# Improving the iGNSS-R Ocean Altimetric Precision Based on the Coherent Integration Time Optimization Model

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## Abstract

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## 1. Introduction

## 2. Signal Processing and Height Inversion

#### 2.1. Signal Processing

#### 2.1.1. Data Fetch

#### 2.1.2. Coherent Integration

#### 2.1.3. Retracking and Incoherent Average

#### 2.2. Height Inversion

#### 2.2.1. Delay Estimation and Error Correction

#### 2.2.2. Height Retrieval and Precision Calculation

## 3. Construction of Coherent Integration Time Optimization Model

#### 3.1. The Reconstruction of Altimetric Precision Model

#### 3.2. The Relationship between Model Parameters and Coherent Integration Time

#### 3.2.1. Altimetric Sensitivity

#### 3.2.2. Effective Incoherent Average Number

## 4. Results and Application

#### 4.1. Validation of Coherent Integration Time Optimization Model

#### 4.1.1. Altimetric Sensitivity

#### 4.1.2. Effective Incoherent Average Number

#### 4.2. Application of Coherent Integration Time Optimization Model

#### 4.2.1. Model Application: Airborne Experiment Scenario

#### 4.2.2. Model Application: Extrapolation to Spaceborne Scenario

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Appendix A

## Appendix B

## References

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**Figure 2.**Examples of measured and simulated power waveforms. (

**A**) Normalized measured power waveform processed from the IF data. The ${\tau}_{\mathrm{DER}}^{\mathrm{obs}}$ is the maximum point of the first derivative. (

**B**) Normalized simulated power waveform generated based on the Z-V model. The ${\tau}_{\mathrm{DER}}^{\mathrm{m}}$ is the maximum point of the first derivative. The ${\tau}_{\mathrm{sp}}^{\mathrm{m}}$ is the nominal SP calculated based on the WGS-84 reference ellipsoid.

**Figure 3.**Sea surface contribution area corresponding to $\langle y(n{T}_{c},\tau ){y}^{*}(n{T}_{c}+\tilde{n}{T}_{c},\tau )\rangle $.

**Figure 4.**Experimental area and IF data processing results. (

**A**) The specular points tracks of the raw IF data. (

**B**) Measured SSH relative to the WGS84 reference ellipsoid. (

**C**) SSH residual after subtracting the fitted value.

**Figure 5.**The variation of the precision with coherent integration time under different conditions. Red indicates the measured result. Black indicates the model result considering the correlation between the waveforms. Blue indicates the model result without considering the correlation between the waveforms.

**Figure 6.**A comparison of the reciprocal value of altimetric sensitivity between measured (red) and simulated data (black).

**Figure 7.**Comparison of the effective incoherent average number (black) and the incoherent average number (red).

**Figure 9.**The simulation variation curves of the estimated altimetric precision with the coherent integration time under different conditions. (

**A**) When the wind speed is 6 m/s, the elevation is 60°, the antenna gain is 15 dBi, and the orbit altitude varies. (

**B**) When the orbit altitude is 600 km, the elevation is 60°, the antenna gain is 15 dBi, and the wind speed varies. (

**C**) When the orbit altitude is 600 km, the wind speed is 6 m/s, the antenna gain is 15 dBi, and the elevation varies. (

**D**) When the orbit altitude is 600 km, the wind speed is 6 m/s, the elevation is 60°, and the antenna gain varies.

**Table 1.**Air-based simulation parameters that are consistent with those for the experiment described in Section 2.

Design Parameter | Value |
---|---|

Receiver height | ~3000 m |

Transmitter altitude | 20,200 km |

Receiver velocity | 50 m/s |

Antenna temperature | 200 K |

Antenna gain | 15 dBi |

Elevation angle | 70° |

Processing interval | 39,102–40,721 |

Power waveform processing time | 1 s |

Wind speed | 7 m/s |

Process method | iGNSS-R |

Sampling frequency | 80 MHz |

Carrier frequency | 1575.42 MHz (GPS L1) |

Receiver bandwidth | 35 MHz |

Waveform retracking method | DER |

Filter bandwidth | 12 MHz |

Design Parameter | Value |
---|---|

Receiver bandwidth | 35 MHz |

Antenna temperature | 200 K |

Transmitter altitude | 20,200 km |

Process method | iGNSS-R |

Sampling rate | 80 MHz |

Carrier frequency | 1575.42 MHz (GPS L1) |

Waveform retracking method | DER |

Filter bandwidth | 12 MHz |

Power waveform processing time | 1 s |

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**MDPI and ACS Style**

Sun, X.; Zheng, W.; Wu, F.; Liu, Z.
Improving the iGNSS-R Ocean Altimetric Precision Based on the Coherent Integration Time Optimization Model. *Remote Sens.* **2021**, *13*, 4715.
https://doi.org/10.3390/rs13224715

**AMA Style**

Sun X, Zheng W, Wu F, Liu Z.
Improving the iGNSS-R Ocean Altimetric Precision Based on the Coherent Integration Time Optimization Model. *Remote Sensing*. 2021; 13(22):4715.
https://doi.org/10.3390/rs13224715

**Chicago/Turabian Style**

Sun, Xuezhi, Wei Zheng, Fan Wu, and Zongqiang Liu.
2021. "Improving the iGNSS-R Ocean Altimetric Precision Based on the Coherent Integration Time Optimization Model" *Remote Sensing* 13, no. 22: 4715.
https://doi.org/10.3390/rs13224715