If you are looking for equipment to conduct aerial surveys, the Emlid Reach RS2+ receiver makes a great base station for both DJI and some Autel RTK drones. For a detailed workflow on integrating Reach RS2+ with a DJI RTK drone, you can refer to this tutorial. If you are working with Autel drones – then have a read of this tutorial.
However, what still seems to be unknown to many drone mapping operators is that Emlid receivers can also be efficiently employed on Post Processed Kinematics (PPK) workflows. In fact, PPK is Mangoesmapping’s preferred aerial surveying technique, given the characteristics of the typical sites we work on.
Choosing a PPK workflow
In order to capture centimetre accurate aerial images or LiDAR point clouds we generally use a PPK (Post-Processed Kinematic) workflow. Some advantages of PPK include:
- Flexibility and Remote Operations: PPK eliminates the need for real-time data links or communication with a base station during the data collection flights. This allows aerial surveying missions to be flown in remote or challenging areas without the need for a reliable internet connection.
- Robustness and Reliability: PPK is not susceptible to real-time signal disruptions or outages, making it more reliable in challenging environment conditions. This robustness ensures more consistent and accurate data collection throughout the entire surveying process.
- Simpler workflow. PPK does not require a constant data connection between the drone and the base station. This makes the workflow simpler and easier to manage. For example, when running an RTK drone in RTK mode, it will generally pause the flight whenever it looses the RTK fix solution and wait (hover) until it recovers to resume the operation. With PPK, this simply does not happen, and you can enjoy faster and uninterrupted flights throughout your entire survey operation.
A typical field workflow for PPK is as follows:
- Pre-survey site visit – If possible, getting on site before the main survey event helps to evaluate the landscape, identify potential hazards and make some decisions about where to set up a base station and GCP’s (Ground Control Points) during the aerial survey. However, we understand this step to be a luxury we can not always enjoy, given the very remote nature of the majority of our work in Australia. As a result, we generally start our PPK workflows from the next step below.
- Establish a base station – We establish our Emlid Reach RS2+ base station at the start of the survey over a physical reference point. This may be a known point (if it exists) or over a point that we establish ourselves. We then start logging RINEX data. This data is a critical piece in the PPK puzzle, as it can serve multiple purposes such as static post processing (when we are establishing a new reference point in the site), and accurate georeferencing of the aerial datasets to be captured. We treat this step very seriously.
- Set out GCPs and QCPs – You might hear that, with RTK and PPK, you don’t need to establish Ground Control Points (GCPs) and Quality Check Points (QCPs). Well, that is not really correct. It is true that, depending on the conditions and requirements of an aerial survey operation, you can considerably reduce the number of GCPs and QCPs surveyed on the site. However, you probably need a few GCPs to help your processing software to inject that extra accuracy into your survey products. More importantly, QCPs are the main instrument we use to ensure our survey deliverables are within the job specifications. So, we usually survey a good amount of QCPs.The shape, colour and size of our GCPs and QCPs depend on the type of data we are collecting, where we are collecting it and the actual flight height, but we usually go with white crosses or back & white checkerboards. The actual coordinates of these ground targets can be surveyed at any time with a precision survey instrument (such as another Emlid Reach RS2+ unit) – so if we want to maximise the number of flights in daylight, we’ll mark the targets before the flights, but survey them only after the aerial capture. Please find more details on options to survey GCPs and QCPs with Emlid Reach RS2+ receivers below.
- Fly the site – While capturing images or LiDAR data, the RTK drone will automatically log relevant flight data, including its own RINEX and timestamp events (for when photos are part of the collected dataset). These are the second critical piece of the PPK puzzle, and will be required when you are back in the office to post process the flight results. Make sure to backup these extra logs files with the same care you backup the actual images and LiDAR point clouds.
- Save base station RINEX – Once all the flights are complete, stop logging on the Reach RS2+ base station and save the base RINEX file. You are now ready to initiate the PPK data processing.
Surveying Ground Control Points (GCP’s)
There are a few options available to survey ground control points with an Emlid Reach RS2+ receiver.
If you have access to cellular network coverage, or internet through a WiFi network, you can use a single RS2+ receiver with NTRIP corrections to capture your points.
No cellular access? In this case you can utilise one Reach RS2+ receiver as your base station, placing it over a known point. Then, you can broadcast corrections via LoRa to another Reach RS2+ (rover) to survey the ground control points with centimetre accuracy.
Stop & Go with Emlid Flow – If the rover cannot connect to the base via LoRa, an alternative method to record the control points involves using Stop & Go. This method allows the rover to capture the data without receiving corrections and then correcting the position after collection using the RINEX log captured by the Reach base station. This is another possible use for the RINEX data captured on step #2 of the field workflow above, and can be done in parallel with the PPK aerial survey operation. This is very useful on large and/or very complex survey sites (hilly area, dense vegetation between base and rover units, long base lines etc.).
PPK Data Processing
It is now time to combine the base station RINEX, drone RINEX, trajectory, timestamp and drone datasets to perform PPK. The software to use for that will depend on the type of the dataset you are post processing and/or the manufacturer of the sensor used when capturing it.
For example, if post processing aerial imagery, Emlid Studio is a free and reliable option. Read more about using Emlid Studio for the drone data processing here. If you are looking for more functionality, such as the ability to export ready-to-process Pix4Dmapper or Agisoft Metashape photogrammetry projects straight out of the PPK process (using reference to the image files rather than creating copies of them), our recommendation is the RedCatch RedToolBox . It’s useful if you have a large dataset that you don’t want another copy of your pictures or overwrite them.
If, however, you are post processing a LiDAR dataset captured with the DJI L1 LiDAR sensor, you will need to use DJI Terra.
Regardless of the software you employ, the PPK dynamics is always the same. The software will request the base station RINEX data, its coordinates (reference point, see step #2 above) and, for each flight, the RINEX data recorded by the drone and the raw dataset. The output will be a positionally accurate dataset (centimetre accurate in relation to the provided reference point), ready to be processed as you would normally do with an RTK-corrected dataset.
Here’s a link to a video we created showing an aerial survey on a remote mine site using the method described above.