# Augmenting GPS with Geolocated Fiducials to Improve Accuracy for Mobile Robot Applications
## 1 Introduction  
GPS (Global Positioning System) is a global absolute navigation system which uses one-way time of arrival (TOA) ranging from a constellation of satellites to compute a reasonably accurate three-dimensional position [1]. Since the removal of selective availability in 2000 (which significantly improved accuracy) and the inclusion of GPS receivers in most smartphones, GPS has become a near ubiquitous system, particularly when paired with GIS (Global Information Systems) for navigation and mapping services. GPS is also used as a precision timing reference across a variety of industries including synchronization of cell towers, timestamping financial transactions and synchronising scientific instruments [2]. GPS (originally known as NAVSTAR GPS) is the oldest Global Navigation Satellite System (GNSS) with planning and development starting in the early 1960's leading to full operational capacity, with a constellation of 24 satellites, being reached in 1995. A number of alternate systems exist or are being developed including the Russian Global Navigation Satellite System (GLONASS), the European GALILEO system and the Chinese BeiDou Navigation Test System (BINTS). Although the majority of low cost receivers currently only support traditional GPS, the proliferation of these additional constellations and the prospect of improved positional accuracy has lead to the development of GNSS receivers which can provide positional estimates by augmenting data across a range of different satellite constellations [3]. GPS has several sources of error which effect accuracy including changing atmospheric conditions, multipath, clock discrepancies and receiver noise [4; 5; 6; 7; 8]. These error sources each contribute with different time horizons. For example multipath effects (where the receiver receives both direct and reflected signals) may be relatively static in nature but then may change rapidly in the case of moving receivers and significant changes in the environment (e.g., the closing of a stadium roof) [7]. In contrast, atmospheric errors (ionospheric and tropospheric) tend to change more slowly. Atmospheric errors are caused by changes in atmospheric conditions (e.g., temperature, humidity,pressure) which contribute to varying degrees based on satellite elevation [8]. Likewise orbital errors and satellite clock errors also have relatively slow changes over time [6; 8]. Besides using other constellations there are a number of technologies used in the augmentation of traditional GPS to improve performance. These technologies include Assisted GPS (AGPS) to improve cold start performance, Differential GPS (DGPS) to reduce error and Inertial Navigation Systems (INS) to provide navigation in GPS denied areas [9; 10]. An alternate scheme to DGPS (but not as accurate), WAAS (Wide Area Augmentation Schemes) involves geostationary satellites broadcasting correction data based on a network of widely dispersed ground based stations [1]. Another correction approach is to use Real Time Kinematic GPS (RTK-GPS) which produces highly accurate navigation with a close (within 10 km) base station transmitting correction data over a radio link [11]. DGPS systems have seen widespread use where greater precision is required than traditional GPS. This includes applications such as precision farming (with GPS guided machinery) and marine navigation [12]. Fiducial markers are artificial landmarks which are added to a scene to facilitate registration of points in images or between images and a known model [13]. Fiducial markers are commonly used with pose estimation for robot navigation, biomedical image registration, augmented reality applications and industrial applications (e.g., PCB fabrication) [14; 15; 16]. Fiducials have previously been used in GPS-denied environments to augment odometry and inertial navigation systems (INS) to facilitate more accurate navigation through association with a ground-truth [17; 18; 19].