Software-In-The-Loop (SITL) concept in Drone Programming - Step-by-Step Execution

Concept Flow - Software-In-The-Loop (SITL) concept

Write drone control code

↓

Run code in SITL simulator

↓

Simulator mimics drone hardware

↓

Code sends commands to simulator

↓

Simulator updates drone state

↓

Feedback sent back to code

↓

Code adjusts commands based on feedback

↓

Repeat loop

↓

Test complete or code ready

SITL runs your drone code inside a simulator that acts like the real drone hardware, letting you test commands and responses in a loop.

Execution Sample

Drone Programming

while True:
    send_command_to_simulator()
    state = get_simulator_feedback()
    if mission_complete(state):
        break

This loop sends commands to the simulator, receives drone state feedback, and stops when the mission is complete.

Execution Table

Step	Action	Command Sent	Simulator State	Feedback Received	Loop Continues?
1	Send initial command	Takeoff	On ground	Acknowledged	Yes
2	Receive feedback	Takeoff	Ascending	Altitude rising	Yes
3	Send next command	Move forward	Ascending	Moving forward	Yes
4	Receive feedback	Move forward	Forward motion	Position updated	Yes
5	Send next command	Land	Forward motion	Landing started	Yes
6	Receive feedback	Land	Landing	Landed	No

💡 Mission complete detected at step 6, loop exits.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
command	None	Takeoff	Takeoff	Move forward	Move forward	Land	Land
simulator_state	On ground	On ground	Ascending	Forward motion	Forward motion	Landing	Landed
feedback	None	Acknowledged	Altitude rising	Moving forward	Position updated	Landing started	Landed

Key Moments - 3 Insights

Why does the loop keep running even after sending the 'Takeoff' command?

Because the simulator state shows the drone is still ascending, so the mission is not complete yet (see execution_table rows 1-3).

How does the code know when to stop the loop?

It checks the feedback from the simulator for mission completion, which happens when the drone has landed (see execution_table row 6).

Is the simulator hardware or software?

The simulator is software that mimics hardware behavior, allowing the code to run as if controlling a real drone (concept_flow).

Visual Quiz - 3 Questions

Look at the execution_table, what is the simulator state at step 3?

A Forward motion

B On ground

C Ascending

D Landing

💡 Hint

Check the 'Simulator State' column at step 3 in the execution_table.

At which step does the loop stop running?

A Step 4

B Step 6

C Step 2

D Step 5

💡 Hint

Look at the 'Loop Continues?' column and find where it says 'No'.

If the feedback at step 5 was 'Still flying', what would happen?

A Loop would continue

B Loop would stop

C Simulator would crash

D Command would change to 'Takeoff'

💡 Hint

Refer to the 'Loop Continues?' logic in the execution_table where feedback controls loop continuation.

Concept Snapshot

Software-In-The-Loop (SITL) lets you run drone code inside a software simulator.
The simulator acts like real drone hardware.
Code sends commands, receives feedback, and adjusts in a loop.
This helps test drone behavior safely before flying.
Loop runs until mission completion feedback is received.

Full Transcript

Software-In-The-Loop or SITL is a way to test drone control code by running it inside a software simulator that behaves like the real drone hardware. The code sends commands to the simulator, which updates the drone's state and sends feedback back. This loop continues until the mission is complete, such as after landing. This method helps developers safely test and debug drone software without needing a physical drone. The execution table shows each step where commands are sent, feedback is received, and the loop continues or stops based on mission status.

Practice

1.

What is the main purpose of Software-In-The-Loop (SITL) in drone programming?

easy

A. To replace drone software with manual control

B. To run drone software on a computer without using real hardware

C. To manufacture drone hardware components

D. To control a drone physically using remote control

Solution

1. Step 1: Understand SITL purpose

   SITL allows running drone software on a computer instead of real drone hardware.

2. Step 2: Compare options

   Only To run drone software on a computer without using real hardware describes running software on a computer without hardware, matching SITL's purpose.

3. Final Answer:

   To run drone software on a computer without using real hardware -> Option B

4. Quick Check:

   SITL = run software on computer [OK]

Hint: SITL means software runs on PC, not real drone [OK]

Common Mistakes:

• Confusing SITL with physical drone control
• Thinking SITL involves hardware manufacturing
• Assuming SITL replaces software with manual control

2.

