Speckle reduction for Sentinel-1A SAR images in the Semi-arid caatinga region, Brazil

Jesus, Janisson Batista de; Kuplich, Tatiana Mora; Barreto, Íkaro Daniel de Carvalho; Hillebrand, Fernando Luis; Rosa, Cristiano Niederauer da

doi:10.1590/s1982-21702023000300007

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ORIGINAL ARTICLE • Bol. Ciênc. Geod. 29 (3) • 2023 • https://doi.org/10.1590/s1982-21702023000300007 copy

Speckle reduction for Sentinel-1A SAR images in the Semi-arid caatinga region, Brazil

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract:

Due to the absence of studies related to the digital processing of synthetic aperture radar (SAR) images in the semi-arid region of Brazil, the aim of this study was to test different filters for reducing speckle noise in SAR images, serving as a reference for choosing the most suitable filter for different studies in this vegetation. The filters: Gamma Map, Lee Sigma, Median, Frost and Refined Lee in different window sizes were tested on the VV, VH and VH/VV polarizations in the Sentinel-1A images, verifying the responses under the influence of the dry and post-rainy period in the Caatinga vegetation. In the state of Sergipe, Brazil, 30 samples of Caatinga fragments obtained from Sentinel-1A images for the dry and post-rainy season were selected. For all images evaluated, the values of the averages of the equivalent number of looks (ENL) were compared by the Tukey test at 5% significance. The Gamma filter showed the highest amount of means (22) with the highest ENL values, followed by Median (5). The generation of ENL results and their comparison, considering all variables used, was essential to serve as a basis for choosing the filtering method in studies that use data from Sentinel-1A in the Caatinga region.

Keywords:
Digital image processing; Noise; Synthetic aperture radar; Tropical dry forest

Pol	W	No Filter	Filters
Pol	W	No Filter	Frost	Gamma	Lee Sigma	Median	Refined Lee
VH	No Filter	92,3 (44,1)	^A	^A	^A	^A
	5 x 5		1003,8 ^A,B (1197,9)	2041,8 ^A (5964,1)	2013,4 ^A,B (5974,1)	1048,3 ^A (1048,2)	405,6 (349,1)
	7 x 7		2557,9 ^A,B,C (5139,6)	1889,7 ^A (2673,9)	1708,9 ^A,B (2522,2)	2076,7 ^A (3241,3)	405,6 (349,1)
	9 x 9		3545,3 ^{b B,C} (6230,8)	3754,4 ^{b A} (3764,2)	2912,6 ^{a,b A,B} (3105)	3357,4 ^{b A} (3285,9)	405,6 ^a (349,1)
	11 x 11		4332,1 ^{b B,C} (5385,5)	5665,4 ^{b A} (4945,1)	4824,8 ^{b B,C} (5034,5)	5961,8 ^{b A} (5497,9)	405,6 ^a (349,1)
	15 x 15		5046,4 ^{a,b C} (5183,6)	17126,1 ^{c B} (16646,7)	9393,6 ^{b,c C} (12659,5)	16486,1 ^{c B} (18146,3)	405,6 ^a (349,1)
VV	No Filter	44,8 (23,4)	^A	^A	^A	^A
	5 x 5		353 ^{b A,B} (261,2)	416,2 ^{b A} (312,8)	376,7 ^{b A,B} (336,5)	373,7 ^{b A,B} (246,8)	162,6 ^a (110,4)
	7 x 7		860,3 ^{b A,B} (1006,6)	999,5 ^{b A} (1097,1)	865,7 ^{b A,B} (1156,1)	870,9 ^{b A,B} (640,5)	162,6 ^a (110,4)
	9 x 9		1332,1 ^{b A,B} (1483,5)	1785,9 ^{b A,B} (1393)	1437,7 ^{b A,B} (1448,9)	2029,7 ^{b A,B} (1832,3)	162,6 ^a (110,4)
	11 x 11		1897,2 ^{a,b B,C} (2308,8)	4810,3 ^{a,b B,C} (6919,2)	3188,1 ^{b B} (3382,5)	5722,8 ^{b B,C} (9986,6)	162,6 ^a (110,4)
	15 x 15		3172,4 ^{a,b C} (5029,9)	8248,5 ^{b,c C} (9410,1)	6162,5 ^{a,b,c C} (8846)	9676 ^{c C} (14767)	162,6 ^a (110,4)
VH/VV	No Filter	3 (3,3)	^A	^A	^A	^A
	5 x 5		13,9 ^{b A,B} (11,3)	16,3 ^{b A} (14,7)	13,4 ^{a,b A} (12,8)	12,8 ^{a,b A} (8,2)	5,8 ^a (4,3)
	7 x 7		23,5 ^{b A,B} (14,8)	33,4 ^{b A,B} (27,2)	24,5 ^{b A} (25,8)	29,6 ^{b A,B} (23,5)	5,8 ^a (4,3)
	9 x 9		38,2 ^{b B,C} (25)	59,2 ^{b A,B} (36,5)	44,8 ^{b A,B} (52,2)	56,5 ^{b A,B} (45,1)	5,8 ^a (4,3)
	11 x 11		54,7 ^{a,b C} (38,3)	113 ^{c B} (87,4)	71,7 ^{c B} (86,9)	106,8 ^{b,c B} (93,6)	5,8 ^a (4,3)
	15 x 15		88,3 ^{a D} (82,1)	311,1 ^{b C} (298,8)	133,2 ^{a C} (112)	273,4 ^{b C} (264,2)	5,8 ^a (4,3)

