## 6 Instrument description
### Description of the different units  
As shown on the block diagram (see Figure 1), the instrument is made of 5 units.  
* The chirp board (see Figure 2) contains the digital chirp generator, the sequencer (ASIC), and the time base which converts the 10 MHz reference from Doris to a 160 MHz reference. There is a high recurrence from the ESA CRYOSAT/SIRAL interferometric altimeter instrument for this board.
- The altimeter processing board is very similar to the POSEIDON-3 one to be flown on Jason-2. The main elements are an ADC (Analog to Digital Converter), a DAPD (Digital Amplitude Phase  
Figure 2: Chirp / sequencer / time base board.  
Demodulator), the FFT 128 points ASIC, the RAM to be used for high data rate mode, the time averaging function (accumulation of elementary waveforms in order to reduce the speckle noise)  
- The DSP board which contains the DSP21020 board (SIRAL recurrence)  
- The DC/DC board (based on SIRAL)  
- The Radiometer processing board is a new development  
* The Altimeter Radiofrequency Unit (ARFU)  
The Altimeter Radiofrequency Unit is based on POSEIDON-2/3 modules (with no C-band ).  
- The bandwidth multiplier module is adapted to 500 MHz bandwidth from POSEIDON-3.  
The transmission (Tx) module structure (Tx Ku + Tx Ka + SSPA module and EPC + Ka LO (Local Oscillator) is recurrent from POSEIDON-3 with some internal adaptations. The SSPA (Solid State Power Amplifier) is a new development: indeed, a 2 Watt High Power Amplifier (HPA) is needed for the link budget, which represents a state of the art device.  
- The reception (Rx) module (Rx Ku + Rx Ka + reference chain + LO Ku + deramp function) is recurrent from POSEIDON-3 with some internal adaptations.  
- The Intermediate Frequency (IF) module (Digital gain application + anti-aliasing filter + LO) is fully recurrent POSEIDON.  
- DC/DC module is based on POSEIDON-3 module  
- The duplexer (see Figure 3) is the interface between the antenna, the transmission module (Tx) and the reception module (Rx). It is similar to POSEIDON-3 Ku duplexer.  
Figure 3: Altimeter Ka-band duplexer.  
* Antenna The antenna is composed of a fixed offset reflector with a 1 m aperture, a 0.7 m focal length and a 0.1 m offset, a three-band feed (35.5-36 GHz; 23.6-24 GHz; 36.6-37 GHz) and a sky horn pointing to deep space, thus providing a cold temperature reference to the radiometer. Separation of the radiometer polarization from the altimeter polarization, and separation of Ka altimeter and radiometer channels through discrimination of polarization have been achieved by designing dedicated polarisers.  
The main antenna performances can be summarized as follows:  
- Altimeter: gain >49.3 dB and 3-dB aperture <0.6\({}^{\circ}\)  
- Radiometer 3-dB aperture : 0.8\({}^{\circ}\) at 23.8 GHz and 0.6\({}^{\circ}\) at 36.8 GHz  
SLL <-25 dB leading to a beam efficiency of 96 to 98 %
* Radiometer Radiofrequency unit The radiometer is a total power radiometer with direct detection. The radiometer radiofrequency unit consists of two RF receivers. These receivers will be the same as the ones used in the framework of the MEGHA-TROPIQUES mission on the MADRAS radiometer. They are developed by ASTRIUM-F.  
Figure 4: Radiometer measurement cycle (A) and Radiometer calibration unit (B).  
* Radiometer calibration unit The radiometer must be switched-off during radar altimeter emission. The radiometer measurement cycle is shown on the left panel of Figure 4. In the nominal mode, radiometer receivers measure antenna temperatures (A1 to A5). In the internal calibration mode, receivers are either connected to a sky horn pointing to deep space (cold reference) (CS) or to a load at ambient temperature (hot reference) (HS). This internal calibration can be performed every few seconds. Measured temperatures are averaged over 200 ms. The radiometer calibration unit (right panel of Figure 4) performs altimeter bandwidth filtering and commutation to calibration sources.