Written By Daniel Sanfelice
With over 20 years of combined experience as sUAS operators, the flight testers at ModalAI sure know their way around a drone. In their new series, learn about five essential components of high flying drones you should be familiar with in order to have a successful flight every time.
- Radios and Datalinks
- IMUs & Magnetometers
During my time as a military sUAS operator, I remember when I had to perform a test for our leadership team demonstrating that we could successfully fly our drone and relay live video back to our headquarters. As I was setting up my aircraft like I had done a million times before, I found that I had a very serious problem- I could not get any connection between the aircraft and the ground station. I tried normal troubleshooting: moving the aircraft, power cycles, checking and rechecking all the settings to no avail. Then, in a “can’t see the forest for the trees” moment, I realized that I was about 20m from our HQ communications tent, which had a plethora of communication antennas and dishes jutting out from its roof. I picked up my equipment and moved to the other side of the camp and like a miracle, the aircraft connected right away. I realized right then that my data link, broadcasting at 1 watt, was being drastically drowned out by the high-powered military radios that were broadcasting right over the top of me. Luckily, I was able to successfully take flight, and always remembered to be mindful of frequency congestion. This practical guide will walk you through common nuances that can affect your drone’s radio and data link connection, so you can be prepared the next time you fly your drone.
What are Radios and Data Links
Radio Transmitters and Data Links make up the Radio Frequency (RF) system for autonomous vehicles. RF systems are more commonplace than you might think; bluetooth IoT devices, WiFi routers, cell phones and AM/FM radio all use radio frequencies to communicate and send data. These make up just a small fraction of the RF devices that our world depends on.
The Radio Transmitter (sometimes referred to as Radio Controller, or RC) and the Radio Receiver make up the basic radio control link between the pilot and the aircraft. Typically, these radio links use simple communication protocols and use tried and tested technology that has been around for decades. The radio link is designed to give very basic Pulse Width Modulation (PWM) commands straight to the autopilot to control the aircraft’s orientation and navigations. You can have basic control over the aircraft such as throttle control, pitch/roll/yaw, changing flight modes, and initiating a return to launch. These transmitters can also do other things such as control servos, cameras, gimbals or other capabilities on the aircraft. The most common radio transmitters are Spectrum and FR Sky and both of these are already integrated into the PX4 autopilot ecosystem.
The data link side normally has more bandwidth and can transmit a more robust amount of data such as aircraft telemetry and live video. However, these added capabilities come at the expense of adding complexity and price to the systems. The data link will connect to your computer and allow you to use your ground station to send commands directly to the aircraft. The most basic telemetry radio, like a Sik Radio can send telemetry like GPS coordinates, heading, orientation and status messages from the autopilot. More advanced data links have more bandwidth and can transmit other things such as HD video and have downlink speeds up to 20mbs. In addition, with a data link, you can bypass the hand held RC transmitter and just use a USB joystick to send navigational RC transmitter commands over the data link connection. We recommend starting with a few simple, close proximity tests with a Sik or the built in VOXL wifi as a basic telemetry radio. These simple data links can perform a variety of missions and are plug and play ready.
Why are Radio and Data Links Important to Autonomous Vehicles?
Without a radio or data link, an autonomous vehicle has no way of communicating with the pilot and no way of receiving pilot commands, sending status messages, or telling you its location. Losing your radio and data link is a loss of control over your aircraft. Consequences of losing connection can result in something as simple as losing control of a camera payload, to something more dangerous like not being able to maneuver your aircraft and avoid a collision. Flying an aircraft without a radio or data link is dangerous because you have no way of controlling the aircraft, knowing the status of the autopilot, its location, or where it is going. If you were to fly beyond visual line of sight (BVLOS), your aircraft would be completely out of view and out of your control if an emergency arises.
How ModalAI autopilots use Radios and Data Links:
The ModalAI VOXL m500 uses the radio and data links to give the pilot navigational control and to relay data and important information to and from the aircraft. Here at ModalAI, we think of the data link as the primary connection between the ground control station and the aircraft, and we think of the radio link as the backup connection in case of emergency. We use the data link (such as the ModalAI LTE or MicroHard communications boards) as the primary because it can transfer the most amount of telemetry data between the pilot and aircraft. We can use the data link to update mission plans, monitor its GPS location and receive important safety information about the status of the aircraft. For more information on compatible data links for the m500, you can visit our radios page and PX4's technical documentation.
