## 2 Related Works
### Hybrid Methods  
There are other papers where the authors propose hybrid systems. In [24], the authors propose a radiolocation scheme based on the AOA and the TOA in multipath environments with a single basestation. This scheme is used in macrocellular networks (such as the Code Division Multiple Access cellular network) and Global System for Mobile communications (GSM) networks. In [25], the authors combine RSS measurements and the TDOA measurements. This model is sturdy to variations of measurement noise and quantization. The error is lower than ones based on individual measures.  
There are few works related with the hybrid location techniques in sensors networks. In reference [26] Cramer-Rao Bound (CRB) is used for location estimation using of two different hybrid schemes: TOA/RSS and TDOA/RSS. These techniques provide improved location accuracy with respect to TOA and TDOA schemes for networks with devices having communications ranges of 30 meters or less.  
In [27] [PERSON] and [PERSON] developed a hybrid location estimation scheme for heterogeneous WSNs with unsynchronized short range simple relays and mobile sensor nodes, and synchronized stations. In this work, the authors use RSS measurements as well as TOA and TDOA measurements. These measurements are used to filter out the clock offset that appears due to the lack of synchronization. They quantify the estimation accuracy of the scheme by deriving the Cramer-Rao bound (CRB), and discuss the performance trade-off between the number of synchronized and non-synchronized devices involved. This work takes into account the heterogeneity of sensor networks, in terms of communication range, time synchronization and routing capabilities of network devices.  
If we analyze the hybrid location systems in WLAN, regardless of whether they are sensor networks or any network, there are several proposed systems. In [28] the authors propose a hybrid location system based on three stages. Firstly, it establishes a database that contains a distance-signal strength map. Next, the system uses the database to obtain the distances between mobile terminal and base stations. Thirdly, this proposal applies trilateration to calculate the mobile terminal position. In this case, we see that the hybrid modeling has better accuracy than propagation modeling.  
Finally, another hybrid location method is proposed in [29]. This hybrid method has two stages. In the first stage, it uses the fingerprinting method with a fast training phase to obtain an estimate of the mobile user (MU) position. In the second stage, trilateration is used to compute the MU location more accurately. The result shows their proposed method is better than the simple trilateration method based on general propagation mode, but worse than the fingerprinting method with a medium training phase.  
The Euclidean models are optimum when there are multiple access points. Although some works show that the statistical properties of the RSS signal is stationary under certain circumstances, the distribution of the RSS is not usually Gaussian, it is often left-skewed and the standard deviation varies according to the signal level. Signals from multiple APs are mostly independent and the interference from other APs using the same frequency does not have a significant impact on the RSS pattern. Consequently, the coverage areas can be grouped together as a group of clusters. More than one cluster may represent one location because of the multimodal distribution of the RSS. In such a case, using a simple Euclidean distance to determine the location may easily classify some patterns into a wrong location. Our proposal combines the advantages of the deductive and inductive methods in order to provide more accurate measurements in hard environments (few base stations and/or few trained points).