AMSAT Linear Transponder Module

AMSAT Engineering · Flight Hardware

Linear Transponder Module

A flight-proven radio system that gives satellite builders command, telemetry, and science-data downlink in one PC104 package, and gives amateur radio operators worldwide a new linear transponder to work every time it flies.

Flown on HuskySat-1 (HO-107) · Fox-1E (AO-109) · MESAT1 (MO-122)

The Linear Transponder Module (LTM) is how AMSAT turns partner missions into new satellites for everyone. A university team gets a proven communications package and the support of an organization that has been flying amateur satellites since 1969. The amateur community gets another transponder on orbit. And the volunteers who design, build, test, and operate the module get to put their skills into space. Developed from the Fox-1E transponder design, the LTM lineage has flown three times and is the communications backbone of AMSAT’s Fox-Plus and GOLF-TEE spacecraft.

Who It Serves

One Module, Two Communities

For Satellite Builders

A complete communications system for your CubeSat, proven on orbit and delivered with the software and support to use it.

  • Command uplink, telemetry, and science-data downlink in one 130 g PC104 module
  • Flight software, boot loader, ground checkout tools, command software, and FoxTelem included
  • Integration, test, and operations support from the AMSAT engineering team
  • A worldwide network of amateur stations receiving your telemetry from the first pass
  • In return, the transponder is opened to amateurs when your mission is not using it
How a partnership works →

For the Amateur Radio Community

Every LTM that flies is a new linear transponder for all of us, and a mission you can help run.

  • A 30 kHz V/u SSB and CW transponder that works well with modest stations
  • Telemetry you can decode and forward with FoxTelem, real data the mission team relies on
  • New birds from partner missions and from AMSAT’s own Fox-Plus and GOLF-TEE
  • Designed, built, tested, and operated by volunteers, with room for more
Working an LTM satellite →  ·  Volunteering →

The Hardware

Three Boards, One Radio System

Latest version flying on Fox-Plus and GOLF-TEE

The LTM is a compact stack of three AMSAT-designed boards, transponder on the bottom, command receiver in the middle, and host interface on top, that together handle every radio function a small spacecraft needs.

Transponder

The RF heart of the module: a 30 kHz-wide linear transponder in the Amateur Satellite Service, with a 144 MHz band uplink and an inverting 432 MHz band downlink, so multiple SSB and CW contacts can share the passband at once. RF output is 450 mW with telemetry and transponder running together.

ICR · Improved Command Receiver

Receives commands on a 144 MHz band uplink from AMSAT and from the host mission’s ground station, giving partner teams command and control of their spacecraft through the module. AMSAT’s uplink command software is provided.

LTI · Linear Transponder Interface

The module’s microcontroller and its connection to the host spacecraft: a PC104 bus connector, CAN bus driver, and a USB 2.0 Mini-B umbilical to a serial terminal. It runs the flight software and boot loader, handles command and data handling, and assembles the 1200 bps BPSK telemetry beacon on its own 432 MHz band downlink frequency. In AMSAT’s own Fox-Plus and GOLF-TEE spacecraft, this slot is filled by the Legacy Internal Housekeeping Unit (LIHU).

Module functions

  • V/u linear transponding
  • Command reception
  • Telemetry downlink
  • Command & data handling

How It Integrates

The LTM is designed as a “black box” radio that drops into a host spacecraft: the host connects over the PC104 bus with a CAN interface and supplies 5–7 V, and the module regulates its own 3.6 V and brings out its own transmit and receive antenna leads. In Fox-Plus it mates to the ISISpace 1U bus through AMSAT’s Power Signal Adapter (FOX-ME-204) and Mechanical Adapter (FOX-ME-203); in GOLF-TEE it rides in the transverse stack beside the radiation-tolerant RT-IHU computer and the software-defined radio. Before either flight, the module was proven on the University of Washington’s HuskySat-1 flat-sat and on AMSAT’s own bench test beds.

LTM-1 Partner Specifications

By the Numbers

Spec sheet Rev. 4 · 16 March 2020

The figures below are from AMSAT’s LTM-1 partner specification sheet. Prospective partners should confirm the current revision with AMSAT Engineering when planning a mission.

General

Dimensions
92 × 90 × 33 mm (L × W × H)
Mass
130 g
Host DC input
5–7 V nominal
Onboard regulation
3.6 V
Safe mode
40 mA, no transmit
Safe mode, TLM burst
310 mA at ~11% duty cycle
Transponder mode
300–650 mA, 100% TLM duty cycle
Bus connector
PC104

RF

Transponder
30 kHz wide, Amateur Satellite Service
Transponder uplink
144 MHz band · 2K40X3WXF
Transponder downlink
432 MHz band · 30K0X8WXF
Telemetry downlink
Separate 432 MHz band frequency, 1200 bps BPSK · 2K40G1DCN
Command uplink
144 MHz band · 20K0F1DXX
RF output
100 mW telemetry only · 450 mW telemetry + transponder

Interface

Flight software
Boot loader with ground checkout, testing & integration code
Umbilical
USB 2.0 Mini-B to serial terminal
Bus driver
CAN
Command software
AMSAT uplink command software
Telemetry software
FoxTelem decoder

Transponder-mode current varies with the number of concurrent transponder users. ITU emission designators are listed to support frequency coordination and licensing.

