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3.2 RA-2 Instrument Characteristics and Performance

3.2.1 Pre-flight Characteristics and expected performance

A simplified RA-2 block diagram is sketched in the following figure: to reduce impacts on instrument complexity, the same hardware is used (on a time-shared basis) as much as possible for both the Ku and S band channels. All the subsystems with the exception of the antenna are redounded according to ENVISAT project system level requirements. The key system parameters of the instrument are listed in Table 3.2-1.

RA-2 is composed by the following sub-systems: Antenna, Ku-Band Front End Electronics (KFEE), S-Band Front End Electronics (SFEE), Ku-Band Transmitter (KTx), S-Band Transmitter (STx), Microwave Subsystem (MR), Frequency Generation and Conversions Unit (FGCU), Chirp generator (CG), Signal Processor Subassembly (SPSA), Low Voltage Power Supply (LVPS) and the Instrument Control Unit (ICU).

 


 

 

Table 3.2 RA-2 key system parameters

Orbit Range

764 - 825 Km

Operative Frequency

13.575 GHz (Ku)

3.2 GHz (S)

Pulse Length

20 μs

Ku Tx pulse Bandwidth

320 - 80 - 20 MHz - CW

S Tx pulse Bandwidth

160 MHz

Tx Peak Power

60 W (Ku)

60 W (S)

Pulse Repetition Interval

557 ms (Ku)

2228 ms (S)

Antenna Gain

41.6 dBi (Ku)

29.2 dBi (S)

Antenna -3 dB Beamwidth

1.35 deg (Ku)

5.5 deg (S)

IF centre frequencies

1223 MHz / 75 MHz

RF Losses

1.8 dB (Ku)

1.7 dB (S)

Receiver Maximum Gain

107 dB

AGC dynamic range

60 dB

programmable in 1 dB step

Receiver Noise Bandwidth

6.4 MHz

Receiver Noise Figure

3.0 dB (Ku)

2.5 dB (S)

USO frequency & stability

10 MHz

(10-9 over 24 Hours)

FFT Processor

128 points

A/D conversion (no. bits)

8 + 8

Data Rate

100 Kbit/s

3.2.1.1 Chirp Generator

The Chirp Generator (CG) unit provides the chirp signals at 450 MHz centre frequency employed for the generation of the transmission signal and for the LO signal used in the deramping process, for both Ku and S band chains. The three chirp bandwidths that can be synthesised are 160 MHz, 40 MHz and 10 MHz. The CG uses SAW RAC (Surface Acoustic Wave - Reflective Array Compressor) technology. To achieve the 35% bandwidth needed for the 160 MHz LFM signal, a LiNbO3 substrate has been used; this design provides a very tight control of the amplitude and phase ripples required to keep sidelobes of the compressed signal down to -30 dB.

3.2.1.2 Frequency Generation and Conversion Unit

The Frequency Generation and Conversion Unit (FGCU) has two major tasks: to provide all the reference frequencies required by the instrument and to up-convert the chirp signal from the CG. All frequencies are synthesised from a reference 10 MHz Ultra Stable Oscillator (USO) and from a stable local oscillator 101.937 MHz frequency. The Ku band up-converter chain includes a frequency doubler and provides 320 MHz, 80 MHz and 20 MHz chirp signals at 13.575 GHz centre frequency to the Ku band transmitter and to the Microwave Receiver, where, after a further translation to 14.798 GHz, is used in the LO branch of the DRM. A single tone pulse for the Tx and a CW LO can also be provided for the acquisition phase. The S band up-converter operates only with the 160 MHz chirp which is translated to a centre frequency of 3.2 GHz; a second chain derives the 4.423 GHz for the LO branch of the S band DRM in the receiver.

3.2.1.3 Transmitter

The Ku band signal is amplified by the Ku band transmitter, which uses a TWT amplifier to provide 60 W peak power at 3.6 % duty cycle, the amplified signal is sent to the antenna through the Ku band Front End, a high isolation switch that uses switched circulators to perform the Tx/Rx duplexing function.

