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
I’ve learned a lot from my 10+ years as a drone operator in U.S. government and civilian environments. As part of the flight testing operations team at ModalAI, I’ve had to recall many important learning opportunities from my past to share with the team and avoid mistakes. In this first series, I’ll walk you through the basics of GPS and how it applies to your drone.
On one of my deployments with the U.S. Navy as a UAS operator, I recall how I spent 30 minutes outside in 115 degree heat with no GPS signal lock; meaning that my drone couldn’t find a GPS signal to connect to. One of my peers advised me to position the drone at least 30 meters from the nearest building because GPS may take longer to acquire in a new location. Once I had this tip, I was able to acquire a GPS signal in about 10 minutes and never had this issue again. This rookie mistake could have been avoided had I read a guide like this.
What is GPS?
The term “GPS” is a colloquialism for a satellite system that we use for terrestrial, airborne and waterborne navigation on Earth. The Global Positioning System (GPS) is actually one of four Global Navigation Satellite Systems (GNSS) and is the American system of 24 satellites that orbit the earth and assist with navigation. Other countries that have their own GNSS are Russia, the European Union and China, known as GLONASS, Galileo and BeiDou, respectively. The orbiting satellites broadcast signals with extremely accurate location and time data, and the GPS receiver on the ground uses the signal and timing to calculate its exact location (latitude, longitude and altitude). While the technology is now several decades old, only in the last few years have GPS receivers become a ubiquitous part of our lives. Almost everyone carries a GPS receiver in their pocket in the form of a cell phone or smart watch on their wrist. Even our automobiles and the IoT devices now have GPS receivers embedded in them.
How does the VOXL® m500 use GPS?
Any drone using an autopilot for autonomous navigation will use its suite of sensors to determine its position and orientation in its local environment. The VOXL® m500 with VOXL® Flight onboard uses the GPS location to determine its latitude and longitude, along with a barometer (for altitude) and compass (for magnetic heading) to get an accurate orientation and position of where it is located in space. It also uses this data to determine the location of other objects such as mission waypoints, geofences and the home position.
Why is GPS Signal Important to Autonomous Vehicles?
The strength of your GPS signal determines the accuracy of your navigation. Degradation or loss of the GPS signal will decrease the accuracy of your location data and possibly force the vehicle to terminate its navigation until a human intervenes (or it crashes). Additionally, you cannot fly in any GPS-based flight mode or use the autonomous failsafe functions until the GPS acquires 4 or more satellites, also known as a GPS fix or GPS lock. Typically, your ground station software will alert you if you are losing GPS accuracy or you don't have enough of a GPS lock for GPS guided flight. A telltale sign that you may have a degraded GPS signal is if your aircraft drifts horizontally greater than 4 meters (US GPS Standard accuracy) while in a GPS guided hover.
If you are using QGroundControl (QGC) as your ground station software, you can check your GPS accuracy by clicking on the GPS status icon at the top of the Fly View screen in QGC. A GPS receiver with full signal will typically have 12 or more satellites, less than 1 meter horizontal position accuracy and less than 2 meters vertical accuracy. The most accurate GPS location data for the m500 will have a 3D DGPS (Differential GPS) lock in the GPS status menu. If you want to get down into centimeter grade accuracy, you can upgrade to a Real Time Kinematic (RTK) GPS which is also supported by PX4.
NOTE: While you can increase and decrease the threshold for minimum GPS horizontal and vertical accuracy, we recommend keeping the PX4 default parameters to maintain the best combination of safety and reliability.
Factors that affect GPS signal strength
Powering on a GPS receiver in a new area: It may take several minutes for a GPS receiver to acquire satellites in a new area. Movement helps a GPS receiver get a better view of the sky, so you may try walking with the aircraft to help it get a GPS fix more quickly. Trees and buildings can also block satellite signals which will slow down the initial acquisition.
GPS receiver positioning: When mounting a GPS receiver, the best positioning is facing upwards towards the sky with the most unobstructed view possible. Mounting it in other orientations or with other objects obstructing its view of the sky can cause the signal strength to degrade.
Signal noise: ESCs, cameras, and other electronic components can create electromagnetic interference (EMI) which will jam the GPS receiver. If you have trouble getting a strong GPS signal, make sure to shield onboard components and wires. Keep the GPS receiver far away from noise sources (the m500 comes with a GPS mast keeping the receiver away from frame). You can also use the GPS Noise and Jamming plots in PX4 logs to determine if you have issues with EMI.
Navigating indoors or near buildings: GPS works best with an unobstructed view of the sky. Navigating indoors or in close proximity of buildings can be very challenging for autonomous vehicles because the buildings block and reflect GPS satellites from communicating with the receiver. Even slightly degraded satellite strength can cause position accuracy to decrease rapidly. A significantly unstable or total loss of GPS signal will force the vehicle into altitude mode (no horizontal position hold) or even terminate the navigation. Without knowing exactly where it is in space and which way to move, the autonomous vehicle becomes unpredictable and unsafe to operate. Therefore, it is not recommended to fly indoors with GPS.
GPS Denied Navigation is Possible with VIO
In situations with a degraded GPS signal, you will have trouble navigating waypoints, hovering in place and returning to the home point in an emergency situation. Without a GPS signal, an autonomous vehicle cannot orient itself in space and, therefore, cannot operate on its own. This is true in most cases, but if your drone is equipped with Visual Inertial Odometry (VIO), it can operate autonomously without GPS.
While almost every other autonomous vehicle on the market cannot operate without GPS, our VIO drone, the VOXL® m500, allows you to use the integrated vision system to autonomously navigate in GPS-denied environments. All VOXL® Flight systems come standard with the ability to navigate using Visual Inertial Odometry (VIO). VIO uses visual and inertial data to measure distance travelled and position in a localized environment without the need to have a GPS signal. What makes VIO unique is that it can measure the position of the aircraft in 3D space and acts as the foundation of many other computer vision functions such as obstacle avoidance, position control and autonomous navigation in a GPS denied environment. For more information on how to configure and test this VIO on VOXL® products, follow our publicly available documentation here.
As a drone operator, it’s important to understand the essential components of whatever autonomous vehicles you are using. 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 radios and datalinks.
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