Using a affordable RTK GPS receiver, a GPS antenna, and a Bluetooth serial, one can have this high-precision RTK GPS for smartphone under $100.
Below shows simple hardware hookup needed:
green wire: 3.3V
white wire: ground
yellow wire: NMEA from RTK GPS to Bluetooth serial
blue wire: base station data from Bluetooth serial to RTK GPS
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On software side:
Install Lefebure NTRIP Client from the Android Play Store
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Select Settings Option
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Select NTRIP Settings.
Set Network Protocol to NTRIP v1.0
Set up IP, COM port, user name, password of accessible base station providing RTK service. Select suitable data stream available that match user name and password entered.
A article on available public RTK base station in USA is available here: http://gpsworld.com/finally-a-list-of-public-rtk-base-stations-in-the-u-s/
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When done, change to Receiver Settings page.
Receiver Connection choose External via Bluetooth. Before selecting Bluetooth Device, enable smartphone Bluetooth, scan if the Bluetooth serial device has not paired before. After pairing Bluetooth, for Bluetooth Device you can select Bluetooth serial used for RTK. You can decide if to save GPS NMEA data to file or save NTRIP base station data to file. Select GPS Mock Locations to allow other App to use RTK GPS.
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When done, change to Display Settings page.
Change Info Box1 and Box2 to show longitude and Latitide so you can see position coordinate changing.
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When done, go to top level screen and click Connect. NTRIP Client first try to connect with base station over Internet. When connected, no GPS fix available it shows Invalid and zero for latitude and longitude
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After getting position, it first show normal meter-level accuracy GPS position fix, with 10 satellite in this case.
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After 10 seconds it shows FloatRTK, meaning RTK receiver is trying to resolve integer cycle count number for each satellite in order to get centimeter-level accuracy.
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After getting to centimeter-level accuracy, it shows RTK,
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If you run GPS application, such as GPS Essentials, in the Satellites window, one can see position accuracy estimate is 0.02 yard
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If using internal GPS of Samsung Galaxy Note 3, position accuracy estimate is 3.3 yard
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Now one can proudly own survey grade centimeter-level accuracy RTK receiver working with his smartphone, like those couple thousand dollar Bluetooth RTK receiver on the market: http://www.terragotech.com/products/terrago-edge/gps-receivers !
This testing is done on roof of building. 3.3V from USB Serial breakout board used to power RTK GPS and Bluetooth serial.
For ease of use, 3 breakout board can be put into protective 3D printed enclosure with RF connector for signal input like those professional Bluetooth RTK receivers, to have option of using better antenna if needed.
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At this high precision, GPS position measurement can be precise and accurate. Do you know typical GPS receiver we use daily use can be off by more than 5 meters?
Last week road test was done on open sky road driving back and forth 4 times keeping vehicle on road center, in morning and afternoon, logging position using popular brand GPS/GLONASS receiver, result is red and green tracks. Using RTK GPS receiver to log similarly, result is yellow and white tracks. As seen, normal GPS/GLONASS receiver deviate more than 5 meters, while RTK GPS deviate around 0.5 meters mostly due to human driving error. Such error usually not seen on maps due to map now show GPS position to nearest road, masking such position error.
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Lefebure NTRIP Client
https://play.google.com/store/apps/details?id=com.lefebure.ntripclient&hl=en
GPS Essential
https://play.google.com/store/apps/details?id=com.mictale.gpsessentials&hl=en