|OBSERVATION OF PHENOMENA IN THE OCEAN USING THE ALMAZ-1 SAR
Ivanov A.Yu. , and K.Ts. Litovchenko
Center ALMAZ, NPO Mashinostroenia
Gagarina 33, Reutov, Moscow Region, 143952, Russia
Space Research Institute, Russian Academy of Sciences
Profsoyuznaya 84/32, Moscow, 117810, Russia
The potential of the S-band Synthetic Aperture Radar (SAR) aboard the Russian Satellite ALMAZ-1 in observing the ocean phenomena has been studied during the ALMAZ-1 mission in 1991-1992 and after it end. ALMAZ-1 SAR images acquired over the ocean show different ocean phenomena such as, surface and internal waves, oceanic fronts, upwellings, bottom topography, sea ice, and oil patches, as well as different atmospheric phenomena, ships and ship’s wakes. This review paper presents the final oceanological results of the ALMAZ-1 mission and evaluates the effectiveness of the ALMAZ-1 SAR for ocean studies and monitoring. Examples of interpreted ALMAZ-1 SAR images are presented.
The Russian Satellite ALMAZ-1 was launched on March 31, 1991. It carried a SAR capable of providing high resolution images of ocean surface. The ALMAZ-1 SAR was operating at a frequency of 3.1 GHz (S-band) using horizontal polarization (HH) for transmittion and reception. The specification of SAR aboard the ALMAZ-1 are summarized in [7,10,11]. The incidence angle of the ALMAZ-1 SAR could be varied between 20 and 60 by rotation the satellite around its axis. The backscatter signal received by the antenna was digitized aboard ALMAZ-1 and stored by onboard recorder, then the data were transmitted via a relay satellite to the ground station in Moscow Region, Russia. This method permits to acquire SAR images of any region of the Earth. The ALMAZ-1 SAR had two operational modes with swath width of 30 and 40 km. The maximum possible duration of the SAR data takes was about 60 s (usually 30 s) due to storage capacity limitation of the onboard digital recorder. The minimum time between two data takes was 3 min. On Oct.17, 1992 ALMAZ-1 ended its mission.
Although the ALMAZ-1 SAR wasn't primary oceanological tool it was valuable for oceanological applications. ALMAZ-1 SAR was a source of data about sea ice cover and edge, it provided information on the nature, extent and motion of sea ice. ALMAZ-1 SAR was also valuable source of information on such ocean features as surface and internal waves, bottom topography, oil spills, ships and ship wakes. Its advantages to other analogous are a low orbit, SAR's high resolution, horizontal polarization and variable look angle. Due to high resolution it could see small ships, small oil spills and short period ocean waves. The variable look angle has allowed to study radar backscatter from ocean surface under different incidence angles. Horizontally polarized signal doesn't respond to short period fluctuations of short surface waves which in many cases mask the surface manifestations of ocean phenomena. All these peculiarities of ALMAZ-1 SAR allowed to observe and study an interaction of the Gulf Stream and swell, to study oil spills, to monitor the area of reseach vessel (RV) MICHAIL SOMOV drift in heavy Antarctic ice and to map with details the shallows during low tide. ALMAZ-1 SAR could accurately measure changes in wave fields including wavelength and direction. ALMAZ-1/ERS-1 experiment conducted in the North Atlantic has shown an utility of S-band SAR for ocean surface wave imaging due to low R/V ratio (R denotes the distance between ocean surface, and the SAR antenna, slant range, and V the spacecraft velocity).
The results of Russian space campaigns and radar imagery are not well-known in the west. Three years after the end of ALMAZ-1 mission were the years of detailed analysis of the ALMAZ-1 data. This review paper discusses the possibitities of the ALMAZ-1 SAR, techniques and results of SAR image interpretation of a number of ocean phenomena. The objective of this paper is also to provide an evaluation of the oceanological capabilities of the S-band SAR in preparation of future programs of ocean investigation from space.The preliminary oceanographic results obtained from the analysis of ALMAZ-1 data can be found in [1-8].
