## 1 Introduction  
Wireless sensor networks (WSNs) are widely deployed in the Internet of Things (IoT) to monitor valuable objects such as rare animals or armies in battlefields [1]. Once an object is detected, the sensor nearest to the object, _i.e._, the source, would periodically send information about the object to the base station [2]. Generally, an attacker could trace the source and then capture the valuable object by localizing the source, therefore, many protocols for source location protection are proposed to protect the object by preventing attackers from localizing the source.  
A typical source location protection model is the panda-hunter model [3, 4]. In this model, sensors are deployed to monitor pandas. Once a panda is monitored, the source would periodically generate and transmit packets through the sensors one by one to the base station. In order to capture the panda, the hunter starts from the base station and tries to locate the source by tracing the packets back hop by hop. The essential idea of existing source location protection protocols includes two major steps. First, a packet is transmitted from the source randomly to other sensors for _h_-hops, before it arrives at a location, called the phantom location. Second, the packet is transmitted from the phantom location to the base station through flooding or the shortest path routing [5, 6, 7]. The first step should generate phantom locations that are not only widely distributed, but also far away from the source [8], so that it would be more difficult for an attacker to trace back to the source even if it has located the phantom location. However, the phantom locations generated by existing works [3, 4, 9] usually gather in two regions (we will prove this point in Section 2) and they are not guaranteed to be far away from the source. As a result, the random _h_-hops in the first step cannot effectively misguide an attacker. To address the above limitations, we first propose a protocol for source location protection based on limited flooding, named SLP, which improves source location security with low communication cost. Considering more powerful attackers with wider fields of vision, we further propose an enhanced protocol for source location protection, named SLP-E. In a nutshell, our contributions are mainly twofold: first, we propose a source location protection protocol named SLP to generate phantom locations that are not only far away from the source, but also widely distributed. Both theoretical analysis and simulation results show that SLP provides more secure source locations than typical source location protection protocols [4, 9]. The average safety period is increased by nearly an order of magnitude.  
Second, we propose an enhanced source location protection protocol based on SLP, named SLP-E, to protect against more powerful attackers with wider fields of vision. SLP-E further improves the source location security with low communication cost. The performance of our SLP-E is validated by theory and simulation, respectively.  
The remainder of this paper is organized as follows: in Section 2, we review the previous work about source location protection in WSNs and investigate the limitations of previous works. Section 3 provides the problem definition. Sections 4 and 5 introduce in detail SLP and SLP-E, respectively. The communication cost and security performance for our protocols are analyzed in Section 6. Simulation results are given in Section 7. Finally, Section 8 concludes this paper.