As far as the S band RF chain, it is similar to the Ku chain but uses a solid state transmitter and a PIN diode duplexer switch in the transmission section.

The S band solid state technology amplifier uses class C bipolar transistor in the final and driver stages and provides 60 W peak power at 1 % duty cycle.

3.2.1.4 Antenna

The antenna is an onset 1.2 meters diameter parabola using a coaxial Ku/S band feed, developed by ALS.

3.2.1.5 Microwave Receiver

The signal received from the antenna is supplied, through the Ku FEE, to the Microwave Receiver (MR), which after low noise amplification, performs the deramping function. The 14.798 GHz LO is generated in the MR from the same 13.575 GHz used in the transmission chain through the 1.223 MHz frequency which, together with the 75 MHz, are used as the reference frequencies to down convert the received signals. The MR performs also the AGC and band-limiting functions providing as output the in-phase and quadrature components of the received signals. Also, separate low noise amplifier and deramping mixer are used in the MR: all the electronics following the deramping function is instead common to the Ku and S band chains.

For the S FEE is concerned, need for a compact design but allowing at the same time good insertion loss and isolation figures, have led to develop an innovative design approach, using PIN diode switches in shunt configuration on a Squared Coax transmission line.

3.2.1.6 Digital Processing Subassembly Unit

In-Phase and Quadrature samples of the received signals as provided by the MR are digitally converted in 8+8 bit format by the Signal Processing Subassembly Unit. The 128 I/Q samples thus obtained (a 6.4 MHz clock is used in the A/D conversion process over a time window of 20 μs) are further processed to allow real time on board tracking of the radar echoes by exploitation of the Ku band data. Additional processing tasks are actually performed by the SPSA during the tracking operations like instrument Point Target Response samples collection, Receiver Noise Power Measurement, computation of two additional Ku band waveform samples in user's selected positions, collection of individual echoes on request from ground. A more detailed description is hereafter available. The SPSA, core of the altimeter instrument has been developed by ALS.

 

The FFT processing board required to implement the digital filter bank for the radar pulse compression through the full deramping technique is based on the TMC2310 DSP device (by TRW-USA), which performs 128 points complex FFT using a block floating point arithmetic in 16 bit format. Echoes accumulation over multiple radar sweeps (100 for Ku and 25 for S band) is performed using a 24 bit internal adder while averaged waveforms are returned in 16 bit unsigned integer format.

 

The tracker processing board is instead based on the general purpose microprocessor MA61750.

 

Most of the tracker processing functions are implemented in 32 bit floating point arithmetic, but for the range measurement related processing tasks which are implemented in 48 bit extended floating point format to guarantee for the best precision of the data. Approximately 70 parameters are used to optimise operations in the Measurement mode. All parameters are modifiable from ground using specific macrocommands. More generally the entire SPSA SW is patchable from ground via the ICU.

 

The whole instrument timing is generated by the SPSA and relies on the 80 MHz reference signal synthesised by the FGCU starting from the 10 MHz USO frequency. Tracking is accomplished by shifting within the PRI the position of the LO chirp pulse in steps of 12.5 nsec. Fine adjustments of the tracker window position within the 12.5 nsec are obtained by shift rotating the echo samples through complex multiply by a phase varying reference function before FFT execution. Shifts as fine as 1/256 of the 12.5 nsec clock period are made possible by the implemented technique.

3.2.1.7 Low Voltage Power Supply and Instrument Control Unit

The last two units in the block diagram are the Low Voltage Power Supply (LVPS) which provides regulated voltages lines to the other units and the Instrument Control Unit (ICU).

 

The ICU is in charge of controlling and monitoring the instrument health and operations and interfaces with the platform. The ICU architecture is still based on the MA61750 general purpose microprocessor and similarly to SPSA, ICU SW is fully patchable from ground.


Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry