Key Notes:

• The word “Transponder” was created in the mid 1940’s by simply combining the words “TRANSmit” and “resPONDER”. It’s just a system that listens and reacts to information it receives.

• Technically, a transponder can transmit and respond to anything like electric signals, smells, and sound. It gets whatever kind of information it’s designed for and it does something in response. You can be a transponder by going to the kitchen when you hear someone say they have cookies or laughing when you read a joke. In the space industry, a transponder almost always means it’s listening and reacting to photons of some sort.
  
• A Transponder is different from a radio because “Radios Radiate” (hence their name) and only send information while a Transponder can both send and receive information. A transponder system in space almost always includes a radio system in addition to other things.
  

The Three Primary Functions of a Satellite Transponder

This is sometimes confusing in space because different satellites use transponders in different ways. The three most often seen use cases are:

1. To receive commands and send telemetry: Nearly every spacecraft ever launched has had a transponder system installed for this purpose. It Responds to the commands and Transmits data.
   
2. To rebroadcast information: For instance, satellite television works by sending one type of signal to the satellite, which then converts the signal to something that home users with their satellite dishes can receive. It Responds to the signal by Transmitting an altered version of the original signal to a huge area. A transponder can be designed to be very fast at converting one type of signal to another.
   
3. To figure out where you are in space: There are a few ways to use transponders to determine your position. We’ll talk about the methods in a section below. Many spacecraft continue to rely on this function but some have been moving to use GPS signals and/or use ground-based radar.

Commands & Telemetry

The number one use case for transponders is to listen to commands and report the results or take some action. However, similar to how different people will respond differently to being told what to do, transponders can be designed to respond to commands in different ways. This can lead to confusion about what exactly is the transponder on a satellite, which has led to the industry not being consistent in their technical solutions or naming conventions.

Technically, every satellite that receives commands and responds to them does so with a transponder system. But only some companies and programs call that system a “Transponder System”. Often, they’ll just call it the “Communications System” or the “Communications Stack” or “The Comms”. You’ll also see it called the Telemetry, Tracking, & Command system, or TTC System for short. Some will even call it the “Radio System”, even though that’s only correct in a limited way since the word “radio” technically implies information only goes one way. 

The worst part, though, is that all of these names are correct in their own way, especially when you consider how commands and responses are calculated, which may or may not include the flight computers.

The figure below shows a version of what a Command & Telemetry Transponder logic chain might look like. There’s a receiver radio, something that parses what it’s being asked to do, something that does what it’s asked to, something that reports the results to the transmitting radio, and then the transmitting radio.

Most spacecraft have a Transponder system that listens for commands, runs them, and sends back the results to the ground.

In the next image, we also show a transponder system. But this time, the transmitting and receiving radio are the same hardware and it can respond without bogging down the flight computer.

Simple spacecraft may have a transponder that performs an important role but has limited ability to do anything else.

There are lots of ways to design these systems. How you design your system depends on things like the frequencies you’re allowed to use, if your radio is half or full duplex, how you design your flight computer system, and how centralized or decentralized your avionics and sensors are. CubeSats will be dead simple while large systems may require years of development to get things set up correctly.

Rebroadcasting Information

In some ways, this is the purest form of transponder and closest to how most of the rest of the world thinks of them. For instance, airplanes have transponders that hear a chirp on one frequency and they chirp back on another frequency. Air traffic control asks “Where are you?” and the airplane yells out “I’m over here!”. Kind of like the swimming pool game of “Marco Polo”.

The diagram below shows how a satellite TV sends its video streams to its customers. They send up a stream of photons on one frequency. Then the satellite rapidly converts that to a frequency the customers can listen to and sends down the streams of the new photons. It works kind of like a parrot that hears you something and then repeats it back but in its particular parrot voice and probably a lot louder. 

Satellites that stream television across the planet are rapidly converting one signal to another

The frequency conversion needs to happen as fast as possible. Consider a live sporting event, for example, where customers want things in real time. So the transponder has hardware that does the conversion without going through a bunch of software pondering how to react. The satellite engineers may use analog devices that convert photons to electricity that then get filtered, shifted, and boosted, and then convert the result back to photons. Or they may convert the photons to a digital signal, use a Field Programmable Gate Array (FPGA) to shift the frequencies and weave in other data, then convert the digital output signal to photons. It’s a near certainty that the data is not being converted to a digital signal and processed with a normal flight computer with its stack of operating system and software layers as that would be far too slow.

Figuring out Where you Are

For decades, many satellites used either a special transponder or a special function within their communications system that’s job was to help mission operators figure out where the satellite was in space. Many systems still include them for backup, but the trend most recently for satellites around the Earth is to use ground-based radar or GPS signals.

A transponder helps you figure out where the satellite is by providing range data. If you get a series of ranges, then you can plot them over time to see the trajectory the satellite is on. Depending on the accuracy you need, you can collect a few points to get an idea of your orbit shape, while collecting ten points gives you more certainty. 

You use this plot to predict where you are and where you will be. Note that there are lots of little things that will change your trajectory over time like gravity level differences, drag, and solar pressure, so mission operators will regularly update their actual orbit. The next figure shows a simple version of how this works.

If you know how far away your satellite is from your ground system, you can plot the data and figure out what your orbit shape is.

