RTK SURVEY USING COMBINED GPS+GLONASS L1/L2 CARRIER PHASES 
Crocetto N. (i) - Gatti M. (ii) - Marchesini M. (iii) - Negroni F. (iv) - Russo P. (ii) 
(i) Department of Civil Engineering. Second University of Naples, via Roma 29, Aversa (Italy), 
(ii) Department of Engineering. University of Ferrara, via Saragat 1, Ferrara (Italy). 
(iii) Guido Veronesi s.r.l., via Caselle 46, Bologna (Italy). 
(iv) Studio Negroni, via Salvador Allende 32/34, Bologna (Italy). 
ISPRS 
WG VI/3 International Co-Operation and Technology Transfer and 
WG IV/3 Temporal Aspects and Topographic Database Maintenance 
KEY WORDS: GPS, GLONASS, RTK, CARRIER PHASE. 
ABSTRACT. This paper describes some experiments carried out with the RTK technique (Real Time Kinematic 
Survey) to combine GPS and GLONASS measurements on carrier phases LI and L2. These experiments involved: 
1) marking out transverse sections of a road embankment; 2) localising reference points in woodland for a water supply 
tunnel. The measurement technique is described and compared, in terms of productivity and precision, with similar 
techniques using conventional topographical instruments and satellite instruments with GPS measurements only. 
RESUME. Ce document décrit quelques expériences réalisées à l’aide de la technique de cinématique temps réel 
combinant les mesures GPS et GLONASS des phases de support L1 et L2. Ces expériences concernent: 
1) le marquage des sections transversales d’un remblai de route ; 2) la localisation de points de référence en terrain boisé 
pour la construction d’une conduite d’eau. La technique de mesure est illustrée en termes de production et de précision, 
notamment dans le cadre d’expériences similaires faisant appel à des instruments topographiques traditionnels et des 
instruments satellites utilisant uniquement des mesures GPS. 
INTRODUCTION 
The GPS "Real Time Kinematic Survey" (RTK) 
technique differs from the classical GPS (Continuous 
Kinematic) technique in that the relative position of one 
receiver (Rover receiver) with respect to another (Master 
receiver) can be calculated instantaneously (Real Time). 
This technique, tested in Italy since the mid-1990s (Bitelli 
et al., 1994; Gatti et ah, 1995), allows us to extend the 
applications of the GPS to the measurements of 
traditional field surveyor. However, some factors have 
inhibited its more widespread use: 
1. the use of radio modem apparatuses approved for use 
in Italy with maximum power in the UHF band of 1 
Watt; 
2. the GPS satellite configuration, which is limited to 
the contemporaneous presence of 7-8 satellites for 
the measurements and values of PDOP greater than 
2. 
3. the number of components of the signal observed and 
the quality of the acquired signal. 
The range of a single radio link device, at the indicated 
power, is a little more than 1.5 km from the Master 
receiver. This can considerably limit the operational area 
of measurement '. The satellite configuration can instead 
increase the times of static initialisation and dynamic re 
initialisation of the Rover receiver * 2 . The components of 
The range of the radio link device can obviously be increased if one 
uses another radio link as a radio bridge (radio repeater). 
2 For example, with at least 6 visible satellites and a PDOP less than 3, 
the static initialisation with only GPS measurements is performed after 
the signal and its quality can affect the instantaneous 
resolution of a double difference. In unfavourable 
operating conditions or in the presence of electromagnetic 
fields (Gatti et al., 1998), it can be difficult to perform the 
positioning in real time with GPS receivers alone. The 
application of the method is thus limited to open and 
sparsely inhabited zones, i.e. areas in which it is possible 
to receive both the radio link signal and the GPS signal 
from at least 6 satellites without interruptions of the signal 
in one or more components. 
The introduction on the market of receivers designed to 
acquire signals on LI and L2 carrier phases from both 
GPS and GLONASS satellites has greatly increased the 
possibilities of employment of the RTK technique. The 
acquisition of the GLONASS signal combined with the 
GPS signal has led to the following advantages: 
• an increased number of operational satellites, from the 
usually 7-8 for the GPS system to around 10-11 for 
the GPS+GLONASS system (Fig. 1) 
• improved reception of the signal even in the presence 
of electromagnetic disturbances poorly tolerated by 
the GPS receivers alone (Misra, 1993; 1996); 
• decrease of the PDOP (Fig. 2-a and 2-b) (Contreras, 
1998); 
at least 5 minutes of reception without interruption of the signal; the re 
initialisation, which in the GPS receivers with double frequency is 
performed in movement (so-called dynamic initialisation or O.T.F.), 
occurs after 2 minutes of reception, i.e. after having received with 
continuity at least 120 periods after the loss of the signal. If the number 
of satellites decreases or the PDOP changes, the indicated times can 
increase considerably. 
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