1.5. ERS系列产品介绍¶
1.5.1. 雷达参数与工作模式¶
ERS1 与 ERS2 是欧洲航天局 (European Space Agency, ESA) 研发的两颗卫星, 分别于1991年与1995年发射, 两颗卫星搭载的相同参数的雷达. 雷达工作在C波段, 采用HH极化收发方式, 轨道重访周期为35天. 在ERS1与ERS2同时工作的期间, ESA通过进行轨道调整和控制, 使得ERS1与ERS2间满足干涉测量要求, 从而使系统具备干涉测量的能力. 具体雷达参数可以在 这里 与 这里 获得. 总结如下:
参数 |
符号 |
值 |
单位 |
|---|---|---|---|
平台高度 |
\(H\) |
786070 |
m |
平台速度 |
\(V\) |
7098.0194 |
m/s |
距离向天线长度 |
\(L_r\) |
1 |
m |
方位向天线长度 |
\(L_a\) |
10 |
m |
雷达波长 |
\(\lambda\) |
0.05657 |
m |
雷达载频 |
\(f_c\) |
5.3 |
GHz |
脉冲宽度 |
\(T_p\) |
37.12 |
\(μs\) |
距离向调频率 |
\(K_r\) |
4.18989015e+11 |
Hz/s |
距离向带宽 |
\(B_r\) |
15.55±0.01 |
MHz |
距离向采样率 |
\(F_{rs}\) |
18.962468e+6 |
Hz |
距离向采样数 |
\(N_{r}\) |
5616 |
|
方位向调频率 |
\(K_a\) |
2122.96 |
Hz/s |
方位向采样率 |
\(F_{as}=PRF\) |
1640-1720, 1679.902 |
Hz |
方位向采样数 |
\(N_{a}\) |
不定 |
|
多普勒中心频率 |
\(f_{\eta_c}\) |
1257.769 |
Hz |
成像分辨率 |
\(\Delta_a × \Delta_r\) |
\(5 × 24.6\) |
m |
幅宽 |
80400 or 102500 |
m |
|
入射角 |
\(\theta_i\) |
23 (at mid-swath) |
\(°\) |
斜视角 |
\(\theta_s\) |
0 |
\(°\) |
1.5.2. 数据获取¶
从 ASF 上可以下载到ERS卫星SAR数据, 解压下载的 zip 数据文件, 可以得到 .raw .vol .ldr 等文件, 此种格式使用 SNAP, NEST 这些软件都无法打开, 可以使用 GMTSAR 来读取数据或者本书作者开发的 iprs 工具读取.
1.5.3. 原始回波数据读取¶
ERS原始SAR回波数据及单视复数产品文件结构描述均可以在 这里 看到, 下面简要介绍原始数据的文件结构及数据提取. 本节以从ASF上下载的 E2_84690_STD_L0_F137.zip 原始回波数据文件和 E2_84690_STD_F137.zip 单视复数数据文件为例进行讲解.
