Sih

AZHAI Firmware — Software-Defined Sonar Transmitter

PS 26058 — Low-Power, Real-Time Adaptive Software-Defined Sonar Transmitter Payload for AUVs

1. Overview

AZHAI is a software-defined sonar transmitter firmware designed for autonomous underwater vehicles (AUVs).

Underwater sonar is used for applications such as:

• Seafloor mapping
• Underwater object detection
• Depth measurement
• Navigation
• Marine habitat monitoring
• Underwater infrastructure inspection

Traditional sonar systems often use fixed transmission parameters. However, underwater conditions continuously change due to factors such as temperature, salinity, depth, pressure, and turbidity. These environmental changes affect acoustic propagation, attenuation, and detection performance.

AZHAI addresses this by dynamically adapting the transmitted waveform and its parameters according to the current underwater environment.

2. Core Idea

The system follows the pipeline:

Environmental Sensors
        ↓
ADC Acquisition
        ↓
Physical Parameter Calculation
        ↓
Acoustic Propagation Model
        ↓
Adaptive Parameter Selection
        ↓
Waveform Selection
        ↓
Waveform Generation
        ↓
DAC → Analog Signal Conditioning
        ↓
Sonar Transducer

The firmware dynamically determines parameters such as:

• Center frequency
• Bandwidth
• Pulse duration
• Transmit amplitude / power
• Repetition rate
• Waveform type

This allows the transmitter to generate a waveform suited to the current underwater conditions instead of using one fixed configuration.

3. Firmware Architecture

The firmware is divided into independent modules so that the physics calculations, waveform generation, and hardware control can be developed and tested separately.

4. Environmental Inputs

The firmware receives environmental information through the ADC.

ADC Pin Mapping

The raw ADC values are converted into physical quantities before being passed to the acoustic propagation model.

ADC Raw Values
      ↓
Calibration / Conversion
      ↓
Temperature
Salinity
Depth / Pressure
Turbidity
Battery Level

5. Physics Engine

environment_calc.c contains the main environmental and acoustic propagation calculations.

The engine performs:

5.1 ADC → Physical Units

Sensor ADC readings are converted into meaningful physical quantities such as:

• Temperature in °C
• Salinity in PSU
• Depth in metres
• Turbidity in NTU
• Battery percentage

5.2 Speed of Sound

The firmware estimates the speed of sound in seawater using the Mackenzie equation, considering environmental conditions such as:

• Temperature
• Salinity
• Depth

5.3 Acoustic Absorption

The firmware estimates frequency-dependent acoustic absorption using the Francois-Garrison model.

This helps determine how much acoustic energy is lost during propagation.

5.4 Turbidity Scattering

An additional turbidity-dependent scattering model is used to estimate signal losses caused by suspended particles.

The turbidity scattering coefficient is represented by the tunable constant:

K_TURB

This value must be calibrated or validated experimentally.

5.5 Transmission Loss

The propagation model combines spreading and absorption/scattering losses to estimate the total transmission loss:

Transmission Loss
        =
Geometric Spreading Loss
        +
Absorption Loss
        +
Turbidity Scattering Loss

The estimated loss is then used for adaptive transmitter parameter selection.

6. Adaptive Parameter Selection

Based on the estimated underwater propagation conditions, the physics engine derives suitable transmission parameters.

These include:

• Center frequency (Fc)
• Bandwidth (BW)
• Pulse duration (T)
• Transmit amplitude
• Waveform type

The system attempts to balance:

Detection Performance
        ↕
Acoustic Propagation Loss
        ↕
Transmitter Power
        ↕
Transducer Bandwidth

The final parameters are passed to the selected waveform generator.

7. Supported Waveforms

AZHAI supports multiple transmit waveform types.

7.1 LFM Chirp

Linear Frequency Modulation (LFM) sweeps frequency linearly over the selected bandwidth.

Frequency
   ^
   |        /
   |       /
   |      /
   |     /
   |____/____________> Time

Advantages:

• Large time-bandwidth product
• Good range resolution
• Pulse-compression capability
• Suitable for underwater detection

Implementation:

lfm.c

7.2 Geometric Sweep

The geometric sweep changes frequency exponentially rather than linearly.

Frequency
   ^
   |          /
   |        /
   |      /
   |    /
   |___/____________> Time

Implementation:

gs.c

The sweep parameters are generated according to the selected center frequency and bandwidth.

7.3 Barker-13 Phase-Coded Waveform

The phase-coded waveform uses a Barker-13 sequence to encode the transmitted pulse.

+ + + + + + - - + + - + -

The phase transitions provide correlation properties useful for pulse compression and detection.

Implementation:

pcm.c

7.4 Chaotic Frequency Modulation

The CFM waveform uses a chaotic sequence to vary the instantaneous frequency.

The chaotic sequence is generated using the selected chaotic-map parameters and then converted into frequency modulation.

Implementation:

cfm.c

Important: The CFM sidelobe characteristic does not have a simple closed-form value. The parameter SIDELOBE_CFM_DB must therefore be determined through offline simulation or measurement.

8. Waveform Selection

The physics engine evaluates the available waveform options and selects the appropriate waveform using an argmax-based selection mechanism.