In order to study the imaging of ocean waves by the SAR aboard the ALMAZ-1 satellite a special joint ALMAZ-1/ERS-1 experiment was carried out [8,10,11]. The SAR images were acquired quiasi-simultaneously over same ocean areas in the North Atlantic near the Iceland. Since the decision for taking these joint experiment was made only a few days before the experiment, no simultaneous in situ measurements could be arranged. However, instead of carrying out in situ wave measurements we hindcasted the ocean wave spectrum by using the third generation wave prediction model (WAM). As input for this model we use ocean surface wind, so-called "analyzed wind" referenced to a height of 10 m above the sea surface provided by the European Centre for Medium Range Weather Forecasts (ECMWF) in Reading, UK. The ALMAZ-1 and ERS-1 satellites had quite different orbital altitudes: one had average orbital altitude of 350 km during the mission and another one with 785 km. This has important consequence on SAR imaging of ocean waves since the degree of non-linearity of the SAR imaging mechanism depends on R/V ratio. For ALMAZ-1 this ratio is approximately 60 s while for ERS-1 is 130 s. Due to permature end of the ALMAZ-1 mission on Oct. 17, 1992 we were able to obtain only two successful joint data on Oct.6 and Oct.8. On both days ALMAZ-1 was in an ascending and ERS-1 in a descending orbits. The ERS-1 SAR images was taken at 12:49 and 13:18 UTC, and the ALMAZ-1 images at 11:51 and 12:30 UTC. Measured and simulated ALMAZ-1 and ERS-1 image spectra acquired have been compared. It was shown that the wavelength and direction of the spectral peaks of measured and simulated SAR image spectra agree quite satisfactory.
Moreover, another quasi-simultaneous ALMAZ-1/ERS-1 coincidences were studied [12,13]. The image spectra were calculated over 6 collocated image pairs taken quasi-simultaneously. In general 16 wave systems were identified in these 6 experiments, 13 of them being presented in both ERS-1 and ALMAZ-1 spectra. Because of absence of in situ data the analysis is restricted by comparison only between two satellite data. In [8,10,11] the data of WAM were used as base, more close to in situ data while for other days such data were not available. For this reason we again used as base the ALMAZ-1 data since ALMAZ-1 SAR, as shown in [10,11], much less distorts the spectral parameters than the ERS-1 SAR. The 13 wave systems covered almost all possible mutual directions of wave propagation and satellite flight (azimuth). In general, the correspondence between ALMAZ-1 and ERS-1 estimates is satisfactory. The discrepancy can be explained by different parameters of two SARs. The ERS-1 and ALMAZ-1 SAR image spectra shown in Fig.1a give different wavelength and wave direction when waves were propagating close to the azimuth direction: 262 m and 4 (ERS-1), 208 m and 23(ALMAZ-1). Both these SAR images were acquired when satellites were in a descending orbit and , angle between the azimuth and wave propagation direction, was almost identical: 172 for ERS-1 and 177 for ALMAZ-1. The analysis, in which ALMAZ-1 SAR estimates (wavelegths and directions) of dominant waves have been compared with those of the ERS-1 by means of scatter plots, has been also carried out. Comparisons of wavelengths and wave directions measured in SAR image spectra of ERS-1 and ALMAZ-1 are presented in the Fig.1b and 1c. ERS-1 SAR estimates of wavelength are biased with respect to the ALMAZ-1 ones. Nevertheless, there is no high bias for wave directions. The maximum percent difference between the two instruments is 52% (the average is 24%) while for wave directions maximum and average differences are 19 and 6, respectively. The first distribution may be explained by the fact that the velocity bunching mechanism is the strongest for azimuth travelling waves (=0,180). As previously was stated theorertically and experimentally the main factors that determine the SAR image spectrum distortions are the wave propagation direction in range-azimuth coordinates and R/V ratio. It is shown that these distortions are more for azimuth travelling waves and grow with R/V increase. It is also shown that due to higher (appox. 