The Radar altimeter is essentially an instrument for determining the two-way delay of the radar echo from the Earth's surface to a very high precision (less than a nanosecond) level. It also measures the power and the shape of the reflected radar pulses.
In RA-2, a clear distinction is made between the robust collection of accurately quantified radar echo data (this tracking is a primary function of the instrument), and the interpretation of these data as meaningful geophysical quantities (this is only the task of the on-ground processor).
Previous altimeters have merged these functions in the on-board tracker, thus increasing the constraints on its optimisation.
Interpretation of the measured radar echoes can be performed with more or less accuracy according to the surface characteristics. The best results are obtained over the ocean, which is spatially homogeneous, and with a surface which follows known statistics. Surfaces which are not homogeneous, and with discontinuities or strong slopes such as some land surfaces, allow less accurate interpretation. RA-2 transmits microwave pulses which propagate at a speed determined by the refractive index (see below) of the propagation medium, which is close to unity. The time elapsed from the transmission of a pulse to the reception of its echo reflected from the Earth's surface is proportional to the satellite's altitude. The magnitude and shape of the echoes contain information on the characteristics of the surface which caused the reflection.
The RA-2 measures the power level and time delay of 128 samples of echoes from ocean, ice, and land surfaces. It is optimised to maintain the leading edge of this echo in the range window. This is achieved by one of the new features on RA-2 compared to ERS-1: A model-free tracker in the on board signal processor. Window position and resolution are controlled by algorithms developed to suit the tracking conditions. Adaptive height resolution operation is implemented by selecting the transmitted bandwidth. As a result, measurements over ocean are carried out with high accuracy at the highest resolution. Over land or ice or during transitions from one kind of surface to another the tracking is maintained accepting sometimes a certain degradation of the height resolution.
Echo samples are processed on ground to account for correction and calibration data. Altitude, surface topography, and reflection coefficients of all kinds of Earth surfaces can be derived. Algorithms based on models are used to estimate altitude over various surfaces, and wind speed and significant wave height over the oceans.
RA-2 is a nadir-pointing, pulse-limited radar altimeter which transmits frequency modulated pulses (chirp) pulses. This frequency modulation is a coding of the signal which spreads the energy of a short pulse over a longer time interval, thus allowing reduced peak power in the pulse.
The transmit pulses, which are generated in the chirp generator (by means of SAW RAC) are amplified by either Ku-band or S-Band amplifiers, depending on the selected transmit frequency. The Ku-band HPA uses a traveling wave tube whilst the S-band transmitter applies the solid state technology. The Ku-band front end electronics (KFEE) or the S-band front end electronics (SFEE) feed the signals to be transmitted to the antenna which is designed as a dual-frequency parabolic antenna. In the block diagram only the Ku-band chain is shown, for simplicity.
The antenna is designed as a dual-frequency parabolic antenna
The radar echo is composed a large number of replicas of the transmitted chirp signal, reflected from the facets of the surface below. When the echo from the surface is expected to return (about five ms later, depending of the altitude of the satellite), the chirp generator is activated again. This time the chirp signal has a different path: it is up-converted and routed to the first mixer in the receiver, where it becomes the intermediate frequency signal (IF signal). In mixing the return echo signal with the IF signal, time delays in the echo are mixed down to constant frequency tones. Thus, mapping of the return pulses from the time domain to the frequency domain is achieved. This full-deramp concept.
The resulting signal is filtered, amplified and down-converted. A phase detector produces the baseband in-phase (I) and quadrature (Q) components. These signals are fed to the signal processing subassembly (SPSA) where they are sampled and digitised. Then, a 128-point complex fast Fourier transform, square-modulus extraction and averaging (over 100 echoes) are applied to the samples to produce an averaged signal power spectrum.
In the SPSA software a robust tracking algorithm keeps the leading part of the echo spectra (independent of their shapes) within the sampling window. It does this by appropriate timing of the deramping chirps such that they will coincide with the echo signals and, if necessary, by appropriate adaptation of the chirp bandwidth and thereby of the sampling window resolution. This allows to track all kinds of Earth surfaces with an optimum adapted height resolution. There are three chirp bandwidths available: 320MHz, 80MHz and 20MHz. In general, the tracking is performed with the highest possible resolution (corresponding to 320MHz). A second tracking algorithm controls the receiver gain.
