GPS World, January 2016

Type Order a1 Bandwidth Hz 1 IIR 1 0005 080 2 IIR 1 0001 016 3 IIR 1 00005 008 4 IIR fwd bwd 1 0001 010 5 BC 2048 0001 012 6 BC GDC 2048 0001 012 7 BW 2048 0001 014 JANUARY 2016 WWW GPSWORLD COM GPS WORLD 57 temperature range 40 to 85 C for pseudorange code positioning However high accuracy carrier phase positioning techniques require uninterrupted carrierphase tracking producing more challenging requirements for the receivers oscillator Here we extend that research to demonstrate the feasibility of using a TSMO for carrier phase positioning BACKGROUND The MEMS resonator used here has an approximately 150 ppm frequency drift over the temperature range of 40 to 85 C which is about three to five times greater compared to a standard crystal The integrated temperature sensor provides very good thermal coupling with the resonator enabling accurate frequency estimation once the frequency versus temperature function FT polynomial is estimated This FT polynomial can be estimated by periodically measuring the frequency and temperature at different temperatures and fitting the FT polynomial to the measurements After this calibration stage the oscillator frequency error can be estimated using the temperature measurement and the polynomial only This frequency error can aid the GNSS receiver for acquiring and tracking signals As the temperature measurements are affected by noise which is also amplified by the FT polynomial producing frequency noise in the receiver the temperature measurements can be filtered accordingly to reduce noise METHODOLOGY Temperature compensation of the oscillator frequency can be beneficial in scenarios with fast changes in temperature and therefore fast changes in frequency or when operating the oscillator at extreme temperatures where temperature sensitivity is more pronounced The TSMO implements an onchip integrated temperature sensor in close proximity to the resonator and provides an accurate estimate of its temperature We first examine more complex and non real time capable filters to assess performance improvement and limits of bandwidth reduction For the second part of this research where the TSMO based GNSS receivers measurements are used for RTK positioning none of the conditions requiring temperature compensation fast changes or extreme temperatures are met and therefore temperature compensation was not applied Temperature Measurements Filtering When temperature compensation is applied filtering of the chip integrated temperature sensor measurements is performed to reduce measurement noise introduced by the temperature measurement circuit As the signal frequency and phase from the satellite can under negligible ionospheric scintillation conditions be assumed significantly more accurate and stable than the local oscillators carrier replica common errors in the received signals carrier frequencies can predominantly be accredited to the local oscillator Therefore under the condition of a defined tracking loop estimated frequency accuracy and phase tracking stability are suitable measures of the local oscillators short term frequency and phase stability as well as the influence of the temperature compensation The temperature compensation method is being digitally applied to the digitized IF signal as a first stage in the software receiver FIGURE 1 For generating this signal a filtered version of the raw temperature measurements is generated and a function temperature compensation or FT polynomial to convert those temperature measurements to local oscillator frequency estimates is applied The digitized IF samples of the received signal as well as the frequency estimates from the temperature measurements are then processed by the GSNRx software GNSS receiver developed at the University of Calgary Satellite specific phase lock indicators PLI as well as FIGURE 1 Temperature compensation and signal processing structure TABLE 1 Filter implementations for temperature measurements