目录及文件格式¶
ERS产品采用CEOS(参见 CEOS 小节)格式存储数据,
在本书作者开发的 iprs 工具包中, 定义的SAR信号数据映射字典如 代码 1.9, 如记录长度('Length of this record')字段的起止地址为 (9,12), 占4个字节, 类型为 1B4, 即一个四字节的数. 又如字段 'Raw Data', 表示采样数据, 第一个样本的起止地址为 (413,414), 占2个字节, 类型为 2B4, 即包含两个单字节整数.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 | SarDataFileSignalDataRecordERS = {
'Record sequence number': [(1, 4), '1B4', 0],
'l-st record sub-type code': [(5, 5), '1B1', 0],
'Record type code': [(6, 6), '1B1', 0],
'2-nd record sub-type code': [(7, 7), '1B1', 0],
'3-rd record sub-type code': [(8, 8), '1B1', 0],
'Length of this record': [(9, 12), '1B4', 0],
# PREFIX DATA - GENERAL INFORMATION
'SAR image data line number': [(13, 16), '1B4', 0],
'SAR image data record index (indicates the record sequence number of the image line)': [(17, 20), '1B4', 0],
'Actual count of left-fill pixels': [(21, 24), '1B4', 0],
'Actual count of data pixels (samples)': [(25, 28), '1B4', 0],
'Actual count of right-fill pixels': [(29, 32), '1B4', 0],
'Reserved1': [(33, 84), '1I52', 0],
'Spare1': [(85, 88), '1I4', 0],
'Spare2': [(89, 92), '1I4', 0],
'Reserved2': [(93, 124), '1I32', 0],
'Spare3': [(125, 128), '1I4', 0],
# PREFIX DATA PLATFORM REFERENCE INFORMATION
'Platform information': [(129, 192), '1B64', 0],
# PREFIX DATA - SENSOR/FACILITY SPECIFIC, AUXILIARY DATA
'Fixed code = AA in Hexadecimal notation': [(193, 193), '1B1', 0],
'OGRC/OBRC flag (1 or 0)': [(194, 194), '1B1', 0],
'ICU on board time': [(195, 198), '1B4', 0],
'Activity task': [(199, 200), '1B2', 0],
'Image format counter': [(201, 204), '1B4', 0],
'Sampling window start time': [(205, 206), '1B2', 0],
'Pulse repetition interval': [(207, 208), '1B2', 0],
'Calibration attenuation setting': [(209, 209), '1B1', 0],
'Receiver gain attenuation setting': [(210, 210), '1B1', 0],
'Spare4': [(211, 340), '130B1', 0],
'36 calibration pulses as (4bit spare 6bit Q 6bit I from MSB down to LSB)': [(341, 412), '36B2', 0],
# SAR RAW SIGNAL DATA
'Raw Data': [(-1, -1), '2B1', 0] # auto infer, (413, )
}
|
数据读取与解析¶
采用本书作者开发的 iprs 工具包可以读取ERS原始SAR回波数据, 使用 iprs 读取 E2_84690_STD_L0_F137.zip 中的 E2_84690_STD_L0_F137.000.raw 文件, 这里给出读取的SAR数据文件文件描述记录, 参见 代码 1.10.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 | Record sequence number [(1, 4), '1B4', [1]]
l-st record sub-type code [(5, 5), '1B1', [63]]
Record type code [(6, 6), '1B1', [192]]
2-nd record sub-type code [(7, 7), '1B1', [18]]
3-rd record sub-type code [(8, 8), '1B1', [18]]
Length of this record [(9, 12), '1B4', [11644]]
ASCII/EBCDIC flag [(13, 14), '1A2', ['A ']]
1Blanks [(15, 16), '1A2', [' ']]
Format control document ID [(17, 28), '1A12', ['CEOS-SAR-CCT']]
Format control document revision level [(29, 30), '1A2', ['B ']]
File design descriptor revision letter [(31, 32), '1A2', ['B ']]
Generating software release and revision level [(33, 44), '1A12', ['SKY 5.4.8 ']]
File number [(45, 48), '1I4', [2]]
File name [(49, 64), '1A16', ['ERS2.SAR.RAWIMGY']]
Record sequence and location type flag [(65, 68), '1A4', ['FSEQ']]
Sequence number location [(69, 76), '1I8', [1]]
Sequence number field length [(77, 80), '1I4', [4]]
Record code and location type flag [(81, 84), '1A4', ['FTYP']]
Record code location [(85, 92), '1I8', [5]]
Record code field length [(93, 96), '1I4', [4]]
Record length and location type flag [(97, 100), '1A4', ['FLGT']]
