Independent System Calibration of Sentinel-1B
"> Figure 1
<p>SAR system calibration procedures required for absolute radiometric calibration of SAR data products.</p> "> Figure 2
<p>Permanently installed, remote-controllable geometric and radiometric SAR calibration measurement standards. (<b>a</b>) DLR C-band “Kalibri” transponder; (<b>b</b>) DLR remote controlled corner reflector.</p> "> Figure 3
<p>DLR’s calibration site with three 2.8 m corner reflectors (D38, D42, D43) and three C-band transponders (D39, D40, D41) in South Germany with approximate center coordinate 48°0′N, 10°42′E. This calibration site is optimized for the Sentinel-1 mission, whereby the blue hatched areas indicate the Sentinel-1 coverage of all beams being selected for in-flight measurements (SM1, SM2, IW1, EW3, IW3, SM5, WV1).</p> "> Figure 4
<p>Sentinel-1 SAR image across the DLR calibration field in StripMap operation (SM5) for the co-polar channel VV. The regions around the deployed reference targets are zoomed out for the corresponding polarization channel (VV, VH).</p> "> Figure 5
<p>Sentinel-1B front-end temperature drift (<b>top plot</b>); instrument drift in amplitude and in phase (<b>middle and bottom plot</b>) of the co-polar channel (VV) for a 25 min long IW data take acquired during the commissioning phase on 17 May 2016.</p> "> Figure 6
<p>Sentinel-1B instrument drift within a DT in amplitude (<b>top plot</b>) and phase (<b>bottom plot</b>) during the three months of CP from June to August 2016. Different colors indicate different acquisition modes.</p> "> Figure 7
<p>Sentinel-1B instrument internal delay for the co- and cross-polar channels observed during the CP. Calibration pulses from the preamble (<b>top plot</b>) and postamble (<b>bottom plot</b>) have been evaluated on a daily basis during the CP.</p> "> Figure 8
<p>Sentinel-1B PG Product amplitude and phase evaluation results during the Commissioning Phase.</p> "> Figure 9
<p>Sentinel-1B Front-End mean temperature for the DTs analyzed during the CP.</p> "> Figure 10
<p>Tx/Rx chain settings measured for individual Sentinel-1B front-end TRMs between June and August 2016. The blue curve represents the mean value of 76 RFC pairs per TRM for the V polarization.</p> "> Figure 11
<p>Sentinel-1B front-end mean temperature during the 24 h of continuous RFC acquisitions, as part of the 24 h RFC campaign performed on 15 May 2016.</p> "> Figure 12
<p>Sentinel-1B amplitude and phase deviations for a total of 42 H-pol RFC measurements acquired during the 24 h RFC campaign in May 2016. The two top plots show the deviations for the Tx-chain (amplitude and phase); the two bottom plots show the deviations for the Rx chain.</p> "> Figure 13
<p>Sentinel-1B amplitude and phase deviations w.r.t. the in-orbit reference for a total of 76 V-pol RFC measurements acquired between June and August 2016 during the CP. The two top plots show the deviations for the Tx-chain; the two bottom plots show the deviations for the Rx chain.</p> "> Figure 14
<p>Azimuth offset for all acquired overpasses derived from corner reflectors for SM (blue), IW (red) and EW mode (green).</p> "> Figure 15
<p>Slant-range offset as a function of look angle derived from corner reflectors for the SM (blue), the IW (red) and the EW-mode (green); the instrument delay and propagation effects are already corrected.</p> "> Figure 16
<p>Squint angle in azimuth as a function of the antenna look angle. The two dashed lines designate two separate fits to data which were acquired with two different star tracker configurations. Here, the change in configuration resulted in an offset in squint angle. N1–N6: azimuth notch acquisition with different look angles.</p> "> Figure 17
<p>Exemplary elevation notch beam evaluation for HH-Pol acquired on 19 June 2016. The blue curve in the plot on the left shows the full measured notch gamma profile. Marked in red is the central part of the notch which is used for minimum position determination. The plot on the right shows a detailed view of the central notch part (blue dots), a polynomial fit (polynomial of seventh order) based on this central part (red line) and the retrieved minimum position (red triangle) for HH-Pol.</p> "> Figure 18