Ranging with time

There are a number of ways the satellite’s transponder can help you figure out how far away it is. The simplest is shown in the next figure. You need to know two times:

1. The total time it takes to send and receive the signal
2. The time it takes for your satellite to respond to a request

To calculate the distance, you subtract the response time from the total time, divide by two (since you want the one-way range), and then multiply by the speed of light. For example, if you ping your satellite and get a ping back in 5 milliseconds and you know it takes your satellite one millisecond to respond, then your range is:

(0.005s – 0.001s) * 0.5 * 299792 km/s = 600 kilometers

For most satellites, the time it takes for the spacecraft to process and reply to the ping request is a handful of microseconds, but that can still be important.

For the response time, it may only take one millisecond to respond but, at the speed of light, that’s 300 kilometers for a satellite shooting directly away from you, which is a lot to be wrong by! Most ranging transponders respond quicker than a millisecond but, like so many things in space, “it depends”. Note that you also want the response time to be exactly the same every time. Even a tenth of a millisecond variation between requests means 30 meters of uncertainty. For some satellites, that’s a big deal.

Without trying to get too deep into the details, another consideration is that you probably aren’t instantly generating and processing the signal on the ground, so you may have delays there that you need to subtract off.

Velocity and Ranging with Doppler Frequency Shift

The bottom line here is that you can get how fast your satellite is moving by measuring the frequency changes the signal and you get range with pseudo random noise, similar to the way global positioning systems work.

This a whole topic on its own so, for now, the key thing to know is that the transmit frequency of a satellite is almost always (very) precisely known both for practical reasons and because it’s the law. It’s already chaotic enough with all the satellite signals clogging up the available frequencies, imagine if everyone was allowed to let their radios wander around the spectrum.

Because the frequency is precisely known, any differences in the frequency you receive on the ground must be caused by something else and the most dominant reason tends to be the speed at which the satellite is moving. The satellite is transmitting at the correct frequency but, like an ambulance driving down the road, it sounds to the ground station like the frequency is higher when the satellite is approaching and lower when the satellite is moving away.

Delayed Response with Timestamp

Another way satellite transponders may work is to report the time it receives a request. You don’t get a ping back, you get a whole message back that says, “Hey, I heard your request and I got it at this specific time.” This is useful for a few reasons, including:

1. Minimizing errors from the actual travel time between going and coming photons: there can be tiny variations in the time that photons require to travel to and from a satellite. The atmosphere has molecules and electrons that can mess with the photons. You’ve also got Einstein’s theory of relativity, which does matter at these speeds. And if you’re far from the Earth, maybe around Jupiter, these effects can be larger. By instead asking for the time the request was received, you can cut these errors to just the variations created by sending the photons.

2. Allowing for requests to be sent without waiting for an immediate response: Some missions may want a bunch of data very close together so they can average results to remove some of the uncertainties we talked about above. Or you may have limited time to talk with your satellite and you don’t want to waste precious seconds. In each of these cases, sending a response, waiting for it, receiving it, then repeating, especially for a spacecraft that’s far away is challenging.
   
   A third reason for this is that many spacecraft can only talk with one or maybe a couple of ground stations around the world for legal and financial reasons. But simple ranging requests may be sent by hundreds, perhaps thousands, of different locations. You can set things up to transmit the ranging request, then download the results later and match the timestamps to figure out the range at that time.
   
   For all of these, being able to download data “later” (milliseconds or hours ) when you want it gives you more control.
   

Wrapping Up

The term “Transponder” means different things for different spacecraft. Different companies and programs may not even use the word. It is perhaps one of the most often confused topics, even amongst people with decades of experience in the space industry. Your best bet is to consider them at their most fundamental level: they listen and react. They always do “something” when asked to.

Curated Videos

We’ve watched these videos and think they may help you to understand a bit more about this topic:

1. https://youtu.be/687660yAp3s “Satellite Communications – Transponders”
   This video covers the type of transponder application used in, for example, satellite TV systems. It mostly covers the analog approach but it also talks briefly at the end about how a processor might be included in the stack. Many people will assume “Transponder” means these types of use cases.
   
2. https://www.youtube.com/watch?v=zaDrAr0Wz74 “Satellite Communication – Telemetry, Tracking, & Command (TT&C) System”
   Some satellites consider their whole TT&C the “Transponder Subsystem” because it hears a signal, does something, and transmits a response. This use of the word Transponder used to be more common but nowadays, most satellites seem to be reserving the word for just the thing that repeats or converts signals.
   
3. https://www.youtube.com/watch?v=J3dzqsxU1No
   This covers a number of ways to get your range and distance and does so in a straightforward way. It’s a good option for an overview.

Curated Links

We’ve reviewed these external sources and think they may you”

1. https://www.amsat.org/amsat/articles/g3ruh/123.html
   This page covers both simple pulse ranging and the coded pulse ranging approaches. Well worth a read, if you’re looking for details.
   
2. https://www.satcatalog.com/component/c-tt-520-transponder/
   https://www.satnow.com/products/transponders/space-micro/115-1219-xbt
   These are two example transponders used for both command and control and for ranging.
   
3. https://ntrs.nasa.gov/api/citations/19660010159/downloads/19660010159.pdf
   Old school NASA published high quality papers filled with practical information. This one covers the range and velocity calculations. Even though it was published in the 60’s, it’s still highly relevant today.
   
4. https://descanso.jpl.nasa.gov/monograph/series1/Descanso1_all.pdf
   If you want to go deep into radio navigation, this is a chapter published by JPL on the topic. Reading this and the many references it includes would make you quite knowledgeable on the topic.