B2. Istanbul 2004
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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
Public Internet
Cellular network
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Firewall
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Figure 3. Collecting GPS corrections via GPRS
Server of RTK corrections
2.5 Towards a mobile NTRIP
The implementation of the broadcasted CMR+ messages via
Internet is based on the condition that the server belongs to a
local area network. To carry out an entirely mobile solution
implies two simultaneous connections of GPRS modules that
are equipping the base and rover stations with Internet access.
Unfortunately, this is difficult to achieve as the cellular phone
operators dynamically distribute private IP addresses to SIM
cards that consequently do not accept any entering connections.
To overcome this drawback, two solutions are foreseeable:
e Come to an agreement with a cell phone service
provider in order to obtain routable IP addresses for
SIM cards that require GPRS communications.
e Carry out a GPRS connection of the base and rovers
to the fixed IP address of an Internet server, which
will authorize the exchange of GPS data.
The tests of the first solution have led to some promising results
as some cellular providers are interested in maximizing the flow
of broadcasted information over their network.
2.6 Field tests
The cell phone operators allocate a portion of their
infrastructure to GPRS, which decreases the voice capacity and
thus challenges the service quality. As opposed to voice, data
transmission is particularly sensitive to the network design
since each of its bytes is equally meaningful.
An interesting indicator of the availability of the cellular
network is the Signal Quality Measure, as defined in GSM07.07
recommendation. Ranging from 0 (no signal) to 32 (excellent
reception), such measures can be obtained by a periodic
invocation of the Hayes command A7+CSQ. The results
reflected in Figure 4 illustrate the constant coverage of
Lausanne and its neighbourhood.
Consequently, the GSM network in Switzerland is highly
suitable for the broadcast of RTK solutions, since there is no
significant degradation of performance compared to use of
other transportation media (Figure 5). Nevertheless, particular
attention should be paid to the conversion of serial GPS
messages to TCP datagrams, as its misconfiguration enables
only DGPS-code positioning accuracy.
Index
20 /
Signal Quality Measure
-- Frame Error Fate
;
SP RP PP ? P? er. P? LP € » e
BS SW SEF Fy oq qd X WX od
Figure 4. GSM quality in poorly served areas in Lausanne
MEM XT = Ah [DILE
Figure 5. NTRIP results at EPFL (baseline « 1 km)
3. ENHANCING THE IMAGING COMPONENT BY
CMOS TECHNOLOGY
3.1 CCD cameras in mobile mapping
Traditionally, the vehicle-based mobile mapping systems use
CCD cameras as the imaging component. For references,
consult GPS-Van'" (Bossler et al, 1991), VISAT (El-Sheimy et
al, 1999), TruckMap™ (Pottle, 1995), KiSS (Hock et al, 1995),
GPS Vision (He, 1996) and GeoMaster (Tamura et al, 1998).
Specifically developed for imaging applications, CCD (charge-
coupled device) technology and fabrication processes are
optimized for obtaining the best possible optical properties and
image quality. A CCD comprises photosites that are arranged in
an X-Y matrix of rows and columns.
Serial |
Clocks jj. dy Output
Amplifier
— €
Parallel
Clocks |...
Photodiode
shift _~
Register i EH 5 3 Hn ul =
(masked) 3 T | Direction of
n cm i. Parallel Shift
Figure 6. CCD structure
Each photosite incorporates a photodiode and an adjacent
charge holding region, which is shielded from light. These
photodiodes converts light (photons) into charge (electrons).
The number of electrons collected is proportional to the light
intensity. Typically, light is collected simultaneously over the