2 times) R/V ratio the spectra of the ERS-1 are affected by a strong azimuth cut-off compared with those of the ALMAZ-1 and that the imaging of ocean waves by ALMAZ-1 SAR due to the lower orbital altitude is more linear than by ERS-1 but ALMAZ-1 SAR images are noisier. The direct comparison of the ALMAZ-1 and ERS-1 SAR image spectra shows a strong influence of R/V ratio and viewing geometry on the imaging of ocean waves. The obtained results establish that the ALMAZ-1 SAR could observe the dominant waves and provide wave measurements as a finding of SAR image spectrum peaks. Also, a comparison of ALMAZ-1 and ERS-1 imagery has been conducted by Beal and Tilley . So, an optimal SAR for mapping of ocean waves should have a low R/V ratio as well as a low incidence angles. The analysis ALMAZ-1/ERS-1 pairs allows to conclude that for measuring ocean waves better use a high resolution and HH-polarized SAR with R/V between 35 s and 60 s, but such a SAR will not be all-purpose applied.
The capability of the ALMAZ-1 SAR to observe large ocean areas and measure dominant ocean waves can be illustated by observations of spacial evolution of surface waves in the strong ocean currents. ALMAZ-1 SAR imagery of the Gulf Stream (GS) from five overpasses and subsatellite contact measurements have been collected in the end of August and the beginning of September, 1991 and examinated to determine the capability of the ALMAZ-1 SAR to detect and monitor dynamic boundaries of the oceanic currents. Some weak linear features found in ALMAZ-1 SAR imagery correspond to ocean water masses interaction near the south wall of the GS. Surface water temperature data based on contact measurements and NOAA AVHHR imagery showed that the water in the GS core was wamer of 4-5C than those in the Sargasso Sea. RV AKADEMIK VERNADSKY was made the oceanographic cuts during ALMAZ-1 imaging over the GS. In suggestion that current boundaries closely corresspond to thermal ones, we have analyzed a SAR image spectra variability in the situation when long surface waves propagated across the GS boundary. In study by Grodsky et al. [14a], an evolution of wave system crossing the GS imaged by the ALMAZ-1 SAR, has been analyzed. The SAR image in Fig.2a covered an area across the GS and shows a presence of two component long wave system. In that time the wind of 7-10 m/s was blowing from the west. Surface waves incoming the GS jet had wave number component directed in along current direction. SAR image spectra have been calculated from the full image and revealed different wave fields in the upwave and downwave GS current sides. The averaged spectra calculated from SAR image scenes on 9x9 km grid along the azimuth are shown in Fig.2b. The SAR image spectra exhibit two wave number peaks in the upwave GS current slope. Only one wave system was observed in the downwave part of the GS. These changes in wave directionality may be explained using a concept of reflection by a fair current. By using contact measurements and wave/current interaction model, simulated spectra were calculated and then compared with spectra derived from the SAR image. As input for the model the contact measurements of the current profile across the GS and GS thermal front location from NOAA data have been used. As shows Fig.2b a comparison gives a good result. So, by making a number of assumption about the incoming wave system and current field one can approximately determine the boundaries of current using only the spacial evolution of the wave field. Computational results also support an idea that complicated wave field in the GS area may be produced by unimodal background waves due to their reflection by shear fair current [14a]. This study also shows that the ALMAZ-1 SAR can be effectively used for observations of reflection of surface waves as well as their refraction and diffraction. Moreover, such methods permits to hope that high resolution SAR will be useful tool for monitoring ocean currents and fronts via an observation of wave/current interaction.
