# Augmenting GPS with Geolocated Fiducials to Improve Accuracy for Mobile Robot Applications
## 3 Proposed Technique  
The FAGPS technique proposed in this paper is to augment the GPS measurement with a georeferenced fiducial marker which a robot may observe within the environment and perform pose-estimation on. Hence, once calibrated, for a short time interval, the robot can use this augmentation to reduce positional error. Unlike DGPS which requires a costly DGPS receiver along with a differential beacon the proposed FAGPS scheme requires no hardware on the ground, doesn't require access to power and uses the same low cost commercial receiver used by the robot itself. As the robot needs to directly observe (and perform pose estimation on) the geolocated fiducial, this limits the effective range the robot can be away from the reference point based on how quickly the robot can travel within the increased accuracy timeframe. To expand the area of operation a series of geolocated fiducials could be deployed--when the robot needs to re-acquire a fiducial it can go to the closest one. The FAGPS approach we have proposed operates in two modes: a calibration mode and a compensated mode as shown in Figure 2. The calibration mode is where the FAGPS approach captures and aligns a GPS position (Latitude, Longitude, Altitude) with a visual reference point. In a practical robot system this could involve a ground robot driving up to a fiducial marker or a helicopter robot hovering and performing pose estimation above a fiducial marker (or a natural visual feature like the corner of a fence) and capturing the GPS coordinates at this point. The calibration mode first involves locating the fiducial marker. As the uncompensated GPS measurement will suffer from significant inaccuracies, the robot (or measurement platform) should approach the approximate location at a distance of 10's of meters and use machine vision try to locate the position of the fiducial. Once this operation has been performed the robot can approach the fiducial more closely (Euclidean distance less than 5 m) to facilitate a more accurate pose estimation  
Figure 2: FAGPS Operational Flow Diagram. measurement. Once this pose estimation has been performed, GPS calibration coordinates can be captured and calibration offsets will be calculated as specified in Equations (1) and (2). \[x_{tc}=x_{t}-(x-x_{0}) \tag{1}\]  
\[y_{tc}=y_{t}-(y-y_{0}) \tag{2}\]  
where \(x\) and \(y\) refer to the respective latitude and longitude of the robot computed with pose estimation from the geolocated reference point. \(x_{0}\) and \(y_{0}\) refer to the GPS position received when pose estimation occurred. \(x_{t}\) and \(y_{t}\) refers to the current received GPS position at time t. \(x_{tc}\) and \(y_{tc}\) refers to the FAGPS corrected coordinates at time t.  
Once these calibration coordinates have been captured the robot will operate in the compensated mode as outlined in Figure 3. In the compensated mode the FAGPS acquired calibration offsets are applied to all new GPS data to improve accuracy. It is expected that the area of operation will be relatively small (as this is primarily designed to improve the navigation performance for small unmanned robots), so an area of operation within 10 km should be reasonable. The other problem relates to the time interval before reacquiring a FAGPS compensated point as changing atmospheric conditions will increase uncertainty over time. The process of reacquisition is easy and efficient to do in a This has been improved in the introduction with references and descriptions of the sources of error over time relating to drift over time and why this causes the aging of the correction values. DGPS scheme (typically performed at most every few minutes) but for the proposed FAGPS technique it requires going back to and reacquiring calibration data from a fiducial point. One way to improve efficiency of calibration reacquisition would be to have a series of fiducial points and the robot can each time go to the closest point to reacquire data.