# High Performance Computing in Satellite SAR Interferometry: A Critical Perspective

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## Abstract

**:**

## 1. Introduction

^{15}FLOPS). Current supercomputing capacity is reported in detail in the TOP500 list of supercomputers [1].

## 2. SAR Interferometric Processing

#### 2.1. SAR Interferometry Fundamentals

#### 2.2. SAR Raw Data Focusing

#### 2.3. Image Coregistration

#### 2.4. Interferograms Formation and Filtering

#### 2.5. Phase Unwrapping Operations

#### 2.6. Multi-Temporal Interferometric SAR Techniques

## 3. Operational SAR Systems and Applications

#### 3.1. New-Generation and Forthcoming Spaceborne SAR Sensors

#### 3.2. InSAR Applications and Products

## 4. High Performance Computing: Fundamentals Concepts and Models

#### 4.1. Parallel Computing Architectures

#### 4.2. Parallel Programming Models for HPC Systems

#### 4.3. Performance Metrics

#### 4.4. Cloud Computing vs. HPC

## 5. Selected HPC Approaches in InSAR Fundamental Functional Stages

#### 5.1. SAR Data Focusing

#### 5.2. SAR Image Coregistration

#### 5.3. InSAR Filtering

#### 5.4. Phase Unwrapping

## 6. Selected MT-InSAR Techniques Using HPC or Cloud-Based Platforms

#### 6.1. MT-InSAR Processing Using HPC

#### 6.2. MT-InSAR Processing via Cloud-Based Platforms and Services

## 7. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Sketch of the interferometric SAR acquisition geometry: ${S}_{1}$ and ${S}_{2}$ represent the two sensor satellites, $b$ is the (spatial) baseline (i.e., the distance between the two satellites), ${b}_{\perp}$ is the perpendicular baseline, and $z$ is the topographic height. The dashed line represents the topographic profile after a ground displacement has happened. The segment highlighted in red represents the projection of the three-dimensional displacement vector along the line-of-sight (LOS).

**Figure 2.**Conceptual InSAR computational scheme for deformation measurements: block diagram of the main processing stages.

**Figure 3.**Sketch of the SAR acquisition mode geometry: (

**a**) Stripmap: the antenna beam is fixed and describe a “strip” on the terrain that corresponds to the imaged area; (

**b**) Terrain Observation by Progressive Scans (TOPS): the acquisition is achieved in bursts by switching the antenna beam among sub-swaths; during each burst the antenna beam also rotates along the azimuth.

**Figure 4.**SAR interferogram shows the ground deformations associated with the Quinghai Mw 7.4 earthquake on 21 May 2021. The interferogram is geocoded and imported into Google Earth.

**Figure 5.**A three-dimensional representation of the vertical (

**left**) and east–west (

**right**) component of the ground displacement related to the Wolf volcano eruption occurred on 25 May 2015. Time-series of the corresponding deformation (cm/year) are also depicted; the red bar indicates the start of the eruption.

**Figure 7.**Hybrid systems: multicore CPUs and GPU (graphical processing unit), with node-to-node connection via the network.

**Figure 8.**Theoretical speedup (Amdahl’s law) as a function of the number of the processing elements for different values of the parallel portion.

**Figure 9.**P-SBAS algorithm applied to COSMO Sky-Med dataset (from [205]): (

**a**) deformation mean-velocity map relevant to the Napoli Bay area is depicted. The graph of the displacement time-series pertinent to a specific pixel located in the area of maximum deformation is also shown; (

**b**) speedup as a function of the number of engaged processors.

Sensor | Frequency | Agency/Country | Access | Revisit-Time | Resolution | Polarization | Frame Size | Year |
---|---|---|---|---|---|---|---|---|

TanDEM-L | L-band (1275 GHz, λ =24.6 cm) | German Aerospace Centre (DLR) | Free and open | 16 days | 7 m | Single, dual, quad mode | dual-pol mode 350 km quad-pol mode: 175 km | ≥2024 |

BIOMASS | P band (435 MHz, λ = 70 cm) | European Space Agency (ESA) | Free and open | 17 days | 60 × 50 m | quad-pol | 50 × 50 km | ≥2023 |

NISAR | S-band (3.2 GHz, λ = 9 cm) | NASA ISRO | Free and open | 12 days | 3–10 m | Single: HH,VV | >240 km | ≥2023 |

Dual: HH/HV, VV/VH | ||||||||

Compact: RH/RV | ||||||||

Quasi-Quad: HH/HV, VH/VV | ||||||||

L-band (1.26 GHz, λ = 24 cm) | Single: HH, VV Dual: HH/HV, VV/VH Compact: RH/RV Quad: HH/HV/VH/VV | |||||||

RCM | C-band (5.4 GHz, λ = 5.6 cm) | Canadian Space Agency | Commercial | Satellite: 12 days Constellation: 4 days | 3–100 m | Single: HH, VV, VH, HV Dual: HH/HV, VV/VH, HH/VV Compact Quad | 20 × 20–500 × 500 km | 2019– |

PAZ SAR | X band (9.65 GHz, λ = 3.5 cm) | Space Agency of Spain | Commercial | 11 days | Stripmap: 3–6 m ScanSAR: 16 × 6 m spotlight: 1–2 m | HH/VV/HV/VH (single or dual) | Stripmap: 30–2000 × 30 km ScanSAR: spotlight: 10 × 10 km | 2018– |