Here we present the results of study of upwelling region using the ALMAZ-1 SAR. Although, the upwelling processes have been studied early using contact oceanographic techniques, its features remain to be investigated. In this sense, SAR imagery allows us to obtain additional information on this phenomenon. The ALMAZ-1 SAR image presented in Fig.3a was taken over the North Atlantic to the southwest of England on July 5, 1991 at 06:42 UTC. The areas covered by slicks which can be identified on the SAR image due to low backscatter (dark areas) occupied approximately 20% of the full image. Fig.3b shows the weather maps on July 5, 1991 at 00:00 and 06:00 UTC. From the midnight to the morning the wind of 10-12 m/s was blowing from the NE and its speed has been sharply decreased in the morning. This led to the water skimming away at the surface and replacing by water from the layers beneath the thermocline. Water in the upwelling area is usually colder and more nutrient than the surface surrounding water. Moreover, it is characterized by a high content of biogenic substances that is favourable to biological activity.
Another interesting feature of the image is numerous internal wave (IW) manifestations. These IW were generated by the upwelling and located at the continental shelf slope between 200 and 300 m isobathes. A mechanism for explanation of IW generation by an upwelling has firstly suggested by A.V.Smirnov (private communication). Although, the tidal IW are predominant among other types of internal waves which occur at the shelf edge or in condition with sharp depth changes and which are frequently visible in SAR imagery, another types are infrequent [14b]. Using the ALMAZ-1 SAR we could know about existence of another types of generation like generation by atmospheric forcing  and by upwelling. Fig.3c shows a scheme explaining a possible generation mechanism.
On one hand relative low radar backscatter in the upwelling regions can be attributed to stable atmospheric layer over cold water which may affect the generation of atmospheric turbulence and result in reduced wind stress. But, on other hand enhanced biological activity and productivity in upwelling regions produces surface active films and forms surface slicks. SARs can observe these areas due to the fact that surface films or active substances damp the short surface waves and these areas become visible on SAR imagery as areas with low radar backscatter. Moreover, under weak winds these two effects may play an important role in reducing a backscatter simultaneously. Therefore, at present there are no accurate experimental evidences what effect is leading, and we cann't disriminate between them. In literature the backscatter reduction of 2-5 dB in upwelling regions was presented . The generation of ripples in upwelling region is lowered both by increased concentration of surface active substances and/or by stable atmospheric boundary layer over cold water. A dissipation of short waves also becomes higher than in the surrounding water partially because of subsurface turbulence associated with upwelling and partially due to increasing of the viscosity and surface tension with decreasing of surface temperature. The last factor may give a significant contribution to reduction radar cross section at significant temperature contrasts . Evidently, that an upwelling forming the large low backscatter areas and appearing due to this fact in SAR imagery is sufficiently complex phenomenon and future studies have to be done.
COASTAL PROCESSES AND UNDERWATER TOPOGRAPHY
Experimental studies of recent years have shown that radar imagery of shelves permits to obtain operative information and data on conditions of coastal waters, erosion and sedimentaion, and remotly define some bottom topography features. It is well-known the first observations of bottom topography were made in the end of 1960's using real aperture radar (RAR) and presented by de Loor  while the simple theoretical explanations of imaging mechanism were given later [19,20,21]. Although, SAR images containing bottom topography signatures are published very frequently and have already became classical some features of such SAR imagery are not explained. Nevertheless, ALMAZ-1 SAR had a potential for detection and mapping of coastal and tidal processes as well as bottom topography as demonstrated in . The ALMAZ-1 SAR image (Fig.4a) was taken over the Elbe estuary and German Bight on July 29, 1991 at 6:31 UTC, at the time of low tide and fixed unique picture of dried watte (tidal flats). It is evident that emptying and filling of the estuarine area according regular tidal cycle allows to study bottom geomorphological structures including rapidly changing processes. The constant dynamic changes in the area caused by strong tidal currents. Under this extremely dynamic regime erosion, sedimentation and sediment redistribution usually produce changes in the bottom topography: shoals and islands often change their location, bottom tidal channels change their direction. The lowest tide completely exposes the channel and microchannel network on the tidal flat. Due to high resolution the most coastal and estuarine features can be observed in details. Also, the ALMAZ-1 SAR image acquired over the German Bight has very clearly demonstrated the sharp boundaries between a water surface and dried shoals as well as bottom structures expressed by water flows in channels. The characteristic network of tidal channels formed jointly under the influence of river and tides has been studied. The location and morphology of tidal channels allows to pick out the plots interpreted as recent morphostuctures (denoted as A and B in Fig.4b) and other geomorphological features (for legend see ). In general, a comparison of the SAR image, existing bathimetric maps and published data shows that such mapping of bottom topography was not provided with such details and accuracy by other remote sensing sensors. For example, we could define the sixth-order network microchannels in the SAR image against fourth-order ones in visible imagery of the same scale.
