Let us acquire. Let us record. Let Us Drone. Have you ever wondered just how many satellites you should have when flying your DJI drone? Maybe you have wondered what satellites even are and why they are necessary when we fly our drones. What's the difference between GPS and GLONASS, and why do our DJI drones use both systems? There seems to be a lot of confusion surrounding this topic, so I've spent many hours gathering data to provide you with a thorough (but understandable) run-down.
When it comes to recording your home point before takeoff, here are the minimum number of satellites needed for each DJI drone:
Mavic Series: 10-11
Phantom 4 Series: 6-7
Inspire Series: 6-7
There are many other factors that affect our satellite signal, but “what is the minimum amount of satellites I need to record my home point” is a very common question that I wanted to address. Now, let's dive into this beautifully complex system our DJI drones use to keep them safely returning home to us time and time again.
Why do our drones need GPS
One of the many incredible features our DJI drones come equipped with is the ability to return to our location with a simple press of a button. How does the drone know where to go? GPS (and GLONASS), my friend. Barring any software or hardware issues while in flight, we will know where our drone is at all times if sufficient satellite signal is obtained.
There is a receiver in our drone that communicates with satellites in space which provide feedback on the drones location by way of triangulation.
Ever wonder how your drone is able to remain so stable while in flight and hover in place? Your DJI drone is using the GPS and GLONASS satellites to maintain its position. If you are flying below 30 feet and the Vision Positioning System (VPS) is activated, then the sensors on the drone are playing a part in keeping you stable; but anything above that 30-foot mark will leave the stabilization to the navigational satellites around us in space.
In addition to advancements with the onboard IMU, compass, and flight controllers, the GPS module in DJI drones has helped propel the company to the forefront of the consumer drone industry. Flying a drone used to take a whole lot of skill and concentration, and you could forget about trying to get stabilized aerial video. Today, drones basically fly themselves, which is why there has been such an interest in the once “hobbyist-only” flying machines, as their practical applications have expanded exponentially.
To sum it up, here are a few reasons why having GPS (and GLONASS) on our drones is so important:
- GPS allows the drone to autonomously return safely to your location with a simple press of a button.
- GPS helps keep the drone stable while in flight.
- GPS allows you to see where your drone is on a map.
- GPS can assist you in finding a crashed drone.
- GPS allows you plan autonomous flights with an intelligent flight mode like Waypoints.
What is the difference between GPS and GLONASS?
GPS (Global Positioning System) is a navigational system in which a receiver (inside our drones) uses satellites to obtain a fix on its location by means of triangulation. Generally speaking, the more satellites the receiver has a fix on, the better the “GPS signal” will be. The Global Positioning System is run and operated by the United States. There are no less than 24 operational satellites in space at any given time.
GLONASS (Global Navigation Satellite System) is another navigational system in which a receiver obtains a fix from satellites that are orbiting earth. GLONASS is run and operated by Russia. As with GPS, there are currently 24 active GLONASS satellites.
So why use both GPS and GLONASS?
There are a couple of reasons why using both GPS and GLONASS benefits us DJI drone users.
- GPS satellites run on a different frequency than those of GLONASS. If the frequency of one type of satellite is interrupted in our current location we have the other satellites running on a different frequency to give us a better chance of maintaining a strong signal.
- They are spread out. GPS satellites are at a different altitude and are on different orbiting planes than the satellites of GLONASS. What does this mean for us? Having access to both GPS and GLONASS satellites gives us the best possible signal due to the multidirectional satellite fixes we can obtain.
- The more the better. As previously mentioned, there are no fewer than 24 active GPS satellites in space at any given time and a total of 24 active GLONASS satellites. Having access to both satellite systems simply gives us the opportunity to receive the best possible signal.
You may see the acronym GNSS, which stands for Global Network Satellite System. This term is used to describe the combination of multiple satellite systems; which applies to us since our DJI drones use both GPS and GLONASS.
To learn more about GPS, click here.
To learn more about GLONASS, click here.
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Does DOP matter with drones?
Dilution of Precision (DOP) is a navigational term that speaks to navigation satellite position in reference to a receiver (using geometry), and how it affects one signal. There are a few different measurements that are used when referencing DOP.
- HDOP – horizontal dilution of precision
- VDOP – vertical dilution of precision
- PDOP – position (3D) dilution of precision
- TDOP – time dilution of precision
- GDOP – geometric dilution of precision
Without getting too in-depth on something that may bore many of you, here's a quick summary on how this applies to us drone users.
If the satellites our drones acquire are close together in the sky, the Dilution of Precision value is “high”, which means that our signal is weaker. Alternatively, if the acquired satellites are spread out in the sky, our DOP value is “low”, and our signal will be stronger. In laymen's terms, the geometry is a lot better when the satellites are spread out, meaning that we will receive a better and more reliable signal.
What does this mean for us? It means that we can't just look at the acquired satellites on our screens and think that just because the number is high we will have a strong signal.
Yes, generally speaking, the more satellites you have the better your signal will be; but you may have just acquired 12 satellites relatively close together and only 2 that are spread apart in the sky, making the DOP value high = weaker signal.
