Altimeter waveform

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Altimeter waveform

Quartly, G. Srokosz, and A. Oceanic Technol.

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This is the first paper to analyze waveform data from a number of altimeters in a consistent manner. Mean shapes and various statistical properties bin-to-bin correlations, number of independent samples were determined and the authors comment on their anomalies. The analyses were performed for data over the deep ocean, as that is the best understood surface.

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However, the determined functional characteristics of the individual altimeters are applicable to their operation over all surfaces. The implications of the existence of these anomalies for the retrieval of geophysical parameters from radar altimeter data are discussed. It is argued that the need for physically based theories, in order to understand radar altimeter returns from the ocean or indeed any other surface, implies a need for the engineering and software design of the instrument to be such as to avoid spurious anomalies in the waveforms.

Spaceborne radar altimeters have been in use for more than 25 yr, and the concepts underlying their operation have been discussed very thoroughly Chelton et al.

In essence, the instrument emits a short electromagnetic pulse, which is reflected off the earth's surface, and the waveform, that is, the shape and strength of the resultant echo, is recorded on the satellite. The waveform, expressed as a power signal as a function of time, P tis the convolution of three terms Brown ; Hayne :.

The emitted pulse may be approximated by a Gaussian function, and if the PDF of reflecting facets is also Gaussian, the waveform should be as illustrated in Fig. Short-duration large-amplitude pulses would produce too high a peak power for the amplifiers; in practice, all recent altimeters have used long-duration chirps with the same frequency content as the idealized pulse, but a much lower peak power. Figure 1a shows an ideal waveform; those recorded in practice are somewhat different Fig.

For each bin of a single pulse, the received power will be governed by a negative exponential PDF. For an ideal altimeter traversing a surface of constant geophysical properties the expected power in each waveform bin should be similar to that depicted in Fig.

If the bin separation is greater than the effective pulse width, the power received in successive bins will come from different reflecting facets and should be independent. On that basis, the anomalies i. The purpose of this paper is to examine waveform data from a number of recent altimeters and evaluate these three properties—shape of mean waveform, effective number of independent pulses, and bin-to-bin correlation of anomalies.

Although some of these anomalies are documented [e. Here we provide a comparison of the waveform quality from a number of recent instruments. The methodology is detailed in section 3with the results in the succeeding section. However, first we discuss the various uses of waveform data, and their sensitivity to waveform artifacts. Although our analyses are only of deep ocean data, section 2 summarizes applications over lakes, rivers, rain, ice, and land since our results pertain to the operation of altimeters over all terrains.

Radar altimeters have been designed with the aim of measuring sea surface height as accurately as possible. The simple PDF of reflecting facets and local homogeneity lead to a tractable form for the average waveform see Fig.

The Brown model has been the basis of all altimeter algorithms to date. It has been modified to allow for nonlinear waves and therefore non-Gaussian statistics by Lipa and Barrick and Srokoszwhich allows other wave parameters, such as skewness, to be derived from the shape of the returns Tokmakian et al.

5.1 How altimetry works

The derivation of geophysical parameters from the radar return takes one of three forms: an empirical, a semi-empirical, or a physically based algorithm. Lipa and Barrick have attempted to determine the skewness by a direct deconvolution of the waveform data, minimizing the effects of fading noise by averaging data in 6- or s segments.

Ideally, a physically based algorithm should be used, as this provides a correct understanding of the relationship between the geophysical parameter of interest and the radar return from the ocean surface. In practice, the algorithms used for deriving geophysical parameters from radar altimeter data are of all three types mentioned previously see, e. The desire for increasingly accurate estimates of geophysical parameters has led to a greater emphasis on physically based algorithms.A radar altimeter measures surface characteristics with a high degree of accuracy.

This implies high precision in the radar measurements and therefore requires performance far greater than a conventional radar, in particular for classic radar range measurements.

Some theoretical details of the principle of radar applied to altimetry will help better understand the different behaviours and characteristics of the pulse in function of irregularities on the surface encountered. The magnitude and shape of the echoes or waveforms also contain information about the characteristics of the surface which caused the reflection. The best results are obtained over the ocean, which is spatially homogeneous and has a surface which conforms with known statistics.

