*Question E2D07:
What digital protocol is used by APRS?*  

*Correct Answer is  C: AX.25*  

### AI Response: (may contain errors)  

APRS, which stands for Automatic Packet Reporting System, is a digital communication protocol used by amateur radio operators to transmit real-time data such as position, weather, and messages. The correct answer to the question is:

**C: AX.25**

### Explanation:

1. **Why the correct answer is correct:**
   - **AX.25** is a data link layer protocol derived from the X.25 protocol suite, specifically adapted for amateur radio use. It is the standard protocol used for packet radio, including APRS. AX.25 allows for the transmission of digital data over radio frequencies, which is essential for APRS operations.

2. **Why the other choices are incorrect:**
   - **A: PACTOR** is a protocol used for HF digital communication, primarily for sending emails and data over long distances. It is not used for APRS.
   - **B: QAM (Quadrature Amplitude Modulation)** is a modulation scheme used in various digital communication systems, including cable modems and Wi-Fi, but it is not a protocol used for APRS.
   - **D: AMTOR** is an older digital mode used for text communication over HF bands, similar to RTTY, and is not used for APRS.

### Useful Information:

- **APRS** uses the AX.25 protocol to send packets of information, which can include GPS coordinates, weather data, and text messages.
- **AX.25** supports features like error detection and correction, which are crucial for reliable data transmission in the variable conditions of radio communication.

Understanding the role of AX.25 in APRS helps clarify why it is the correct choice for this question.