The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008
* GPS Antenna Stability
■ Multi-path
■ Easy access
■ Network Geometry
■ Electrical Power and Communications for Real Time
Kinematic (RTK) Surveys
■ Equipment Protection
According to the island topography, the distance between
stations of the network must be under 35 Km, while the height
could vary from 100 m up to 1900 m.
In addition to the aforementioned key factors, other important
details like:
■ Monument construction
■ Antenna installation
■ Water proofing
should be taken into account.
Special attention should be given to the radio noise sources and
in general the interference which can be assumed as originated
from these sources. As it is known, the strength of the GPS
satellite signal is very low, so nearby sources of electrical or
radio noise can cause significant problems by interfering with
the GPS signals. These types of noise may originate from:
■ Electrical transmission lines
■ Nearby commercial radio or television broadcast stations
■ Radio dispatch stations
■ Police, fire & other emergency services
■ Taxi services
■ Pick up and delivery services.
■ Airports
All these parameters and restrictions have been taken into
account while designing the PSI network and quality of signal
tests were performed (Leica 2006). The designed network
geometry is depicted in Fig.l
3. NETWORK ASSESSMENT
3.1 The Challenge of VRS
Recent developments in differential GPS services are focusing
mainly on the reduction of the number of permanent reference
stations, required to cover a certain area, and on the extension
of baseline lengths between reference and rover receivers. The
most advanced technique nowadays is the Virtual Reference
Station networks. The name of this method results from the fact
that artificial observations for a (non existing) ‘virtual’ GPS
station are created, using the real observations of a multiple
reference station network.
The concept idea is that a user is activated inside a network of
permanent reference stations, that are connected with a central
control station. User receiver transmits its navigated position to
the central station. The communication is usually performed
with cellular phones (GSM) and in future with Universal
Mobile Telecommunication Service (Hada et.al, 1999,2000).
The contribution of the network is the knowledge of the errors
behaviour, specifically of those that are distance dependent. So,
ionospheric and tropospheric refraction and orbit errors are
modelled and can be interpolated at any position inside the
network. The artificial observations for the user approximate
location are created and then transferred (through RTCM
format) back to the user (rover station). On the rover side,
standard RTK or DGPS algorithms are used to obtain the
correct position. The result is the increment of baseline lengths
and the reduction of initialization times (Rizos, 2002, Fotiou
and Pikridas 2006).
During these projects, the PSI network was established and the
VRS method was tested with C/A code data using custom-
developed software. The results showed always a positional
accuracy of better than 1 m.
3.2 Description of Applications
In the next paragraphs, a number of test cases, based on the PSI
network, are presented. These span a wide spectrum of different
geomatics problems, while the main focus remains the proof of
the productivity of the network and the evaluation of its
operation.
For this purpose, different techniques, hardware and software
components have been used. The applications cover also a wide
range of accuracy requirements (from several cm to several m)
and all of them have been performed in Cyprus.
3.2.1 Case 1: Application of VRS Technique
In order to test the accuracy achievements of the virtual
reference station concept, a test network, consisting of three
dual frequency GPS receivers, was temporary established in the
broader area of Pafos. The network was first measured and
adjusted in order to determine accurate coordinates. Inside the
area (triangle) of the three permanent stations a low cost (less
than 3.000 €) rover receiver was also activated to perform a
kinematic chain of about 2 hour’s duration. The survey took
place almost 20 Km away from the closest station. A user
friendly VRS software called “3VRS” was developed and
artificial C/A code observations were created for the first
navigated position of the rover receiver using the data of the
closest station. Investigating the effectiveness of the VRS
technique three different solutions were derived.
■ Phase solution with fix ambiguities
■ DGPS solution with post-processing
■ DGPS solution using VRS data
The results from the comparisons between the various solutions
show a precision for the X and Y components better then 0.5 m
and about 1.5 times worse for the Z component (Table 1).
Therefore our tests prove that an accuracy increment for this
kind of baseline lengths can be obtained when the VRS method
is applied.
Difference of
components
Maximum
value (m)
Minimum
value (m)
Mean
value (m)
AX
0.62
0.14
0.26
AY
0.74
0.16
0.32
AZ
0.84
0.01
0.65
AS (position)
1.28
0.21
0.75
Table 1. Characteristic values of the differences of the
component and position between the VRS (DGPS) solution
(C/A code and LI frequency processed only) and the one
derived from the phase measurements
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