Surfaces which are not homogeneous, which contain discontinuities or significant slopes, such as some land surfaces, make accurate interpretation more difficult. Even in the best case the oceanthe pulse should last no longer than 70 picoseconds to achieve an accuracy of a few centimetres. Technically, this means that the emission power should be greater than kW and that the radar will have to switch every few nanoseconds. These problems are solved by the full deramp techniquemaking it possible to use only 5 W for emission.

altimeter waveform

The range resolution of the altimeter is about half a metre 3. This is achieved by fitting the shape of the sampled echo waveform to a model function which represents the form of the echo. Poseidon -2 Jason-1 altimeter first waveforms. Onboard reception and tracking Full deramp techniqueor how to work as if you had a high-power short-pulse by using a low-power long modulated pulse.

The return echoes differ with respect to the surface: over ocean: Ocean waveforms over continental ice: Ice waveforms Land waveforms The Envisat RA-2 individual echoes or Burst modeFootprint size.The first follow-on to Jason -3, now part of the EU Copernicus programme under the name of Sentinel-6 is at launch site, for a lift-off planned on Nov.

We are pleased to announce that the abstract submission is now open until October 16th midnight UTC. Thanks to the good quality of the HaiYang-2B measurements, the integration of the mission in the operational system DUACS has been made on July 7th bringing to 6 the number of altimeter missions processed in Near-Real-Time in the system, which is unprecedented!

In the current situation, like in nearly all space agencies over the world, CNES has taken action in order to secure all critical activities, and ensure the safety of all its agents at the same time.

It continues a major contribution to Ocean monitoring as well as climate change studies. All the satellite, instruments and ground segments elements are functioning well. The ENSO index is computed with preprocessed altimeter sea level maps, which are corrected for the annual, seasonal and day signals.

However, the sea level maps are not corrected for the long-term trend, which directly affects the final indicator. In Januarythe processing of the ENSO index has been updated so that the altimeter sea level maps are now corrected for the trend in each grid point. In addition, the reference period used to estimate the periodic signals has been upgraded and We are pleased to announce that Thanks to the good quality of the HaiYang-2B measurements, the integration of the mission in the operational system DUACS has been made on July 7th bringing to 6 the In the current situation, like in nearly all space agencies over the world, CNES has taken action in order to secure all critical activities, and ensure the safety of all All the satellite, instruments and ground segments elements are functionin However, the sea level maps are no Customizable home, reading tools, products search guide, etc.

Hide a widget with the cross at the top right. Reset or add a widget with the "wrench" icon. The "User" and "Public" profiles offer a predefined organization. For more readiness of the page contents, you can fold and unfold the browsing menu by clicking on the tab.

At any time you can know where you are thanks to the browsing menu or the breadcrum at the top of the page.Search this site:. AVISO: 3. CSIRO: 3. NOAA: 3. What determines the x-intercept i. Is sedimentation in the oceans accounted for in the GMSL estimate? How often are the global mean sea level estimates updated? What is glacial isostatic adjustment GIAand why do you correct for it? Improved retracking algorithm for oceanic altimeter waveforms.

Luand Y. In the case of complex altimeter waveforms corrupted due to a variety of reasons, the processor on the satellite cannot properly determine the center of the leading edge, and range observations can be in error.

5.1.2 From radar pulse to altimetry measurements

As an efficacious method to improve the precision of those altimeter observations with complex waveforms, waveform retracking is required to reprocess the original returning pulse. Based on basic altimeter theory and the geometric feature of altimeter waveforms, we developed a new altimeter waveform retracker, which is valid for all altimeter waveforms once there exists a reasonable returning signal.

The performances of the existing Beta-5 retracker, threshold retracker, improved threshold retracker, and the new retracker are assessed in the experimental regions China Seas and its adjacent regionsand the improvements in the accuracy of sea surface height are investigated by the difference between retracked altimeter observations and referenced geoid. The comparisons denote that the new algorithm gives the best performance in both the open ocean and coastal regions.

Also, the new retracker presents a uniform performance in the whole test region. Besides, there is a significant improvement in the short-wavelength precision and the spatial resolution of sea surface height after retracking process. DOI Our Publications Climate-change—driven accelerated sea-level rise detected in the altimeter era.

