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Understanding Piksi location data outputs

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Piksi Location Data

The Piksi provides real-time GPS location data outputs of two different types:

  • Single point position: This is a standard precision (meter accurate) solution that is an absolute location on the Earth.
  • RTK baseline: This is a high precision (centimeter accurate) solution that is a relative vector between two Piksi units. It comes in two stages:
    • Float solution: This is a high precision, relative solution that Piksi enters as it searches for a the higher precision fixed solution.
    • Fixed solution: This is a high precision, relative solution where Piksi achieves its highest accuracy.

This data has different levels of accuracies and assumes some basic configuration parameters. This section will help you understand Piksi data, so that you can consider the ways in which you can design it into your application..

Single Point Position


The diagram below illustrates the relationship between these two different types of solutions. A single point position is our terminology for a standard GPS position. This is a position that tells you where you are on earth. The grid in the background of this diagram illustrates this location within the context of latitude and longitude.

A single point GPS position is not a high precision location, so it is accurate to within a few meters. You can envision an accuracy envelope around the true position of Piksi. We know with some level of confidence that the reported position of Piksi will be inside the accuracy envelope around the true position.

The typical way to specify this accuracy in the GPS world for horizontal data is circular error probability (CEP). A CEP test tries to determine whether or not a specific number of the reported locations fall within a certain distance from the true location. These are typically quoted with the radius of the circle and the percentage of the measured positions that occur within that radius. For example, a CEP50 5m accuracy would mean that 50% of the measured positions will be 5m or less distance from the true position. CEP50 is the standard test in the industry and is often simply abbreviated CEP.

Fixed RTK Baseline


Imagine that you have two Piksi's, one a Base and one a Rover. Imagine that these two Piksis are communicating corrections to each other, and that you are above them and are looking down at the data that they are generating. The orange dots and lines shown in the diagram below represent the RTK relative solution (also sometimes referred to as the baseline or the fixed solution). This is a different position solution than a single point position solution in a few ways.

First, the position solution - often referred to as a baseline - is much more precise: on the order of centimeters, rather than meters. Second, the position solution is relative, not absolute. This means that you do not know, from the RTK baseline, where you are on Earth. The figure to the left illustrates this by not showing the latitude longitude grid behind the Piksi.

Instead you know two things: the precise distance between your two Piksis, and the vector (relative to the earth) between them. Often, this is expressed as a [North East, Down] vector. This vector is illustrated in the graphic below. In fact, you know a little bit more about the two Piksis, you know

All of the same accuracy conventions that we described above are exactly

Combined Positions

Imagine that you have two Piksi's, one a Base and one a Rover. Imagine that you are above them and are looking down at the data that they are generating. The dots in this case are the data. Gray dots represent measured single point positions and the gray circle represents the a level of certainty about the single point location. Orange dots represent possible measured RTK baselines and the line between them represents the directional baseline vector. The single point position is an absolute position that is less accurate. The RTK baseline is a relative position between the base and the rover that is more accurate.

The single point precision GPS solution, which is an absolute solution, with respect to the earth. The RTK baseline is a relative baseline between the base and the rover, which has no relationship to the earth. The diagrams below illustrates this difference. All of the single point position solutions are fixed, relative to the earth.

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