The mapping of the bottom topography in the sea with ALMAZ-1 SAR was also possible. The SAR image in Fig.5a taken over southwestern part of the North Sea shows main bottom structures (sand banks) of the sea bed including secondary features on the bank slopes, so-called sand waves. The image was acquired on July 29,1991 at 15:39 UTC (2 hours after low tide at the England coast). Tidal current of 1.2-1.9 kts during ALMAZ-1 overpass was directed from the NW. Wind of 7-10 m/s was blowing from the east. The wakes from the gas drilling platforms (bright small spots on the image) also show the current direction. This underwater area northeastward the coast of England are covered by sand banks which are located at depths of 20-30 m and have distance from 5 to 10 km between them. The Hewed Ridges Banks, Smith's Knoll Bank, Leman Bank, Our Bank, Inner Bank, Well Bank and Broken Bank can be found in the image (Fig.5a) and in the map (Fig.5b). According to a theory they were imaged due to interaction of underwater relief, short surface waves and tidal flow. But, it should be noted, that existing theories and models are not capable to explane all aspects of imaging mechanism. Most probably, S- or L-band imaging radars, as discusses, are more suitable for this purpose because they may observe a bottom topography under weather/sea conditions of a wider scale. To sum it up, we can conclude that ALMAZ-1 SAR could really provide a high resolution mapping of tidal flats and bottom topography than previous satellite radars.
In order to evaluate capabilities of space imaging radars to detect oil spills in the sea and to sudy damping effect of oil films on short waves, a series of experiments has been recently conducted using X-,C-,L-band SARs aboard SIR-B, SIR-C/X and ERS-1. However, these data sets didn't include data obtained by S-band SAR aboard Russian Satellite ALMAZ-1. To supply the lack we have analyzed the ALMAZ-1 SAR imagery taken during the Dedicated Oil Spill Experiment-91 (DOSE-91). The experiment was performed in the Norwegian Sea, 100 km off the western coast of Norway around the Haltenbanken. Three artificial slicks have been produced by Norwegian research vessel on August 21, 24 and 27, 1991 (18:00 UTC) that also has collected data on sea and weather conditions near the test area. From August 22 to August 29 once or twice per day the test area was imaged by the ALMAZ-1 SAR under different look angles and weather conditions. ERS-1 has also imaged the oil spills on Aug. 22, 24 and 25, 1991 quasi-simultaneously with the ALMAZ-1. The results obtained from analysis of ERS-1 data and other detailed information on the DOSE-91 are presented in . Here we present and discuss the data collected during the experiment with use the ALMAZ-1 SAR. Fig.6a shows the ALMAZ-1 SAR scene of Aug.22 acquired at 16:46 UTC, approximately a one day later the first oil spill. Black spot is the slick area occupied by oil film. The relative damping of backscatter power is 3 dB. The ERS-1 SAR image taken six hours early shows a weak signature that can only conjectively associated with oil slick. Wind speed of 5-8 m/s from the NW and long waves of 2.8 m waveheight and 11 s period as reported , have been observed. The next SAR image was acquired on Aug.23 at 10:35 UTC is presented in Fig.6b. The other ALMAZ-1 SAR image of Aug.25 acquired simultaneously with the ERS-1 one (10:44/10:48 UTC) delineates the second oil spill (wind speed of 5-6 m/s). The good signature of oil slick in the ERS-1 image (see Fig.2 in ) looks weak in the ALMAZ-1 SAR image, may be, due to viewing geometry (high incidence angle). The ALMAZ-1 SAR image of the second oil spill taken on Aug.27 at 15:29 UTC (Fig.6c) shows a good oil slick affected by long waves about 2 days (wind speed of 10-12 m/s).
