Study of external path delay correction techniques for high accuracy height determination with GPS

For specific applications such as permanent GPS network calibration and nat ional leveling network surveying, a vertical accuracy of similar to 1mm for observing durations of a few hours to a few days at maximum in 10-100-km b aselines would be required. To achieve a 1-mm accuracy in height determinat ions with differential-GPS measurements, path delay must be corrected with an accuracy of similar to0.3 mm. This level of accuracy is not achievable w ith standard GPS data analysis procedures. External correction from a water vapor remote sensing technique is therefore necessary. Microwave radiomete rs, which have been most extensively used for this purpose, solar spectrome ters, DIAL and Raman lidars are considered in this paper. The principle and performance of these techniques is reviewed in the context of wet path del ay retrieving. Namely, we evaluate the errors arising during the conversion of raw measurements to wet path delay, using retrieval coefficients or sta ndard profiles. It is shown that changes in temperature profiles can produc e errors of up to cm in wet path delay with microwave radiometers. Similarl y, mismodeled temperature profiles can produce errors of 2-3 mm in wet path delay with DIAL and Raman lidars. Raman lidar offers the possibility to re trieve the temperature profile from total air density. Assuming that absolu te concentrations of water vapor and dry gases can be retrieved, the accura cy would be unbiased. In addition, Raman lidar would also allow for the cor rection of hydrostatic path delay without requiring the use of mapping func tions. This might reduce the residual errors due to horizontal pressure and temperature gradients. This technique will therefore be investigated in mo re details in a future study. (C) 2001 Elsevier Science Ltd. All rights res erved.