Proven on Orbit

Flight Heritage

Three missions flown · more in the pipeline

First LTM Flight · 2019–2020

HuskySat-1 (HO-107) · University of Washington

The pilot-production LTM and the first radio module AMSAT designed and built for a host CubeSat. The Husky Satellite Lab commanded the spacecraft and received its telemetry through the module under an FCC Part 5 experimental license, then handed the transponder to the amateur community in May 2020. It operated for more than three months before a thermal anomaly in full sun ended transponder operations, experience that fed directly into the units that followed.

Parent Design on Orbit · 2021

Fox-1E (AO-109) · AMSAT & Vanderbilt University

The fifth and final Fox-1 satellite, RadFxSat-2/Fox-1E, replaced the series’ FM transponder with the 30 kHz V/u linear transponder and 1200 bps BPSK telemetry design the LTM is built on, carrying Vanderbilt University radiation-effects experiments. Launched January 17, 2021 aboard Virgin Orbit’s LauncherOne and designated AMSAT-OSCAR 109, it reached orbit with its transponder working but at very low signal levels; AMSAT opened it for amateur use in July 2021 for CW and efficient digital modes, and it re-entered in 2024.

Second LTM Flight · 2024–2025

MESAT1 (MO-122) · University of Maine

Maine’s first CubeSat, built by the WiSe-Net Lab and selected under NASA’s CubeSat Launch Initiative, launched July 4, 2024 on a Firefly Alpha from Vandenberg. AMSAT supplied the LTM with integration and operational support; it carried the mission’s command uplink and telemetry downlink and opened a strong linear transponder to amateurs in September 2024, earning the MESAT1-OSCAR 122 designation. MESAT1 fell silent in August 2025, and the University of Maine announced the end of the mission in March 2026.

Next · In Development

Fox-Plus, GOLF-TEE, and a Third University

The latest LTM is the payload of the first two Fox-Plus 1U spacecraft and flies in GOLF-TEE’s transverse stack as the legacy V/u transponder, command receiver, and C&DH. A third university mission carrying the current version is under construction. See the AMSAT project status page for the latest.

Each flight has shaped the module that flies next: HuskySat-1’s thermal experience, AO-109’s signal investigation, and MO-122’s power and eclipse management all inform today’s LTM and the guidance AMSAT gives partner teams.

For Operators

Working an LTM Satellite

An LTM transponder is inverting, 30 kHz wide, and sensitive: a small directional antenna and 5–10 watts on 2 meters is typically plenty, with the downlink on 70 centimeters. Correct for Doppler by tuning your uplink while holding the downlink frequency steady, work SSB or CW, and keep your downlink no stronger than the telemetry beacon so everyone can share the passband.

Collect Telemetry, Help the Mission

The 1200 bps BPSK beacon is decoded by AMSAT’s FoxTelem software, which forwards received frames to AMSAT’s servers where mission teams and AMSAT engineers use them to monitor spacecraft health and experiments. A receive-only station with an inexpensive SDR is enough to contribute, and during commissioning and anomalies those frames are often the most valuable thing a volunteer can send.

Heritage frequency plans

  • HuskySat-1 & MESAT1: up 145.910–145.940 · down 435.810–435.840 · TLM 435.800 MHz
  • AO-109: up 145.860–145.890 · down 435.760–435.790 · TLM 435.750 MHz

All three missions have concluded. Frequencies for upcoming LTM satellites, including Fox-Plus, are coordinated through the IARU and announced in the AMSAT News Service bulletins.

For Satellite Builders

Fly With AMSAT

Available to educational CubeSat missions

AMSAT develops the LTM and related payloads specifically for integration into university CubeSat missions. Partner institutions use the hardware for their own mission communications, and when it is not needed for mission operations the transponder is opened to amateur radio operators around the world: a true win-win for STEM education and the amateur community.

  1. Start the conversation early. Share your mission concept, orbit, bus, and schedule with AMSAT Engineering so the module can be planned into your design from the beginning.
  2. Coordinate spectrum and licensing. Work through IARU amateur satellite frequency coordination and the licensing path for your experimental and amateur operations.
  3. Integrate and test. AMSAT supports integration on your flat-sat and bench, along with ground station and telemetry setup.
  4. Fly, commission, and open the transponder. Run your mission, then enable the transponder for worldwide amateur use.
University Participation Partner with AMSAT

Questions? Reach AMSAT Vice President – Engineering Jerry Buxton, N0JY, through the AMSAT contact page.

Learn More

Get Involved

Built by Volunteers. Room for More.

Volunteers welcome

The LTM exists because volunteers designed its boards, wrote its flight software, ran the flat-sat tests, and staffed the command and telemetry stations that flew it. The next version, and the missions that will carry it, need the same kind of help.

Skills that fly

  • RF & analog design
  • Embedded firmware
  • Flight & ground software
  • PCB layout & assembly
  • Mechanical & thermal
  • Test & integration
  • Command station operations
  • Telemetry collection
  • Documentation
  • Project management

Not an engineer? Telemetry collection, documentation, and outreach are just as essential, and every AMSAT member helps fund the next flight.

Volunteer for AMSAT Meet the Team at the 2026 Symposium

Keep Amateur Radio in Space

Every LTM is designed, built, and tested by AMSAT volunteers and funded by members and donors. Your membership and contributions put more transponders on orbit, and you can support GOLF and Fox-Plus directly through the 2026 AMSAT President’s Club.

Join or Renew AMSAT Donate