Is the detection of accelerated sea level rise imminent? Uncovering an anthropogenic sea-level rise signal in the Pacific Ocean.

Recent Articles Climate-change—driven accelerated sea-level rise detected in the altimeter era. Some pitfalls of the semi-empirical method used to project sea level.

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We would like to thank JPL and G. Kruizinga for providing the altimetry database software. Bao, L.Altimetry satellites basically determine the distance from the satellite to a target surface by measuring the satellite-to-surface round-trip time of a radar pulse. However, this is not the only measurement made in the process, and a lot of other information can be extracted from altimetry. The principle is that the altimeter emits a radar wave and analyses the return signal that bounces off the surface.

If the altimeter emits in two frequencies, the comparison between the signals, with respect to the frequencies used, can also generate interesting results rain rate over the oceans, detection of crevasses over ice shelves, etc.

Thus several locating systems are usually carried on-board altimetry satellites. Any interference with the radar signal also needs to be taken into account. Water vapour and electrons in the atmosphere, sea state and a range of other parameters can affect the signal round-trip time, thus distorting range measurements.

We can correct for these interference effects on the altimeter signal by measuring them with supporting instruments, or at several different frequencies, or by modelling them.

Altimetry thus requires a lot of information to be taken into account before being able to use the data. Data processing is also a major part of altimetryproducing data of different levels optimised for different uses at the highest levels. Ries, B. Haines, L. Fu, P. Fu and A.

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Cazenave Ed.Tournadre, J. Chapron, and N. Oceanic Technol.

How does altimeter work?

This paper presents a new method to analyze high-resolution altimeter waveforms in terms of surface backscatter. Over the ocean, a basic assumption of modeling altimeter echo waveforms is to consider a homogeneous sea surface within the altimeter footprint that can be described by a mean backscatter coefficient. When the surface backscatter varies strongly at scales smaller than the altimeter footprint size, such as in the presence of surface slicks, rain, small islands, and altimeter echoes can be interpreted as high-resolution images of the surface whose geometry is annular and not rectangular.

The method is tested using synthetic waveforms for different surface backscatter fields and is shown to be unbiased and accurate.

Several applications can be foreseen to refine the analysis of rain patterns, surface slicks, and lake surfaces. The authors choose here to focus on the small-scale variability of backscatter induced by a submerged reef smaller than the altimeter footprint as the function of tide, significant wave height, and wind.

All satellite altimeters use pulse-limited geometry and a full deramp technique to measure the power backscatter by the ocean surface as a function of time [i. Over an ocean surface, the waveform has a characteristic shape that can be described analytically using in general the classical Brown model Brown The altimeter geophysical parameters—epoch rangesurface backscatter, and significant wave height swh as well as the satellite off-nadir angle—are estimated by fitting the theoretical model to the measured waveforms.

The basic assumption of the echo waveform models is that the distribution of sea surface roughness within the altimeter footprint is homogeneous and can be described with a mean value.

However, previous studies such as TournadreQuartly et al. Under such occurrences, an altimeter can be seen as an imager of the sea surface backscatter whose imaging process is more complex than a classical one in the sense that pixels are not rectangular but annular. The imaging process of the sea surface i. We present a method to invert the measured waveforms in terms of surface backscatter at a resolution on the order of the along-track Hz resolution m for Jason.

The method is validated using simulated waveforms and is shown to be unbiased and to have an rms on the order of 0. Several applications for which small-scale variability of backscatter is of interest can be envisioned, such as the fine analysis of rain patterns or surface slicks.

altimeter waveform

We choose here to focus on the analysis of the change of roughness over a small submerged reef, which is still quite poorly documented Hearn Satellite altimeters are nadir-looking radar that emit short pulses reflected by the sea surface and that measure the backscattered power as a function of time the altimeter pulse echo waveform.

Figure 1 shows a pulse being reflected from a flat surface. As the pulse advances, the illuminated area grows rapidly from a point to a disk, as does the returned power. Eventually, an annulus is formed and the geometry is such that the annulus area remains constant as the diameter increases. The returned signal strength, which depends on the reflecting area, grows rapidly until the annulus is formed and remains constant until the growing annulus reaches the edge of the radar beam, where it starts to diminish.

In an agitated sea, a change of the surface elevation larger than the pulse width causes the returned pulse to be distorted and stretched.

