The routines for ground effect and engine installation effect developed in this project improve calculation accuracy for the aircraft noise models NORTIM and GMTIM. For a sample of nearly 70.000 measurements at Gardermoen, the overall improvement is in the order of 1 dB for equivalent noise levels (SEL). Noise data extracted from the measurement program at Gardermoen in 2001 further improves the accuracy by 0.5 dB for SEL. It is recommended that updated versions of the two programs should be used in noise calculations as an interim solution until a revised recommendation (either from ECAC or SAE) for lateral attenuation appears. An international co-operative work is needed to supply noise data from normal operations for all aircraft in the database. It is recommended that new noise data be implemented as they occur.