TerraSAR-X Tandem-X | X band (9.65 GHz λ = 3.5 cm) | German Aerospace Centre (DLR) | Commercial; limited proposal-based scientific | 11 days | Spotlight: 0.2 × 1.0–1.7 × 3.5 m Stripmap: 3 × 3 m ScanSAR: 18–40 m | Single: HH, VV Dual: HH/VV, HH/HV, VV/VH Twin: HH/VV, HH/VH,VV/VH | Spotlight: 3–10 km Stripmap: 50 × 3 0 km ScanSAR: 150 × 100- 200 × 200 km | 2007–2010– |

COSMO-SkyMed | X band (9.6 GHz λ = 3.5 cm) | Italian Space Agency (ASI) | Commercial; limited proposal-based scientific | Satellite: 16 days Constellation: 1–8 days | Spotlight: ≤1 m Stripmap: 3–15 m ScanSAR: 30–100 m | Single: HH, VV, HV, VH Dual: HH/HV, HH/VV, VV/VH | Spotlight: 10 × 10 km Stripmap: 40 × 40 km ScanSAR: 100 × 100–200 × 200 km | 2007– |

Sentinel-1 | C band (5.4 GHz, λ = 5.6 cm) | European Space Agency (ESA) | Free and open | Satellite: 12 days Constellation: 6 days | Stripmap: 5 × 5 m Interferometric Wide Swath (IW): 5 × 2 0m Extra Wide Swath (EW): 20–40 m | Single: HH, VV Dual: HH/HV, VV/VH | Stripmap: 375 km IW: 250 km EW: 400 km | 2014– |

RADARSAT-2 | C band (5.4 GHz, λ = 5.6 cm) | Canadian Space Agency | Commercial | 24 days | Spotlight: ~1.5 m Stripmap: ~3 × 3–25 × 25 m ScanSAR: 35 × 35–100 × 100 m | Single: HH, VV, HV, VH Dual: HH/HV, VV/VH Quad: HH/HV/VH/VV | Spotlight: 18 × 8 km Stripmap: 20–17 0 m ScanSAR: 300 × 300–500 × 500 km | 2007– |

SAOCOM | L band (1275 GHz, λ = 24.6 cm) | Argentina National Space Activities Commission (CONAE) | Commercial; limited proposal-based scientific | Satellite: 16 days Constellation: 8 day | Stripmap: 10 × 10 m TopSAR: 100 × 100 m | Single: HH, VV Dual: HH/HV, VV/VH Quad: HH/HV/VH/VV | Stripmap: >65 km TopSAR: 320 km | 2018– |

ALOS-2PALSAR-2 | L band (1275 GHz, λ = 24.6 cm) | Japan Aerospace Exploration Agency (JAXA) | Commercial; limited proposal-based scientific | 14 days | Spotlight: 1 × 3 m Stripmap: 3–10 m ScanSAR: 25–100 m | Single: HH, VV, HV, VH Dual: HH/HV, VV/VH Quad: HH/HV/VH/VV | Spotlight: 25 × 25 km Stripmap: 55 × 70–70 × 70 km ScanSAR: 355 × 355 km | 2014– |

Processing Stage | Selected Works | Parallelism | Tools | Year |
---|---|---|---|---|

Focusing | [181,182] | Multithreading | OpenMP | 2016 |

[183,184] | GPU-based | OpenCL | 2019 | |

[185] | GPU-based | CUDA | 2020 | |

[188] | GPU-based | CUDA | 2014 | |

[186] | GPUs + CPUs | CUDA/OpenMP | 2016 | |

[189] | multi-GPU | CUDA/MPI | 2018 | |

[187] | GPU-based | CUDA | 2016 | |

Coregistration | [190] | Multiprocessing | MPI | 2002 |

[49] | Multithreading | OpenMP | 2021 | |

[191] | GPU-based | CUDA | 2014 | |

[192] | GPU-based | CUDA | 2020 | |

Phase Unwrapping | [198] | Dual-level | MPI/OpenMP | 2015 |

[196,197] | Dual-level | MP/OpenMP | 2015 | |

[199] | Multithreading | OpenMP | 2015 | |

[200] | GPU-based | CUDA | 2014 | |

[201] | GPU-based | CUDA | 2017 | |

InSAR Filtering | [190] | Multiprocessing | MPI | 2015 |

[194] | GPU-based | CUDA | 2016 | |

[195] | GPU-based | OpenCL | 2020 |

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**MDPI and ACS Style**

Imperatore, P.; Pepe, A.; Sansosti, E.
High Performance Computing in Satellite SAR Interferometry: A Critical Perspective. *Remote Sens.* **2021**, *13*, 4756.
https://doi.org/10.3390/rs13234756

**AMA Style**

Imperatore P, Pepe A, Sansosti E.
High Performance Computing in Satellite SAR Interferometry: A Critical Perspective. *Remote Sensing*. 2021; 13(23):4756.
https://doi.org/10.3390/rs13234756

**Chicago/Turabian Style**

Imperatore, Pasquale, Antonio Pepe, and Eugenio Sansosti.
2021. "High Performance Computing in Satellite SAR Interferometry: A Critical Perspective" *Remote Sensing* 13, no. 23: 4756.
https://doi.org/10.3390/rs13234756