# Augmenting GPS with Geolocated Fiducials to Improve Accuracy for Mobile Robot Applications
## 2 Related Research  
This section provides background to existing technologies used to improve GPS performance and fiducial techniques. Assisted-GPS (AGPS) is a technique which uses a non-GPS radio signals (e.g., WIFI locations, cell tower beacons) to significantly improve GPS cold start time as these radio signals help coarsely localise the GPS receiver. Once the navigation unit knows its approximate location (within a few kilometres) the GPS receiver can more rapidly determine the precise position [20]. An additional advantage of AGPS is that it helps guard against GPS spoofing (when a GPS signals are maliciously spoofed) as all of the relevant cell tower beacons or WIFI locations would also have to be spoofed. Differential-GPS (DGPS) uses a fixed ground based GPS beacon as a reference point [21]. The beacon computes the error measurement between the measured GPS point and the known beacon position. This error is predominately due to atmospheric conditions and hence can be used by local DGPS receivers to correct for errors. The distance to the beacon is an important factor with each increase of 100 km resulting in an error of 0.2 m [10] and so for best performance the DGPS receiver should be relatively close to the beacon. DGPS receives regular beacon updates (typically in the order of 5 min) to ensure that recent data is being used to correct GPS signals. Like traditional GPS, DGPS is susceptible to spoofing and jamming. For applications which require the precision of DGPS (e.g., autonomous farming, marine navigation) this reduction in accuracy may have catastrophic consequences [22]. One more recent development is internet based DGPS, which is targeted to applications where a GPS receiver is connected to the internet and can receive differential update information online. Such systems are well suited to mobile phone applications where GPS and 4G are tightly coupled together [23; 24]. Table 1 summarises positional error for different GPS augmentation schemes. Although the DGPS scheme provides the most accuracy it is also the highest cost (receivers are several orders of magnitude more expensive than traditional GPS receivers). Central to the concept of FAGPS is the pose estimation based on georeferenced landmarks or fiducial points. In terms of fiducial points, the proposed FAGPS system could conceivably operate both with traditional 2D fiducial images and with so-called real-world fiducials. 2D Fiducial images are high contrast, black and white images of known size and content which are placed in an environment to facilitate pose estimation (as shown in Figure 1) [26]. Real-world fiducials are natural or man-made structures not designed for the purpose of machine vision waypoints, but can be used for this task [27]. Some examples of real-world fiducials include: a corner in a fence or building and key features in a path or structure which have been accurately georeferenced to a map. These real-world fiducials are generally difficult to automatically detect reliably (even artificial fiducials can be difficult to detect in real-world environments) [28]. The constrained search size (expectation that a fiducial is within a certain area given the non-compensated GPS margin of error) should help improve detection and reduce false positives. The other issue with many of these real-world fiducials is the quality of pose-estimation, which can be very accurate given a completely flat, high contrast 2D artificial fiducial (Figure 1), but less so for a real-world fiducial not optimised for post-estimation. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Data Point** & **Horizontal Error** & **Notes** \\ \hline
**Global Positioning System (GPS)** & 10–15 m [25] & No augmentation \\
**Wide Area Augmentation System (WAAS)** & 2–5 m [25] & Availability subject to location \\
**Differential Global Positioning System (DGPS)** & \textless{}1 m [10] & Accuracy depends on distance to beacon \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of horizontal error for different augmentation techniques.