So, the ALMAZ-1 SAR imagery made possible detailed identification and localization of oil spills as well as furnishing with information on the dimensions, structure and dynamics of the oil slick. It is found that for oil spill detection with use of the ALMAZ-1 SAR is more suitable incidence angles of 30-40. The spectral analysis of SAR image scenes has shown an image spectrum variability both around the oil slick and within the slick. The analysis of data has also shown that the high resolution of the ALMAZ-1 SAR is more suitable for local oil slicks detection. The area occupied by oil is found depends on sea surface conditions. Slicks and oil patches equal 0.15 x 0.15 km were easily detected by visual inspection of the images at favourable wind conditions (i.e. wind speed from 3 to 6 m/s). It was possible to study also the wave components relaxed by oil and wave/oil interaction. Unlike VV-polarized SARs, the ALMAZ-1 SAR permitted to identify oil spills under moderate and strong winds. The results show that wind speed upper limits determining the oil spill detectability are various for different SARs .
ALMAZ-1 has also contributed to mapping sea ice cover. The ALMAZ-1 SAR permited detailed mapping of ice types and structures revealing the processes of ice growth, motion and decay (melting). The determination of common ice types (firstyear, multiyear, pancake, grease, park ice etc.) using ALMAZ-1 SAR imagery was also possible. Due to HH polarization is expected the SAR can provide better discrimination between open water and ice, between ice types and better determination the location of ice edge including high sea conditions than VV-polarized SARs. The sensitivity of the SAR to surface roughness and the resolution permitted to detect the most of structure and texture ice features such as fractures, polynyas, leads, ice hummocks and ships in ice cover. At present it is well-known that ice classification or cocentration estimation methods based on data from single frequency and polarization SAR may lead to a problem of determination ice types. Nevertheless it was useful to evaluate an ability S-band ALMAZ-1 SAR for these purposes.
Operational sea ice mapping with use of ALMAZ-1 SAR was performed and validated in Antarctica in winter 1991 when Russian RV MICHAIL SOMOV was blocked by heavy sea ice off the MOLODEZHNAYA Antarctic Station. From July 24 to Sept. 4, 1991 the area of RV drift was operationally imaged by ALMAZ-1 SAR during polar night and under all-weather conditions. Collected imagery made possible to assess the SAR ability and to make single correct solution. During this experiment it was found that ALMAZ-1 SAR is capable to delineate important parameters of sea ice cover such as total concentration, ice edge location, size of icebergs, ice drift velocity and direction, other ice cover features. HH-polarized SAR at high incidence angles had provided a better discrimination between water and sea ice because of less sensitivity to wind stress. A series of consequent SAR images were analyzed to estimate parameteres of ice motion as well as ice thickness. In Fig.7a and Fig.7b the ALMAZ-1 SAR images taken during the experiment are shown. The typical grounded icebergs having a high brightness and immovable during the experiment can be found in both images.The circle indicates the location of RV MICHAIL SOMOV in Fig.7a. These consecutive images were used to follow the displacement ice floes, location of fast ice boundary and RV drift in sea ice. Scheme of the RV dift extracted from the SAR imagery is shown in Fig.7c. Thus, the experiment has clearly demonstrated the capability of ALMAZ-1 SAR not only map and quantify sea ice types/parameters but also to detect ship or icebreakers in sea ice cover. Minimum size (i.e. width or diameter) of 30 m of floes, icebergs, fractures or leads which the ALMAZ-1 SAR was capable to detect, was established. Such advantages of ALMAZ-1 SAR as high resolution and HH polarization were fully practised during the experiment.
