To appreciate Unmanned Aircraft Systems (UAS) operations better, it helps to understand the communication technology that enables effective drone flight. Three critical pieces of communication are typically required: 1) the command link from the controller to the drone, 2) the positional data computed directly from the Global Positioning System (GPS) satellite communications sent to the drone, and 3) the data sent back to the pilot from the drone.
The first step in any mission driven UAS flight is the development of a flight plan. The pilot needs to think about the data to be collected and how to position the drone to effectively collect this data. For large scale UAS operations, multiple flights may be needed to achieve this, but we will focus on the details for a single flight to keep this discussion concise.
The flight plan needs to focus on both the range and the “look angle” of its primary sensor. The sensor needs to have the object or terrain in its field of view to collect the data, and it also needs enough detail (pixels resolution) in the image for an effective analysis. These details combined will result in a flight path at a certain altitude. Now the pilot needs to take into consideration other environmental factors such as buildings, light poles, telephone or electrical lines, or the presence of people, all of which could complicate the flight path or cause adjustments to the flight altitude. This also includes any proximity to local airports and the safety regulations associated with the presence of manned flight operations in the area. Once the pilot has a plan in mind, the controller is used to initiate the flight and send this guiding information (heading, speed, altitude changes, drone aspect, and sensor gimbal commands) to the drone throughout the flight.
This information is sent to the drone as a series of Radio Frequency (RF) communication commands that convert the flight control rod positions and other controller settings into flight or sensor commands. These uplink signals for many small commercial quadcopters are commonly sent at 2.4 GHz and allow the pilot to control their drone out to about one mile. However, other drones and other controllers can also use the 433 MHz, 900 MHz and 5.8 GHz frequencies as well. The tradeoff is that the higher frequencies provide higher data rates at shorter ranges. The lower MHz frequencies can provide longer communication ranges at a slower data rate.
Most drones now also have GPS antennas and GPS processing chips onboard the drone to determine their specific geographic location. GPS systems use the signals sent from a GPS satellite constellation in orbit around the earth. The United States GPS system is a network of 30 satellites orbiting at 12,000 miles above the earth. Using RF transmissions from at least 3 GPS satellites, the GPS receiver can pinpoint your location with an accuracy around 1-20 ft. The U.S. GPS signals are transmitted on two frequencies; the L1 signal at 1575 MHz and the L2 signal at 1227 MHz.
The U.S. is not the only country that realizes the value of geographic location systems, but we were the first country with the first GPS satellite launch back in 1978. Just a few years after this in 1982, Russia launched their first GLONASS satellite. The GLONASS constellation uses 26 satellites for an accuracy of 9-24 ft. Much later, both China and the European Union launched their own satellite-based positioning systems. The Chinese BeiDou Navigation Satellite System uses 35 satellites for a 32 ft accuracy. The first BeiDou satellite was launched in 2000. The European Galileo global navigation satellite system uses 30 satellites for a 3 ft accuracy. The first Galileo satellite was not launched until 2011; it takes advantage of many years of improved technology and precision location algorithms. It is also interesting to know that many of these constellations also support encrypted data signals solely for their government or military to provide increased location accuracy, even down to half-inch resolution.
Understanding there are multiple geolocation satellite constellations is important because many drones today can use public location signals from more than one constellation. For example, the DJI Phantom 3, Phantom 4, and Mavic models use both GPS and GLONASS signals simultaneously. This enables UAS hovering performance to achieve a 1.5 ft vertical accuracy and a 5 ft horizontal accuracy. The flight controllers today for many drones will indicate how many satellites are in view during startup and flight. Generally, the more satellites in view of the UAS will result in increased position accuracy, hence producing smoother and more stable flights. Many pilots recommend having at least six satellites in view for stable flight, and if your GPS receiver can receive dual signals, 11+ satellites are recommended for highly stable flight.
Lastly, drone flight data sent back from the airframe to the controller closes the flight control loop. The downlink include telemetry data from the UAS such as altitude, speed, heading, and even battery level. Without this telemetry data, the controller commands would cause flight instabilities. This same data link also transmits the much higher volume video data.
To explore data transmission further, we will discuss the DJI drone transmission systems since these are the more common drone models you will likely encounter today. Depending on the model, DJI commonly uses Wi-Fi, or the proprietary Lightbridge and OcuSync transmissions systems. Each of these transmission system choices can vary the drone’s communication range, video transmission resolution and frame rate, transmission latency, and available control frequencies:
- Wi-Fi transmissions will have a reduced range over the other options (0.3-0.4 mile), but this is a cost-effective method of control that was used on earlier DJI models (Phantom 2 and Spark). Wi-Fi transmissions can typically support a video resolution of up to 720p (1280 x720, HD) at 30 fps.
- Because Wi-Fi transmissions are less reliable due to interference from other common Wi-Fi devices, the Lightbridge transmission system was introduced with the later Phantom 3 models. Lightbridge can support a 2 mile range with a higher video resolution of 1080p (1920 x 1080, Full HD) at up to 60 fps. Lightbridge transmission provides a reliable connection between the drone and the controller, and master and slave connections are available allowing separate drone and gimbal control. Where this might be helpful is to have one operator control the drone while the second operator controls the camera gimbal.
- OcuSync has a slightly superior transmission range (2.3-2.5 miles) to Lightbridge and can handle even higher camera resolutions of 2.7K (1080p at 30fps) at 60 fps or 4K (3840 x 2160, Ultra HD) at 30 fps. OcuSync also allows multiple remote controllers to connect to the drone at one time allowing a split between drone and gimbal control if desired. DJI has also introduced a newer Lightbridge 2 transmission system with range out to 3-4 miles supporting high frame rates and high pixel resolutions with low transmission latency.
In summary, most drone flights are dependent on data uplinks and downlinks between the drone and the controller. If this link is lost, there is no way to reliably control the drone. In addition, without the critical geolocation information constantly being sent from satellite constellations, quadrotor drone flights can be erratic and unstable even with a strong controller link.
Successful UAS operations are critically dependent on drone communications. Understanding the data content, specifications, and limitations of these communication streams will help to improve all UAS flights and missions.
In this blog, we have not included any discussion on the unique case of pre-programmed UAS flight plans, where there is limited communications with the drone. Pre-programmed flights are possible because an electronic device called an Inertial Measurement Unit (IMU) is part of the drone that detects flight attributes such as changes in pitch, roll, and yaw using accelerometers and gyroscopes. However, there are many limitations on pre-programmed flights. This will be a separate discussion for a later blog post.
Now that you have a solid understanding of UAS transmission systems and geolocation systems, we can move to our next topic, CUAS: “Why is this difficult?”, and, “What can go wrong?” There are many ways to disrupt a drone flight by disrupting its communications--we will explore some of these concepts in more depth in our next blog.
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