Evidential problems with GPS accuracy: device testing
In the last few decades there has been an exceptional growth in the use of the GPS (Global Positioning System). The usage of GPS navigation has escalated from the military area and commercialised for civilian use. Since the Selective Availability was turned off in the year 2000, the accuracy of civilian GPS has improved from 100 meters to 20 meters. In the last decade, several improvements to the GPS have been implemented, including new signals for civil use and better accuracy and integrity for all users. As a result, the GPS applications are now widespread in aviation, rail, marine, and particularly in passenger cars and personal handheld devices. This research builds on research reported in relevant publications and focuses on the immediate variables external to a GPS device such as cloud cover, weather, obstructions, split signals and user preferences, and tests the accuracy of three GPS devices. The testing was conducted in three types of weather conditions (sunny, cloudy and rainy) and four environmental conditions were selected (tree canopies area, a suburban area, a city street with tall buildings and an indoor environment). Based on the literature reviewed these represented a 100% tree canopy, a varied percentage of canopy open and a closed canopy. The proposed testing methods therefore measure the travelled distance for each device in each weather and canopy combination, to analyse the accuracy and evaluate the possible factors that influence the accuracy. The research project showed that the environmental conditions, GPS technique employed and the speed of movement all influence the GPS accuracy. These results confirm the results found in the literature regarding the impact of different canopy types on GPS accuracy. The findings of this study have implications for the way a digital investigator must audit and report evidence extracted from GPS devices. Future research is also required to further explore other factors that are found to influence GPS accuracy by other studies. Findings from such research will further the understanding of GPS accuracy in digital forensic investigations and will improve the presentation of GPS related evidence in courts.