Phantom 4 RTK changes everything! The 5 reasons why

Phantom 4 RTK in use

The Phantom 4 RTK (Real-Time Kinematic) is an enterprise version of the popular prosumer DJI Phantom 4 Pro. With a price tag of $6,500, the Phantom 4 RTK may seem expensive to the uninitiated. In reality, this system comes at a price that is about a third of the cost of existing precision drone mapping and surveying solutions. It also looks like the Phantom 4 RTK will outperform existing solutions in many ways. As a result, DJI is poised not only to dominate the drone mapping market. More importantly, the Phantom 4 RTK will accelerate the adoption of drone mapping and the growth of a disruptive technology. The Phantom 4 RTK changes everything and here are the 5 reasons why!

Accuracy for one low price

At $6,500, the RTK is an extra $5000 over the price of the Phantom 4 Pro ($1,499). This may seem expensive, but DJI just undercut the competition by a factor of 2 or 3. Existing precision mapping systems typically involve a drone that costs over $10,000 and a base station that also costs more than $10,000. Do the math, the Phantom 4 RTK is actually a great deal.

The Phantom 4 RTK package includes the D-RTK 2 ground station. Typically, an RTK system, whether on a drone or not, consists of a fixed ground station and a rover. The rover is the part of the system that is moved around by the operator. In this case, the rover is the Phantom. All of this comes in a single integrated system, which is pretty amazing given the alternatives available. The Phantom 4 RTK already seems like the clear winner, but there is even more to this story.

The Phantom 4 RTK with the D-RTK 2 Base station (on the right)

What is RTK?

Before we dive too deep, let’s get a couple of definitions out of the way. RTK stands for “Real Time Kinematic,” which doesn’t really tell you much. It is basically a hack of the standard GPS system, where extra precision is pulled out of the GPS signal by analyzing not just the data in the signal, but also the signal carrier wave itself.

Accuracy is dramatically improved by looking at two frequencies, allowing the system to eliminate what is known as ”integer ambiguity”. This is some pretty nerdy stuff, but it all contributes to the magic of an RTK rover knowing its location to ridiculous centimeter-level accuracy.

What is PPK?

PPK (Post Process Kinematic) is a very similar technology to RTK. PPK processes the precise location of the rover, the Phantom 4 RTK in this case, based on stored data. In contrast, RTK does the math in real time. PPK can be more robust in noisy RF (radio frequency) environments like cities. The good news is that the Phantom 4 RTK is capable of performing both RTK and PPK missions. And don’t worry if the drone loses signal during your RTK flight, DJI has an answer for that.

If the RTK signal is weak and differential data cannot be transmitted during Photogrammetry or Waypoint Flight operation, users can read the raw satellite observations recorded in the microSD card in the aircraft after the flight, and then us PPD technology to achieve centimeter-level positioning. – Phantom 4 RTK User Manual

The Phantom 4 RTK with an all-new remote

Two ways to connect the Phantom 4 RTK

There is more than one way to get centimeter-level accuracy from the Phantom 4 RTK. Construction company Strabag indicates that the most accurate solution is to use the D-RTK 2 station, particularly when a wireless network connection is spotty or not available. DJI provided a second solution for providing accurate positioning, a Network RTK solution that integrates with the remote control.

The Phantom 4 RTK has a built-in DJI Onboard D-RTK, which provides more accurate data for centimeter-level positioning to improve operation precision when using with the DJI D-RTK 2 High-Precision GNSS Mobile Station or Network RTK service…The Network RTK service uses the remote controller instead of the base station to connect to an approved Network RTK server for differential data. – Phantom 4 RTK User Manual

Phantom 4 RTK’s new remote

The remote for the Phantom 4 RTK has a host of upgrades. Unfortunately, the drone will not be compatible with the Phantom 4 Pro remote, but industrial pilots will be happy to switch because of the new features. Here are all of the improvements to the Phantom 4 RTK remote:

  • A micro-SD card slot
  • A built-in speaker
  • Hot-swappable batteries
  • Network RTK with a 4G Dongle
  • GS RTK App is pre-installed in each remote controller

The Phantom 4 RTK remote accepts a network dongle

Faster workflow with the Phantom RTK

Time is money, and creating maps just got cheaper. The benefits of a drone with RTK capability are huge. Today, pilots need several known locations on the ground, typically 5 to 10, in order to achieve accurate maps. These ground control points (GPCs) are expensive to set up, as it needs to be done manually. Setup of GPCs is typically done with an RTK rover, the same thing that DJI just put on the Phantom 4 RTK!

