 This paper examines the carrier-to-noise ratio, CNR, of global positioning system, GPS, satellites in relation to multi-path errors caused by floods. It demonstrates how the CNR can be used to detect and track floods over time. The paper also shows how the CNR can be used to accurately measure the amplitude of multi-path errors and flooded environments. This article was authored by Ji Feng Tong, Ming Kuen Su, Fu Jing and others.