Performance Analysis of Doppler Aided Tracking Loops in Modernized GPS Receivers
The GPS modernization program deploys new generations of satellites designated Block IIR-M, Block IIF and Block III-A, equipped to transmit multiple civil signals. The coexistence of multiple civilian signals in a GPS receiver has been a focus in recent research activities. In the literature to date, correlator outputs of individual channels are combined in ways either to increase the power available for processing or to reduce the error in tracking measurements. These combinations are typically made at the discriminator level, after the discriminator or after the loop filter. The combinations have been considered both for channels operating on the same carrier frequency (e.g. data and pilot channels of L5) as well as channels across different carrier frequencies. In a modernized GPS receiver, tracking multiple civil signals, the relationship between Doppler frequencies (proportional to the corresponding carrier frequencies) of all signals is known and therefore Doppler estimates of one PLL can be utilized by another PLL tracking a different frequency. While such benefits are conventionally achieved through ultra-tight GPS/INS coupling, this paper presents performance analysis of a Doppler-aided tracking loop architecture, where the Doppler assistance is obtained from another loop in the same receiver. L1 C/A and L2C are the two civil signals selected for the analysis, because this pair of signals will be available long before any other combination. The analysis, however, is relevant to any pair of signals from a single satellite. The L2C carrier, having the pilot channel and relatively slower dynamics, is used as aiding source in the considered arrangement. The L2C PLL, operated with a wide loop bandwidth, absorbs the dynamic stress and provides Doppler estimates to the L1 PLL (affected by wide-band interference) operating at narrow-band. The tracking error associated with such collaboration between two tracking loops is analyzed. It is shown that a 7 dB-Hz RFI margin (equivalent to that achieved with a moderate quality inertial sensor), can be achieved through the local Doppler-assistance.
Receiver Algorithms for the New Civil GPS Signals
The United States is planning to implement two new civil
signals on the GPS satellites in the next few years. The L2
Civil (L2C) signal will be implemented beginning with
the first Block IIR-M satellite anticipated to be launched
in 2003. The L5 signal will be implemented beginning
with the first Block IIF satellite anticipated to be launched
in 2005. Both new signals include design features that
make them very different from the only existing civil GPS
signal – the L1 C/A-code. This paper describes, in great
detail, receiver algorithms for processing the L2C and L5
signals including code and carrier tracking loop
implementations and digital pulse blanking
mechanizations (for L5). The paper also provides
performance evaluations for these algorithms, based upon
analysis and simulation results.
New Low-Profile Applicators for Local Heating of Tissues
A new type of low-profile applicator is described giving efficient power transfer to tissue with low leakage. It consists of a resonant element spaced from a ground plane by low-loss material having high relative permittivity and some relative permeability. This combination is covered by similar material which can be placed in direct contact with the tissue to be heated, or separated from it by a water bolus as required. Laboratory measurements using phantom materials show that these applicators are well matched and produce well defined and relatively uniform heating profiles. Applicators having maximum dimensions of 3-15 cm covering the frequency range 1000-100 MHz, respectively, are described. Penetration depths of at least 4 cm at 100-200 MHz are achieved. Tests on anesthetised pigs and some clinical assessment have confirmed the usefulness of this compact low-frequency type of applicator and have shown that performance compares favorably with that of other recently reported types. The design can be optimized for treatment of specific sites, and suitable choice of dimensions and electrical characteristics of the layer material allow the low-profile concept to be extended over a wide range of operating frequencies.