Introduction

Inmarsat operates a global network of geostationary satellites that provide communication services for aviation, maritime, and other remote industries. Unlike terrestrial networks, geostationary satellites remain fixed over a specific point on the Earth's surface, allowing continuous coverage of a region. This makes them ideal for applications where real-time communications are critical, such as aircraft operations and ship navigation.

In the aviation sector, airlines and air traffic services rely on Inmarsat satellites to exchange messages, voice calls, and data between aircraft and ground stations. These communications cover everything from routine position reporting and flight planning to weather updates, maintenance alerts, and safety-critical messages.

ACARS — a digital datalink that allows aircraft to automatically send and receive operational messages. ACARS messages are transported over the Inmarsat AERO data link using packetized digital channels, typically including information such as the aircraft identifier, message type, and content. The system reduces reliance on voice communication over VHF or HF and ensures timely delivery of flight-critical data.

Inmarsat AERO channels are the satellite-based extension of ACARS. These channels provide global coverage by relaying ACARS messages through Inmarsat satellites:

Aircraft transmit data to the satellite using an L-band uplink (~1.6 GHz)
The satellite relays the signal back to a ground station, which forwards the message to airline operations centers or air traffic control

Simplified Signal Path

      Aircraft
      
      L-band uplink
      
      Inmarsat Satellite
    
↓

      Satellite
      
      L-band downlink
      
      Ground Receiver

Ground-to-aircraft transmissions also occur in the L-band around 1.5 GHz, which makes them relatively accessible to radio enthusiasts using inexpensive Software-Defined Radio hardware.

In addition to basic ACARS data, Inmarsat AERO channels can carry higher-bitrate transmissions for voice communications (such as pilot-to-ground calls) and more data-intensive applications, making them a rich source of real-time aviation telemetry for SDR enthusiasts.

Satellite Coverage

The Indian Ocean region (IOR) is served by the geostationary satellite located at 143°E longitude, currently the Alphasat-4 F1. Geostationary satellites orbit at approximately 35,786 km above the equator, allowing them to remain fixed relative to the Earth's surface. This provides continuous coverage of a large area, unlike low-Earth orbit satellites, which move quickly across the sky.

Coverage Area

The 143°E satellite provides:

Primary Coverage
India, Southeast Asia, Middle East, eastern Africa, Australia

Secondary Coverage
Portions of the Indian Ocean, South Asia, and adjacent maritime regions

The exact coverage depends on the satellite's transponder beam footprint, which is designed to maximize service in regions with dense aviation and maritime traffic.

Inmarsat geostationary satellite coverage (IOR region)

Historical Context

The Inmarsat I-4 series, including I-4 F1, represents the second-generation geostationary satellites replacing earlier I-1 and I-3 generations. Key improvements include:

Greater transponder capacity, supporting more ACARS channels and voice traffic
Enhanced beam shaping, allowing reliable communication in both dense and remote regions
Extended lifespan, typically 15 years, ensuring continuous service for aviation and maritime operations

AERO ACARS Channel Types

Inmarsat AERO uses L-band channels to transmit ACARS messages and other data. Each channel has a defined bitrate and typical usage, which affects both traffic volume and decoding difficulty.

Bitrate	Typical Usage	Notes
600 bps	Low-rate ACARS messages	Usually used for routine position reports and simple operational messages. Easy to decode even with a small directional antenna.
1200 bps	Standard ACARS messages	Higher throughput for more frequent messages or slightly larger payloads. Easy to decode even with a small directional antenna.
8400 bps	Voice communications (AERO phone)	Primarily used for pilot-to-ground voice calls or satellite phone traffic. Frequencies may shift dynamically, making reception unpredictable.
10500 bps	High-speed data	Used for bulk data transmissions, maintenance logs, or airline operations systems. Requires good SNR and clean reception.

Higher bitrate channels carry more traffic but require better signal-to-noise ratio to decode reliably.

AERO ACARS Frequencies (143°E IOR)

The Indian Ocean region satellite (143°E) provides multiple ACARS channels across different bitrates.

600 bps Channels

Channel	Frequency (MHz)
600bps-01	`1545.0032`
600bps-02	`1545.1131`
600bps-03	`1545.1182`
600bps-04	`1545.1283`
600bps-05	`1545.1582`
600bps-06	`1545.1634`
600bps-07	`1545.1834`
600bps-08	`1545.1884`
600bps-09	`1545.2131`
600bps-10	`1545.2182`
600bps-11	`1545.2232`

600 bps channels are typically the most stable and easiest to decode with small patch antennas and low-cost SDRs.

1200 bps Channels

Channel	Frequency (MHz)
1200bps-01	`1545.1233`

1200 bps channels are slightly busier than 600 bps, carrying more frequent or larger ACARS messages, and are easy to decode with small patch antennas and low-cost SDRs.

