The advancement of GPS/GNSS-enabled relative positioning—the control of two or more objects relative to each other—has gained momentum in recent years, particularly in the agricultural, mining and construction communities. The idea that two or more autonomous vehicles, such as a combine and a grain cart, can run side-by-side or in-line to harvest and collect grain, with precision holds promise for safer, more efficient operations.
Mine operators are using relative positioning capabilities to manage the synchronized operation of an excavator and the multiple haul trucks that are used to carry the material from the mine site. In this case, the excavator and all of the haul trucks ‘talk’ to each other as they move in and out of position in a synchronized way as the shovel fills each truck.
On the construction side, a contractor recently used relative positioning to operate paving equipment along a highway to form curbs and shoulders. In the past, this same task would require a surveyor to spot and stake the line, which the paving machine would follow. With relative positioning, the surveyor sends the stake position to the paver with a push of a
button. The paver orients itself to the position and continues along the line to the next position for greater efficiency and improved safety for the construction team.
As highway and heavy equipment applications continue to expand, it’s important to understand the key elements of an effective relative positioning system and the limitations.
There are many considerations that go into selecting the optimal relative positioning solution including accuracy, signal availability, solution rate including system dynamics, installation constraints and, of course, price.
Remember, relative positioning is all about spatial awareness, which begins with a master receiver and a rover(s) combination. The master receiver is located in the lead vehicle, and the rover in the ‘slave’ vehicle. The vehicles typically
exchange position information through a radio link or NTRIP through the Internet.
Depending on the system, an operator can position the slave vehicle to the master vehicle to the centimetre level. In terms of signal availability, relative positioning works very much like RTK. There needs to be sufficient common satellites tracked between vehicles to produce a relative solution. Therefore, the vehicles should be operating in close proximity to each other.
In some cases, such as the agriculture application noted earlier, you’ll also need full roll, pitch and
heading between the master and rover vehicles (not antennas). Therefore, some relative positioning solutions
also incorporate a GNSS capability with inertial measurements. With an IMU, a relative positioning solution not only provides a continuous positioning capability, but also full 3D attitude (roll, pitch and heading), even during short periods of satellite outages.
Pricing varies considerably. Today’s market includes a wide range of options from low cost GNSS—only solution to an integrated GNSS/INS solution with various grades of IMUs. Customers have many options depending on the complexity of
their applications and their budget. Talk to a relative positioning expert to determine the best choice for your application.