Here is a little more information on setting up GPCs from Paul Aitken, Chief Pilot at DroneU and Instructor of Photogrammetry. DroneU is a leader in drone education and training.

“In common drone mapping [before the Phantom 4 RTK], typically GCP’s are 2 parts. The first part is a physical marker on the ground, the second part is a precise GPS location to “shoot” the point on the ground.

The physical marker on the ground is just as imperative as the GPS unit used to mark said point. The GCP marker should have a highly discernable center point with highly contrasting colors. How many GCP’s should we use? Typically the answer is 5 GCP’s and 2 Checkpoints for a total of 7 markers.”

Paul goes on to confirm just how powerful the Phantom 4 RTK can be.

“This is fundamentally why the P4P RTK can provide so much value. If the Phantom 4 RTK can acquire centimeter accurate geotags for the images, you wouldn’t need GCP’s or to mark them in the software. This alleviates a massive amount of time in the field and time spent at the computer post-processing data.”

Paul was having such a hard time finding a good GPC to put on the ground that the DroneU created their own. You can buy a 5 pack here. Paul stresses the need to use checkpoints in any drone mapping mission where accuracy is important. A checkpoint is basically an extra GPC, but it is not used to build the map. Instead, checkpoints are used to verify that the map is accurate. Whether you are mapping with an RTK-enabled drone or mapping with traditional GCPs, checkpoints should be used.

Some practical testing from European construction company Strabag also indicates success in dramatically reducing the number of GPCs used. Before the Phantom 4 RTK, they reported needing 20 to 40 GPCs per square kilometer. Now they are using only 3-5 GPCs to achieve their desired accuracy, for a 7X to 8X reduction in the number of GPCs.

The DroneU landing pad (right, orange and blue), which doubles as a ground control point. The GPC is being marked in the image on the left. Credit: DroneU

Is the Phantom 4 RTK accurate?

Thanks to some excellent work by Pix4D, we already have a pretty good answer to this question. Pix4D is currently the leading drone mapping software company and a rising star for its parent company, Parrot. Pix4D ran 2 scenarios, one in a city environment, and one in open fields, to test the accuracy of RTK, PPK, GPCs, and flying with regular vanilla GPS.

Accuracy of Field and Urban Mapping Missions with Various Methods
Values are RMS error in centimeters
no GPCs)
poorly distributed
well distributed
Field 625 65 6.6 2.7 NOT TESTED
Urban NOT TESTED NOT TESTED 4.8 8.1 6.7

Table compiled by the author. Data credit: Pix4D

The results speak for themselves. With GPS alone, the accuracy was only good to 358 centimeters, or about 12 feet. That’s not very good. According to Paul Aitken, this accuracy is useless for most mapping applications.

Accuracy using the traditional method of GPCs was highly related to the skill of the person placing the ground points. In one example, GPCs produced an RMS error of 65 cm (2 feet).

The accuracy of maps captured using RTK, PPK, and well-placed GPCs was always better than 10 cm (4 inches). The best result was when using RTK over a field, where an accuracy of 2.7 cm RMS was achieved. Overall, the results are pretty overwhelming. RTK looks to be a major time saver for drone mapping professionals once it gains more widespread use.

There is some additional practical information in accuracy provided by the construction company Strabag. In DJI’s Promotional video, Strabag reports achieving 3-centimeter horizontal accuracy without the use on any ground control points. With the increased affordability of RTK, the laborious task of setting up several GPCs may be on its way out.

