Evaluating RTK GPS for UAV


Background:

Real Time Kinematic (RTK) Global Positioning Systems (GPS) have been around since the late 1990s. RTK GPS offers centimeter level precision versus meter level precision of normal GPS. RTK GPS’ that can reasonably fit on an Unmanned Aerial Vehicle have been available since late 2013, and we decided to explore how ready and powerful they really are.

We explored the following systems. Most require a non-grounded ground plane some provide a ground plane in their kits. All systems require ground station GPS latitude, longitude, and altitude to be configured in the ground station receiver. The RTK GPS system requires two GPS receivers. As a consequence there is a trade off between momentary high precision followed by normal GPS precision when a packet is delayed or lost and reduced average precision (due to reduced accuracy of ground station position) with perfect communication between receivers (wired connection on the vehicle).

We purchased the two most affordable kits, NS-HP and Reach, but as you will see, neither is quite ready to use right out of the box.

Testing:

Direct replacement of existing GPS receivers with a RTK GPS receiver is not trivial. Additional hardware and software are required to provide basic position estimates. Including additional hardware and software into a system increases the amount of possible failure points and may effect overall system performance such as signal interference.

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Various communication paths realized in software were created to facilitate communication between base and rover receivers. Hardware in the communication paths included at most a laptop, paired XBee Pros, base and rover receivers, and an Odroid-xu3/ZyXel router–post processing only.

State machines were created to decode the generated GPS solutions for “real time” position estimate access.

Hardware was added onto an Ascending Technologies Hummingbird to construct a stable platform for the base and rover antennas.

Cables were constructed to facilitate direct connection between receivers while providing a serial output path as well.

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After this was completed, generated position estimates for a stationary Ascending Technologies Hummingbird with propellers rotating was not centimeter level  and required the router to access the solution. A cable was constructed as described above to replace the onboard GPS unit, but the Hummingbird did not recognize the data being transferred. Full integration with the Ascending Technologies Hummingbird was not achieved.

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Within the bullseye are position estimates for a stationary Hummingbird with propellers rotating. The outer ring has a radius of 2 meters. The dots leaving the bullseye in the upper left corner are position estimates for when the vehicle was in flight.

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We now provide more details about these systems.

NS-HP ($394):

NavSpark manufactures the NS-HP with separate modules for 1 Hz, 5 Hz, and 10 Hz update rates. The NS-HP is 40 × 38 mm and weighs 6 g.  A windows application, GNSS Viewer, is available to verify receiver operation. Requirements to operate:

  • 7 satellites with signal level over 38 dB/Hz
  • soldering and additional cables to access the generated solution
  • electrical characteristics of UART are 70mA @ 3.3v and is not tolerant to higher voltage levels
  • 57600 baud rate
  • careful handling of rover kit antenna

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REACH ($570):

Emlid manufactures the Reach with 1 Hz, 5 Hz, and 10 Hz adjustable update rates (gps_glonass_5hz is the only update rate fully tested). The Reach is 45.5 × 27 mm and weighs 14 g. A wiki is provided for setup and a community forum is available. ReachView is Emlid’s application to verify receiver operation.

Requirements to operate:

  • 5 satellites with signal level over 45 dB/Hz — need non-grounded 100 mm diameter ground planes to achieve this signal level
  • an IP address generator, normal access — an internet connection to update firmware
  • logic levels of UARTs are 3.3v with 5v tolerance
  • supports most standard baud rates
  • fine tuning configuration settings

Jenny_reach_rtkgps

SMARTNAV ($932):

Drotek manufactures the SMARTNAV with a 10 Hz update rate. The SMARTNAV is 110 × 110 mm and weighs 200 g. A user guide is provided for setup. These receivers are designed to be used with a Dropix or pixhawk. RTKLIB is the application to verify receiver operation though the more user friendly u-center from u-Blox may be used as well.

Requirements to operate:

  • 5 satellites with signal level over 45 dB/Hz —  ground planes are provided to achieve this signal level
  • logic levels of UARTs are 3.3v with 5v tolerance
  • supports most standard baud rates

smartnav

Piksi ($995):

Swiftnav manufactures the Piksi with a 10 Hz update rate. The Piksi is 53 × 53 mm and weighs 32 g. A combined wiki and community forum is provided for setup. Virtual machine use is not recommended. Utilizes a Swift Navigation Binary Protocol. Piksi Console is the application to verify receiver operation. A simulation mode is available for indoor operations prior to testing outdoors.

Requirements to operate:

  • 5 satellites with signal level over 33 dB/Hz —  ground planes are provided to achieve this signal level
  • protocol conversion for compatibility with existing systems
  • logic levels of UARTs are 3.3v with 5.5v tolerance
  • supports most standard baud rates

piksi

NV08C-RTK ($1100 > $? > $1500):

NVS Technologies AG manufactures the NV08C-RTK with 1 Hz, 2 Hz, 5 Hz, and 10 Hz adjustable update rates.The NV08C-RTK is 71 × 46 × 8.1 mm and weighs 21 g. No user guide is available.

Requirements to operate:

  • 4 satellites with signal level over 33 dB/Hz
  • logic levels of UARTs are 3.3v with 5.5v tolerance
  • supports baud rates from 4800 to 460800

nv08c_rtknnv08c-rtk

Precis-BX305 ($1999):

Tersus manufactures the Precis-BX305 with a 20 Hz update rate.The Precis-BX305 is 130 × 60 × 18 mm and weighs 48 g. A user guide is available. Tersus GNSS Center is the application used to verify results.

Requirements to operate:

  • 5 satellites with signal level over 33 dB/Hz
  • logic levels of UARTs are 3.3v with 5v tolerance
  • supports baud rates up to115200

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About Willy Wells

I am Semi-Nomadic by upbringing. I am a child of a military veteran and was raised within a community in which abrupt change was a normal fact of life. Ethnically, I am African, Apache, Cherokee, English, and German. I have a Bachelors of Science in Computer Engineering from the University of Tennessee, Knoxville. I am currently pursuing a Masters of Science in Computer Science with a concentration in Computer Engineering. I served six years in the Navy as a Naval Nuclear Electronic Technician, which consisted of two years of training and four years of being assigned to the U. S. S. Ronald Reagan. I was born in Yokosuka, Japan, raised in San Diego, California, and educated in Knoxville, Tennessee. I am the second oldest of six full siblings.

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