2.9 Altimetric waveforms to monitor lakes level: Lake Issyk-kul

Results and comments

On the first kind of graph, Y=F(X), each waveform can be seen individually by browsing one per one, or all the waveforms selection by animating them. This animation can't be saved, but each frame/waveform can be. We have selected and saved 5 waveforms per cycle, which are closest for a given location from one cycle to another (altimetry measurements are not taken exactly at the same point from one cycle to another). We observe many waveform shapes along the lake: Brown echoes, specular, multipeaked, etc (see From radar pulse to altimetry measurements). But the waveform shapes are also quite different for a same location, due to the kind of tracker used or due to the change of surface conditions between the cycles.

Individual waveforms along five given points over the lake Issyk-kul for the three first cycles of OSTM/Jason-2 (maps from Google Earth). X-axis represents the 104 waveforms samples (gates) in Ku-band, so the time. The Y-axis represents the return echo power.

On the second kind of graph, Z=F(X,Y), we observe a set of juxtaposed wafeforms in function of latitude, as if seen from above. The segment pass is covering approximatively 520 km, from 41°N to 45°N, so a long segment before and after the lake.
The altimetric signal is not present all over the segment, there are a lot of missing values (white areas on the graph) corresponding to the mountainous land. The signal loss is different in function of the tracker used : the Median tracker (middle) seems to be locked and gives a signal more often.
The Southern boundary of the Lake Issyk-kul is detected for the three trackers at the same time, around 42.15°N. But the Median tracker (middle) seems to give a longer tracking over (or beyond) the Northern boundary of the lake.
Over the lake, the thermal noise of the waveforms corresponds to black areas. The leading edge position corresponds to the black/dark blue transition and follows the thermal noise. The leading edge position varies a lot over the lake for the trackers SGT and Median: a difference of 20 gates can be seen for the Median tracker, corresponding to a range difference of almost 10 m. Such discrepancies are corrected using the adapted retracking algorithm.
Another feature can be underlined with the red area measured over the lake by all the three trackers and at the same time, around 42.7°N. This feature might correpond to a highly reflecting surface nearly the Northern coast.

Set of juxtaposed wafeforms in function of latitude (41°N - 45°N, pass #131), as seen from above for the three first cycles of OSTM/Jason-2. Each cycle corresponds to a different tracker: SGT for cycle 001, Median tracker for cycle 002 and Diode+MNT for cycle 003.