Record length location [(101, 108), '1I8', [9]]
Record length field length [(109, 112), '1I4', [4]]
Reserved1 [(113, 113), '1I1', [[]]]
Reserved4 [(116, 116), '1I1', [[]]]
Reserved segment [(117, 180), '1A64', [' ']]
Number of SAR DATA records (nominal) [(181, 186), '1I6', [28603]]
SAR DATA record length (bytes) [(187, 192), '1I6', [11644]]
Reserved1 (blanks) [(193, 216), '1A4', [' ']]
Number of bits per sample [(217, 220), '1I4', [16]]
Number of samples per data group (or pixels) [(221, 224), '1I4', [1]]
Number of bytes per data group (or pixel) [(225, 228), '1I4', [2]]
Justification and order of samples within data group [(229, 232), '1A4', [' ']]
Number of SAR channels in this file [(233, 236), '1I4', [1]]
Number of lines per data set (nominal) [(237, 244), '1I8', [28603]]
Number of left border pixels per line [(245, 248), '1I4', [0]]
Total number of data groups per line per SAR channel [(249, 256), '1I8', [5616]]
Number of right border pixels per line [(257, 260), '1I4', [0]]
Number of top border lines [(261, 264), '1I4', [0]]
Number of bottom border lines [(265, 268), '1I4', [0]]
Interleaving indicator [(269, 272), '1A4', ['BSQ ']]
Number of physical records per line [(273, 274), '1I2', [1]]
Number of physical records per multi-channel line [(275, 276), '1I2', [[]]]
Number of bytes of prefix data per record [(277, 280), '1I4', [412]]
Number of bytes of SAR data (or pixel data) per record (nominal) [(281, 288), '1I8', [11232]]
Number of bytes of suffix data per record [(289, 292), '1I4', [0]]
Reserved2 [(293, 340), '1A48', [' 1 4PB 37 2PB 33 4PB 9 4PB 17 4PB ']]
Blanks [(341, 368), '1A28', [' ']]
Reserved3 [(369, 400), '1A32', [' ']]
SAR Data format type identifier [(401, 428), '1A28', ['COMPLEX SIGNED INTEGER*2 ']]
SAR Data format type code [(429, 432), '1A4', ['CIS2']]
Number of left fill bits within pixel [(433, 436), '1I4', [0]]
Number of right fill bits within pixel [(437, 440), '1I4', [0]]
Maximum data range of pixel (max-min value for I and Q) [(441, 448), '1I8', [65535]]
|
由 代码 1.10 知, 数据存储格式为 CEOS-SAR-CCT, 该数据共 \(28603\) 行脉冲数据, 每行为一次脉冲回波, 一个脉冲含 \(5616\) 个距离单元, 每个像素含实部(I)虚部(Q)共2个字节, 故每行脉冲含11232个字节. 该记录长度为11644字节, 后面的SAR数据记录长度亦为11644字节.
1.5.4. 单视复数数据读取¶
代码 1.11 给出了ERS合成孔径雷达单视复数数据文件的记录格式, 按照该格式读取数据即可, 本书作者开发的 iprs 工具包可以读取ERS单视复数数据, 读取函数为 read_ers_sar_slc(), iprs 中定义的用于读取处理后的SAR数据映射字典如下:
SarDataFileProcessedDataRecordERS = {
'Record sequence number': [(1, 4), '1B4', 0],
'l-st record sub-type code': [(5, 5), '1B1', 0],
'Record type code': [(6, 6), '1B1', 0],
'2-nd record sub-type code': [(7, 7), '1B1', 0],
'3-rd record sub-type code': [(8, 8), '1B1', 0],
'Length of this record': [(9, 12), '1B4', 0],
# SAR PROCESSED DATA
'Processed Data': [(13, 16), '1C4', 0] # '2F2' for [real, imag], or '1C4' for real + 1j*imag
}
警告
需要注意的是, 下载到的单视复数数据可能仅仅是幅度图, 这种情况下, 读取到的 SAR Data format type code 值可能为 IU1 即一字节无符号整数, 而不是 代码 1.11 中所列的 CI*4. 此时将映射表中的 'Processed Data' 段的类型改为 '1B' 即可.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 | ===DATA SET FILE FORMAT DEFINITION
---1 SAR DATA FILE - FILE DESCRIPTOR RECORD (FIXED SEGMENT)
FIELD BYTES FORMAT DESCRIPTION CONTENT
1 1-4 B4 Record sequence number (1)
2 5 Bl 1-st record sub-type code (63)
3 6 Bl Record type code (192)
4 7 Bl 2-nd record sub-type code (18)
