Ever since the advent of drone technology being released on the world, it has worked in conjunction with another major technological feat of man. That is the Global Positioning System or GPS for short. GPS is what took that easily crashable toy quadcopter and made it a viable piece of kit.
We wouldn’t be anywhere though without what makes GPS, well GPS and that’s satellites. If it wasn’t for that network of objects orbiting our globe, GPS wouldn’t be possible and as such neither would a stabilized hover be possible nor the Return to Home function.
With the average satellite being around 1200 miles from the earth’s surface, some are a bit closer and some further. There are nearly 5,000 currently in orbit, and there are multiple networks available today for positioning, such as GLONASS or Global Navigation Satellite System or BeiDou, the Chinese Positioning system.
Due to the distance and the location of the satellites’ orbit, more than one is needed to triangulate a position.
To simply find your position, you need a minimum of four satellites, but on average you need around 7-12 satellites for a solid stable connection to a drone. Depending on what drone you’re flying, you also need 7-12 for the Home Point to record accurately.
Say you’re hiking a trail and wanted to know your location. Just acquiring four satellites, in this case, would give you a good idea of your location.
Our cellphones are a great example of a device we carry every day that is using GPS to always know the phone’s location. Don’t fret, there’s plenty of tin foil to go around if we need to make hats.
Drones, although similar, move at higher rates of speed and can change their altitudes, making it a bit harder to know a position from just 4 satellites. And then to have the highly accurate positioning to maintain a hover, well 7 to 12 seems about right.
How many satellites to fly?
This raises the question, out of those nearly 5,000 satellites, just how many do I need to fly my drone and why so many?
One of the best and most beneficial features built into all DJI drones is the Return To Home function. This function will return the craft to its landing zone if the connection is lost with the controller or some other reason that you have set the drone for.
GPS also provides the real-time location of the craft and the controller. Without these sorts of flying aids, flying a drone would be very different. Kind of like lighting a light bulb with a Bagdad Battery.
So, recording your Homepoint prior to lift-off is important. In order for the Home Point to be recorded, you will need a minimum number of satellites for each of these DJI drones:
|DJI Drone Model||Minimum Satellites Required for Lift-off|
|Phantom 4 Series||6-7|
Important reasons for having GPS
Today’s DJI drones are known for their quality and their flight abilities. From a plethora of Intelligent Flight Modes to just holding a rock steady position, it’s GPS that makes it all possible.
It’s what keeps that Mavic 2 Pro in that stable position so you can get the perfect shot. Without the GPS unit, you would not know the drone’s location on a map, relative to you own position.
With out GPS, there would be no such thing as Orthomosiac Mapping and that wonderful Phantom 4 Pro will have a lot less work to perform.
Drones would not have formed into a new industry as it has, without GPS. Oh no, we’d still be getting sticky fingers from building balsam wood models and watching them crash repeatedly while attempting to fly them.
Can a drone be flown without GPS? Sure. Where’s the fun in fighting the flight though? If I had to choose, I’d keep the GPS.
Background of GPS
Much like our industry, GPS has had worldwide effects on nearly every aspect of our lives. After all, did you see the latest? There’s a company now offering drone advertising, like flying billboard sort of stuff. The drone is programmed to maintain a flight and position in a DroneZone area at events and such, keeping event participants and the drones comfortably from each other.
This just shows that there are so many new and creative uses for drones we haven’t even thought of yet. Without GPS, this type of flight could never have happened. Of course, the other side of the prop is, who needs commercials flying around? Am I right?!
GPS had its inception when the Department of Defense was looking for a robust, reliable satellite navigation system in the 1970’s. They first brought an early version online in 1978. That system was known as NAVSTAR or Navigational System with Timing and Ranging, but it was not available for civil use.
The system we know today as GPS is available and used by many entities. Still with a heavy background around military systems, it is also used for civil, commercial, and scientific needs. Today’s GPS system uses 24 satellites, and came online in 1993.
The current GPS system provides two levels of service. The one we use and are most familiar with is the Standard Positioning Service or SPS for short. This SPS system is available on a continuous, worldwide basis and is free to the end-users.
The other side of the current GPS system, known as Precise Positioning System or PPS, is restricted to governmental use. GPS is owned by the United States and operated by the US Air Force.
Similar to the causes for the United States to create such a system, other countries have done the same. Russia has their own system that is called GLONASS or Global Navigation Satellite System.
Much like the American GPS system, the GLONASS follows a similar timeline of creation and reasoning, with its creation as a military system that eventually moved to being available to the general public.
Also, like the GPS system, the GLONASS system is available to everyone globally at no cost to the end-user. GLONASS, like GPS, uses a system of 24 satellites.
BeiDou Navigation Satellite System
A third system operated by the Chinese Government is the BeiDou Navigation Satellite System, BDS for the short version. This system became globally operational in 2018 and has a larger array with 35 total satellites in its configuration over the 24 found in the other systems.
With the additional satellites, it is expected that the BDS system will be capable of millimeter level accuracy.
You can find other Global Positioning systems out there such as the Galileo Positioning System. However, DJI though uses these three for their positioning systems, with the BeiDou system being found on the new Mavic 3 for the first time.
So how does it all work?
Within your DJI Drone as well as the controller is a GPS unit, a transmitter. This unit is constantly sending out a signal. The satellites within range of that signal will respond. Here’s where it can get really complicated really quick.
You may receive a signal from a satellite that is just going out of range, or over the horizon. After all, the typical satellite is trucking along at 17,000 mph (27,400 kph) at an altitude of 150 miles (242 km).
