Justin is an electrical engineer working for ZenWorks Navigation, a Global Positioning device manufacturing company based in Las Vegas. Justin and a team of other engineers are working on the latest GPS handheld system for the company. ZenWorks previously only produced GPS systems for airplanes, but now wants to branch out to the individual consumer market. Currently, Justin is trying to work out errors the devices are experiencing in regards to four variables (latitude, longitude, altitude, and time) on the accuracy of a three-dimensional fix. Until this issue is resolved, the new devices cannot be finished. What GPS-related issue is Justin currently working on?