ardupilot/libraries/AP_Scripting/drivers/LTE_modem.md

LTE Modem Driver

This driver implements support for LTE modems for establishing cellular data connections. It provides either PPP or a transparent TCP connectivity to a remote server through the LTE modem, allowing network communication over LTE networks without using a companion computer.

The driver best paired with with the ArduPilot remote support server https://support.ardupilot.org, but can also be used for any other network service.

If you don't have access to the ArduPilot support server you can install your own using https://github.com/ArduPilot/UDPProxy

Supported Hardware

The driver currently supports:

• all SimCom SIM7600 variants
• SimCom A7670
• Quectel EC200
• Air780

There are some limitations:

• the Air780 does not support signal level monitoring
• the SimCom A7670 only supports TCP connections, not PPP

Parameters

The script uses the following parameters:

LTE_ENABLE

This must be set to 1 to enable the driver. Set to 0 to disable the LTE modem driver.

LTE_PROTOCOL

This controls if a PPP connection will be used or a raw TCP connection with MAVLink2.

Set LTE_PROTOCOL to 48 for PPP and enable ArduPilot networking with the NET_ENABLE=1 parameter.

Set LTE_PROTOCOL to 2 for MAVLink2 and enable the LTE_SERVER parameters for the TCP server you want to connect to.

Note that the LTE_PROTOCOL parameter must match the value of the SCR_SDEVn_PROTO parameter.

LTE_SERPORT

This sets the serial port to use for the LTE modem. This is the index of the SERIALn_ ports that are set to 28 for "scripting".

LTE_SCRPORT

This sets the scripting serial port to use for the LTE modem. This is the index of the SCR_SDEVn ports that are set to 2 for "MAVLink2". This port handles the MAVLink data that will be transmitted over the LTE connection. You must first set SCR_SDEV_EN to 1 to enable scripting serial devices.

LTE_SERVER_IP0

This is the first octet of the server IP address to connect to. The full IP address is constructed from LTE_SERVER_IP0 through LTE_SERVER_IP3. Range: 0-255. This is not used with PPP.

LTE_SERVER_IP1

This is the second octet of the server IP address to connect to. Range: 0-255. This is not used with PPP.

LTE_SERVER_IP2

This is the third octet of the server IP address to connect to. Range: 0-255. This is not used with PPP.

LTE_SERVER_IP3

This is the fourth octet of the server IP address to connect to. Range: 0-255. This is not used with PPP.

LTE_SERVER_PORT

This sets the IPv4 port of the server to connect to. This should match the port that your ground control station or server is listening on. Range: 1-65525. This is not used with PPP.

LTE_BAUD

This sets the baud rate for the serial port to the LTE modem to use for data transfer. Common values are 115200 or 921600. Default: 115200.

LTE_IBAUD

The initial baud rate when the modem is powered on. This is normally 115200 but can be changed in the modem using the AT+IREX terminal command.

LTE_TIMEOUT

This sets the timeout in seconds for the LTE connection. If no data is received for this time, the connection will be reset and the driver will attempt to reconnect. Range: 1-60 seconds. Default: 10 seconds.

LTE_OPTIONS

This sets options for debugging and data display

Operation

This driver should be loaded by placing the lua script in the APM/SCRIPTS directory on the microSD card, which can be done either directly or via MAVFTP. The following key parameters should be set:

• SCR_ENABLE should be set to 1
• A SERIALn_PROTOCOL should be set to 28 (Scripting) where n matches LTE_SERPORT
• SCR_SDEV should be set to 48 (PPP) or 2 (MAVLink2) for the port matching LTE_SCRPORT

If using PPP then you also need to ensure PPP support is compiled into your firmware (you can use https://custom.ardupilot.org to do that) and set NET_ENABLE=1. You will likely also want to setup some outgoing UDP or TCP ports with the NET_Pn parameters.

Then the flight controller should be rebooted and parameters should be refreshed.

Once loaded, the LTE_ parameters will appear and should be configured according to the parameter list above. The server IP address and port must be set to match your ground control station or telemetry server.