Which of the following commands correctly starts a SITL simulation for a quadcopter using PX4 firmware?

?

easy

A. start_sitl quadcopter

B. run_sitl --type=hexacopter

C. make px4_sitl gazebo_quad

D. launch_sitl drone_sim

Solution

1. Step 1: Identify correct SITL start command

   PX4 uses 'make px4_sitl gazebo_quad' to start SITL for a quadcopter with Gazebo simulator.

2. Step 2: Check other options

   The other options are not standard PX4 SITL commands.

3. Final Answer:

   make px4_sitl gazebo_quad -> Option C

4. Quick Check:

   PX4 SITL start = make px4_sitl gazebo_quad [OK]

Hint: PX4 SITL starts with 'make px4_sitl' command [OK]

Common Mistakes:

• Using non-PX4 commands to start SITL
• Confusing drone type in command
• Missing 'make' keyword in command

3.

Consider this Python snippet controlling a SITL drone simulation:

for i in range(3):
    print(f"Starting SITL simulation {i+1}")
    # simulate start
print("All simulations done")

What will be the output when this code runs?

medium

A. Starting SITL simulation 1
Starting SITL simulation 2
Starting SITL simulation 3
All simulations done

B. Starting SITL simulation 0
Starting SITL simulation 1
Starting SITL simulation 2
All simulations done

C. Starting SITL simulation 1
Starting SITL simulation 2
All simulations done

D. All simulations done
Starting SITL simulation 1
Starting SITL simulation 2
Starting SITL simulation 3

Solution

1. Step 1: Analyze loop iterations

   The loop runs 3 times with i values 0, 1, 2. The print uses i+1, so outputs 1, 2, 3.

2. Step 2: Check output order

   Each iteration prints 'Starting SITL simulation X' then after loop prints 'All simulations done'.

3. Final Answer:

   Starting SITL simulation 1 Starting SITL simulation 2 Starting SITL simulation 3 All simulations done -> Option A

4. Quick Check:

   Loop 3 times, print i+1, then done [OK]

Hint: Remember range(3) loops 0,1,2; add 1 for output [OK]

Common Mistakes:

• Printing i instead of i+1
• Missing last print outside loop
• Confusing loop count with output numbers

4.

Identify the error in this SITL launch script snippet:

def start_sitl():
    print("Starting SITL")
    sitl_process = subprocess.Popen(["make", "px4_sitl", "gazebo_quad"])
    sitl_process.wait()

start_sitl()

What is the main issue?

medium

A. Using Popen instead of run

B. Incorrect command arguments for SITL

C. No error, script runs fine

D. Missing import of subprocess module

Solution

1. Step 1: Check imports

   The script uses subprocess.Popen but does not import subprocess module.

2. Step 2: Verify command and usage

   The command and Popen usage are correct; missing import causes NameError.

3. Final Answer:

   Missing import of subprocess module -> Option D

4. Quick Check:

   Using subprocess without import causes error [OK]

Hint: Always import modules before using them [OK]

Common Mistakes:

• Assuming command args are wrong
• Thinking Popen is incorrect usage
• Ignoring missing import errors

5.

You want to simulate a drone mission in SITL that changes drone type dynamically. Which approach is best?

1. Restart SITL with new drone type each time
2. Change drone type variable inside running SITL
3. Use one SITL instance for all drone types simultaneously
4. Modify SITL source code to support multiple drones

hard

A. Restart SITL with new drone type each time

B. Change drone type variable inside running SITL

C. Use one SITL instance for all drone types simultaneously

D. Modify SITL source code to support multiple drones

Solution

1. Step 1: Understand SITL drone type handling

   SITL simulates one drone type per instance; changing type requires restart.

2. Step 2: Evaluate options

   Restart SITL with new drone type each time is standard practice; options B and C are not supported; D is complex and unnecessary.

3. Final Answer:

   Restart SITL with new drone type each time -> Option A

4. Quick Check:

   Change drone type by restarting SITL [OK]

Hint: Restart SITL to switch drone types safely [OK]

Common Mistakes:

• Trying to change drone type on running SITL
• Assuming one SITL can simulate multiple drones
• Thinking source code change is easiest