APJ File

What are APJ files and how to open them

Are you having problems opening an APJ file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your APJ files.

What is an APJ file?

APJ files have multiple uses, and ArduPilot JSON Firmware is one of them. Read more about the other uses further down the page.

ArduPilot JSON Firmware

ArduPilot JSON Firmware is a file format used by the ArduPilot autopilot ecosystem to bundle flight control firmware together with metadata in a single JSON-based file. It is produced when building firmware for flight controller boards using the ChibiOS build system and is used for uploading and verifying firmware via ArduPilot-compatible tools.

The format is created by ArduPilot's build process, which compresses the binary firmware image (and optional external flash sections) using zlib, encodes that compressed data in base64, and embeds it along with metadata such as board-type, version, flash size, and git commit identity. This metadata enables ground station software or bootloaders to check whether a given firmware is suitable for a particular hardware board, whether there is sufficient free flash, and whether the version matches expected limits.

Within a manifest of available builds on ArduPilot's firmware server, firmware files in this format are listed with extension "apj", which is the current name for what were once called "px4" files; both use the same structure. These files are among the pre-built binaries offered (stable, beta, latest), meant to be downloaded and applied to boards via tools that understand the ArduPilot firmware upload protocol.

Internally, the file is valid JSON containing fields such as magic identifier (e.g. "magic": "APJFWv1"), board_id, description, image (base64-encoded compressed firmware), summary, version, image_size, possibly extf_image size, USBID, git_identity, board_revision, and build_time. Byte signatures include the ASCII header "{\n \"board_id\": " at offset 0, and presence of specific JSON keys like BOARD_REVISION, GIT_IDENTITY, DESCRIPTION, IMAGE_SIZE, BOARD_ID, SUMMARY, VERSION, MAGIC.

How to open APJ files

Important: Different programs may use files with the APJ file extension for different purposes, so unless you are sure which format your APJ file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested ten different APJ openers which you will find listed below.

Last updated: August 18, 2026

All known file formats using extension .APJ

While ArduPilot JSON Firmware is a popular type of APJ-file, we know of 2 different uses of the .APJ file extension. Different software may use files with the same extension for different types of data.

ARM Project Manager Project

ARM Project Manager Project files are used by ARM Project Manager (APM), a tool provided by ARM Ltd under the ARM Software Development Toolkit. These files define how a software project is built, including which source files, include files, sub-projects, compiler/assembler/linker tools, variants (such as Debug or Release), partitions (like Sources, Objects, Libraries), and output types (typically an executable image or library) belong to the project.

APM writes these project files. They store the project template and variant settings, variable definitions (e.g. for tool paths or project name), and references to source and derived files without moving them from their original locations; file paths may be relative to the project directory. Building the project runs build step patterns defined in the project template to generate outputs like object files, libraries, or executable images. Sub-projects are simply other APM projects included in a larger project.

These files begin with a signature: the first three bytes are ASCII "APJ" followed by a null byte; at offset 26 the ASCII string "ARMExecutableImage" appears.

Various apps that use files with this extension

These apps are known to open certain types of APJ files. Remember, different programs may use APJ files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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