We use the RC transmitter as the backup because it gives us a very reliable connection to some basic aircraft functions that can help us in a pinch or if we have issues with the (sometimes) more complex data link connection. If your aircraft is returning for an autonomous landing and is a bit off target, you can use the RC to quickly take over to maneuver the aircraft to a safe landing spot.
Note: Be aware of how the different radio links might overwrite the commands of the other. For example, if you initiate an autonomous Return to Launch (RTL) from the ground station computer but accidentally hit one of the sticks on the RC transmitter, then the aircraft will stop and hover in position mode. Similar actions on the RC transmitter, such as changing flight modes or sending changes in the PWM signal, can override autonomous commands of the autopilot or data link.
How to Get the Most Out of Your RF System
Frequency and bandwidth are important:
There are a few key things to keep in mind when choosing what kind of transmitters or data links to use. The most important thing is to understand the bandwidth and range requirements of your mission.
- For basic tests such as autonomous navigation or carrying a payload that you don’t need to monitor, then you will be fine with basic telemetry radios such as a Sik Radio or even just the RC transmitter with telemetry.
- For more complicated tests such as transmitting HD video or long range flights, then you will need more robust radios that have the range and bandwidth to carry all that information. Visit our radios page to discover which radio option works best for your use case.
The frequency of the radios will also determine how much bandwidth they have and what range they are effective at (there are many factors that affect range, more on this later). Also, remember that as the range increases, the signal strength and bandwidth decrease (the same way your WiFi speeds are faster next to your router versus at the very edge of the signal), so be sure to have extra room in your bandwidth budget if you plan on conducting long range flights. It is important to do your research and ensure the frequency and bandwidth are suitable for your application before purchasing any radios or data links.
Understand the type of antenna and power requirements for your operation:
Some of the most important factors in RF performance are choosing the right antenna and selecting the appropriate radio output power for your operation.
- Omnidirectional Antennas: The most common type of antenna is omnidirectional which emits a 360 degree signal from your ground station. These are great for the majority of short range unmanned flights (1km or less), because you don’t have to constantly adjust the antenna or worry about the aircraft losing reception as easily. The downside to omnidirectional antennas is that radio output power is split between 360 degrees, so some bandwidth and decibels may be transmitted in areas the aircraft is not.
- Directional Antennas: In scenarios where you plan on flying a very long range or in one general direction, a directional antenna is the most appropriate. These antennas take the power output of the radio and concentrate it over a limited area, which allows you to fly and transmit more data farther. The downside of this is that the aircraft must be pointing towards the antenna's main lobe (or propagation direction), which means you need to adjust the antenna as the aircraft navigates or have a tracking antenna setup - which can increase the cost and complexity of your operation.
- Radio Power Output: The power output of your radio is also very important. For radios, power and range are directly related, so having a more powerful radio can lead to an increase in range. A more powerful radio does have downsides, as it increases power consumption onboard the aircraft and can be more expensive than standard radios. Be aware that the FCC requires special permissions and licenses to operate above 1w (30 dBm) of transmitting power. Lastly, never leave a radio powered on without the antennas connected. As the radio is transmitting, it is creating heat from all the power it is creating. If you do not have antennas on the radio to transmit that power, they can burn up the radio or even cause a fire.
Besides choosing the correct antenna type, it is important to note that not all antennas are created the same. Having the correct gain and db strength is critical for successfully sending telemetry and data over long distances. This is an incredibly complex topic (and you can find more information here). It’s best to consult with your radio and antenna suppliers to ensure you have the correct combination for your requirements.
Note: Different countries have different rules and regulations around what frequencies and power outputs are allowed to be used on unmanned vehicles. If traveling to different countries with an aircraft, always do your research to ensure you comply with local radio frequency laws.
Antenna placement is key:
Antenna placement is incredibly important in having high RF performance.
- Ground control station: Make sure your antenna is as high as possible and always has a clear line of sight (LOS) to the aircraft with no obstructions during your flight. Make sure that there are no other sources of signal noise that could jam or reduce the signal to and from your ground station.