<p>Azimuth pattern measurement for SM2 on 29 June 2016, shifted to coincide at the maximum of the main beam. The top plot shows the sidelobes as a function of azimuth angles. The bottom plot shows a zoom of the measurement/model difference over the main lobe area and the deviation between the averaged measured patterns; the calculated reference is shown in black.</p> "> Figure 19
<p>Averaged IW gamma profiles measured after star tracker realignment shown without antenna pattern compensation (solid lines) for co-pol (<b>top plot</b>) and cross-pol (<b>bottom plot</b>) and the corresponding antenna model predictions (dashed lines). The antenna model prediction has been adjusted using an offset defined by the difference between the mean modeled and mean measured IW3 gain. Hence, the beam-to-beam differences can be compared with respect to IW3 for both model and measurement.</p> "> Figure 20
<p>IW3-mean-removed difference between averaged measured IW data and the corresponding antenna model prediction after star tracker realignment.</p> "> Figure 21
<p>Plot analogue to <a href="#remotesensing-09-00511-f019" class="html-fig">Figure 19</a> for averaged EW measurements. Here, the antenna model prediction has been adjusted using the offset defined by the difference between the mean modeled and mean measured EW3 gain. Hence, the beam-to-beam differences can be compared with respect to EW3 for both model and measurement.</p> "> Figure 22
<p>EW3-mean-removed difference between averaged measured EW data and the corresponding antenna model prediction. The distinct monotonous variation over elevation for each sub-swath signifies a potential influence of antenna mispointing.</p> "> Figure 23
<p>Timeline of the standard deviation of the absolute calibration factor for each overpass for all acquired products (blue: SM, red: IW, green: EW) for co-polarization channels. The number of covered targets within the scene is denoted in the circle center.</p> "> Figure 24
<p>Absolute calibration factor for co-polarized channels using different targets (triangle: corner reflectors, square: transponders) acquired for different modes (blue: SM, red: IW, green: EW).</p> "> Figure 25
<p>Channel imbalance between the cross- and co-polar channel derived from the transponder IRF for H- (filled diamonds HV-HH in dB) and V-polarization (open diamonds VH-VV in dB) on transmit for different modes (blue: SM, red: IW, green: EW).</p> "> Figure 26
<p>Channel imbalance between the cross- and co-polar channel derived from rainforest EW mode acquisitions for H on transmit (black) and V on transmit (blue). The channel imbalance is plotted for two cases: using the standard SLC products including the antenna pattern correction (blue and black lines) and without antenna pattern correction (green and purple lines).</p> "> Figure 27
<p>Cross-talk derived from DLR corner reflectors for different modes (blue: SM, red: IW, green: EW).</p> "> Figure 28
<p>Mean Doppler centroid frequencies of Sentinel-1A and 1B units during the Sentinel-1B commissioning phase.</p> "> Figure 29
<p>InSAR baselines, burst mis-synchronization and final common Doppler bandwidth for the combination of S1B–S1B (<b>first column</b>) and S1B–S1A (<b>second column</b>) InSAR data pairs. The first row shows the physical baseline (B), perpendicular baseline (<math display="inline"> <semantics> <msub> <mi>B</mi> <mrow> <mi>p</mi> <mi>e</mi> <mi>r</mi> <mi>p</mi> </mrow> </msub> </semantics> </math>), and parallel baseline (<math display="inline"> <semantics> <msub> <mi>B</mi> <mrow> <mi>p</mi> <mi>a</mi> <mi>r</mi> </mrow> </msub> </semantics> </math>) for all analyzed pairs, as a function of the geographic latitude. The second row shows the burst mis-synchronization values of the analyzed pairs. The third row shows the percentage of the final common Doppler bandwidth with the master acquisition date.</p> "> Figure 30
<p>Sentinel-1A/B cross-interferogram acquired over east Europe for a long data take (range is vertical, azimuth is horizontal): (<b>top</b>) Reflectivity image; (<b>middle</b>) coherence and (<b>bottom</b>) DEM-flattened interferometric phase. The Sentinel-1A/B images were acquired on 13 June (S1A) and 19 June 2016 (S1B). The data take consists of eight slices with a total of 72 bursts and is about 1400 km long. The data take duration was 3 min and 20 s. The residual fringes observed in the interferogram are mainly related to atmospheric artifacts.</p> "> Figure 30 Cont.