Here is a full set of required parameter settings assuming you want to use PPP and make a TCP connection to the ArduPilot support server on port 20001. It assumes you have the modem on Telem1 (SERIAL1)

• SCR_ENABLE 1
• SCR_SDEV_EN 1
• SCR_SDEV1_PROTO 48
• SERIAL1_PROTOCOL 28
• LTE_PROTOCOL 48
• NET_ENABLE 1
• NET_P1_TYPE 3
• NET_P1_IP0 157
• NET_P1_IP1 245
• NET_P1_IP2 83
• NET_P1_IP3 174
• NET_P1_PORT 20001

here is an alternative configuration where a direct TCP connection to the support server is used and modem is attached on SERAL1 (no PPP used)

• SCR_ENABLE 1
• SCR_SDEV_EN 1
• SCR_SDEV1_PROTO 2
• SERIAL1_PROTOCOL 28
• LTE_PROTOCOL 2
• LTE_SERVER_IP0 157
• LTE_SERVER_IP1 245
• LTE_SERVER_IP2 83
• LTE_SERVER_IP3 174
• LTE_SERVER_PORT 20001

Status Messages

The driver provides status messages through the GCS indicating the current connection state:

• "LTE_modem: starting" - Driver initialization
• "LTE_modem: found modem" - Modem detected and responding
• "LTE_modem: CREG OK" - Network registration successful
• "LTE_modem: transparent mode set" - Modem configured for transparent operation
• "LTE_modem: network opened" - Network stack ready
• "LTE_modem: connected" - TCP connection established
• "LTE_modem: connection closed, reconnecting" - Connection lost, attempting reconnection
• "LTE_modem: timeout" - No data received within timeout period
• "LTE_modem: error response from modem" - Modem returned an error

Physical Connections

The modem should be connected to a flight controller serial port. You may also want to use a serial port with hardware flow control support and set BRD_SERn_RTSCRS to 1 for that port.

Note that the modems can be quite sensitive to power supply issues. The power from the serial port will likely not be sufficient.

Troubleshooting

The driver creates a log file "LTE_modem.log" on the SD card that contains all communication with the modem. This log can be useful for debugging connection issues.

Common issues:

• Ensure the SIM card is properly inserted and activated
• Check that the cellular antenna is properly connected
• Verify network coverage at the operating location
• Confirm the server IP address and port are correct and reachable
• Check that the serial port configuration matches between the flight controller and modem

If the connection fails or is lost, the driver will automatically attempt to reconnect by restarting the connection sequence.

Logging

A LTE log message is saved in the onboard log. That has signal strength information and data transfer statistics.

A NAMED_VALUE_FLOAT MAVLink message "LTE_RSSI" is sent with the RSSI signal strength.

Notes on specific modems

Each modem has it's own unique behaviour. Some of the key differences are listed below.

SIM7070G

The SIM7070G is a CAT-M modem, meaning you must have a special CAT-M capable SIM. A normal data SIM won't work at all. I recommend the SIMs from https://hologram.io

The baud rate of the SIM7070G as set by the AP+IPR AT command is "sticky", it persists over a power cycle. This means if you change the LTE_BAUD or LTE_IBAUD parameters then you can get stuck where you can't connect as the script won't be trying the right baudrate. You should connect on USB with a terminal program and use:

• AT+IPR?
• AT+IPR=115200

that will reset the baud rate. Note that the USB port on the SIM7070G does not care what baud rate you connect on, so you can always use it for recovery.

Some SIM7070G modems also have a problem with the level shifting of the UART which can cause issues with reliable communcation. I have found some modems only work at a max of 230400 baud.

You may also need to enable roaming using:

• AT+CREG=2
• AT+CGREG=2

may need NET_OPTIONS=64 as high load prevents LCP echo replies

Also note that all the SIM7070G modems I have (and all SimCom modems I've tried) have reversed TX/RX labels on the UART. You need to connect the flight controller TX to the SIM7070G TX and RX to RX.

SIM7600

The SIM7600 is a CAT-1 modem, so it can use any data SIM. I recommend using a multi-carrier SIM (eg. the SIMs from https://hologram.io ) to maximimise the number of cell towers you can use.

Depending on your SIM and carrier you may need to enable roaming using:

• AT+CREG=2
• AT+CGREG=2

Also note that all the SIM7600 modems I have (and all SimCom modems I've tried) have reversed TX/RX labels on the UART. You need to connect the flight controller TX to the SIM7070G TX and RX to RX.