RTK GPS System Notes

Here is where all the technical notes go about building a Survey grade RTK GPS system. This would support the collection of accurate OCF GIS data.

The driving goal of this system is to make it useful to as many crews as possible. For example, even though we currently do have some GIS layers for different crews, the camping crew can't participate because the current accuracy is only within about 20 feet, and they need it down to about 1 foot or less.

GPS systems work by triangulating distances from satellites and sometimes ground based stations. The more satellites you can see the more accurate your fix (point) is. The US put up the first set of GPS satellites in the 70s, but other countries have followed suit. Collectively these comprise the Global Navigation Satellite System or GNSS. Modern day receivers are built to take advantage of everyone's satellites and hence can achieve more accurate fixes on the order of a few meters of uncertainty around where you're standing. Over time several GNSS augmentation systems have evolved that can reduce the fix uncertainty down more and more until at today's state of the art we can get it down to about 2 centimeters or an inch around the point your trying to measure. That technology is called Real-time kinematic (RTK) GPS which is form of Differential GPS.

RTK GPS requires three things; A fixed GPS station at a precisely known point; a roving GPS station to survey new points; and a way to send correction data from the fixed station to the rover. One of the choices the fair needs to make is should we use just our own radios to send this data, or should we subscribe to a Cellular data service like T-mobile to help us cover more area.

The rest of these notes are pretty jumbled for now, sorry. smile

Feature Lists:

  • Settable Epoch rate, perhaps less than 1 sec.
  • Allow data collection without RTK collection for post processing later.
    • Aways store enough data to post process later, even if you have the RTCM stream.
  • Warn below accepted accuracy level.
    • Use only RTK FIXED solutions
    • Running standard deviation calculations?
    • Have FEC for data radios, and warn of packet delays / outages.
      • Lora has FEC
    • Constantly display estimated error. Look for an app that can do this.
    • Allow time window averaging.
  • Mask angle filtering in SW
  • Have Sessions reprogrammed?
  • Have an accelerometer on antenna platform to calculate angle from vertical for tall antenna heights.
  • A transceiver is more desirable for a data radio. Possibly a Ripi hat / Arduino shield
    • don't Spam the spectrum.
    • Cellular data is a possibility. Could use a hot spot for nearby surveyors.
  • The Survey Pole:
    • Should be of adjustable height.
    • Uniform so that it can hang straight down, perhaps made of carbon fiber so that it won't dent easily.
    • Have apps that launch automatically on boot up.
        • Have a form to fill out when you start that has name of person, crew, height of antenna etc...
        • The antenna height should be clear and easy to enter into the system.
    • Do sanity check on RTK data signal strength etc. Re-init either chip if needed.
    • Have a 3D printed case that holds the tablet at a convenient angle, perhaps with an optional snap on sun shade.
    • The instrument head. (3D case) should be mounted, and clamped on the pole with enough antenna cable that it can slide up and down so that it's easy to see.
    • Have a circular bubble level on the top of the case, mounted with three screws with springs and washers under them.
    • Have a small removable side panel that snaps into place after you put a small amount of Loctite on each screw after calibration.
    • There could be some Velcro straps to bind the cable to the pole.
    • Try to find some plastic with a low coefficient of expansion, and design the box so that it does not tilt when the plastic expands and contracts.
    • Do we want a keypad?
    • Have everything USB powered with one battery powering the hub.
      • The tablet may not need a battery.
      • External charge via USB or barrel connector, with voltage printed next to it.
    • Consider making tablet removable
    • A built in Camera? Or a BT connection to allow a phone to take pictures and send them to the tablet.
    • Need an antenna to camera tripod adaptor for testing.

Reference Station:

  • Nice ground plane under antenna, research proper diameter.
  • * Spec in Auger rental, ditch witch $225 / day $175 half day, auger $55/day with 6" or 9" bit.
  • Tower out of 20 foot 2" Dia .154? thick pipe, 73#
    • Hmmm, or does it need to be shorter and fatter? research...
    • Research best practices for CORS or RTN stations.
    • Should it be painted inside, and or water tight?
    • 3D print plug with 5/8"-11 All thread in it. Have painted ground plane disk with extra hole for antenna cable. perhaps ground plane to pole.
    • Have Rpi box on pole under ground plane, and PoE Ethernet up from 6x6 pole. Have 48V PoE pass through extender like at Art Barn.
    • Power from Peach Power? PoE on both towers, perhaps small internal battery with each SBC for backup.
    • Flags on tower
  • Note that PP must be live all year and MC box must have the two RG11's connected with a double female F connector.
  • Get very precise level 10' ?? or bubble / sprint level.
  • Battery backup for the PoE injector located at Peach Power
  • Pull new one piece Cat 6 into Tickets West conduit to keep it water proof.
  • Probably want an SSD with load balancing on Rpi for long term logging.

