Full text: Proceedings, XXth congress (Part 7)

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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
  
Monitoring System by Joint Research Centre) in the FGI were 
based on weather data, historical crop yield statistics and optical 
low-resolution satellite images, and it turned out that cloudiness 
almost completely hampered the use of optical satellite images 
in Finland. The launch of ERS-1 satellite in 1991 activated 
vivid research on agricultural remote sensing using satellite 
SAR images (ESA, 1995). The excellent radiometric stability of 
the ERS-1 and ERS-2 SAR images enabled a long-term 
continuous monitoring of the agricultural fields. In 1997 
agricultural SAR research was started in the FGI, firstly for crop 
species classification purposes, but later also for crop damage 
assessments and yield estimation purposes. 
In 2001 FGI started a study in the test area near the city of 
Seindjoki in Finland. The objective was to evaluate the 
potential of high-resolution high-repetition multitemporal SAR 
images in agricultural monitoring including aspects of the crop 
yield estimation and assessment of the crop yield damages. For 
the growing season in 2001 Radarsat-1 Fine beam satellite SAR 
images were used. The image delivery from the Tromsg 
Satellite Station took only few days, so near real-time 
agricultural monitoring was possible. Field surveys were made 
simultaneously with each of the image acquisition. The results 
were promising in some cases, for example flooded areas and 
cultivation practises were detected well, but the crop growth 
caused only weak change to the SAR backscattering and yield 
damages caused by lodging were detected only in very rare 
cases. Conclusion was that the HH polarization of the Radarsat- 
| is not sensitive to the changes of biomass of small leaved 
crops, but on the other hand, it is sensitive to variation of the 
surface roughness and soil surface moisture. Example of the 
detection of the flooded area caused by snowmelt in the end of 
April is represented in Figure |. 
  
Figure 1. Snowmelt caused a minor flooding in April 2001. 
Original data © distributed by — Radarsat 
International/TSS/Novosat Ltd. Processed by FGI. 
In 2003 the study continued using Envisat SAR images, which 
were obtained in the framework of ESA’s Envisat 
announcement of opportunity (AOE-488). We choose to have 
Envisat SAR images in VV/VH alternating polarization mode, 
which means that both VV and VH images are acquired 
simultaneously with some loss in the radiometric accuracy 
compared to one polarization imaging mode. Although two 
polarizations give more information about the research subject, 
a drawback with Envisat SAR in comparison with Radarsat-1 
Fine beam SAR images is significantly worse spatial resolution. 
Envisat SAR has 30 m and Radarsat-1 SAR has 8 m spatial 
resolution. This paper describes our experiences of using 
satellite SAR images in the remote sensing of agricultural fields 
in Finland. 
2. TEST AREA AND DATA 
2.1 The test area and satellite images 
The test area near the city of Seinäjoki in Finland is located at 
the latitude of 63 degrees North and it is one of the 
northernmost consistent agricultural areas in the world. 
Percentages of different crop species in the arable land in 2003 
were: oats 25%, barley 25%, grass silage 18%, fallow land 
10%, turnip rape 5%, wheat 3%, rye 1%, potato 3% and the rest 
10% comprises of sugar beet, grassland, pasture, garden etc 
(Tike, 2003). Cereal crops are mostly spring-sown varieties, 
except small amount of rye and autumn wheat. In 2003 the 
average crop yields in the Seinäjoki region were: oats 3.6 t/ha, 
barley 3.8 t/ha, wheat 3.5 t/ha and rye 2.8 t/ha (Tike, 2003). 
Crop yields are low compared with the yields in the southern 
regions of Europe. The river of Kyrójoki, which has been 
embanked in the last centuries, divides the test area, and 
nowadays there are no regular flooding in the springtime, but in 
some places floodwater from melting snow can stay on the low- 
lying fields for few weeks. The growing season in the test area 
lasts from 150 to 160 days a year. Sowing date 1s usually in the 
end of May or at the latest in the beginning of June. The 
harvesting of food crops occurs at the end of August or early 
September. 
In 2001 altogether 20 Radarsat-1 SAR Fine beam images were 
used in the agricultural monitoring study. The description of the 
Radarsat-1 SAR images is given in Karjalainen et al. (2003). 
In 2003 Envisat alternating polarization SAR were used. 
Altogether 16 SAR images were ordered in early spring 2003, 
but due to the problems in Envisat satellite and conflicts 
between commercial image orders, we were able to have 12 
SAR images. All images were acquired from descending orbit, 
thus the local time was around 12:00 at the time of the 
acquisition. The SAR incidence angle varies from 23 degrees to 
41 degrees. On the average, the time interval between image 
acquisitions was approximately two weeks, but in the most 
intensive growing period in the beginning of July there were 
even two image acquisitions in a week. The list of Envisat SAR 
images used is represented in Table 1. 
Table 1. The list of Envisat alternating polarization (VV/VH) 
SAR images in 2003. 
  
  
  
  
  
  
  
  
  
  
  
Image # Date PAF Swath 
] 15 June 2003 D-PAC 4 
2 18 June 2003 I-PAC 3 
3 21 June 2003 D-PAC 2 
4 28 June 2003 I-PAC 6 
5 04 July 2003 I-PAC 
6 07 July 2003 UK-PAC 3 
7 14 July 2003 I-PAC 6 
8 23 July 2003 I-PAC 3 
  
  
  
  
 
	        
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