Altimeter Design

Introduction

The Radar Altimeter is a Ku-band (13.8 GHz) nadir-pointing active microwave sensor designed to measure the time return echoes from ocean and ice surfaces. Functioning in one of two operational modes (ocean or ice) the Radar Altimeter provides information on significant wave height; surface wind speed; sea surface elevation, which relates to ocean currents, the surface geoid and tides; and various parameters over sea ice and ice sheets.

The Radar Altimeter operates by timing the two-way delay for a short duration radio frequency pulse, transmitted vertically downwards. The required level of accuracy in range measurement (better than 10 cm) calls for a pulse compression (chirp) technique. In ocean mode a chirped pulse of 20 micro-s duration is generated with a band width of 330 MHz. For tracking in ice mode an increased dynamic range is used, obtained by reducing the chirp bandwidth by a factor of four to 82.5 MHz, though resulting in a coarser resolution.


 

Functional Description

The Radar Altimeter antenna is shown in the figure and a block diagram illustrating the functioning of the Radar Altimeter is shown in the figure.

The Frequency Generator units provide the transmit signal at a frequency of 450 MHz to the chirp generator. This generates a chirped output with a bandwidth of 165 MHz (ocean) and 41.25 MHz (ice), gated within a pulse of 20 micro-s. This signal is up-converted and multiplied (using C- and L-band LO signals) to 13.8 GHz, with 330 MHz (ocean) and 82.5 MHz (ice) bandwidths. The required power output level (42 dBm) is generated by the High Power Amplifier (HPA), which is realised as a Travelling Wave Tube and Electronic Power Conditioner (TWT/EPC) combination. A harmonic filter at the TWT output attenuates the harmonics of the RF signal. The transmitter signal is fed to the antenna.

The returned signal is routed to the receiver via the Front End Electronics , with an insertion loss of approximately 1.6 dB. The received chirp signal is deramped by mixing it with the LO chirp at a frequency of 15.025 GHz. The deramped output (first IF) is at 1.225 GHz. The signal is then amplified to recover the conversion loss, filtered and mixed with a second LO chirp (1.3 GHz) to provide a second IF of 75 MHz. The second IF signal is filtered, using a surface acoustic wave (SAW) device with a bandwidth of 3.2 MHz and passed via a step attenuator. This provides an overall gain adjustment over a 62 dB range, implemented as two 31 dB step attentuators with a step size of 1 dB. The output is then coherently detected by a quadrature IF mixer to obtain the I- and Q-components of the received signal.

The Processor and Data Handling Sub-system (PDHSS) performs tracking and the necessary processing of the radar echoes in order to maintain the echo within the radar range-window. In summary, the main instrument parameters and technical characteristics of the Radar Altimeter are listed below:
 
 

Mass: <= 96 kg
Antenna diameter: 1.2 m
DC power: <=134.5 W
Data rate: <= 15 kbit/sec
RF frequency: 13.8 GHz (Ku band)
Bandwidth:
ocean mode : 330 MHz
ice mode : 82.5 MHz
Pulse repetition frequency: 1020 Hz
RF transmit power: 50 W
Pulse length: 20 micro-s chirp
Altitude measurement: 10 cm (1s, SWH = 16 m)
Significant wave height: 0.5 m or 10% (1s) whichever is smaller
Backscatter coefficient: 0.7 dB (1s)
Echo waveform samples: 64 x 16 bits at 20 Hz
Beam width: 1.3deg.
Foot Print: 16 to 20 km (depending on sea state)

Measurement Objectives

The Radar Altimeter for ERS-1 has been designed to meet very demanding constraints and has the following major objectives:

Measurement Principles