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The warning “Strong Interference Detected” simply means strong radio interference being received at either the RC unit or the aircraft. It does not mean magnetic interference from metal structures or magnetic fields from power lines or large transformers, nor does it normally mean radio interference from cell towers. While you are at it, you can also rule out large metal bridges, iron ore deposits, big ships and starcruisers. There is a lot of wrong information being written in forums about this, often by people who do not have a clue. At the same time, some owners are now using their drones to inspect high voltage power lines and they do not suffer from interference. To be more specific, the warning means radio transmissions in the 2.4 or 5.8 Ghz radio band, and there are plenty of them. These bands are used all over the world for transmitting data of a non-essential nature, meaning they are not protected radio bands like, for example cell phone bands, military or broadcast bands. The 2.4/5.8 Ghz bands are unregulated, and can be used by almost any manufacturer to link together parts of a product for domestic use. Examples are cordless phones, garage door openers, baby monitors, security cameras, doorbells, remote weather monitors… the list is almost endless. The important thing about this band is that it is up to the makers of the equipment to make it resistant to interference by similar devices, and at the same time, take steps to try and minimise interference being caused to other devices. Therefore they have to transmit clean signals, with power outputs limited to levels that are determined by the local radio-frequency regulation body. In the US, that body is the FCC, and most countries have their own local regulators.
In earlier models of their drones, DJI made RC Units that had adjustable power outputs to set the transmit power output according to the rules in various countries. The mechanism for doing this was using the GPS receiver in the aircraft to work out what country it was located in, and setting the power output according to the rules for that country. Presumably because some owners sought to defeat this setting and wanting to raising their output level, it appears that DJI now make specific models of RC Units set for specific regions and countries, that cannot change their output by software.
Within each band, 2.4 Ghz or 5.8 Ghz, there are a number of available channels that can be used, the number of channels being set by each regulatory authority. It is not the same world-wide, because some countries allocate part of those bands to other purposes and some of those are protected. One example is the US where the FCC has allocated space in the 2.4 Ghz band to cell-phone service providers for the new 5G services.
But, to their credit, DJI have for some time now been using frequency agile or Frequency Hopping Spread Spectrum (FHSS) radio links in the RC Unit and the aircraft to attempt to make the link more reliable. This is a technique often only implemented by top end manufacturers. The link controller continually examines the reception quality of the chosen channel to see if it is interference free, if not, then the receiver negotiates with the transmitter to select another channel, and both of them simultaneously change to the new channel. If the new channel is, or becomes, polluted by other devices, the transmitter and receiver pair then negotiates yet another channel to switch to. This procedure may continue until either a free channel is located, or none of the available channels are free, at which time the receiving device sends the infamous warning “Strong Interference Detected”. At the same time, each transmission, which is purely digital in nature, also contains additional data bits for the purpose of “Forward Error Correction”. This simply means extra data bits in the control stream that can be used to recover bits lost by interference to the signal.
All in all, the transmission system used by DJI is pretty rugged and reliable. But it still has its limitations. In the forums you will often hear members offering advice about how to improve the reliability of the control process, advocating changing from 2.4 Ghz to 5.8 Ghz or, in models that support it, locking the channel to one channel only or even using patches to the code to increase the number of available channels. Only one of these pieces of advice is technically sound, and that is changing bands from 2.4 Ghz to 5.8 Ghz. In some regions this can work quite well, but beware the 5.8 Ghz signals do not work quite as well over long distances, 2.4 Ghz is better suited to longer range operation. Locking the frequency hopping system to one channel is not advisable, as you are defeating one of the most useful technologies used in the radio system. If some other user fires up on the band you have chosen, and you are within range, you may completely lose control of your aircraft. The frequency hopping system was put in the equipment for good reason, and now it cannot take action to restore the radio link. Despite the claims made by forum members, who in many cases do not know how the system works, the link is best left in automatic mode. Increasing the channels by using patches is equally as ill-advised. The number of available channels is set to those channels that are able to be used for your country. If you increase the number of channels, you may simply be providing yourself with a bunch of channels that are being used by high power systems and will not be of any use to you. Worse still, your regulatory authority may come after you and confiscate your equipment. When flying, it is important to understand what problems you can experience with the radio link and how to deal with them. Modern receivers are technically very advanced, and will not suffer reception problems from strong signals on adjacent channels. But when another device is transmitting on the same channel, it can be difficult to decode the signals correctly. In the past, with older receivers, strong adjacent channel signals could ‘swamp’ a receiver with radio frequency energy. These days the receivers are far more selective.