Conceptually:

Environmental Conditions
          ↓
Propagation Model
          ↓
Candidate Waveforms
          ↓
Performance Estimation
          ↓
       ARGMAX
          ↓
Selected Waveform

The selected waveform configuration is then passed to the corresponding generator.

9. Real-Time Waveform Generation

The firmware uses a DMA-driven ping-pong buffer architecture for continuous waveform transmission.

             ┌──────────────┐
             │ Waveform     │
             │ Generator    │
             └──────┬───────┘
                    ↓
          ┌───────────────────┐
          │ Ping-Pong Buffer  │
          └───────┬───────────┘
                  ↓
                 DMA
                  ↓
                 DAC
                  ↓
        Analog Signal Conditioning
                  ↓
            Sonar Transducer

While one buffer is being transmitted by DMA, the other buffer can be prepared with the next waveform samples.

This helps maintain continuous real-time transmission without requiring the CPU to generate every sample at the exact transmission instant.

10. Sampling and Timer Configuration

The waveform sample rate is defined by:

FS_HZ

The reference implementation assumes a 500 kHz sample rate.

TIM6 is configured based on the timer clock.

If the TIM6 timer clock is 90 MHz:

Period = (90,000,000 / 500,000) - 1
       = 179

Therefore:

TIM6 Period = 179

Important

The 90 MHz assumption must be verified against the actual STM32 clock tree before flashing.

Check:

SystemClock_Config()

and the project's:

.ioc

file.

If the actual TIM6 clock is different, recompute:

Period = (timer_clock_hz / 500000) - 1

11. Tunable Design Constants

The constants in the TUNABLE DESIGN CONSTANTS section of:

Src/environment_calc.c

are currently placeholders and must be reviewed before final deployment.

Important parameters include:

These values should be obtained from:

• Transducer datasheets
• Tank/field measurements
• Acoustic simulations
• Calibration experiments
• Offline waveform analysis

Refer to:

AZHAI_Physics_Engine_Equations.md

for the meaning and derivation of each parameter.

12. Main Firmware Flow

The main control loop follows this sequence:

        START
          │
          ↓
 Initialize GPIO
 Initialize ADC
 Initialize TIM6
 Initialize DAC
 Initialize DMA
          │
          ↓
   Read Environment
          │
          ↓
 Convert ADC Values
          │
          ↓
 Run Physics Engine
          │
          ↓
 Calculate:
 Fc / BW / Pulse / Amplitude
          │
          ↓
 Select Waveform

13. Telemetry

The firmware provides telemetry for monitoring the current operating state.

Typical information includes:

• Temperature
• Salinity
• Depth
• Turbidity
• Battery level
• Estimated speed of sound
• Transmission loss
• Selected waveform
• Center frequency
• Bandwidth
• Pulse duration
• Amplitude / transmit level

This allows the adaptive behaviour of the sonar transmitter to be monitored during testing.

14. Project Structure

AZHAI/
│
├── Inc/
│   ├── sonar_types.h
│   ├── environment_calc.h
│   ├── lfm.h
│   ├── gs.h
│   ├── pcm.h
│   └── cfm.h
│
├── Src/
│   ├── main.c
│   ├── hann_window.c
│   ├── environment_calc.c
│   ├── lfm.c
│   ├── gs.c
│   ├── pcm.c
│   └── cfm.c
│
├── AZHAI_Physics_Engine_Equations.md
│
└── README.md

15. Before Flashing — Verification Checklist

Before programming the STM32, verify the following.

Clock

• Confirm the actual TIM6 timer clock.
• Confirm the sample rate is 500 kHz.
• Recalculate the TIM6 period if the clock is not 90 MHz.

ADC

• Verify PA0 → Turbidity
• Verify PA1 → Depth
• Verify PB0 → Temperature
• Verify PB1 → Salinity
• Verify PA5 → Battery

Physics Model

• Review target range.
• Review transmission-loss budget.
• Verify transducer bandwidth limits.
• Calibrate K_TURB.
• Determine SIDELOBE_CFM_DB experimentally/offline.
• Verify all sensor conversion equations.

Waveform

• Verify frequency limits.
• Verify bandwidth limits.
• Verify pulse-duration limits.
• Verify DAC amplitude scaling.
• Verify waveform samples remain within DAC range.

Hardware

• Verify DAC interface.
• Verify analog filter.
• Verify amplifier/driver stage.
• Verify transducer impedance and operating range.
• Verify power-supply limits.

16. Presentation Summary

Problem

AUVs operate in changing underwater environments. Temperature, salinity, depth and turbidity affect acoustic propagation, so a fixed sonar waveform may not always provide efficient transmission.

Solution

AZHAI uses environmental sensor data and an acoustic propagation model to dynamically determine suitable sonar transmission parameters and waveform types.

Key Idea

Sense → Model → Adapt → Generate → Transmit

Instead of:

Fixed Parameters → Fixed Waveform → Transmit

AZHAI provides:

Changing Environment
        ↓
Adaptive Parameters
        ↓
Adaptive Waveform
        ↓
Efficient Sonar Transmission

The goal is to enable a low-power, real-time, software-defined sonar transmitter that can adapt its transmission strategy to the underwater environment experienced by the AUV.