Developments are known under the name SERIES [Mc
Doran, 1983],
4. GPS receivers
Currently two types of receiving systems for GPS signals
are available which can be used for geodetic purposes.
These are the Tl 4100 Navstar Navigator from Texas In
struments and the Macrometer V-1000 from Macrome-
tric. Possibly still in 1985, also Magnavox will come out
with a geodetic receiver.
Fig. 5. Tl 4100 Navstar Navigator equipment.
The Tl 4100 (see fig. 5) is a multiplex receiver with one
channel which switches within 20 milliseconds between
four different satellites on both frequencies. By this,
pseudoranges, Dopplershift and carrier phase can be
measured with up to four satellites „quasi" simulta
neously. In addition the satellite message can be de
coded so that real time navigation is possible. An on-line
navigation solution is computed in the built-in micro
processor through a Kalman filter technique, and can be
displayed on a hand held „Control Display Unit" (CDU).
This solution is updated all four seconds and can be used
for navigational purposes in low and medium dynamics.
In addition all data can be transferred via an RS-232 in
terface to an external data recording device or an exter
nal computer for independent (post-) processing.
Through all these features, the Tl 4100 is a flexible multi
purpose receiver, which can be used for precise navi
gation, precise geodetic positioning and research.
However, code availability is essential for Tl 4100 opera
tion. Currently the receiver is working with P-code and
C/A-code. After the completion of the whole GPS in
1989 possibly only the less accurate C/A-code may be
available. The system is ruggedized, rather light weighty
(24 kg) and portable in special transportation cases. The
antenna weights only 2 kg and can be installed on a
simple tripod.
The Macrometer Model V-1000 consists of an antenna
unit with preamplifier, an electronic unit which has to be
transported in a car and a special processor P-1000. A
complete equipment includes at least two receiving units
and one computer. The observable is the phase diffe
rence between the arriving satellite signal, processed by
squaring techniques without knowledge of the code,
and a reference signal in the receiver. The main charac
teristic advantage is the fact, that no code availability is
required. On the other hand, it is necessary however, to
get orbital information from some external source in
order to operate the receivers and to process the data.
The phase ambiguity is resolved by special differencing
techniques through simultaneous measurements from at
least two observing sites to at least four satellites. The
instrument enables simultaneous measurements of up to
six satellites on six channels.
Since 1983 Macrometers have been widely used for
ground control in the United States and also in Germany
[Hothem and Fronczek, 1983, Lindstrot, 1984], The
accuracy prospects for relative positioning with 1 106
could mostly be achieved for distances up to 50 km, al
though no ionospheric corrections could be applied for
single-frequency instruments. For the near future the
availability of two-frequency Macrometers has been an
nounced. This should lead to further improvements.
Within the next few years a wide variety of GPS-
receivers will come to the market. Most of them will be
C/A-code receivers for navigation. Until the end of this
decade the development will lead to low-cost receivers
at the price level of current navigational equipment. All
these receivers are not usable for geodetic purposes.
The essential feature for geodetic application will be the
access to phase observables and if possible to both
carrier frequencies. Also for this we can expect some
interesting new developments within the next few years
which should lead to considerably lower prices.
5. GPS applications in geodesy and navigation
The capability of GPS receivers to provide 3D co
ordinates and co-ordinate differences with high accura
cy with a rather short observation period in static and
dynamic mode offers a wide range of possible applica
tions in geodesy and navigation. The following list gives
some indications:
applications for geodetic control networks;
applications for the determination of control points in
photogrammetry, inertial geodesy, geophysical sur
veys, engineering surveying;
applications in marine geodesy;
applications in polar research.
With respect to geodetic control networks, GPS will not
only be used for the installation and improvement of first
and second order, but will also be used in lower order
networks and operational detailed surveying. One li
miting factor will be the question, whether 1 cm-
accuracy is available for distances of a few km. If this
can be attained by differential techniques, GPS will de
velop to be a standard surveying tool with one reference
receiver being operated continuously by some official in-
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