Phase Modulated DGPS Transmitter Implemented with a Clamped-Mode Resonant Converter SERGIO A. GONZ ´ ALEZ MARÍA IN ´ ES VALLA, Senior Member, IEEE CARLOS H. MURAVCHIK, Senior Member, IEEE UNLP Argentina A clamped-mode resonant converter (CMRC) is proposed to be used as a transmitter which broadcasts correction signals in a differential Global Positioning System (DGPS). The digital information of the DGPS modulates the carrier with different modulation methods minimum shift keying (MSK) (recommended for the marine radiobeacon band), stepped binary phase shift keying (S-BPSK) and BPSK. The first two methods allow keeping the converter’s operation in soft switching mode. Extensive trials under impedance mismatch and modulation conditions confirm that the soft switching mode is preserved. Manuscript received March 20, 2003; revised December 8, 2003 and March 1, 2004; released for publication March 30, 2004. IEEE Log No. T-AES/40/3/835888. Refereeing of this contribution was handled by W. M. Polivka. This work was supported by National University of La Plata (UNLP), Agency for the Promotion of Science and Technology (ANPCYT), National Research Council (CONICET), and Buenos Aires Province Research Council (CICPBA). Authors’ address: Laboratory of Industrial Electronics, Control and Instrumentation (LEICI), Electrical Engineering Dept., UNLP, CC 91, 1900 La Plata, Argentina, E-mail: (sag@ing.unlp.edu.ar). M. I. Valla and S. A. Gonz ´ alez are also with CONICET. C. H. Muravchik is also with CICPBA. 0018-9251/04/$17.00 c ° 2004 IEEE I. INTRODUCTION The differential Global Positioning System (DGPS) concept was created to improve the position estimates for several civil applications (land transportation, precision agriculture, maritime traffic, civil aviation, etc.). In DGPS, Earth stations whose positions are accurately known measure the difference between their actual positions and the GPS measurements. With this information they generate and transmit a correction signal to a GPS receiver in an unknown location (within radii of a few hundreds of kilometers to keep correlated GPS errors). This system is also employed in several countries to enhance maritime safety. In particular, U.S. Coast Guard (USCG) provides the transmission of the differential correction signal in the radiobeacon band (285 kHz—325 kHz). This service operates from about 50 remote broadcast sites [1—4]. A DGPS-radiobeacon broadcast network is attractive since it does not interfere with the marine and aviation direction finders. The radiobeacon band is available at low cost and it propagates in ground wave mode, so it reaches distances which are well beyond the radio horizon. It is also very attractive for applications other than navigation, like precise surveying and geological studies. Different kinds of RF power amplifiers can be employed to broadcast the correction signal from the known location (Earth station) to the unknown location (GPS receiver) in the radiobeacon band. The power amplifier is required to have low energy consumption and harmonic distortion. To reach an efficiency higher than 80%, it is necessary to use tuned power amplifiers class D, E, F [6—9]. The main characteristics of the resonant amplifiers are as follows. 1) They require a high quality factor (Q), in order to obtain a sinusoidal voltage on the load from a square wave with a given mean value at the input of the amplifier. In resonance, the peak voltage on the elements of the resonant network is Q times the source voltage. 2) In low frequency amplifiers the required reactive components are large and heavy, increasing the power loss involved in this components. 3) Optimal design of the resonant amplifiers does not tolerate load variations. With impedance mismatch, high voltage or high current on the power switches appear. One way to improve the design of the amplifier is to obtain a voltage with low harmonic distortion at the input of the resonant circuit. In this way it is possible to use a low-pass filter to minimize the remaining harmonics. The design of a voltage clamped-mode resonant converter (CMRC) [13—15] as an RF power amplifier is proposed here. This amplifier is built with a bridge power converter which generates a zero mean value voltage at the input of a low pass filter. Besides controlling the “clamped voltage” to zero during a convenient time interval, it can also eliminate or IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 40, NO. 3 JULY 2004 919