The atmosphere is transparent for a SAR signal and,therefore, atmospheric phenomena can be imaged only as signatures or patterns in surface roughness field of the sea surface. The ALMAZ-1 SAR images acquired over the ocean delineate not only ocean phenomena but also different atmospheric phenomena. Such phenomena are also detected in ERS-1 SAR imagery  because of backscatter of the C-band SAR is strongly modulated by variations of surface wind stress. The phenomena detected by the ALMAZ-1 SAR are atmospheric gravity waves, atmospheric boundary layer rolls and convective cells. Atmospheric gravity waves or periodic strips on the sea surface are the most interesting phenomenon of this class was firstly observed in the SAR imagery. Early atmoshperic gravity waves were often observed in the Barents Sea using X-band (VV) real aperture radar of the COSMOS-1500 satellite  and recently imaged by ERS-1 . In the first case atmospheric gravity waves were attributed to orographic or lee waves; in the second the generation was associated with atmospheric temperature inversion during passage of warm front.The ALMAZ-1 SAR image given in Fig.8a shows atmospheric gravity waves to the south off Isl. Novaya Zemlya (July 10, 1991, 20:35 UTC) and Fig.8b shows an example of ALMAZ-1 image containing atmospheric rolls off the coast of California (July 31, 1991, 08:34 UTC). Since many atmospheric features which appeared in SAR imagery are caused by small scale and rapidly changing phenomena it is necessary to have a high revisit time of a sensor to monitor them. Some characteristics of this phenomena can be obtained from SAR imagery but in the most cases researchers must additionally have contact measurements and meteorological data for analysis. Unfortunately, the case study of the ALMAZ-1 SAR image allows to derive only quantative information. Very encouraging results were obtained by simultaneous using HH and VV polarizations of Russian airborne Ku-band SLAR for detection of atmospheric features in a wider range of weather conditions [28,29]. Thus, on one hand the ALMAZ-1 SAR shown a low capability for detection of atmospheric features on the sea surface which could be imaged only in individual cases and under certain viewing geometry. Nevertheless, on other hand the ALMAZ-1 SAR imagery allowed to identify a small-scale phenomena at the ocean/atmosphere interface, such as Langmuir strips [5-7].
SHIPS AND SHIP'S WAKES
Ships and ship's wakes were often seen in ALMAZ-1 SAR imagery. It is well-known that ships are usually imaged by SAR as bright tagets or small spots on the gray sea surface. Unlike ERS-1, ALMAZ-1 SAR could detect ships smaller than 15-20 m at wind speed from 0 to 15 m/s due to higher resolution and less sensitivity to wind stress. The example of ALMAZ-1 SAR image depicted in Fig.9a shows a group of Singaporian ships with length from ~10 m to ~200 m. The dark turbulent wake is expected was the most frequently observed type of a wake feature that also confirmed by analysis of ALMAZ-1 SAR imagery (Fig.9b). Classical Kelvin wake has been observed only in single cases. Dark turbulent wake is sometimes accompanied by one or two bright lines along wake edges, most probably, under calm weather condition and weak winds. Turbulent wakes were observed with any viewing geometry and any look direction. We could detect small ships under moderate and strong winds. Maximum length of turbulent wake of 40 km has been observed. Also, ships moving along the look direction (range) were displaced from their tracks due to Doppler effect. The dark thin strips along the azimuth direction near the ship in all ALMAZ-1 SAR images appeared because of ALMAZ-1 data processing peculiarity. These strips additionally raise a detectability of small ships and fishing vessels. So, carried out study on ship detection has shown that ships of about 10-15 m length can be imaged and seen in the ALMAZ-1 SAR imagery under wind speed between 10 and 15 m/s as well as in calm wind condition 0-3 m/s.
