You could use a GPS receiver if you are working outside. Of course, RSSI is intrinsically unreliable for distance measurement as it is affected by many factors other than distance (i.e. the signal propagation environment). There are some interesting papers published on using timing measurements (averaged over many samples) from 802.11 access points for triangulation:

Yamasaki, R.; Ogino, A.; Tamaki, T.; Uta, T.; Matsuzawa, N.; Kato, T., "TDOA location system for IEEE 802.11b WLAN," Wireless Communications and Networking Conference, 2005 IEEE , pp.2338,2343 Vol. 4, 13-17 March 2005

doi: 10.1109/WCNC.2005.1424880

Thanks alot

I agree with Daniel. RSSI depends on other factors apart from distance. However for a free space environment, it can reasonably be used to model distance.  To ascertain errors in the measurement of distance using RSSI you can  use existing RSSI models which are functions of distance to determine the distance for different RSSI values and then take actual measurement of distance and RSSI simulteneously. You can compare the RMS errors and residuals between the distance obtained using the models and those from your field data. If the RMS errors are small and the residuals alternate about the zero position between  small positive and negative values then the error is small otherwise it is reasonably large. Note you can also statistically develop your models from your measured data which you can then compare with actual data.