NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

also be able to track the broadband signal E5ab without
splitting the spectrum into sub-carriers. The E5-signals
exhibit a particularly small multipath error due to their large
bandwidth. Future combined GPS/GALILEO dualfrequency
(L1/E1, L5/E5a) receivers can be manufactured
without large technological efforts due to these interoperability
features. The Commercial Service will have a
privileged access to the E6-signal featuring a particularly
large data rate for additional services. The Public Regulated
Service will use E6, too, but uses a different signal modulation.
Like the modernised GPS signals, GALILEO will also
offer a data-free pilot channel which simplifies carrier phase
tracking leading to an increased performance regarding
tracking stability (less cycle slips to be expected).
Almost all facets of the GALILEO signal design evolution
are mirrored in the available scientific literature with
contributions by HEIN et al. (2003), HEIN et al. (2004).
Optimisation work followed (HEIRIES et al., 2005; HEIN et
al., 2005a) and culminated in the latest signal design using
MBOC (Modified Binary Offset Carrier) signal modulation
which will also become an option in the GPS modernisation
programme for reasons of interoperability (HEIN et al.,
2006B; HEIN et al., 2006C; AVILA-RODRIGUEZ et al., 2006).
In addition to this research of signal optimisation leading
to an easy use of both GPS and GALILEO civilian signals,
investigations are also carried out to combine the protected
GPS military and GALILEO Public Regulated Service
(PRS; formerly GAS: Government Access Service) as
described in HEIN and AVILA-RODRIGUEZ (2005), HEIN and
AVILA-RODRIGUEZ (2006). Moreover, intensive interference
computations were carried out in order to confirm that all
existing GNSS can co-exist without any significant loss in
service performance (WALLNER et al. 2006; WALLNER et
al., 2005).
Combined GNSS Positioning
As already stressed, an added benefit of the innovations
taking currently place in satellite navigation comes from
the fact that GPS and GALILEO show a high degree of
interoperability and can thus be easily combined in a GPS/
GALILEO receiver. The advantages of both the dual
satellite-constellation and the advances in signal design are
outlined in AVILA-RODRIGUEZ et al. (2005) for a typical
mass-market single-frequency GNSS receiver. Regarding
precise static positioning for scientific purposes, work on
the combined analysis of GPS and GALILEO signals has
been carried out by HEINZE (2006), and with respect to the
modelling of GPS and GALILEO observations by GRUBER
(2006).
Ambiguity Resolution Techniques
Considerable work has been carried out regarding the
improvement of GNSS positioning algorithms, in particular
taking into consideration the expected benefits from the
innovations of satellite navigation in the future.
The key to high-precision GNSS positioning is the quick
and reliable (correct) ambiguity resolution. Future GPS will
offer signals on up to 3 carriers (L1, L2, L5), GALILEO
T. Schüler: GNSS Positioning 109
will offer signals on up to 4 carrier frequencies without any
further costs (Open Service: E1, E5ab and sub-carriers E5a
and E5b) and even 5 signals when the Commercial Service
on E6 is used in addition. Many new signal combinations
can be used to resolve ambiguities. Signals on nearby
frequencies can be combined to form a “Super Wide Lane”
(e.g. E5a and E5b or L5 and L2). This linear combination,
a kind of “virtual signal”, has a particularly large wavelength
and could be easily fixed to its integer value by
combination of code (pseudo-range) and carrier-phase
measurements without any knowledge of the antenna
position (geometry-free ambiguity resolution approach).
Furthermore, noticeable improvements can be expected due
to the fact that the improved signal structures for GALILEO
and also for GPS L5 allow for a better reduction of the
multipath errors by receiver-internal signal processing
algorithms. This statement is particularly true for the broadband
E5ab/E5a/E5b signals, whereas, in general, shortdelay
multipath signals – not infrequent in geodetic
positioning – will still lead to significant errors (SCHÜLER
et al., 2005b).
A number of authors outline the advantages of using
measurements on more than 2 frequencies for ambiguity
resolution. SAUER et al. (2004) report about test results
using 3 (TCAR – triple carrier ambiguity resolution) in
comparison to 4 carriers as offered by GALILEO; a
multiple-carrier ambiguity resolution approach is shown
in VOLLATH and SAUER (2004). Geometry-free resolution
approaches for 3 and more carrier for both GPS and
GALILEO are depicted in WERNER and WINKEL (2003).
Further contributions comprise investigations on the
difficulties in ambiguity resolution under multipath impact
JOOSTEN and IRSIGLER (2003).
Positioning Algorithms
Remarkable efforts have also been devoted to precise
positioning in general, in particular with focus to RTK
(real-time kinematic) positioning, and also taking the
improvements of the new satellite navigation systems into
account. A clear tendency towards precise positioning
utilizing active GNSS reference networks – which are
currently established or already in operation in many
countries – can be noticed (WILLGALIS, 2005; WILLGALIS
et al., 2003).
Potential benefits from GPS modernisation and GALILEO
are illustrated in (LANDAU et al., 2004; VOLLATH et al.,
2004a; LANDAU et al., 2007). The deep impact on network
real-time kinematic positioning is also stressed and questioned
in CHEN et al. (2004). Network processing using
federated sequential filter techniques is demonstrated by
CHEN et al. (2003) in order to process large GNSS networks.
The datum transition in real-time for services is dealt
with in KUHLMANN and KÖTTER (2006). As most of these
techniques require active GNSS networks, please also have
a look on the corresponding section “Permanent GNSS
Networks” of this report.
Ongoing work to improve the estimation models for
satellite positioning is carried out by German scientists, in
particular with respect to a realistic stochastic model of GPS