Where am I?
Patrick Sherman
lucidity@roswellflighttestcrew.com
Every time you are out flying a drone—or even navigating to an unfamiliar address with your smartphone perched on your car’s dashboard—you are relying on a constellation of satellites orbiting 12,000 miles overhead. Today, we take GPS satellites so much for granted that, as a journalist, I don’t even need to explain that those three letters are an abbreviation for Global Positioning System.
While the basic concept is nearly as familiar to most Americans as a microwave oven, drone pilots need to have a better understanding of how
To gain a more nuanced understanding of how the GNSS system is performing in your local area when you intend to fly your drone, consider downloading one of the many apps that are available for Android and iOS devices that allow you to see detailed information.
this technology works, as well as its limitations. If your GPS goes awry while you’re driving to a new restaurant, you’ll be five minutes late for your dinner reservation. If it goes awry while you’re flying your drone, you risk an accident that could hurt someone or damage property.
GNSS Who?
If you’ve closely read the literature put out by drone manufacturers during the past couple of years, you have seen a new acronym associated with this type of technology: GNSS, which stands for Global Navigation Satellite System. This might seem redundant with the already familiar GPS, which we commonly think of as a generic term for satellite positioning.
GPS is actually the official name of a network of satellites owned and operated by the U.S. Air Force. The first of these was launched into Earth’s orbit in 1979, and there are currently 31 in active use, with several older satellites in “residual” status that could be reactivated should the need arise.
This system was an enormous improvement over previous, ground-based radio navigation, providing extremely precise information worldwide and giving the Air Force a huge advantage over its Cold War rivals. Of course, anyone could tap into those same GPS radio signals and achieve similar precision.
Therefore, GPS satellites incorporated a system called Selective Availability, which deliberately degraded the quality of the results for non-US military users. The Air Force could simply flip a switch and disable the network at will.
As a result, other nations felt the need to establish their own independent satellite networks to guarantee this capability for themselves. In 1982, the Soviet Union launched the first of its GLONASS satellites—an abbreviation for Globalnaya Navigatsionnaya Sputnikovaya Sistema. After the fall of the Soviet Union, Russia continued to build and upgrade the network, which currently has 24 satellites in orbit.
The Chinese launched the first of its BeiDou satellites in 2000 and the intent is to build a constellation of 35 satellites by 2020. In 2011, the European Union established the Galileo network, which currently has 13 operational satellites in orbit and two more set to come online soon. Japan and India are building their own regional systems as well, known as QZSS and NAVIC, respectively.
GNSS refers to all of these different systems collectively, and it turns out that these competing networks are an enormous boon for users worldwide. The more satellites that are in view of a GNSS receiver, the more accurate its position fix becomes. That’s why GNSS—not GPS alone—has become the standard for precision position-hold on the latest drones.
Radio Waves From Space
All GNSS satellites work using the same basic principle. Each satellite in a constellation continually transmits a radio signal that contains two pieces of information: the location of the satellite and the precise time that the signal was sent. Because radio waves travel at a set velocity—the speed of light—by knowing the satellite’s location, you can determine how far away the satellite is based on how long it takes the signal to reach you.
Do this simultaneously with a minimum of four satellites and you can determine your position on the surface of the Earth, including your elevation above mean sea level, with a fairly high degree of precision. Add one, two, or 10 more satellites into the equation, and you achieve an extremely accurate fix.
Of course, there are limitations, both in terms of the signals transmitted by the satellites and the ability of the receiver on the ground to detect and interpret those signals. The accuracy of the signals themselves is reported as User Range Error (URE). A typical URE measurement for the GPS constellation is approximately 2½ feet at 95% of the time.
URE is driven by several factors, including Geometric Dilution of Precision (GDOP), which is determined by the position of the satellites overhead when the measurement is taken. If, at a particular moment, all of the satellites are clustered in one corner of the sky, or your view of the sky is partially obscured by surrounding terrain or structures, the same number of satellites will yield a fix that is less accurate than it would be if those same satellites were distributed evenly overhead.
The impact of GDOP on precision is measured on a scale of zero to 20, with any value less than one representing ideal circumstances, and a value of 20 or greater indicating that the location fix could be wrong by more than 1,000 feet.
Imagine engaging the return-to-home function on your drone when, unknown to you, the GDOP in your vicinity is poor. Your drone could move three football fields farther away from you, possibly taking it beyond visual line of sight and any possibility of you regaining control.
In addition to the inherent limitations of the GNSS constellation itself,
The receivers built into drones and smartphones are not especially accurate, adding 12 feet of variability on top of the URE.
The Air Force discontinued Selective Availability in the year 2000 upon the order of President Bill Clinton, allowing non-military users to achieve much higher degrees of precision. However, the required hardware is too bulky and expensive to incorporate into most civilian applications.
Be Prepared
Beyond the inaccuracies that are an inherent part of GNSS, there is also the possibility that you will find yourself flying in an area that further degrades the ability of your drone to achieve a solid fix. For example, if you have ever watched your location jump around on your smartphone while driving among tall buildings, you have seen the effect of the satellite signals reflecting off of the surrounding structures, or being blocked outright.
Never forget that GNSS relies on radio transmissions to work, and that the signal is far weaker than your RC radio controller at any range you are likely to be flying. Although intentional jamming of GPS is illegal in the US, it is still possible that you could encounter a signal on the same frequency, or a harmonic frequency, because of the 1,575 MHz used by GNSS satellites. Solar flares also have the potential to cause disruptions.
As a drone pilot, there are a few steps you can take to mitigate the possibility of an accident caused by a failure of your GNSS system. First, download an app for your smartphone that will give you access to raw data such as the actual position of satellites overhead, the accuracy of a fix taken in your local area, and the current GDOP.
Second, learn to fly without the benefit of GNSS-enabled features, such as position hold, return-to-home, and headless mode. It’s a much more challenging way to fly, but to me, it is what distinguishes true pilots with real skill from people who simply push a drone around the sky, relying on the aircraft to fly itself.
All electronic and mechanical systems will fail at some point, including the GNSS receiver onboard your drone. When that happens, it will be your skills as a pilot that will allow you to bring the aircraft safely home.