<p>Sentinel-1A/B cross-interferogram acquired over east Europe for a long data take (range is vertical, azimuth is horizontal): (<b>top</b>) Reflectivity image; (<b>middle</b>) coherence and (<b>bottom</b>) DEM-flattened interferometric phase. The Sentinel-1A/B images were acquired on 13 June (S1A) and 19 June 2016 (S1B). The data take consists of eight slices with a total of 72 bursts and is about 1400 km long. The data take duration was 3 min and 20 s. The residual fringes observed in the interferogram are mainly related to atmospheric artifacts.</p> "> Figure 31
<p>Measured azimuth mis-registration between master and slave images by using the enhanced spectral diversity (ESD) approach [<a href="#B22-remotesensing-09-00511" class="html-bibr">22</a>] for every overlap area of the data take.</p> "> Figure 32
<p>Absolute calibration factor derived from point targets for the IW mode (triangle: corner reflectors, square: co- transponders, crosses: cross- transponder) and distinguished between transmit polarization (blue: H on transmit, red: V on transmit). (<b>a</b>) for Sentinel-1B of the IW mode; (<b>b</b>) for Sentinel-1A of the IW mode.</p> ">
Abstract
:1. Introduction
- StripMap (SM), with six different look angles (SM1–SM6), each beam covering a swath width of 80 km, spatial resolution 5 m × 5 m,
- Interferometric Wideswath (IW), illuminating a swath width of 250 km by switching between three different subswaths in elevation, spatial resolution 20 m × 5 m,
- Extra Wideswath (EW), covering the complete range of 400 km by switching between five different subswaths in elevation, spatial resolution 40 m × 20 m,
- Wave Mode (WV), by illuminating small vignettes (20 km × 20 km) within a distance of 100 km available for two different look angles, spatial resolution 5 m × 5 m.
2. In-Orbit Calibration Overview
- DLR’s novel C-band transponder, called “Kalibri”, was designed for the Sentinel-1 mission. Despite the compact design, as shown in Figure 2a, “Kalibri” provides a RCS of 60 dBm [7] at a center frequency of 5.405 GHz with a bandwidth of 100 MHz. As “Kalibri” serves as a radiometric reference, significant effort was spent on the radiometric calibration of these transponders. Different methods were analytically compared [8] and conducted in order to increase calibration confidence through cross-comparisons. Thus, a standard uncertainty of 0.2 dB has been achieved [9,10,11].
- DLR’s remote controlled trihedral corner reflector is shown in Figure 2b. This corner reflector is turned up-side down, allowing the opening facing downwards to be rotated when the corner reflector is not operated. This parking position reduces the accumulation of dirt over time and protects the opening from different weather conditions when the corner reflector is not being operated. With an inner leg length of 2.8 , the corner reflector provides a peak RCS of 49.2 dBm at the Sentinel-1 center frequency according to the physical optics approximation [12]. A similar standard uncertainty of 0.2 dB has been achieved by a low shape tolerance. Precise laser-tracker measurements have confirmed mechanical deviations (e.g., plate deformations) from an ideal corner shape, which result in RCS changes that are negligible at below ±1 mm.