10500 bps Channels

Channel	Frequency (MHz)
10500bps-01	`1546.0049`
10500bps-02	`1546.0205`
10500bps-03	`1546.0353`
10500bps-04	`1546.0848`
10500bps-05	`1546.1004`
10500bps-06	`1546.1149`

These high-speed channels require a higher signal-to-noise ratio (SNR) for reliable decoding and are more sensitive to cable loss, interference, and antenna performance.

8400 bps Voice Channels

These channels are mostly used for pilot-to-ground voice or satellite phone calls. Unlike the other fixed ACARS channels, 8400 bps channels are dynamic:

Frequency Range

1546.125 – 1546.200 MHz

Frequencies can shift within this range
Channels may appear simultaneously or overlap, depending on satellite traffic

Because the 8400 bps channels are not fixed, hobbyists often have to scan or monitor the spectrum continuously to catch active voice channels.

Hardware Setup

My setup looked like this:

L-band reception setup

RTL-SDR L-band Active Patch Antenna
Built-in LNA, powered via SDR bias-tee. Good for 600/1200 bps channels.

RTL-SDR L-band Active Patch Antenna works well for decoding 600 bps and 1200 bps channels, but in my experience, it struggles with the higher bitrate channels such as 8400 bps and 10500 bps due to lower signal-to-noise ratio.

Advanced Setup

A more capable setup involves:

Offset Dish
For increased gain and directivity

Helical Feed
3D-printable design for L-band

Filtered LNA
Improve signal SNR

Inmarsat signals use right-hand circular polarization, so when using a dish, the feed must be left-hand circularly polarized.

I used Derekcz's 3D-printable helical scaffolding from Thingiverse. The recommended model for Inmarsat is "1700L_5.5T_0.14S_4D_10-90M.stl" Download here

Complete signal path from antenna to decoder

The Nooelec SAWbird IO series provides low-noise amplification for L-band signals centered at 1.542 GHz, with the standard module offering 20 dB gain and the SAWbird+ IO providing 30 dB gain, both units feature built-in SAW filtering to reduce out-of-band interference.

SAWbird iO vs SAWbird+ iO comparison

Software Setup

Three pieces of software are required:

Purpose	Software
SDR receiver	SDRSharp or SDR++
Audio routing	Virtual Audio Cable
Decoder	JAERO

Basic Decoding Workflow

1. Connect SDR

2. Start SDR Software

3. Tune Frequency

4. Route to VAC

5. Configure JAERO

Connect the SDR to the computer
Start the SDR software
Tune to one of the AERO ACARS frequencies
Route audio output to Virtual Audio Cable
Configure JAERO to use the same virtual audio device

Recommended Bandwidth Settings

Channel Type	Bandwidth
600 / 1200 bps	4 kHz
8400 bps	10 kHz
10500 bps	15 kHz

Once configured correctly, JAERO will immediately begin decoding messages.

JAERO configuration with Virtual Audio Cable routing

Occasionally the decoder may require manually clicking the signal peak in the FFT window to obtain lock. The speed and locking parameters must also be adjusted according to the channel bitrate.

Decoding Multiple Channels Simultaneously

Decoding a single channel works well, but the Inmarsat band contains many active channels. To monitor several channels simultaneously, a different approach is needed.

A useful tool for this purpose is SDRReceiver. This software can demodulate multiple signals from a wideband SDR stream and send each one to its own decoder instance.

I followed a guide created by @thebaldgeek How to use SDRReceiver to send data to JAERO which explains the process in detail.

Configuration File

I created a custom configuration file for the 143°E IOR satellite (Inmarsat-4 F1) with the correct frequencies to decode all channels using SDRReceiver.


      Download 143E.ini

SDRReceiver with multiple JAERO instances

Running multiple decoders can be CPU-intensive, especially when monitoring wideband AERO channels simultaneously. My 8th-generation Intel i7 system was able to decode all 600 bps and 1200 bps channels simultaneously, but struggled when the 10500 bps channels were added.

Voice Channels

The 8400 bps channels typically carry voice communications.

These may include:

Pilot-to-Ground Calls
Operational communications between aircraft and ground stations

Satellite Phone Calls
Passenger or crew satellite telephone communications

Operational Airline Communications
Flight operations and coordination traffic

To decode voice, enable Local Voice Decoding in the Voice Settings tab of JAERO.

The following example shows a decoded voice transmission:

Final Thoughts

Inmarsat AERO ACARS signals are one of the easiest & rewarding satellite communications to receive using inexpensive SDR hardware.

With a modest antenna, low cost SDR, and freely available software, it is possible to monitor aviation data links transmitted thousands of kilometers away from geostationary satellites.

Receiving Inmarsat AERO ACARS