Pol	W	No Filter	Filters
Pol	W	No Filter	Frost	Gamma	Lee Sigma	Median	Refined Lee
VH	No Filter	100,3 (52,8)	^A	^A	^A	^A
	5 x 5		885,2 ^{a,b A} (875,3)	1105,7 ^{b A} (1090,5)	1036,8 ^{b A} (1127,6)	716,1 ^{a,b A} (649,7)	367 ^a (235)
	7 x 7		3442,4 ^A (9089,4)	4434,1 ^A,B (8615,9)	4035,3 ^A (8617,3)	2126,3 ^A,B (1947,3)	367 (235)
	9 x 9		4466 ^{b A,B,C} (5602)	6118,5 ^{b A,B} (7085,7)	5146,7 ^{b A} (5566,3)	3484 ^{a,b A,B} (2730,3)	367 ^a (235)
	11 x 11		5218,8 ^{b B,C} (4542,8)	6140,3 ^{b B} (5877,4)	6015,6 ^{b A} (6366,4)	6412,6 ^{b B} (6687,1)	367 ^a (235)
	15 x 15		7686,7 ^{a,b C} (8544,1)	16765,6 ^{b C} (14516,8)	14018,8 ^{b B} (19257,4)	15057,2 ^{b C} (16431,6)	367 ^a (235)
VV	No Filter	36,9 (15,3)	^A	^A	^A	^A
	5 x 5		440,5 ^{a,b A,B} (419,1)	552,1 ^{b A} (590,9)	499,6 ^{b A} (554,1)	462,6 ^{a,b A,B} (460,3)	148,4 ^a (96,2)
	7 x 7		848,2 ^{b A,B,C} (731,2)	864,4 ^{b A} (609,1)	748,7 ^{b A} (623,5)	758 ^{b A,B} (569,7)	148,4 ^a (96,2)
	9 x 9		1240,1 ^{b B,C} (1233,5)	1330,7 ^{b A} (1327,3)	1197,2 ^{b A} (1378,6)	1205,4 ^{b A,B} (1025,7)	148,4 ^a (96,2)
	11 x 11		1685,4 ^{b C,D} (1641,9)	2097,6 ^{b A} (2303,7)	1809,8 ^{b A} (2310,8)	2063,9 ^{b B} (2516,3)	148,4 ^a (96,2)
	15 x 15		2848,3 ^{a,b D} (3143,3)	6487,8 ^{c B} (7819,4)	4477,3 ^{b,c B} (6152,1)	5058,9 ^{b,c C} (4583,3)	148,4 ^a (96,2)
VH/VV	No Filter	3,1 (2,6)	^A	^A	^A	^A
	5 x 5		21,5 ^{b A,B} (20,6)	26,3 ^{b A} (23,8)	22,8 ^{b A} (22,4)	19,9 ^{a,b A} (15,5)	7,5 ^a (5,7)
	7 x 7		58,2 ^{b A,B,C} (64,3)	75,3 ^{b A} (85,1)	55,5 ^{b A,B} (68,1)	57,9 ^{b A,B} (55,2)	7,5 ^a (5,7)
	9 x 9		101,9 ^{b B,C} (167,5)	76,8 ^{b A} (64,8)	66,2 ^{a,b A,B} (68,5)	72,5 ^{b A,B} (63,5)	7,5 ^a (5,7)
	11 x 11		130,2 ^{b C} (201,8)	145,6 ^{b A} (197,4)	104,9 ^{a,b B} (140,3)	121,2 ^{b B} (122)	7,5 ^a (5,7)
	15 x 15		143,8 ^{a,b C} (146,6)	423,1 ^{c B} (479,5)	197,9 ^{a,b C} (201,5)	290,3 ^{b,c C} (266,1)	7,5 ^a (5,7)

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[TFN1] * In parentheses: Standard Deviation. ^{a, b, c}: Different subgroups for the Tukey test between the filters. ^{A, B, C, D}: Different subgroups for the Tukey test between the applied windows.