The IEEE 802.15.4 standard specifies that a radio's PHY layer
must provide an 8-bit integer value as an estimate of the
received signal power. This value is commonly known as the
Received Signal Strength Indication (RSSI) in the wireless
sensor networks (WSN) community. While many protocols directly
use the measurements that the radios provide, the standard only
requires that the reported RSSI values should be linear and
within ±6 dB of the actual RSSI values. However, ±6 dB is a wide
error margin. For example, Packet Reception Ratio (PRR) can
decrease from 100% to 0% with a 2 or 3 dB difference in the
received signal strength. We examined two 802.15.4 compliant
radios, the widely used TI CC2420 and Atmel AT86RF230, and
discovered that they indeed introduce systematic errors when
providing the RSSI readings. Therefore, we also developed a
calibration scheme to effectively minimize the adverse effects
associated with inaccurate RSSI values.