Impact of GPS delay to geotagging #
GPS or RTK system provides data via serial bus, which delays the position data for some very short time. When GPS or RTK is connected to flightcontroller, and than forwarded to TAG-E or ENTIRE, there is another delay of transmission.
Both TAG-E and ENTIRE calculates automatically offset of this delay between flight controller and the TAG-E or ENTIRE based on precise clock synchronization. Delay of processing and transmission between GPS and flightcontroller needs to be inserted statically.
Mission conditions #
Calculation process is based on test flights and stitching of the data in Pix4D Mapper stitching software. You can use testing version of Pix4D for this calculation.
Calculation process is iterative, it needs to be passed multiple times to get perfect delay correction.
Check all the boxes below and make sure you haven’t missed any of the pre-flight recommendations.
Longer shutterspeeds, like 1/500s may cause motion blur in photos and cause errors in calculation.
Suggested speed is 10m/s and higher.
Natural subjects are hard to stitch precisely and result will not be precise.
Suggested are rocks, buildings, stone quarry or similar places where objects are not moving by the wind.
Flying mission #
Create mission via your favourite program
- Use any software you like, MP or QGC to setup mission for mapping of terget subject space.
- Make sure the drone will fly mission “nose forward” and gimbal is in follow mode.
Mission parameters
- Mission does not need to be large.
- You need four lines, with approx. 10-15 photos in the row.
Fly mission
- Now, fly your mission, collect geotagged JPG data and transfer to your PC with Pix4D Mapper application.
- You can use trial version for this calculation, however you have scale down JPG files to slightly lower resolution. Down-scaling does not have huge impact to delay calculation.
- https://www.pix4d.com/download-software/
Data processing #
Create new project
- Open Pix4D mapper and create new project. Import mission data JPGs to the project.
- Do not include images taken out of the mission, or mistakenly.
Start processing
- In next window select “3D maps” mode and start processing of stitching
Processing output
- Once you will get Quality report window, you can close this and stop model processing.
- Model is not necessary for delay calculation.
Check geotagging errors
- You can now see two types of pyramids with spheres on the top. Green ones are showing calculated position and orientation of the image and blue ones are showing original position of the image saved in geotag data.
Check geotagging errors
- At this point, move the model to see pictures from the top.
- (Press Shift, press and hold left mouse button and move mouse to move model to vew from the top)
Understanding of errors
Difference between blue and green pyramids may help you to calculate GPS delay and also check YAW offset issues.
On this example image:
- Y – Yaw error, this is most likely caused by
- Wrong placement of flightcontroller
- Wrong mount of gimbal QR (not aligned with flightcontroller direction)
- Bad compas calibration in flightcontroller
- D – delay error caused (mostly) because of the GPS transmission delay
- Blue point is present earlier (in flight direction) than actual calculated photo position (green point)
- Important is, that delay error is only in flight direction and should be rawly the same for all photos in a row and have same direction. If direction is changing, or changes its direction, problem is not caused by GPS delay
- Delay error can have slightly different amount (distance between blue and green point) because flight speed vary a bit due to winds
Example photo selection
- Select some of photos around the center of the mission. Click to blue sphere of this photo by right mouse.
- In the right part of the window you will see details of selected photo, including relative coordinates between initial and computed position.
- These coordinates will be used to calculate real distance between initial and computed position.
Coordinate extraction
- Navigate to this page for our tool for distance calculation
- https://airpixel.cz/delaycalc.htm
- Select computed position coordinates by cursor selection and press CTRL+C
- Switch to your browser with our calculator and insert the text
- Select initial position coordinates and press CTRL-C
- Switch back to calculator and insert the line. Make sure coordintes copied correctly (it is tricky sometimes)
- Insert flight speed to the calculator
- For precise calculation we suggest to browse flightcontroller log and check for actual flight speed at the moment of capture
Calculator result
- Press Calculate
- Check for calculated result
- Result of calculator will be always positive number
- You need to select positivity or negativity of this result depending on direction of the error relative to flight direction
- If Initial position is before computed position (relative to flight direction) delay needs to be inserted as negative.
Insert value to geotagger
If there is already configured any value in your device, you need to add or subtract new value!
- For TAG-E
- Use QGC or MissioPlanner and via parameters access MAV_LINK_DELAY
- Insert (add or subtract) calculated value
- Click to Write params
- For ENTIRE
- Navigate to Geotagging->Settings->GPS link delay
- Insert (add or subtract) calculated value
Ready for next iteration
- Now you are done with first iteration, you need to return to point “Flying mission” and start over.
- Next iteration will provide you less distance, between blue and green point, also much less GPS link delay value which needs to be “added” or “subtracted” from the actual value set in your geotagger.
Tips and tricks #
Static photos
Plan and flight your mission as stated in this manual. However, stop mission before return to starting point and:
- Manually fly back to the center of mission field
- Keep the same altitude as the mission was flown
- Stop the drone and keep in hover
- Take a picture (top down same like in the mission)
- Move heading of the drone by some angle (around 45deg) and let it stabilize
- Take another picture
- Continue with different YAW offsets for another 4-6 photos
Include static photos into the stitching process and after processing in Pix4D is done, you can examine if your XYZ antenna offsets are set properly or not.
Because on static imagery geotagging has GPS link delay no imapct at all, all offsets you will see between Initial and Computed position will be most likely caused by XY antenna offset.
Fly backwards
Alternative method to distinguish if error is caused by GPS link delay value or by XYZ offsets is to – Fly backwards:
- Fly a mission as per this manual
- Before return to launch point pause the mission and fly manually one line backward
- Remember this line and after stitching check errors on these photos
- If you see different error than on other lines, you most likely have XYZ offsets wrong
Reaching perfect value
As you will be reaching perfect value of GPS delay for your system, you will see some amounts of error not in flight direction. This is error caused by Y antenna to payload offset configuration. In this case we strongly suggest to re-check all offsets as all may be in wrong direction and GPS delay calcualation may be invalid.
At the image on the right you can see almost perfect GPS delay tuning where:
- Y – YAW error caused by bad compass calibration
- Z – Means bad “Y” antenna – payload offset configuration
- D – Little imperfection of GPS Link delay
Conclusion #
Optimum result
With proper settings you may reach accuracy below 3 cm for your setup, however it may need few iterations of mission/calculation to get right value of GPS link delay. Also it may need few iterations to understand the process and distinguish XYZ offsets errors from GPS link errors.
Feel free to rech us for help!
