Raw GPS L1 SNR (and ancillary) data for an experiment to use a low-cost GPS antenna/receiver to measure water levels using the GNSS - Interferometric Reflectometry technique. The data were recorded at the RNLI lifeboat station in Sligo, Ireland (N 54o 18' 17.8'', W 8o 34' 5.4'' ) using a Globalsat BU353S4 USB puck that uses a SirfStar IV receiver with patch antenna (2018 data) and a Maestro A2200A SirfStar IV module (2019 data). Both systems were mounted to a radio mast at around 16m above sea level. The data are stored in daily files with the naming convention sligDDD0.YY.TNR.gz where DDD is the Day of Year and YY is the year in short format (18,19). Each file is gzipped. The files are flat text files with fixed width columns in the following order 1) PRN GPS satellite code 2) Elevation (degrees) 3) Azimuth (degrees) 4) Seconds of Day 5) change in elevation angle with time (degrees/second) : needed for reflector height change corrections 6) Blank 7) S1 SNR signal (dB-Hz) 8) Blank reserved for S2 SNR signal 9) Blank reserved for S5 SNR signal