NIVELES DE DESHIDROGENASA LÁCTICA (LDH) Y

In 1973 the SKYLAB altimeter was the first to give a continuous, direct measurement of the sea-surface topography. This altimeter w as designed for obtaining the radar measurements necessary for designing the future improved instruments. The GEOS-3 altimeter was actually the first globally applied altimeter system. SEASAT was the third in the series o f altimetry m issions. Since SEASAT several space m issions have been planned which included altimeters: GEOSAT in 1985 and more recently ERS-1 and TOPEX-Poseidon.

An altimeter is a nadir pointing radar instrument designed to make precise measurements o f the echoes from the surface o f the ocean. From these measurements three main quantities can be derived:

• the height from the satellite to the ocean surface • ocean wave height

This study is only concerned with the height measurement

The principle o f altitude measurement by an altimeter is simple and straightforward. A short pulse is transmitted by the radar toward the surface beneath it at a time t%. The pulse propagates toward the surface, at a speed c, is returned by the surface, and an echo is received by the sensor at a time The height from the sensor to the ocean surface is therefore a function o f the time difference A t= t% - ti;

. _ ^At

- “" T (7 .1 )

The accuracy o f time measurement depends mainly on the duration o f the pulse x. The sharper the radar pulse is, the wider the frequency band required to carry it. The value o f

X is determined by the sm allest wavelength to be resolved at the sea surface. For SEA SA T and ERS-1 altimeters x is 3 ns giving a minimum resolved wavelength o f 0.5 m.

When using such a short pulse the problem arises o f how to give it sufficient power so that the strength o f the return signal is measurable. This is achieved by a pulse compression (chirp) technique. A filter is applied to the sharp pulse, dispersing it into a much longer pulse T, and the frequency is swept linearly over range f " to f + such that f+-f' = B, where B is the signal bandwidth. This expanded chirp pulse carries the same information, but possesses more energy. On return, the reflected dispersed pulse is decom pressed by applying an inverse filter, producing an effective pulse width o f x =

1/B. For SEASAT and ERS-1, T = 20 (is, x = 3 ns and B = 300 MHz.

Ideally one would like to have a sharp pulse focused in a small area o f the sea so that all the measured area would be simultaneously illuminated. This is the so-called beam- lim ited geom etry. H ow ever this would require a very large antenna with serious technological disadvantages. In the present altimeters the pulse-limited geometry is used instead, requiring only an antenna o f small diameter (= Im for SEASAT and ERS-1).

The formation o f the footprint is explained in Figure 7.1. This is formed when the pulse radiated from the altimeter antenna, as a spherical shell, intersects the ocean surface and produces a transient illuminated area. The leading edge o f the pulse w ill strike the sea first, just beneath the satellite and then m ove out in a circular front. The trailing edge w ill do the same a fraction o f time later. The illuminated area w ill be first a circle o f growing area and then w ill becom e an annulus. The effective area (footprint) w hose height is sampled is the illuminated circle just before it becom es an annulus. This area also depends on the wave height.

WAWt IO> M IA N UA M tf ACI W A V I IIOUCH ' OlllCTtON O f sfAciCkAn fLICHt (fMCT O f UA tIA fi On fO O IftiN I tAOiUS x l / K - i 0 1.2 I 1.7 S 2.1 10 4 .1 M s .o r ■ fUUIWIOTM « ■ VIIOCITV O f UOHT « «lAOlUt oiiiaiON Of i f ACICM fl aiCMi A V flA C I M lA iU ilM IN T O V f l I t ( b ) (c)

Figure 7.1 - SEASAT altimeter footprint, a) Cross-sectional view; b) and c) Satellite's eye views. Extracted from (JPL, 1980).

For a calm sea, the footprint is a circle o f radius (Robinson, 1985):

^m in ~ V 2 h C T (7.2)

For a rough sea with significant wave height h i/3 this radius increases to:

^inax ~ *^2 h ex'

2 2 16

h]n

T' = T + ---^ ---

(7.3)

For SEASAT and ERS-1, h « 780 km, x = 3 ns and so fmin = 1 2 km and r^ax = 5km for hi/3=5m. Therefore the swath width varies from 2.4 km for a calm sea up to som e 10 to 15 km in high seas.

The return signal is sampled and fitted to a model curve. The shape o f the curve varies with wave height, but for the height measurement the quantity o f interest is the timing o f the mid-point o f the leading edge slope (Figure 7.2). B y comparing it to the known characteristics o f the emitted pulse the travel time can be derived. Curve fitting can lead to errors, but these are reduced by averaging over a certain time interval. For SEA SA T the accuracy for 1 second averaged measurements was « 10 cm. For ERS-1 it is expected to be better than 10 cm.

1 .94 M SWH