In Example 1 below, the drone is one mile away from the pilot and his RC unit, but there is a building nearby to the pilot that has a WiFi system, which is by the way, the most common source of interference to drones. At one mile distant, the signal from the RC Unit is very small, and probably equal to the signal from the house WiFi. And one thing to keep in mind is that while the antennas on the RC Unit can be pointed towards the aircraft, the antennas on the aircraft cannot be pointed towards the RC Unit. The aircraft antennas are omni-directional, meaning they pick up and transmit signal equally in all directions. If you think about it, the aircraft antennas cannot be directional or they would not work when the aircraft is rotated. This example is a pretty obvious one, and the pilot should recognise the situation, and know that it may cause him problems. But unfortunately, few people understand radio transmission well enough to identify the problem. A classic case is where the drone is being flown in a busy city or suburb, and the single building shown in the drawing can be multiplied by hundreds. These days you can expect almost every house in a suburb to have a WiFi system, so you are surrounded by hundreds of devices competing for the limited number of channels of radio space. And by the way, WiFi is simply radio signals on your drone’s band, and your competitor in using the radio spectrum.
In Example 2, if the drone is moving away from the pilot, it is moving toward another source of radio signals that may be very much stronger than the signal it is receiving from the RC Unit in the pilot’s hand. In the example, the drone is equal in distance from each source, but the pilot may very well not be aware of the house on the right side of the diagram because it is out of sight to him. So he thinks he is flying away into a clear area, when all the time, he is approaching an interference source that has a stronger signal than his RC Unit as far as the drone is concerned.
If we now add to the scenario that the WiFi at the house on the right is on the same channel the drone is, we now have a big problem. WiFi systems do not use FHSS or frequency hopping, so it cannot change to another channel, and by now the signal from the pilot’s RC Unit is so weak, the drone and the RC Unit cannot communicate in order to negotiate a switch to an unoccupied channel. The result is that the pilot now receives the “Strong Interference Detected” warning, and the drone goes into RTH mode.
The pilot is now pretty upset, because all he sees is that the aircraft barely got ¾ mile away and lost signal, and remembering the DJI sales pitch that claimed a range of 4 Kms, he feels he got a defective product. But if you followed the previous description, there was nothing wrong with the product, it was the circumstances in which it was flown.
Example 3 shows the pilot in a clear environment, no sources of interference anywhere near him, the aircraft is a mile out, but only ¾ mile from a school, or an industrial complex or a shopping mall. These three sites are notorious for creating extremely high levels of interference. Schools because with every school student carrying around a tablet or laptop, the schools have highly developed WiFi networks that are often using higher transmission powers in order to provide coverage all of the school grounds. Industrial of office complexes are similar, and the larger the area they occupy, the stronger the WiFi signals. Shopping malls are also very bad, because so many retailers offer free WiFi connections to their customers and the mall owners will have security devices all through the complex, some of which are wireless, and guess what frequency band they will be on.
So once again, the pilot is certain he is in a clear area, there are no sources of interference anywhere near him, nor near the drone, but once again the aircraft gives the ominous warning and goes into RTH. WTF? Why is this happening? The answer is the giant mess of radio pollution only ¾ mile away that the pilot cannot see.
So are the DJI claims regarding range pure BS? Yes and no. They are marketing claims, so the marketing people will go for the most optimistic value they can, and achieved under ideal circumstances, but they are not lies, because people have managed that sort of range in practice, me included. But only with:
· Care taken to point the RC Unit antennas with the flat side toward the drone. · Piloted from a good, slightly elevated position. · Out in the countryside, away from any building development. · With reasonable altitude on the drone, not flying in the grass. · No obstacles between the pilot and the drone.
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