UK government study on: Satellite-derived Time and Position - Critical Dependencies https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/676675/satellite-derived-time-and-position-blackett-review.pdf Seems quite comprehensive, and may be of interest to GNSS users. 1.6 MB and 86 pages. Cheers, David -- SatSignal Software - Quality software written to your requirements Web: http://www.satsignal.eu Email: david-taylor@blueyonder.co.uk Twitter: @gm8arv