• Manual on Sea-level Measurements and Interpretation, Volume V: Radar Gauges [includes Supplement: Practical Experiences].

      Woodworth, Philip (UNESCO-IOCParis, France, 2016)
      Therefore, Part 1 of this Volume 5 discusses topics such as how radar gauges can be mounted over the water to measure sea level. It considers how gauges can be calibrated, either in the laboratory before installation or in the field during routine maintenance visits. It describes how radar performs in comparison to other technologies and discusses how the measured radar levels can be biased in the presence of waves and, consequently, what other technologies must be used in parallel. Part 2 of this Volume returns to some topics that have been presented in the previous Volumes 1-4 of the Manual. These are particularly important aspects of tide gauge measurements, and so have been repeated each time, although in different ways. Volume 1 introduced the essential procedures to be followed for maintenance of the datum of the sea level measurements (i.e. the stability of the measurements with respect to benchmarks on the nearby land). Volume 2 described how levelling should be undertaken between a local network of benchmarks and introduced the use of Global Positioning System (GPS) receivers for monitoring vertical land movements. GPS at tide gauges was further discussed in Volumes 3 and 4. These sections were based partly on the insight that had been obtained into the use of GPS in the workshops that led to the two ‘Carter Reports’ (1989 and 1994) and in an important subsequent workshop at the Jet Propulsion Laboratory (1998).2 By this time, GPS at tide gauges was being undertaken using continuous (rather than episodic or campaign) and dual- (rather than single-) frequency receivers, and further research into their use had begun within the TIde GAuge (TIGA) project of the International GNSS Service. The present Volume 5 contains a similar section on the survey methods and benchmark requirements at tide gauges, including the use of GNSS (Global Navigation Satellite System) equipment, and brings up-to-date the recommendations on the use of GNSS at tide gauge sites. 3 Part 2 of the Volume also has updated sections on how tide gauge operators can ensure that their data find their way to centres where they can be used to the maximum extent possible for practical and scientific purposes. For example, it is now inconceivable that gauges installed in the GLOSS network would be without a real-time reporting capability for storm surge and tsunami warning. On the other hand, the data must be of sufficient quality that ‘delayed-mode’ centres can process them into mean sea level values for use in studies of long-term sea level change. These real-time and delayed mode objectives need not be in competition if care is taken to understand the data that are recorded, essential metadata are compiled, and data are transmitted rapidly to the relevant national and international centres. We suggest that new readers of the volumes would benefit from looking at Volumes 1-4 before reading the present Volume 5. Although the earlier volumes date from many years ago, and technology has evolved considerably in the meantime, much of the previous discussion is educational with regard to how the historical sea level data set has been obtained. There are often dangers in exchanging one measuring system for another, in that different systematic methods can be introduced into a long-term time series, so an appreciation of how methods have changed is essential. It is clear that the same kind of mistakes in changing technologies could be occurring now, as radar systems replace others, so we must make attempts to understand them all as well as we can