Engineering model delivered to
For the initialization of the tracking and, after loss of tracking, an acquisition routine detects the radar echoes and presets the tracking parameters in order to allow the start of the tracking. The indication of loss of track with the following acquisition, and transitions between acquisition and tracking are performed autonomously. The acquisition uses unmodulated radar pulses.
Simultaneously to the tracking, the RA-2 periodically performs internal calibration measurements. For this, the transmit pulse is coupled into the receiver by means of a calibration coupler (-100 dB) within the front end electronics. The signal is amplified, deramped, filtered, etc., as the normal radar echo signals are. The power spectrum of this signal, which is extracted by the SPSA, represents the point target response of the instrument thus indicating all residual errors and distortions in the transmit/receive path (except the antenna and the antenna feeders which have to be characterized separately on ground).
All measured data, the radar echo spectrum samples, the point
target response spectrum samples, etc., together with auxiliary
data which are necessary for the evaluation of the measured data,
e.g., the instrument parameter settings, measurement datation data
and others are sent to ground for further processing. On command,
the RA-2 is able to transmit also bursts of single radar echo
samples (ADC outputs) to ground.
Radar pulses transmitted from the satellite are radiated in a spherical shell whose intersection with the surface defines an instantaneous illuminated area.
A flat surface which is rough on the scale of the radar wavelength (of order centimetres) is a diffuse scatterer. Reflected energy from a flat diffuse surface is proportional to the illuminated area. Resulting from the constant area of the expanding annulus, the averaged return exhibits a linear initial rise, followed by a plateau region, which eventually is attenuated by the fall-off of the antenna pattern off-boresight. The resulting form of the mean radar echo as a function of time is shown below. We define the instant at which the leading edge of the spherical shell just touches the surface (or in fact the time this echo arrives in the altimeter) as a time origin (t = 0).
Radar pulses transmitted from the satellite are radiated
in aspherical shell whose intersection with the surface defines an
instantaneously illuminated area
Now consider that the surface is slightly rough due to ocean waves. The roughness scale is a few times (although the same order of magnitude) the pulse length (c . tau; where tau is the compressed pulse). Then the leading edge of the pulse will illuminate the peaks of the roughness earlier than before, and therefore there will be echo energy returned to the altimeter earlier than t = 0. The same reasoning applies for the trailing edge of the pulse, and so the echo strength at time will be slightly less than for a perfect flat surface. The diameter of the footprint is therefore larger, and the duration of the leading edge of the echo is proportional to the surface roughness. A modeling of the echo under these conditions was built by Brown (1977), and this Brown model is widely used in the altimeter community so far. In this model the power envelope of the echo is described by the convolution of the system point target response, the antenna pattern, and the height distribution of surface scatterers. In the Brown model these functions are all taken to be gaussian. Other models developed later, such as the Hayne model (Hayne, 1980), are based on the Brown model but include skewness in the gaussian assumption of the point target response and in the height distribution of scatterers.
Non-ocean surfaces are characterised by a height distribution of scatterers which is not easily described by statistical or analytical models. They can have abrupt altitude changes and large-scale slopes. Over such surfaces the form of the echo is determined by the range to the different scatterers, their reflectivity, and their position in the antenna pattern. This results in echoes of unpredictable shape, which change rapidly. RA-2 offers reliable tracking of such echoes and their interpretation into geophysical results will provide interesting data.
Refractive index corrections
The radar altimeter measures the time delay of the echo. This can be interpreted as range or altitude, knowing the speed of radar wave propagation, which means that the refractive index of the propagation medium must be known. At the primary frequency of 13.575 GHz the main contributors are oxygen, water vapour, and the ionosphere.
The refractive index of oxygen is the largest contributor,
partly because the integrated column density is greater. It
accounts for over two metres of range correction. The effect can be
estimated with an error of about 0.2% from a simple expression
depending only on sea level atmospheric pressure and the local
acceleration due to gravity.
The effect of water vapour is smaller than oxygen, but much more variable. It can range from virtually zero over the high, dry ice caps, to about 40 cm in tropical regions. Over oceans the water vapour column density is measured by the on board microwave radiometer (see MWR in the Instruments section). Over other surfaces the extraction of the water vapour signature from the MWR is less accurate and forecast or analysed fields must be used.
The ionosphere, a low-density cold plasma, has four effects on electromagnetic waves:
phase advance and group delay