Possible Locations:

Sky visibility from the CORS antenna location must be adequate...: https://www.ngs.noaa.gov/PUBS_LIB/GPS_CORS.html
  • Sites with Power
    • Tickets West Post
      • Location: TW-Post, N 44.05519, W 123.37413
      • Horizon obstructions up to: N / 14°, E / 12°, 120° / 16°, 210° / 5°, W / 5°, 300° / 4°
    • Hub communications tower
    • Elmira Elementary School
  • Sites with Solar
    • Miss Piggies Field
      • Location: PGGIES1, N 44.05502, W123.37511
      • Horizon obstructions up to: N 11°, E 8°, 130° / 9°, S 7°, W 9°, 280° / 5°
    • Dead Lot:
      • Location: DEADLOT1, N 44.05434, W 123.38158
      • Horizon obstructions up to: N 7°, 035° / 8°, 060° / 8°, E 7°, 100° / 8°, S 7°, W 8°
    • Winery Field
      • Location: WINERY1, N 44.05241, W 123.40246
      • Horizon obstructions up to: N 11°, 015° / 11°, 60° / 5°, E 4°, 135° / 12°, S 7°, W 6°, 300° / 15°
      • Location: WINERY2, N 44.05255, W123.40167
      • Horizon obstructions up to: N 13°, 65° / 9°, E 5°, S 12°, 260° / 7°, W 5°, 285° / 11°,

Types of GPS augmentation systems:

RTK:

Post processing using log files vs real-time processing using streamed corrections.
RTKlib based apps:

Keywords:

  • DGPS / RTK CPGPS etc...
  • Antenna Reference Point (ARP)
  • L1 phase center
  • Radio Technical Commission for Maritime Services (RTCM) protocol
  • Trimble Compact Measurement Record (CMR), (CMRx)
  • Receiver Independent Exchange Format, (RINEX)
  • NTRIP
  • 3.5G HSDPA
  • Virtual Reference Station (VRS)
  • Observation Space Representation (OSR) (high BW?)
  • Code-Based Positioning vs Carrier-Based Ranging
Possible radio links: LoRa, LTE-CAT M1, NB-IoT, or MURS VHS for radio unlicensed or licensed freq.

Links:

Communications:

Available Software:

Antenna Features:

  • Circularly polarized
  • Phantom powered?
  • Should have ground plane to protect against multi-path
  • Choke ring antenna
  • 15° cutoff or mask angle.
  • Has antenna phase center (APC) datum listed
  • Find best practices for tower mounting
  • how to set GPS antenna height in the data collection app?

Products:

Receivers:

Antennas:

Mounting HW:

Data Radios:

Accessories:

APRS with DGPS:

How much more data in the RTCM packets vs GPS addresses?

Notes:

Would we want to integrate multiple reference stations, and how? Probably use VRS.
Best to reference our system to CORS; LPSB in Eugene, and possibly LFLO in Florence.
What are the requirements for being a CORS station? What is CORS MSOL? RTX vs RTK?

Note that in heavy trees degraded performance, but better antenna plus more constalations...
Research Rx sensitivity comparasons.

Find toolchain for GIS proposal. Try to find a BT solution
Note that origanl proposal did not include RTK and show the average improvment.

Pole Calibration:

  1. Strip the pole bare so it is symmetrical.
  2. 3D print a screw on cap that fits top with a small hole in the center.
  3. Put some string through the hole and screw it on.
  4. Hang it from a fixed point like a medium sized wall bracket.
  5. Have a 2x4 with a symmetrical notch cut out of it that the pole top hangs in front of, just barely touching.
  6. Make two boards with another notch that will restrain the point of the pole from sliding, perhaps with some weight.
  7. Let the pole hang just above the floor and also just in front of the notch.
  8. Once it stops swinging, constrain the point, and gently clamp it to the notched 2x4 without moving it away from vertical.
  9. Fasten the control head back on the pole. Finally we can adjust the three small screws that hold the bubble level until it is perfectly centered.
  10. Put a drop of Loctite on the bottom of each screw where it passes through the nut.
  11. Replace the cover if there is one. And you're done! smile

Possible Configurations:

  • Different types of systems: https://home.csis.u-tokyo.ac.jp/~dinesh/LCHAR.htm
  • SImpleRTK with RPi to provide serial data over the internet. Use Lora for rovers.
  • SImpleRTK rover with Lora and USBOTG for a tablet. Or USBOTG to a tablet / phone with RTN network. Alternatively, we can use a RPi / Arduino to add BT and SD card storage with Ethernet up down load.
  • SparkFun rover with USBOTG and a USB hub to talk to both the GPS and the RTK radio. Have a startup script or app that inits them and routes the RTK stream to the GPS.
  • SparkFun reference with just a RiPi. Could connect with USBOTG or serial. That that provides the RTK data to whomever connects. Maybe with a password?
  • One client would be a RPi / Aurduno with a Lora? radio to just broadcast the RTK data. This could be the hub of the Lora star network.
Sparkfun

RTK vs. RTN (real time Network service) CORS being an example. Label our normalized coordinates by which reference point we're using. Including version and date. Have accurate translations between them so as to preserve the value of old data. Keep the meta data of all collected points so we have the same reference point, version, and date for them. Have a meta data policy for GIS data.

Todo:

  • Do comparative Surveys between Miss Piggies, and the Winery.
    • Perhaps do signal quality tests, (requires some equipment).
    • Compare site costs?
  • Start network upgrade to Tickets West pole.
    • Finish up the following year, if we have to coordenate with Shane's MC project.
    • Survey any rough points Shane needs this year.
    • If any money is leftover, buy some basic parts to experiment with.
  • Check out invar mounting parts.
  • Update GIS with deatials on pole with pictures.

Search terms:

  • rtk gps with "multiple reference (stations OR points)"
  • Access to raw GNSS measurements in Android 7+, 8.0 etc...
  • rtcm data rate

-- ClifCox - 17 Feb 2020

This topic: OCF > WebHome > OCFIT > RtkGpsSystem
Topic revision: 2025 May 14, clif
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