Integration equals accuracy

[pullquote]…the Phantom 4 Pro is already the go-to drone for mapping, it is inexpensive to buy and also inexpensive to keep it in the air.[/pullquote]Existing drones with RTK rovers on board are not only expensive, but they are built by a company that is assembling parts from several suppliers. Many of those parts are likely from DJI, while the camera may come from another source. With the Phantom 4 RTK fully assembled by DJI, there are likely some big benefits in terms of integration and accuracy.

One of these benefits is what DJI is calling TimeSync. Here is how they describe it.

TimeSync ensures each photo uses the most accurate metadata and fixes the positioning data to the center of the CMOS – optimizing the results from photogrammetric methods and letting the image achieve centimeter-level positioning data. – DJI

The key term in the quote above is “image.” Surveyors and mappers want image accuracy, not just drone position accuracy. Most RTK drone systems are happy to brag about the accuracy of the drone’s position. But how accurate is the map?

Integrated sensors provide the most accurate data

To achieve more accurate results, it looks like DJI is going to record not only the position of the rover but also data from the drone’s other sensors, most importantly the IMU (inertial measurement unit).

The exact location of the camera can be calculated by the Phantom 4 RTK based on the gimbal angles and the tilt angle of the drone, as measured by the IMU. The drone’s angle will vary based on flight speed and wind conditions. Can you trust other drone RTK rover solutions to go to this level of detail in achieving accuracy? I’m not so sure.

DJI states that positioning data will be mapped to the “center of the CMOS.” I suspect data is actually mapped to the center of the entrance pupil of the camera. The entrance pupil is where the information is actually gathered, the data is only captured and digitized on the CMOS sensor. DroneDJ is waiting for feedback from DJI on this topic.

The Phantom 4 Pro camera is almost perfect

Before discussing the improvements of the Phantom 4 RTK camera, it is worth looking at why the Phantom 4 Pro camera is already the most practical mapping camera in the business. When it comes to mapping it is all about resolution. The Phantom 4 Pro’s 20 MP sensor is hard to beat.

The Phantom 4 Pro camera (left) has a mechanical global shutter while the Mavic 2 Pro (right) does not.

Living up to the sensor specification is another matter, and many cameras don’t have lenses that can take advantage of all of those pixels. The Phantom 4 Pro and Phantom 4 RTK do have lenses that step up to the challenge. The Phantom 4 Pro camera is just as good as the newer Mavic 2 Pro ($1,499) for mapping, but with one major improvement – a mechanical global shutter.

A global shutter isn’t important for most photography, but it is a major improvement for mapping. Mapping makes use of photographs that are taken while the drone is moving along at a pretty high speed. You could slow down your Mavic 2 Pro to decrease the effects of rolling shutter that it experiences, but that will cost you in batteries, time in the field, and time cleaning up the data in post-processing.

Phantom 4 Pro camera is as sharp as can be

I have measured the camera on the Phantom 4 Pro and found it to be crisp when wide open at f/2.8. Thousands of happy pilots agree. With a pixel size of 2.4 microns, the 20-megapixel sensor is well matched with the lens, which produces a spot size of roughly 3.4 pixels. That means that the light from a given spot in space is spread over just 1.4 pixels. Why can’t lenses focus the light better than this? It is a fundamental problem of physics and a phenomenon called diffraction. Even the best lenses can beat physics.

Phantom 4 Pro Mavic 2 Zoom
f/number f/2.8 (open) f/2.8 (wide) f/3.8 (tele)
Pixel size (microns) 2.41 1.54 1.54
Spot size (microns) 3.42 3.42 4.64
Spot size (pixels) 1.4 2.2 3.0

Contrast the Phantom 4 Pro spot size with the Mavic 2 Zoom at f/3.8 (telephoto mode), where the spot size is 3 times the size of a pixel. As a result, the Mavic 2 Zoom images are a little soft when optically zoomed in at 2X magnification. Soft images are not only less attractive, but they are also less accurate. The Phantom 4 Pro really takes advantage of all of its pixels and puts them to use.

DroneDeploy tests Phantom cameras

A whitepaper by DroneDeploy details the increased mapping accuracy achieved with the Phantom 4 Pro, which has the same camera as the Phantom 4 RTK. DroneDeploy is self-proclaimed to be “… the leading cloud software platform for commercial drones.”

