RTK and PPK GNSS - thomaspingel/advanced_remote_sensing GitHub Wiki

The Emlid RS2, RS3, and M2 are multi-band RTK/PPK GNSS receivers that operate in the L1, L2, and L5 bands. They are capable of 2 cm accuracy. The RS2 and RS3 are generally used for surveying ground control points for UAV flight operations. The M2 is much smaller, and is often integrated with other devices such as drones or lidar. The Emlid GPS devices have several functions: First, they work as data loggers. Anytime the units are on they will record raw data in Rinex 3.03 format (this is configurable). Second, they can both supply and receive corrections. Any unit can function as a base or as a rover. Corrections can be transmitted via WiFi, Bluetooth, serial adapter, LoRa radio signal, or cell network (with a SIM card / data plan).

Emlid has a great YouTube Channel with tutorials and each unit has a documentation page.

Units

First Time Setup

This part is the most difficult of the process; once set up, it's generally quite easy to take RTK points with an Emlid. Set up requires you to start the system in "Broadcasting Wi Fi" mode, configuring it to use your hotspot, and then reboot. It's a little easier said than done.

  • Power on the unit by long-pressing the button. Power-off is also a long-press. If indoors, place the unit near a window. The unit must either see satellites or be on a known Wi-Fi network when you start it.
  • Unless a known WiFi network is in range, it will create its own network with name that starts with "Reach" (with four hexidecimal characters to indicate a unique ID, e.g., "Reach:FA:6F"). Connect to that WiFi with your device (usually smartphone, but also works with a laptop). The default password (not PIN) in emlidreach.
  • Once connected, you can control the unit with the Emlid Flow app (Android or iPhone), or via a webserver at https://192.168.42.1.
  • You'll use your phone as a hotspot, through which the Emlid can receive RTK corrections. Set up your hotspot (enable maximum compatibility mode, if available), and then navigate to the Wi-Fi configuration page and add your hotspot. It will stop its own Wi-Fi network and connect to yours. Refresh your connection. If using a phone, Emlid Flow should find it right away, and indicate its IP address. If using a computer, connect to your phone's hotspot, and then browse to the IP address indicated by Emlid Flow. NEVER CONFIGURE AN EMLID REACH TO USE A SCHOOL WI-FI HOTSPOT. Security settings on the school network make it difficult to control the unit.
  • Under "Correction input", configure the unit to connect to the New York State Spatial Reference Network (CORS / Real Time Network). Once configured, take the unit to where it can see enough satellites to acquire a signal, and verify that corrections are coming in on the status screen. Corrections won't be received if the unit doesn't know its position.
    • Address: rtn.dot.ny.gov (Or whatever your CORS network is)
    • Port: 8080 (Specific to your network)
    • Username / Password
    • mount point: near_msm (Specific to your setup. For NYS network, this uses whatever is closest. You should be within 10 km for single-band units and 60 km for multi-band units like we have [source].)
    • For NYS CORS, the datum is NAD83(2011) EPOCH 2010 MYCS2. Heights produced from this network are related to the reference ellipsoid for NAD83, GRS80. [source]
  • Under "Logging" make sure that Raw Data is being logged at its full rate to RINEX 3.03, that Position track is set to LLH, that Base correction is set to RTCM3, and that recording is active.

RTK for GCPs

  • Start your phone hotspot that you have configured the Emlid to connect to.
  • Attach the Emlid to a 2 meter survey pole outdoors and start it up. Wait a minute or so. It should connect to your hotspot. Verify this in your phone.
  • Start the Emlid Flow app. Check the status to make sure you have a position and are receiving corrections. If using the RS3, enable the IMU at this time if you wish to enable tilt compensation. Verify that you are receiving FIXes and not FLOAT or SINGLE.
  • Click on survey. Create a new project (+ in upper right). Name the project. If your corrections are coming in as WGS84/ITRF, then you can choose output coordinates. If not (as is the case with the NY CORS corrections), you must leave this as-is. Set pole height appropriately. This is generally 2 meters. Set survey settings to FIX ONLY and AVERAGING (10 sec). Set TILT COMPENSATION for RS3 if desired.
  • Sample points.
  • When complete, project can be exported as CSV, SHP, KML, and many other formats. CSV and SHP are recommended at a minimum.
  • If you used the NY State CORS network, your data are in EPSG 6319 (2D version is 6318). For your SHP, you'll need to Define Projection in ArcGIS Pro to "repair" the location, since Emlid will assume it is WGS84 (EPSG 4979, or 4326 for 3D).

PPK for GCPs

  • If RTK is not available or post-processing is required, download Emlid Studio. Choose "Stop and Go with Emlid Flow" as your processing option.
  • You'll need the "Rover" RINEX file. This is the file on the Emlid Reach under Logging -> Record Logs -> And the log that corresponds to the time you gathered data. Download that and transfer it to your PC. Unzip it. You'll drag the "o" file (e.g., Reach_raw_20240204002129.24O) to the "Rover" box in Emlid Studio AND the "p" file (e.g., Reach_raw_20240204002129.24P) to the "Navigation" box.
    • You can also download log files via SFTP with user: reach, pw: emlidreach.
  • Download the CORS data. This is most easily done by browsing the HTTP structure based on the 4-character station name ("NYBS" for Binghamton) and the date (example). Download these files. Unzip and drag the "o" file (e.g., nybh260a.24o) to the "Base" box.
    • Dates correspond to UTC time.
    • Note that RINEX data are initially collected at 1 hour intervals. These are generally aggregated into single whole-UTC-day files the next day. The hourlies are deleted after 2 days.
    • After 30 days the data are downsampled to 30 seconds and daily values after 6 months, so get your data quickly! Data should still be available via NCEI, but this is much more cumbersome.
    • RINEX data can also be downloaded from the cors.dot.ny.gov site. Usefully, the site can create "Virtual RINEX" files for specific intervals.
  • Hit Process to start processing this data.
  • Drag the project CSV file to "Project from Emlid Flow" box and hit Start. The results will be mapped to the screen. You'll likely want to keep only FIX values.
  • Hit "Show Result Files" to see the output data. You'll like want the output CSV file. Remember that these are still in the coordinate system of the CORS correction.

Conversion between WGS84 and NAD83 (2011)

Soler describes the transformation between NAD83 (2011) and WGS84 (G1674) in this NGS paper. We recommend using the Horizontal Time Dependent Positioning (HTDP) tool if you need to do transformations of this kind. It isn't easy! Best to collect your data in NAD83 (2011) if you can. Technically speaking, the WGS84 (EPSG 4326) commonly in use is a dynamic coordinate system, meaning the same point taken years apart will have (slightly) different coordinates due to continental drift. This log illustrates the velocity (Ve, Vn, Vu) of various reference stations. NYBH is drifting at 3.9, -15.7, -0.3 mm per year (e/n/u).

Online Resources

Academic References