3. Internal Calibration
3.1. Instrument Drift within Data Take
3.2. Time Delay
3.3. Instrument Stability
3.4. TRM Characterization
4. Geometric Calibration
4.1. The Pixel Localization Accuracy in Azimuth
4.2. The Pixel Localization Accuracy in Range
5. Antenna Pointing Determination
5.1. Antenna Pointing Determination in Azimuth
5.2. Antenna Pointing Determination in Elevation
6. In-Flight Antenna Model Verification
6.1. Verification in Azimuth
6.2. Verification in Elevation
7. Radiometric Calibration
7.1. Relative Radiometric Accuracy
7.2. Absolute Radiometric Calibration
8. Polarimetric Characterization
8.1. Channel Imbalance
- the delivered standard product which includes a common correction by the antenna pattern (solid lines), and
- rainforest data without the correction of the antenna pattern (dotted lines).
8.2. Cross-Talk
9. Interferometric Verification
- Burst mis-synchronization, i.e., the ability to start interferometric acquisitions at orbital positions with the same argument of latitude, thus observing targets on the ground under the same Doppler centroid.
- Mean Doppler centroid frequency, related to the attitude stability of the satellite for different passes.
- Resulting common Doppler bandwidth, coming from the combination of the two previous points.
- Baseline in the zero Doppler plane, in order to check whether the satellite is kept within the orbital tube and also relevant in terms of common bandwidth range.
- Residual azimuth coregistration error consistency (using SAR Data) for the exploitation of the azimuth shifts for accurate coregistration or geophysical measurements.
9.1. Repeat-Pass Interferometric Consistency Evaluation
9.2. Interferometric Verification of Long Data Takes
10. Radiometric Cross-Check between Sentinel-1A and Sentinel-1B
11. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
References
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IW | SM6 | SM6 | |
---|---|---|---|
Polarization | DV | DH | DV |
Date | 17 May 2016 | 26 May 2016 | 25 May 2016 |
Duration | 25 min | 25 min | 25 min |
Mean Temp. | 14 C | 12 C | 13 C |
Temp. Drift | 23 C | 20 C | 23 C |
Amp. Drift | 0.25 dB | 0.2 dB | 0.23 dB |
Phase Drift | 17.2 | 13.9 | 19.6 |
Data Take | Swath | PREAMBLE | POSTAMBLE | Time Delay |
---|---|---|---|---|
SM DH | SM6 | 433.91 ns | 434.11 ns | 0.2 ns |
100 MHz | 433.70 ns | 433.81 ns | 0.11 ns | |
SM DV | SM6 | 433.46 ns | 433.68 ns | 0.22 ns |
100 MHz | 433.51 ns | 433.65 ns | 0.14 ns | |
Mean Value | 433.64 ns | 433.81 ns | 0.17 ns | |
Standard Deviation | 0.18 ns | 0.18 ns | 0.04 ns |
Mode | Azimuth Offset | Range Offset | ||
---|---|---|---|---|
[m] | [m] | [m] | [m] | |
SM | −2.2 | 0.2 | −0.5 | 0.5 |
IW | −3.0 | 1.2 | −0.7 | 0.2 |
EM | −2.8 | 0.8 | −0.7 | 0.3 |
all | −2.7 | 1.0 | −0.6 | 0.4 |
Datatake ID | Measurement Date | Pass | Polarization | Mispointing [mdeg] |
---|---|---|---|---|
