Geodesy: https://oceanservice.noaa.gov/facts/geodesy.html
Geoid: http://www.esri.com/news/arcuser/0703/geoid1of3.html
UNAVCO tutorial: http://www.unavco.org/education/resources/tutorials-and-handouts/tutorials/geoid-gps-receivers.html
European Space Agency (ESA) Gravity field and steady-state Ocean Explorer (GOCE):
http://www.esa.int/Our_Activities/Observing_the_Earth/GOCE
How GPS works: http://gisgeography.com/trilateration-triangulation-gps/
Tide gauge: https://oceanservice.noaa.gov/facts/tide-gauge.html
https://oceanservice.noaa.gov/podcast/oct14/mw128-taking-measure.html
Argo floats: http://www.argo.ucsd.edu/
Jason satellites: https://www.eumetsat.int/jason/
GRACE satellite: https://earthobservatory.nasa.gov/Features/GRACE/