## 8 Expected altimeter performances
### Measurements objectives and principle
#### Performance of the instrument tracking phase  
Let us now go to the tracking performances, that is evaluating the capability to remain tracked even with land perturbation, and also to detect a useful signal as soon as possible. In this respect, coastal and continental water areas have been addressed with a particular emphasis, based on the coupling with DORIS/DIODE and the embarked along-track DEM. The DEM has been designed a priori in order to be optimized in areas of interest (coasts, lakes, rivers, deserts,...) : to do so, even if the nadir point is above a land surface, the distance found in the DEM corresponds to the distance from the surface of interest. An example of such an optimization is illustrated on Figure 10. The validation and optimization of such an along track DEM is a critical issue as it has to be very accurate, with regards to the range analysis window of 30 meters. (The basic concept is very different from the GLAS instrument that works which much larger windows and may then use less critically accurate DEMs).  
In the classical tracking mode, the input of the loop is a range error calculated from a median discriminator. The principle is to estimate the range gate that corresponds to half of the total power of the echo and then derive the error, the theoretical gate being known. Range instructions are then computed by the loop from the error. Signal to Noise Ratio (SNR) is also calculated and is the basis of the criterion that is used to advertise loss of track (when SNR is too low).  
Transitions from water to land are generally well processed with a median discriminator. Over such transitions, using the algorithm benefiting from DORIS/DIODE and a DEM rather than a classical tracking loop does not improve tracking very much. However, coupling the altimeter with the DORIS/DIODE navigator greatly improves the performance in transitions from land to water areas.  
Figure 9: Simulated sea to land transition echo over a topography known from a DEM.  
The performances of the various tracking algorithms over land to sea transitions can be compared on Figure 11. The loop surveillance criterion is based on the value of the signal to noise ratio. This plot illustrates the improvement reached by using the coupling process with DORIS/DIODE. Indeed, in this case, using DORIS/DIODE in the search phase results in keeping track 1 second more than when using the classical tracking loop algorithm. The result is even better when using DORIS/DIODE and a DEM, as the altimeter then keeps track 2 seconds more. This example is obtained in good environment conditions as the relief along the considered coast is not very sharp: This is illustrated on Figure 12  
Figure 11: Comparison of tracker behavior in a land to sea transition with several options (Vertical axis: SNR in dB, Horizontal axis: time).  
Figure 10: Distance optimization in the on board DEM.  
where the topography is displayed together with the footprint of an echo having 45% of water in it. The corresponding echo is displayed on the left panel of Figure 12, after tracking using Diode and a DEM.  
Most often water must be in at least 30% of water of the radar footprint to detect an echo. However, this depends a lot on the relief in the vicinity of the transitions. It should also be underlined that echoes are often affected by some deformations in transitions areas, such as : a low signal to noise ratio, a mispointing effect, etc.