2283/1 (A495) | 13 August 2016 | asc. | HH | −11 |
2283/2 (3D2F) | 13 August 2016 | asc. | HH | −11 |
2283/3 (F631) | 13 August 2016 | asc. | HH | −13 |
2283/4 (4932) | 13 August 2016 | asc. | HH | −21 |
2380/1 (5279) | 15 August 2016 | desc. | VV | −16 |
2380/2 (9CBA) | 15 August 2016 | desc. | VV | −17 |
25D2/1 (A9C4) | 18 August 2016 | asc. | HH | −13 |
25D2/2 (ED0E) | 18 August 2016 | asc. | HH | −13 |
25D2/3 (05D5) | 18 August 2016 | asc. | HH | −13 |
25D2/4 (B0C9) | 18 August 2016 | asc. | HH | −19 |
26F5/2 (3311) | 20 August 2016 | asc. | VV | −18 |
26F5/3 (52FF) | 20 August 2016 | asc. | VV | −18 |
26F5/4 (DF60) | 20 August 2016 | asc. | VV | −16 |
−15 ± 3 |
Measurement Date | Swath Type | Transmit Polarization | Model-Measured over Main Beam/ [dB] |
---|---|---|---|
29 June 2016 | SM2 | H | 0.035 (3 transponder average) |
6 July 2016 | SM1 | H | 0.031 (1 transponder) |
11 July 2016 | SM2 | V | 0.023 (3 transponder average) |
18 July 2016 | SM1 | V | 0.032 (3 transponder average) |
30 July 2016 | SM1 | H | 0.019 (3 transponder average) |
1 August 2016 | SM5 | H | 0.026 (3 transponder average) |
4 August 2016 | SM2 | H | 0.015 (1 transponder) |
11 August 2016 | SM1 | H | 0.042 (1 transponder) |
13 August 2016 | SM5 | H | 0.008 (3 transponder average) |
Mode | Co-Channels (HH, VV) | Cross-Channels (HV, VH) | All Channels (HH, HV, VV, VH) | |||
---|---|---|---|---|---|---|
[dB] | [dB] | [dB] | [dB] | [dB] | [dB] | |
S1 | 0.39 | 0.45 | 0.49 | 0.35 | 0.44 | 0.41 |
S2 | 1.13 | 0.23 | 1.12 | 0.18 | 1.13 | 0.21 |
S5 | 0.07 | 0.25 | 0.10 | 0.43 | 0.08 | 0.31 |
IW (1,2,3) | 1.40 | 0.23 | 1.17 | 0.33 | 1.31 | 0.29 |
EM (2,3) | 1.40 | 0.30 | 1.54 | 0.14 | 1.45 | 0.26 |
Physical Baseline (B) | Perp. Baseline () | Parallel Baseline () | |
---|---|---|---|
S1B–S1B. Standard deviation | 94.4 m | 81.22 m | 63.67 m |
S1B–S1A. Standard deviation | 105.82 m | 89.43 m | 73.35 m |
Radial Component () | Across-Track Component () | |
---|---|---|
S1B–S1B. Standard deviation (baseline) | 12.5 m | 93.56 m |
S1B–S1B. Standard deviation (single platform) | 8.83 m | 66.16 m |
S1B–S1A. Standard deviation (baseline) | 11 m | 105.24 m |
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Schwerdt, M.; Schmidt, K.; Tous Ramon, N.; Klenk, P.; Yague-Martinez, N.; Prats-Iraola, P.; Zink, M.; Geudtner, D. Independent System Calibration of Sentinel-1B. Remote Sens. 2017, 9, 511. https://doi.org/10.3390/rs9060511
Schwerdt M, Schmidt K, Tous Ramon N, Klenk P, Yague-Martinez N, Prats-Iraola P, Zink M, Geudtner D. Independent System Calibration of Sentinel-1B. Remote Sensing. 2017; 9(6):511. https://doi.org/10.3390/rs9060511
Chicago/Turabian StyleSchwerdt, Marco, Kersten Schmidt, Núria Tous Ramon, Patrick Klenk, Nestor Yague-Martinez, Pau Prats-Iraola, Manfred Zink, and Dirk Geudtner. 2017. "Independent System Calibration of Sentinel-1B" Remote Sensing 9, no. 6: 511. https://doi.org/10.3390/rs9060511