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
