Acquire a GNSS signal

Now it is time to understand better how a GNSS receiver finds a satellite.

If you landed here without prior knowledge, check Chronicle 2 where we explain how a receiver works and how it generate a location replica of the satellite signal.

Here, you will use that idea to acquire a satellite (in a simplified way).

🎮 Goal

Press Start and become the receiver.

A satellite signal just arrived, but you do not know

• what satellites is talking to you
• when the signal was sent (code delay -> chip offset)
• how is shifted in frequency (Doppler frequency)

Your task is to find the correct combination.

🎲 How to play

On the grid on the right, each square represents a combination of:

• Chip offset -> shifts the PRN code in time
• Doppler frequency -> stretches or compresses the signal

Click a square, then double-click or hit space/enter to evaluate how good is the combination that you have selected.

For each selection, the receiver compares the incoming satellite signal with your local replica built based on your selection. That comparison is done by calculating the correlation between both of them. The higher the correlation the stronger is the match.

🏁 Final

Find the combination that gives the highest correlation. When you think you have it, press validate to confirm.

You are searching for the cell where the satellite signal aligns as well as possible with your chosen PRN, chip offset, and Doppler frequency. To make things a bit more interesting, the signal arrives with some noise; therefore, you will not get a perfect match, and the correlation will not reach 1.