component. The autonomous accuracy is about 100 meters 
(2dRMS). These accuracy specifications apply with Selective 
Availability active and when at least 5 satellites are visible, 
PDOP is less than 6, good signal to noise ratio and the satellite 
elevation mask is set at 15 degrees. Accuracy specification may 
change if Selective Availability is deactivated or modified. The 
receiver is capable of receiving RTCM SC-104 standard format 
broadcast from a Trimble reference station or from a provider, 
like OMNISTAR. Ionospheric conditions, multipath signals or 
obstructions of the sky by buildings or heavy tree canopy may 
degrade accuracy by interfering with signal reception. Optimal 
accuracy is obtained by collecting data in an environment that 
is devoid of large reflective surfaces and also has a clear view 
of the sky, as happen along a highway. 
The main advantage of implementing with GPS existing road 
survey systems is the integrability (Greenspan, 1996) with the 
sensors mounted on board, which usually includes an odometer. 
In such cases, the integration allows for establishing a rigorous 
correspondence between the relative mile chainage measured by 
the odometer and the absolute GPS co-ordinates. Furthermore, 
the odometer guarantees the continuity of the positioning when 
interruptions in the GPS signals occur. 
2.1 Differential GPS 
The differential GPS technique is based on two receivers 
operating simultaneously: the one located in a known position 
is called REFERENCE or MASTER STATION, and the second 
one located in the unknown position. If the last one is moving, 
it is called NAVIGATOR o ROVER and the technique is 
named DGPS kinematic. DGPS kinematic method is based on 
the measurements of the length of every segment joining the 
MASTER and the ROVER stations; moreover the angle that 
such segment forms with the North of the reference system and 
the slope of the segment are also measured. Therefore the co 
ordinates of the ROVER station are known relatively to the 
fixed MASTER position. 
It should be noted that the achievable accuracy depends on the 
type of GPS observables (phase or code) which can be acquired 
by the adopted receivers and on the inter-distance between the 
stations (Seeber et al. 1995). 
2.2 Real Time Kinematic Differential GPS 
In case of the two receivers being joined trough a radio link or 
mobile phone communications, the position of the ROVER can 
be determined in real time (Real Time Kinematic). The raw data 
of the stations are immediately elaborated from a software 
installed in one of the two receivers (generally in the 
MASTER) and the corrected position is sent to the other one 
(ROVER) with a maximum delay of a second. Since the radio 
connection use an assigned low frequency band to avoided 
interruptions in the transmission of the corrected data it is 
necessary to maintain the distance between the two stations 
within lCP-20km. In this way the relative accuracy of the RTK 
technique is really high (1 O' 5 10' 6 ) and do not require any 
further post-processing of the data. 
2.3 Post Processing Differential GPS 
Alternatively, a post-processing differential procedure can be 
applied to the raw data downloaded from the memory of the 
two receivers. The post processing solution is not strongly 
limited by the distance between the two receivers and provides 
co-ordinates of high accuracy ( 1 O' 6 10‘ 7 ) (Scwharz et al., 
1989). 
Since most of the highway surveying requires an accuracy 
requirements are within 1 2 meters, both DGPS techniques 
can be adopted even though for different applications. The RTK 
method is a fast procedure only suitable for small range survey 
owing to the limitation of communication link. The PP method 
is more time consuming but it can be applied for positioning 
ROVER receivers at a distance of about 100km. 
This technique can be applied over the entire national highway 
network, provided that an optimised reference station network 
is established. The co-ordinates of the vertices of the network 
have to be previously determined. A limited number of 
MASTER receivers can be moved in the specific area of 
interest. As an example, a network covering the highway 
managed by the Società Autostrade was designed and shown in 
Figure 1, where the centres of the circles correspond to the GPS 
station locations. 
Figure 1 GPS Reference Station Network configuration that 
covers the highway system managed by the Societa Autostrade 
2.4 RTCM-104 Differential GPS