe PDF format.