- Aircraft: Ensure that your antennas won’t be blocked by the aircraft as it turns and moves throughout its flight. An antenna mounted on top of the aircraft might lose signal to the ground control station if the aircraft rolls with the belly towards the GCS. You can mitigate this by mounting the antenna on the bottom of the aircraft or using multiple antennas that will cover for each other as the aircraft moves. Just as the GPS signal could be blocked by signal noise, so can radio and data link signals. Make sure you mount antennas away from wiring or other sources of EMI or else you may induce signal noise and reduce performance.
- Null points: An antenna null point is a spot in the radiation pattern where little to no signal strength is transmitted. Most antennas radiate around the body of the antenna, so you should not point the tip of the antenna at the aircraft to avoid the null point. These null points can vary by the type of antenna you use and should be taken into consideration when mounting on the aircraft or ground station. For more info on the specific null point of an antenna, you can check the spec sheet for its radiation pattern.
Environmental factors are a huge impact on RF performance:
A lot of times, antenna and radio manufacturers will give you a spec sheet with the system’s RF range. While this can be useful in narrowing down what systems will work for your application, it may not be comprehensive of the myriad of environmental factors that can affect RF system performance.
- EMI: The physical environment we operate in is filled with sources of EMI which degrade our RF signal. Things such as powerlines, wireless routers, microwaves and cell phone towers can all cause degradation in RF performance, so it is important to be aware of these items not just in your immediate vicinity but along your entire flight path.
- Frequency congestion: Operating near other radios that transmit on the same frequency can also cause interference with your radio or data link system. Many RF transmitters and receivers work on the 2.4ghz frequency, which also happens to be what is being radiated from almost every single building and internet of things (IoT) device in the world. The 2.4ghz band only has a finite amount of channels for different devices to send data (think lanes of traffic), when these are all full, you can have different signals stepping on each other and causing signal and bandwidth problems. Avoid these problems by making sure you are flying clear of other devices that give off the same frequency you are using such as WiFi hotspots, bluetooth devices or any radio communication towers along your flight path.
- Physical barriers: Lastly, mother nature and the physical environment can have an effect on RF performance as well. Trees, hills, buildings, cars and other physical objects can block signals. Even the weather can have degrading effects on your RF performance, so make sure you take all these into account before deciding on any radio and data link systems.
Picking an RC Transmitter:
RC transmitters and receivers are fairly simple and straightforward, but there are some things to consider before purchasing a setup.
- Choose a brand: Make sure that the transmitter and receiver are the same brand. Different brands may use the same frequency but use different protocols to communicate. A communication protocol is a set of rules and standards so that two pieces of equipment on the same protocol can communicate with each other. Being on the same frequency does not automatically make two transmitters compatible. For example, to heat food, a microwave oven transmits very high power 2.4ghz radio waves, but that doesn’t mean that it can communicate with a 2.4ghz WiFi router. At ModalAI, we use Spektrum and FRsky RC transmitters..
- Check channel availability: You should also ensure that your transmitter and receiver have enough channels for all the functions you wish to utilize. A basic transmitter with 1-8 channels should be enough for most requirements, but if you plan on having a camera with zoom capabilities or extra servos, then you will need RC channels to control all those functions.
- Flying without a GCS: In addition, if you want to fly without a ground station, FRSky radios have the ability to transmit basic telemetry data such as heading, altitude and geolocation straight to your FRSky transmitter. This is incredibly useful but does take a little extra work to get it running on PX4 based systems.
- Ease of Integration: Lastly, it is important to point out that Spektrum and FRSky transmitters are supported right out of the box and Spectrum receivers even have a button in Q Ground Control (QGC) that allows you to start the binding process right from your ground station (Spektrum Bind). Here at ModalAI we recommend using Spektrum and FRsky receivers because of their quality and ease of integration.
Stay Connected and Fly High
Radios and data links are an important component of autonomous drones. Remember to familiarize yourself with the bandwidth and range requirements for your antenna, the different types of antennas and their power requirements, correct antenna placement, environmental factors that could affect RF performance, and the different factors to look out for when picking an RC transmitter. This foundational knowledge of critical system components will help reduce troubleshooting time and ensure you have a safe and successful flight every time. Stay tuned for the next blog in this series to learn about IMUs and Magnetometers.
Subscribe to our newsletter to be notified of new blogs, products and software releases.