RIVER and LAKE


Frequently Asked Questions



Could you indicate if it will be a commercial product or if there will be a PI programme or other research activities associated with it? What is the availability of lake and river altimetry data outside of the sample data regions mentioned at http://earth.esa.int/riverandlake/samples/samples.htm ? Is it planned to consider the region of the Okavango Delta, Botwana (where a hydrological model is currently being developed as part of a development scheme)? I read in your email flyer that "Envisat RA-2 obtains data intermittently from the surrounding terrain, and gets very good data from the bottom of the gorge through which the Yantze river runs." I presume that you are NOT talking about getting to the actual river channel and this means past the walls of the canyon and getting a signal off the top of the water surface. How often would water levels be available (e.g. once a day)? Would a radar image cover a complete river of for example 400 km length? We are interested in water levels during floods when the width of the river is 1-3 km's. Would that width be sufficient for its detection by radar altimetry? Does the method give a 2-dimensional water level map? Comparing side-looking radar with radar altimetry, what are the differences other than the look (incident) angle of the transmitted signal? What accuracy in water level is possible? What is the delay between paasage of the satellite and availability of the water levels (production time)? Could you please indicate if river altimetry data will be available on a worldwide basis and the time frame for processing and receiving such data?


Could you indicate if it will be a commercial product or if there will be a PI programme or other research activities associated with it? All samples currently available on the website are part of a pilot Demonstration project and can be used by any user. In return we expect some feedback on the usefulness, the quality, the accuracy, the latency, the frequency, etc... of the producs used. Any report on comparison with local knowledge and in-situ data is welcome. In the future, RLH and RLA products will follow the ESA Earth Observation data policy (where data delivered by ftp for research purposes are free).


What is the availability of lake and river altimetry data outside of the sample data regions mentioned in the River and Lake website? At present, the samples presented in the website are the only RL products currently available. It is intended that all ERS-1 and ERS-2 data will be reprocessed and the respective RLH products will be produced. Furthermore, another objective to reach after this development is to install in the ENVISAT and Sentinel-3 Ground Segment the River-Lake processor to distribute RLA and RLH products in Near Real Time (<3 h).


Is it planned to consider the region of the Okavango Delta, Botswana (where a hydrological model is currently being developed as part of a development scheme)? No products from this region will be provided since it is a swamp.


I read in your email flyer "Envisat RA-2 obtains data intermittently from the surrounding terrain, and gets very good data from the bottom of the gorge through which the Yantze River runs." I presume that you are NOT talking about getting to the actual river channel and this means past the walls of the canyon and getting a signal off the top of the water surface. There is an example in the River and Lake Website of possible measurements over the Yangtze near the Three Gorge Dam. Since the Radar Altimeter is looking at Nadir, echoes coming from the river can be acquired (even if it is surrounded by steep banks).


How often would water levels be available (e.g. once a day)? The ERS-2 and ENVISAT satellites have a 35-day repeat orbit. Over large lakes more than one altimeter crossing per 35-days may be available, improving the temporal frequency. Further information about the frequency of the ERS-2 and ENVISAT satellite tracking can be done running the free ESOV tool (http://eop-cfi.esa.int/ESOV/index.html)


Would a radar image cover a complete river of for example 400 km length? The radar altimeter will have several measurements over a 400 km length river at different geographical positions and different times. It does not provide an "image" like the SAR.


We are interested in water levels during floods when the width of the river is 1-3 km's. Would that width be sufficient for its detection by radar altimetry? There may be between 3 and 9 altimeter measurements across a river of 1-3 km width (approximately one measurement per 350 meters). A RLH product requires a minimum of 3 successive measurements to generate a reliable value for the crossing point.


Does the method give a 2-dimensional water level map? No, the RLH product is a time series of a particular location (crossing point). It is possible to have several crossing points over a large lake or a long river. The RLH product could be assimilated within a catchment basin model.


Comparing side-looking radar with radar altimetry, what are the differences other than the look (incident) angle of the transmitted signal? The RLH product is not a radar image but a radar range on a point target at Nadir.


What accuracy in water level is possible? It depends on the precision of the satellite measurement, on the width of the river and on the morpholology of the banks. The accuracy is 3 cm over oceans and large lakes and 10 cm over the Amazon. The accuracy may be further reduced over smaller rivers. These are indicative numbers that vary a lot with the target.


What is the delay between passage of the satellite and availability of the water levels (production time)? Currently, we are only producing 7-year samples over defined targets (available in September 2003 and April 2004) but the objective of the project is to product RLH and RLA products in near-real time. This means that the altimeter data will be processed to produce RL products in less than 3 hours after the measurement has been made.


Could you please indicate if river altimetry data will be available on a worldwide basis and the time frame for processing and receiving such data? See FAQ02 and FAQ11. The next selection of samples will be available by the end of April 2004.