5 8 Bl 3-rd record sub-type code (18)
6 9-12 B4 Length of this record (10012)
7 13-14 A2 ASCII/EBCDIC flag A$
8 15-16 A2 blanks $$
9 17-28 A12 Format control document ID for this data file
format CEOS-SAR-CCT
10 29-30 A2 Format control document revision level $B
11 31-32 A2 File design descriptor revision letter $B
12 33-44 A12 Generating software release and revision level <......>
13 45-48 I4 File number $$$2
14 49-64 A16 File name ERS1.SAR.SLCIMGY
15 65-68 A4 Record sequence and location type flag FSEQ
16 69-76 I8 Sequence number location $$$$$$$1
17 77-80 I4 Sequence number field length $$$4
18 81-84 A4 Record code and location type flag FTYP
19 85-92 I8 Record code location $$$$$$$5
20 93-96 I4 Record code field length $$$4
21 97-100 A4 Record length and location type flag FLGT
22 101-108 I8 Record length location $$$$$$$9
23 109-112 I4 Record length field length $$$4
24 113 A1 Reserved <$....$>
25 114 Al Reserved <$....$>
26 115 Al Reserved <$....$>
27 116 Al Reserved <$....$>
28 117-180 A64 Reserved segment <$....$>
---2 SAR DATA IMAGERY OPTIONS FILE FILE DESCRIPTOR RECORD VARIABLE SEGMENT
FIELD BYTES FORMAT DESCRIPTION CONTENT
29 181-186 I6 Number of SAR DATA records (nominal) $15000
30 187-192 I6 SAR DATA record length (bytes) $10012
31 193-216 A24 Reserved (blanks) <$....$>
SAMPLE GROUP DATA
32 217-220 I4 Number of bits per sample $$32
33 221-224 I4 Number of samples per data group (or pixels) $$$1
34 225-228 I4 Number of bytes per data group (or pixel) $$$4
35 229-232 A4 Justification and order of samples within data group <$....$>
SAR RELATED DATA IN THE RECORD
36 233-236 I4 Number of SAR channels in this file $$$1
37 237-244 I8 Number of lines per data set (minimum) $$$15000
38 245-248 I4 Number of left border pixels per line $$$0
39 249-256 I8 Total number of data groups per line per SAR channel $$$$2500
40 257-260 I4 Number of right border pixels per line $$$0
41 261-264 I4 Number of top border lines $$$0
42 265-268 I4 Number of bottom border lines $$$0
43 269-272 A4 Interleaving indicator BSQ$
RECORD DATA IN THE FILE
44 273-274 I2 Number of physical records per line $1
45 275-276 I2 Number of physical records per multi-channel line $$
46 277-280 I4 Number of bytes of prefix data per record $$$0
47 281-288 I8 Number of bytes of SAR data (or pixel data) $$$10000
per record (nominal)
48 289-292 I4 Number of bytes of suffix data per record $$$0
49-55 293-340 A48 reserved <$....$>
56 341-368 A28 blanks <$....$>
57-60 369-400 A32 reserved <$....$>
61 401-428 A28 SAR Data format type identifier COMPLEX$INTEGER$..$
62 429-432 A4 SAR Data format type code CI*4
63 433-436 I4 Number of left fill bits within pixel $$$0
64 437-440 I4 Number of right fill bits within pixel $$$0
65 441-448 I8 Maximum data range of pixel $$$65535
66 449-EOR A15564 spare <$....$>
===DATA RECORD
---1 IMAGERY OPTIONS FILE - PROCESSED DATA RECORD
FIELD BYTES FORMAT DESCRIPTION CONTENT
1 1-4 B4 Record sequence number (n)
2 5 Bl 1-st record sub-type code (50)
3 6 Bl Record type code (11)
4 7 Bl 2-nd record sub-type code (31)
5 8 Bl 3-rd record sub-type code (20)
6 9-12 B4 Length of this record (nominal) (10012)
7 13-16 C4 first sample of image line (n)
8 17-20 C4 second sample of image line (n)
. ..... .. ............................ ..........