The effect of this is to impose an additional slope on the leading edge of the returned signal strength curve Fig. This slope is related to the ocean wave height and the midpoint of this leading edge slope is equivalent to the reflection from the average position of the surface i.

The amplitude of the waveform depends on the mean roughness over the altimeter footprint, which can be empirically related to wind speed. Assuming that the distribution of the sea surface roughness and of the elevation are homogeneous over the altimeter footprint, the backscatter coefficient can be expressed as a convolution product of the radar point target response, the flat sea surface response, and the joint probability density function of slope and elevation of the sea surface Brown ; Barrick and Lipa The first term within the integral represents the effect of the antenna pattern while the second term represents the effect of the variations of the surface elevation.

Equation 1 can be easily integrated and simplifies to the classical Brown model:. Examples of altimeter waveforms for a flat sea and a 5-m significant wave height are presented in Fig. When the surface backscatter strongly varies at scales smaller than the altimeter footprint diameter, the waveform can significantly departs from the Brown model and Eq.A radar altimeter RAradio altimeter RALTelectronic altimeteror reflection altimeter measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft.

This type of altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined vertical datumusually mean sea level. When used on aircraft, it may be known as low-range radio altimeter LRRA. From the legal point of view, a radio altimeter is — according to article 1. The use of radio altimeter equipment is categorised as so-called safety-of-life servicemust be protected for Interferencesand is an essential part of Navigation.

As the name implies, radar ra dio d etection a nd r anging is the underpinning principle of the system. The system transmits radio waves down to the ground and measures the time it takes them to be reflected back up to the aircraft. The altitude above the ground is calculated from the radio waves' travel time and the speed of light.

To do this, the transmitter sends a frequency modulated signal that changes in frequency over time, ramping up and down between two frequency limits, F min and F max over a given time, T.

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In the first units, this was accomplished using an LC tank with a tuning capacitor driven by a small electric motor. The output is then mixed with the radio frequency carrier signal and sent out the transmission antenna. Since the signal takes some time to reach the ground and return, the frequency of the received signal is slightly delayed relative to the signal being sent out at that instant. The difference in these two frequencies can be extracted in a frequency mixerand because the difference in the two signals is due to the delay reaching the ground and back, the resulting output frequency encodes the altitude.

The output is typically on the order of hundreds of cycles per second, not megacycles, and can easily be displayed on analog instruments. Radar altimeters normally work in the E bandK a bandor, for more advanced sea-level measurement, S band. Radar altimeters also provide a reliable and accurate method of measuring height above water, when flying long sea-tracks. These are critical for use when operating to and from oil rigs.

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The altitude specified by the device is not the indicated altitude of the standard barometric altimeter. Absolute altitude is sometimes referred to as height [ citation needed ] because it is the height above the underlying terrain. As ofabout 25, aircraft in the US have at least one radio altimeter. The underlying concept of the radar altimeter was developed independent of the wider radar field, and originates in a study of long-distance telephony at Bell Labs.

altimeter waveform

During the s, Bell Telephone was struggling with the reflection of signals caused by changes in impedance in telephone lines, typically where equipment connected to the wires. This was especially significant at repeater stations, where poorly matched impedances would reflect large amounts of the signal and made long-distance telephony difficult.

Engineers noticed that the reflections appeared to have a "humpy" pattern to them; for any given signal frequency, the problem would only be significant if the devices were located at specific points in the line. This led to the idea of sending a test signal into the line and then changing its frequency until significant echos were seen, and then determining the distance to that device so it could be identified and fixed.

Lloyd Espenschied was working at Bell Labs when he struck upon the idea of using this same phenomenon as a way to measure distances in wire in a more general fashion. One of his first developments in this field was a patent granted [7] on the idea of sending a signal into railway tracks and measuring the distance to discontinuities.

These could be used to look for broken tracks, or if the distance was changing more rapidly than the speed of the train, other trains on the same line. During this same period there was a great debate in physics over the nature of radio propagation. Guglielmo Marconi 's successful trans-Atlantic transmissions appeared to be impossible; studies of radio signals demonstrated they travelled in straight lines, at least over long distances, so the broadcast from Cornwall should have disappeared into space instead of being received in Newfoundland.


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