In order for a position to be known, a minimum of 4 satellite signals need to be acquired, due to the positioning of those satellites and their movement, as well as the craft’s. But you may not be getting a good accurate positioning from just the 4 or even 6.
Using more satellites to increase that accuracy and confirm data sets provides that rock steady hover we take for granted.
DJI likes redundancy and has built-in many safety functions. In this case they have programmed the drone to not only acquire the base reading for a position, but a backup reading that verifies the first, and in some cases even a third.
It is for this reason that most DJI drones will require a minimum number of Satellites in the range of 6 to 12. With this number of satellites providing location data, the aircraft is able to have pinpoint location data.
Issues with GPS
Dilution of Precision (accuracy of position)
When utilizing GPS positioning, there are a few known issues. One of these is the Dilution of Precision, DOP for short. DOP is the measure of the accuracy of the position of the four satellites relative to the receiver.
There’s a value scale for DOP and your DJI drone will have programmed algorithms that will calculate this for you. It is due to this issue that DJI has taken the precaution to require more than just the four minimal satellites needed, as the more connections you have the more accurate your positioning data can be.
The chart below covers the DOP values ranging from 1 to 20.
|1||Ideal||This is the highest possible confidence level to be used for applications demanding the highest possible precision at all times.|
|1-2||Excellent||At this confidence level, positional measurements are considered accurate enough to meet all but the most sensitive applications.|
|2-5||Good||Represents a level that marks the minimum appropriate for making business decisions. Positional measurements could be used to make reliable in-route navigation suggestions to the user.|
|5-10||Moderate||Positional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended.|
|10-20||Fair||Represents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location.|
|>20||Poor||At this level, measurements are inaccurate by as much as 300 meters with a 6 meter accurate device (50 DOP × 6 meters) and should be discarded.|
DOP can be measured in a few ways. There is Horizontal, vertical, Position 3D, Time Dilution, and Geometric. When it comes to measuring DOP, it’s all about the geometry. For us drone users, luckily the craft is doing all that number crunching. Could you imagine?
What does all of that mean then? It means that even though we may have good high numbers in the satellite count, we may not have all that good of signal strength dependent on how those satellites are clustered.
This is because satellites that are further apart will provide better positioning data then satellites that are in close proximity to one another. Triangulation, remember?
Kp-index (Effects of solar activity)
Another known issue that can arise with GPS positioning is what is known as the K-index and its extension the Planetary K- index. This indexing system is used to gage the effects from solar activity, known to most as simply the Kp-index.
This index is the measure of magnetic interference in the Earth’s magnetic field. The Earth’s magnetic field can introduce errors into your drone’s GPS positioning and as such, events that effect that field will also affect that positioning.
Since you’re using satellites that are located in space, space-related events such as solar storms or other magnetic field effecting events need to be considered.
You may have noticed this index if you use weather apps such a UAV Forecast. As a general rule, it is okay to fly if the index reads under 5, so any Kp between 1 and 4 is a green light to flight. The index goes up to 9, with 5 and higher possibly effecting whether you are able to acquire satellites and if those satellites are able to provide good positioning.
Satellite positioning (relative to you)
As we have seen above, the satellite’s position is just as important to having a good signal as having a high number of satellites, as a high number of satellites will still provide a weak signal if they are clustered together.
Acquiring good satellite coverage can also be affected by the area’s terrain that you’re flying in. If you are in a deep valley with rock walls around you, satellites might have a hard time finding you.
Your location on this rock that is orbiting the fiery ball in the sky is also something that can affect your GPS. Since most satellites orbit near the equator, if you are in a high latitude location such as Alaska or northern Canada, you may see more days where the Kp-index is higher.
In areas such as this you can see a unique phenomenon also known as the Northern Lights or the Aurora Borealis. If you’re located in Australia, you could find a similar occurrence known as the Aurora Australis.
Both of these phenomena are seen when the earth is experiencing high levels of solar radiation or solar storms. During these times it’s very possible to experience strong electromagnetic interference. In turn this interference can have an effect on your GPS.
What happens if you lose GPS?
It’s always possible for your GPS signal to drop out on you while flying. This can be a momentary thing or it can be longer-lasting. This event can lead to you having to either fly in ATTI (Attitude) Mode or bringing the craft in and landing.
As a failsafe, all DJI drones are programmed to switch to ATTI mode in the event that the GPS signal is lost. The app view will display warnings of the nature, “Strong Magnetic Inference Calibrate Compass”, “GPS Loss, Switching to Atti Mode”.
In most cases, this is a momentary loss and before you can even react, the GPS has returned. In some cases, you may be near an area that is affecting your signal such as high tension power lines, substations, in a deep narrow valley, etc.
So, the source to the interference is often a ground-based source. As we covered, there are space-based sources as well for interference.
No matter the cause of the signal loss, just know that the craft will enter Atti mode. In this mode, the craft will maintain its set altitude and will fly normally. However, without the GPS, the craft will be subject to the winds that may be in the area and you will have to take into account that drifting while flying.
Flying drones just wouldn’t be the same without our orbiting friends up there making sure we know where we are. GPS is all around us today, but it wasn’t always the case. Prior to GPS, we would plot a course on a map and rely on compass bearings.
Of all the many changes in this modern world, GPS has had a profound effect on how we travel and get around, whether it’s by air, land or sea. For us drone pilots, we just couldn’t do the amazing things we do without it.
Fly Safe, Fly Always, Always Fly Safe!