# Index
- [Papers](#papers)
1) [*A survey of techniques and challenges in underwater localization*](#survey_techiques)
- [Documentation](#docs)

# Papers <a name="papers"></a>

## a) *A survey of techniques and challenges in underwater localization* <a name="survey_techiques"></a>

**1. Challenges towards design localization schemes**
- High accuracy
- Fast convergence
- Wide coverage
- Low communication costs
- Good scalability

**2. Range-based underwater localization**
- Step 1a: Range measurement (within communication range)
- Step 1b: Range measurement (outside communication range)
- Step 2: Location estimation
- Step 3: Calibration

**2.1. Challenges**
- 2.1.1. Underwater environment
- Reference deployment in deep sea
- Node mobility
- Inter-node time synchronization
- Signal reflection due to obstacles and reflective surfaces
- 2.1.2. Underwater acoustic propagation
- Long propagation delay
- Multipath fading and shadowing
- Sound speed variation
- Low bit rate
- Highly unreliable and asymmetric signal-to-noise ratio
**TL($d_{ij}$)=10$\lambda$$log_{10}$$\frac{d_{ij}}{1m}$+$\alpha$$d_{ij}$ dB**
- TL is the transmission loss
- $\lambda$ is the propagation loss parameter
- $\alpha$ is the absorption loss parameter
- $\uparrow$ with carrier frequency
- affected by salinity and water temperature)
- ambient noise level
- depth dependent
- $\downarrow$ with frequency $\rightarrow$ 50-log(f)[dB]

**2.2. Single-stage, static references**
- 2.2.1 UPS
- 2.2.2 Model-based localization

**2.3. Multi-stage, static references**
- 2.3.1. Localization for hierarchical UWSNs
- 2.3.2. Projection-based localization

**2.4. Mobile References**
- 2.4.1. AUV-assisted localization
- 2.4.2. DNR-enabled localization

**3. Range-free underwater localization schemes**
- does not depend on the accuracy of range measurement

**3.1. Schemes based on area localization**
**3.2. Schemes based on directional beaconing**

**4. Commercial underwater positioning system**
- Based on the distance between the baseline stations
**4.1. Long baseline systems**
**4.2. Short baseline systems**
**4.3. Ultra short baseline systems**

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## Documentation <a name="docs"></a>

- [Writing Mathematic Formulas in Markdown](https://csrgxtu.github.io/2015/03/20/Writing-Mathematic-Fomulars-in-Markdown/)
- [List of Greek letters and math symbols](https://www.overleaf.com/learn/latex/List_of_Greek_letters_and_math_symbols)