This paper proposes an improved Time-of-Flight (ToF) estimation algorithm that allows sub-mm position estimation and reduces the power consumption of the receiver. The former is ensured by employing a multicarrier ultra-wide band (UWB) signal. The latter is achieved through sub-Nyquist sampling allowing to decrease the rate of the analog to digital converter (A/D) and digital computational cost. This paper derives the necessary conditions to use sub-Nyquist sampling while keeping the high precision obtained from an UWB signal. These conditions involve ranging signal construction and using a phase-based ranging algorithm adapted to sub-Nyquist sampling. The resulting algorithm achieves sub-mm ranging at 0 dB signal to noise ratio (SNR) by sampling a 8.76 mu s long signal burst of 6 GHz bandwidth with 343 MHz A/D rate. Moreover, an energy scalable receiver architecture based on this algorithm is proposed. The scalability allows dynamicaly adapting the A/D rate and power consumption to the received signal strength meeting a required estimation precision.