Measuring GPS Signals Indoors

Precise measurement of GPS signals inside buildings is made possible by a combination of high resolution analog to digital conversion, a separate reference channel from an external antenna, and long coherent averaging times. We simultaneously downconvert the signals from both outdoor and indoor using identical downconverters with the local oscillators phase locked. These signals are then digitized at 25.6 MHz with 12 bits in records as long as 160 milliseconds, and are stored in RAM. The outdoor signal is then processed via FFT-based cross correlation calculations in MATLAB . These results are used to account for Doppler shift and remove navigational data bits from the indoor signal. After calibrating relative cable delays and path gains, we make precise measurements of the relative signal strengths and times of arrival (TOAs) of the indoor and outdoor signals. We detect indoor signals as much as 60 dB below their signal levels outdoors and can consistently find five or more satellites at any given time. Due to short-delay multipath contamination of the indoor signal, the TOAs contain an average error on the order of 200 ns, which translates to a pseudorange error of nearly 60 meters. In an attempt to remove these errors, TOAs were recalculated by measuring the slope of the leading edge of the cross correlation function. No significant improvement was obtained by this alternate method.