In their testing, DroneDeploy demonstrated an accuracy improvement of 20% to 50% when using the Phantom 4 Pro as compared to the older Phantom 3 Pro. The Phantom 3 Pro uses a smaller 12 MP 1/2.3” sensor.

The resolution of the 20 MP Phantom 4 Pro is better at any altitude compared to the Phantom 3’s smaller 12 MP camera. Credit: Drone Deploy

The Phantom 4 RTK has a “custom” camera

The Phantom 4 RTK takes it to another level again by adding in custom distortion constants for each camera. The RTK uses the same camera like the Phantom 4 Pro, but with an ability to correct distortion to a level never before seen on a flying platform.

The distortion parameters gathered are saved into each image’s metadata, letting post-processing software adjust uniquely for every user. – DJI

I have seen uncorrected distortion in DJI JPEGs before. This is because DJI historically applies a distortion correction that it expects to perform pretty well across all of their drones of a given model. Unfortunately, each camera is a little different, so distortion correction has never been perfect.

The DroneDJ test target as seen from the Mavic 2 Pro. The left side is from the RAW image and is not distortion corrected. The default distortion correction in the JPEG on the right is very good for my Mavic 2 Pro, but it still isn’t perfect.

With individual lens calibration constants on the Phantom 4 RTK, now maps can be even more accurate with pixels that actually line up with where the software assumes they should be.

Hidden costs of mapping

[pullquote]The most exciting aspect of the Phantom 4 RTK is that it will enable a much faster adoption of mapping and a rapid growth in commercial drone opportunities.[/pullquote]Many professionals feel that the Phantom 4 Pro is not a fancy enough drone for mapping. Surely you need a bigger drone with a bigger camera, right? There is a reason why the Phantom 4 Pro is already the go-to drone for mapping, it is inexpensive to buy and also inexpensive to keep it in the air.

Consider the fact that the Phantom 4 has a 30 minute flight time and that batteries cost only $109. In contrast, a pair of Inspire 2 batteries will cost you 3 times as much and the flight time of an Inspire 2 is less than that of a Phantom 4 RTK. If you have a large area to map then you could easily spend over a thousand dollars on batteries alone.

With that math, it’s no wonder that the Phantom 4 Pro is currently the go-to drone for many commercial applications. The Phantom 4 RTK is completely compatible with Phantom 4 Pro and Advanced batteries, so you won’t need to stock up if you already own the Pro.

Phantom 5 RTK?

Should enterprise customers be concerned with their Phantom 4 RTK being obsoleted in 3 months by a Phantom 5 RTK? I don’t think so. The Phantom 5 will almost certainly have interchangeable lenses, which may eliminate the possibility of having custom distortion constants for each image.

Furthermore, the Phantom 5 will likely still use a 1-inch 20 MP sensor, so there is no real reason to upgrade. The Phantom 5 will be all about the lenses and Hasselblad color. The lens on the Phantom 4 Pro is already great for mapping, and the exact color profile you use is not going to have an important impact on map quality.

Phantom 4 RTK changes everything

While the Phantom 4 RTK does not get the same attention as some other products from DJI, it is probably the single most important drone of 2018, 2019, and probably even 2020. Just as DJI has creamed its competition in the consumer market, they are now poised to do the same in the commercial market. But that’s not the exciting part.

The most exciting aspect of the Phantom 4 RTK is that it will enable a much faster adoption of mapping and a rapid growth in commercial drone opportunities. The barrier to entry is dramatically reduced, and I am excited to see what happens next.

Here are the top 5 reasons why the Phantom 4 RTK changes everything

  • At one third the price of existing solutions, RTK drones are finally affordable
  • The Phantom 4 RTK is an integrated solution from the most trusted name in drones. Training, setup time, and accuracy are all likely to be dramatically improved compared to existing solutions.
  • Calibrated image distortion leads to even better accuracy
  • Reduced GPCs mean more jobs in less time, saving customers money
  • Batteries and other costs to operate a Phantom are low, that is one reason why the Phantom 4 Pro already has market dominance in mapping.


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