On one hand the ALMAZ-1 results support an idea that SAR techniques is a valuable tool for observation and monitoring different scale ocean dynamic features at high resolution. The ALMAZ-1 SAR images of the ocean phenomena give strong evidence that spaceborne SAR is a reliable instrument for imaging ocean surface and internal waves, fronts, upwellings, shallow bottom topography, coastal pocesses, surface pollution and sea ice. It should be also noted that ALMAZ-1 SAR was practically weather independent instrument since it was less sensitive to the wind stress fluctuations. The obtained results allow us to hope that the next spacecraft ALMAZ-1B equipped with X,S,P-band SARs and X-band SLR will be at the level of world requirements of marine community.
But, on other hand the recent spaceborne RARs (COSMOS-1500, OKEAN series) and SARs (ALMAZ-1, JERS-1, ERS-1/ERS-2, RADARSAT) are not measuring instruments in the sense that observed field of short waves intensity or radar cross section depends on a number of the phenomena arised on the ocean/atmosphere interface, and, therefore, identification and interpretation of SAR imagery often becomes problematic in a number of case. An necessity of the operational measuring and monitoring such ocean mesoscale phenomena as ocean level, currents and waves (seismical, tidal, internal), and the most phenomena associated with ocean/atmosphere interaction (wind waves, drift currents, atmospheric disturbances etc.) requires appropriate spaceborne techniques. All listed phenomena are associated with small scale variations of an ocean level and surface currents. A spacial resolution of about 1 km, swath width of 500-1000 km and revisit time of the same area from one hour till days are required to study and understand such mesoscale variability of the ocean level and currents.
However, if available imaging techniques for measuring of surface wind (scatterometers) and ocean surface temperature (IR scanners) have characteristics which close to required ones, the imaging radars for measuring of ocean level, currents and orbital velocity of surface waves are absent today. For example, an appearance of the wind scatterometer was extremely due to an enthusiasm of R.Moore and is a pleasant exception. Magnificent results were obtained with use of spaceborne altimeters aboard SEASAT, GEOS and Topex-Poseidon satellites. However, these results fully concern to large scale variability of global ocean circulation and mean ocean level. At the same time, the most of ocean phenomena associated with small variations of ocean level such as tidal, seismical and internal waves, bottom topography etc., radar altimeters cannot observe and measure. The absence of available sensors doesn't yet mean that such radar measurements are principally impossible. In particular, we had undertaken theoretical  and experimental  attempts to use the ALMAZ-1 SAR data (and contact measurements from RV AKADEMIK IOFFE) to reconstruct a current velocity field for a small ocean site. This experimental attempt was successful in a sense that the expected accuracy of measurements has been achieved. However, the obtained results also show that existing spaceborne SARs are not always suitable for such measurements since they don't provide vector data extraction.
Thus, a key problem for operational oceanology is development imaging radars able to measure small scale variations of ocean level and surface currents. Moreover, it is necessary the pixel-by-pixel overlapping of acquired data with data on wind waves, surface wind speed and surface temperature. A dynamic character of interaction of these fields means that they have to be measured simultaneously and independently. The problem of measurement of the small variations of ocean level on a background of wind waves can be decided through various combinations of phasometric and altimetric methods . For restoration of ocean level with required parameters seems is suitable three radar methods: side looking interferometry, side looking two-frequency interferometry and bistatical "quasispecular" phasometry . These methods are known itself, however, with reference to given problem, as far as we know, they were not applied in practice.
We thank S.Egorkin, O.Ivlev and S.Dudkin of NPO Mashinostroenia for processing the SAR images and preparing the figures.
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