tIMestAMPIng At the trAnsMItter
On the nRF24Cheep platform, an advertisement packet is transmitted by loading the TX FIFO and pulling the CE pin to a high state. One of the fields that is pushed into the SPI buffer is the current timestamp value. A second timestamp is also recorded once the TX_DS interrupt is seen by the microcontroller (i.e., when the radio’s IRQ pin is pulled down low), signaling the success of the transmit event. The difference between these two timestamps provides an accurate estimate of the time delays incurred due to send, access, and transmission; and this information is encapsulated into the next advertisement packet. As discussed above, due to the high determinism of send, access, and transmission time delays on the nRF24Cheep beacon platform, our implementation uses a sin- gle broadcast packet (instead of two) to reliably synchronize the transmitter and the receiver. The timestamping characteristics at the transmitter end are shown in Fig. 2, depicted as a histogram show- ing the distribution of the transmitting time interval recorded for 35,000 broadcast packets. The distri- bution appears Gaussian with best fit parameters of = 0.201829 s and a minuscule s2 = 5.19537e–07 s. This latency characterization supports the deter- minism of our approach on the transmitter end.