We can't see the DOP values while we are looking at our screens, but just keep in mind that there is more that goes into what makes a strong satellite signal than just the number of satellites we obtain a fix on.
What we can see is the GPS and GLONASS signal health next to the number of satellites we have. If the satellite number is 14 but we only have 3 out of 5 bars on the health bar, that may be an indication that there is a high Dilution of Precision. It could also indicate that although you have a lock on 14 satellites, there is some type of satellite interference.
How many satellites are needed for an updated home point?
We have all impatiently waited for the home point to be updated before we launched our drones. As I mentioned in the opening paragraph, the question of “how many satellites are needed before the home point is updated” is a common question.
I took a poll that quite a few people participated in. Here is the minimum number of satellites required for the home point to be updated according to those people. There were a few people that reported being able to update the home point with either more or fewer satellites than the numbers I list, but I took the numbers that the majority of the participants selected.
Mavic Series: 10-11
Phantom 4 Series: 6-7
Inspire Series: 6-7
Whatever the number of satellites it may take for you at any given location, it is crucial that you wait for the home point to be recorded and you see “Ready to Go (GPS)” in the green status bar at the top of the screen.
But I had 18 satellites the last time I flew here
A common statement that I see from frustrated drone pilots reads something like “I had 18 satellites the last time I flew at *enter location here*, why do I only have 10 satellites now?”. There are two plausible answers for this.
- GPS and GLONASS satellites are constantly in orbit, so the number of satellites we can acquire at a location will vary at any given time of the day. Here is a helpful website that will give you an idea of how many satellites you may expect to acquire at a specific time/location.
- There may be interference in the form of space weather, radio band emissions, or jamming.
What happens if you lose GPS signal while flying?
If you are flying along and you get an error message stating something like “satellite positioning off. Fly with caution”, or “satellite signal weak”, your first instinct may be to panic. Sure, it's scary to think about a drone flyaway, but try to keep your cool and you'll most likely get that drone back to you in one piece.
As discussed above, there are a number of factors that determine whether a satellite signal will be strong or not. If you happen to lose signal, here are my recommended steps to ensuring the drone returns safely.
- Get eyes on the drone. You should be within visual line of sight (VLOS) with the drone, so make sure there are no immediate nearby threats to the drone (buildings, people, etc.).
- If the drone is above 30 feet in altitude and you lose satellite signal, the drone will enter ATTI mode. If the drone is close enough to you that you can see what's between it and the ground, descend to 30 feet or below so the Vision Positioning System (VPS) can keep the drone steady. Remain below 30 feet as you fly back to your location of the drone does not pick up the satellite signal again.
- If the drone is far enough away from you that you can't see what's between it and the ground, I'd suggest that you fly towards your location while in ATTI mode. When you get close enough to see that there is nothing it may collide with if you descend, lower the drone to 30 feet and resume your flight back to your location.
Mobile device GPS signal weak
If you have ever flown your drone from a boat, then you know how important it is to update your home point periodically, especially if the boat is moving. The GPS from the device you're using to fly with will be used in order to update the home point to your current location.
This is important to remember as some devices are not equipped with GPS, which means that you will not be able to update the home point to your location once the drone is airborne.
There have been reports of people receiving a “mobile device GPS signal weak” error message which prohibits them from updating their home point. Despite various methods of trial and error, some people are unable to ever successfully update their home point because their device doesn't seem to acquire the GPS even though other applications (Google Maps, etc.) are working fine on their device. This seems to be an error with the DJI GO 4 app. If anyone is having this issue, please leave a comment with your experience at the bottom of this post.
- DJI drones use GPS to allow the drone to autonomously return home, keep the drone stable while in flight, and show where the drone is in real time on a map.
- DJI drones have access to both the GPS (Global Positioning System) and GLONASS (Global Navigation Satellite System).
- Generally, the more satellites that are shown on your screen the better your signal will be.
- Dilution of Precision (DOP) refers to the relative location of satellites in space to your drone. Many satellites close together are referred to as a high DOP, meaning a weaker signal. Adversely, satellites that are more spread out have a low DOP, giving you a stronger signal. This means that a high number of satellites being displayed on your screen doesn't necessarily translate to a great satellite signal.
- Satellites in space are constantly in orbit so your satellite number at a location may change on any given time/day.
- If you lose GPS and GLONASS signal while flying the drone will enter ATTI mode (if above 30 feet). Descend to below 30 feet when safe to allow the VPS to keep the drone stable, and fly back with caution.
- Remember that it's important to update your home point frequently if you will be changing locations while flying. In order to successfully do this, your mobile device will need a GPS signal.
GPS not only makes flying enjoyable, but it allows us to use our drones for many professional tasks. I know that I get so used to having GPS on my drone that I momentarily panic when the signal is interrupted and my drone enters ATTI mode. Although generally consistent, the occasional signal loss is why it's important to practice in ATTI mode so you'll be prepared when you're on a boat in 15 MPH winds and you get a GPS error message on your screen. No fun. Have you had trouble with the GPS on your DJI drone? Let us hear about it in the comment section below!