5006 10009-10012 C4 last sample of image line (n)
|
读取的 E2_84690_STD_F137 文件中的数据大小为 \(9182×9182\), 实际有效数据大小为 \(9182×7833\), 从7834到9182列为填补的无效数据. 该数据实际上为幅度图像数据, 而不是复数数据, 由于数据较大, 故进行8倍降采样显示, 显示结果如 _fig-E2_84690_STD_F137 所示, 其中左图未去除无效右边界像素, 右图去除了无效右边界的像素. 此外, 可以看到成像算法处理完后的数据对比度较低, 可以通过增强算法进行调整.
图 1.30 数据 E2_84690_STD_F137 中的图像数据. (左) 未去除无效右边界像素, (右) 去除无效右边界的像素.¶
1.5.5. 原始数据成像¶
本节给出Chirp Scaling Algorithm(CSA)在以上两景ERS数据上的成像结果, 有关CSA算法, 参见 调频变标成像 小节. 由于原始数据较大, 且方位向采样点数一般远大于距离向采样点数, 故这里仅给出降采样的成像结果图以及多视处理后的结果图. 对于数据 E2_84690_STD_L0_F137, 其方位向采样点数为 \(N_a=28603\), 距离向采样点数为 \(N_r=5616\), 成像后的单视复数图像矩阵大小为 \(28603 × 5616\), 将其降采样至 \(1024 × 1000\), 得到 图 1.32 所示结果. 图 1.33 显示的是方位向多视处理结果, 其中视数为4, 处理得到 \(7150 × 5616\) 的数据, 将其降采样至 \(1024 × 1000\) 显示. 图 1.31 显示的是下载的数据 E2_84690_STD_L0_F137 的单视复数图像, 对比可以发现, CSA成像结果正确.
图 1.31 Single Look Complex image of ERS SAR product (E2_84690_STD_SLC_F137)¶
(sa0ea28603sr0er5616)_CSA_Imaging_SLC.png)
图 1.32 Single look imaging result of ERS SAR product (E2_84690_STD_L0_F137) using ChirpScaling Algorithm¶
(sa0ea28603sr0er5616)_CSA_Imaging_MLC4.png)
图 1.33 Multiple look (4) imaging result of ERS SAR product (E2_84690_STD_L0_F137) using ChirpScaling Algorithm¶
同样地, 图 1.34 显示的是下载的数据 E2_84686_STD_L0_F203 的单视复数图像, E2_84686_STD_L0_F203 数据的方位向采样点数为 \(N_a=28659\), 距离向采样点数为 \(N_r=5616\), 成像后的单视复数图像矩阵大小为 \(28659 × 5616\), 将其降采样至 \(1024 × 1000\), 得到 图 1.35 所示结果. 图 1.36 显示的是方位向多视处理结果, 其中视数为4, 处理得到 \(7150 × 5616\) 的数据, 将其降采样至 \(1024 × 1000\) 显示
图 1.34 Single Look Complex image of ERS SAR product (E2_84686_STD_SLC_F203)¶
(sa0ea28659sr0er5616)_CSA_Imaging_SLC.png)
图 1.35 Single look imaging result of ERS SAR product (E2_84686_STD_L0_F203) using ChirpScaling Algorithm¶
(sa0ea28659sr0er5616)_CSA_Imaging_MLC4.png)
图 1.36 Multiple look (4) imaging result of ERS SAR product (E2_84686_STD_L0_F203) using ChirpScaling Algorithm¶