Despite the prevalence of GPS services, they still suffer from intermittent positioning with poor accuracy in partially shadowed regions like urban canyons, flyover shadows, and factories' indoor areas. Existing wisdom relies on hardware modifications of GPS receivers or power-hungry infrastructures requiring continuous plug-in power supply which is hard to provide in outdoor regions and some factories. This paper fills the gap with GPSMirror, the first GPS-strengthening system that works for unmodified smartphones with the assistance of newly-designed GPS backscatter tags. The key enabling techniques in GPSMirror include: (i) a careful hardware design with microwatt-level power consumption that pushes the limit of backscatter sensitivity to re-radiate extremely weak GPS signals with enough coverage approaching the regulation limit; and (ii) a novel GPS positioning algorithm achieving meter-level accuracy in shadowed regions as well as expanding locatable regions under inadequate satellites where conventional algorithms fail. We build a prototype of the GPSMirror tags and conduct comprehensive experiments to evaluate them. Our results show that a GPSMirror tag can provide coverage up to 27.7 m. GPSMirror achieves median positioning accuracy of 3.7 m indoors and 4.6 m in urban canyon environments, respectively.

翻译：尽管GPS服务已广泛普及，但在城市峡谷、立交桥阴影及工厂室内等部分阴影区域中，仍存在定位间歇性中断且精度低下的问题。现有解决方案依赖GPS接收器的硬件改造或需要持续供电的高功耗基础设施，这在户外区域及部分工厂中难以实现。本文通过GPSMirror填补了这一空白——这是首个无需修改智能手机、仅借助新型GPS反向散射标签即可增强GPS定位的系统。GPSMirror的关键技术包括：（i）微瓦级功耗的精细硬件设计，突破反向散射灵敏度极限，使极弱GPS信号在接近法规限值的覆盖范围内得以重新辐射；（ii）在阴影区域实现米级精度的新型GPS定位算法，并可在传统算法因卫星不足而失效时扩展可定位区域。我们构建了GPSMirror标签原型并开展全面实验评估。结果表明：单个GPSMirror标签的覆盖范围可达27.7米，在室内环境与城市峡谷中的定位中位数精度分别达到3.7米与4.6米。