GPS

Global Positioning System (GPS) is a powerful space-based satellite navigation system that provides location and time information anywhere on the planet. These sensors are very common in outdoor Unmanned Ground Vehicle applications. This page will also discuss two modified GPS systems called WAAS and RTK.

Sensor Overview

Measurement: GPS receiver exchanges data with GPS satellite system to obtain time and location information anywhere on the planet. For basic GPS the position accuracy is about 10-20 meters. WAAS can reach an accuracy of under 3 meters and RTK can reach 1 cm.

Ideal operating conditions: Outdoors with an unobstructed view of the sky. Does not work indoors! WAAS only works in the US and requires a nearby public reference station to feed it correctional data. RTK requires sky visibility in a cone at an elevation of ~30 degrees to the horizon and a base station setup very close by for corrections.

Sensor Pros:

• Data is easily processed, allowing the use of cheaper microcontrollers
• Can receive positioning data anywhere in the world (while outside) with no setup. This is a huge positive.
• WAAS and RTK can have decent accuracy with some extra setup

Sensor Cons:

• Slow update rate
• Only works outside
• Generally not accurate enough by itself to be used for robot navigation
• WAAS has reasonable accuracy but only works in the US
• While RTK has impressive accuracy, it is difficult to setup and get working reliably in real-time
• Difficult to align with map data

GPS Receiver Module

GPS is a blending of positional and movement detection. It can return your position along with how fast you are moving. GPS is a space-based satellite navigation system that provides location and time information anywhere on the planet. A GPS receiver calculates its position by timing the signals sent by GPS satellites. The receiver compares the time stamp between multiple satellites. These timestamps are compared and the position of the receiver is then extrapolated by the delays. The expected accuracy of the position is expected to be between 10 and 20 meters depending on various atmospheric conditions and the location of the receiver. Combining the data from an accelerometer, gyroscope, and GPS allows you to know where you are, what direction you are moving and what orientation you are in. These are critical components in many autonomous systems. These features can be found in our 66-Channel LS20031 GPS Receiver.

High Precision RTK GNSS Receiver

When it comes to obtaining accurate location feedback outdoors, an RTK GNSS is hard to beat. Let’s go over what this setup can do, and how it does it. This is a global navigation satellite system (GNSS) configuration that provides positioning accuracy up to 1 cm. Compared to the 10-meter accuracy of normal GPS setups, RTK systems greatly improve the viability of integrating satellite navigation on autonomous robots.

So, how does this work? The current and most popular method for obtaining a real-time kinematic (RTK) lock is to have one GNSS module as the designated “base” station and another as a “rover” station. The base receiver is responsible for calculating error offsets while keeping the rover station updated. The rover station applies this offset to its position readings and provides the user with very precise position feedback. The base station is more interested in the phase of the signal rather than the content of the signal. This is because Earth’s atmosphere (ionosphere and troposphere in particular) is an error source in the form of signal delays and phase changes. Since the base station has to make these precise calculations, it must remain stationary in operation.

Along with the base station remaining stationary, two conditions need to be met to achieve an RTK lock. Both GNSS receivers need to have a 30-degree view of the horizon, and they also need to be linked to at least four of the same satellites with a signal to noise ratio (SNR) of 40 or higher. Of course, the second is more critical than the first. This should make sense given how the error is calculated.

In areas with tall buildings and dense vegetation, it can be very difficult to obtain and keep an RTK lock. However, some modifications will increase overall efficiency. Antenna placement is critical to reducing multipathing errors. Multipathing occurs when the satellite signal is reflected before it reaches the receiver. This causes the signal to take multiple paths and therefore increases the delay since the distance traveled is increased. This is usually the culprit of massive outliers in position data that we see all too often. Mounting the antennas on ground planes can also help with this issue. A ground plane is typically a flat symmetrical metallic plate under the antenna that will create a more consistent reception pattern as well as filtering out satellite signals close to the horizon. The optimal size of the ground plane usually depends on the antenna design itself. Luckily, most RTK GNSS manufacturers make setup rather simple by selling the two modules and corresponding antennas as a package while providing sufficient “how to” documentation.

WAAS Enabled GPS

More often than not, 1 cm accuracy is overkill for applications that require global positioning feedback. Fortunately, SuperDroid Robots now stocks the Garmin 18x GPS modules in 1 Hz and 5 Hz variants. This offers a cheaper and easier to use alternative to the RTK systems that are viable for autonomous development platforms. The driving force behind the 18x series compared to a normal GPS is the Wide Area Augmentation System, or WAAS. By promoting a reasonable cost and less than 3 meter error, this technology is a must have on outdoor autonomous robots.

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The underlying methodology of WAAS is very similar to RTK as the onboard receiver uses correctional data for improved positioning. Multiple ground reference stations positioned across the U.S. that monitor GPS satellite data. Two base stations, located on either coast of the U.S., collect data from the reference stations and create a GPS correction message. The corrected differential message is then broadcast through 1 of 2 geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

Products

Garmin GPS 18x LVC
66-Channel LS20031 GPS Receiver Module (MT3339 Chipset)