342
| An on-line phase solution with fixed
Code solution Phase solution | ambiguities, however, requires the
| transmission of the whole raw data block
a at a typical frequency of 1 Hz. Therefore
Reference Moving Reference Moving | a much higher baud rate (2400-9600
Station Receiver Station Receiver | | bits/sec) is necessary. Such systems are
"EEE voue | + .
| now commercially available. They are
Y Y using small wireless sets able to transmit
Range Raw ef
data over distances up to 5-10 km. They
Corrections Measurements : : :
» | allow a differential on-line GPS-
Differential | positioning at the cm-level but are
Ambiguity strongly limited in range.
Resolution
: : Differential | In order to extend the range to tens or
Meter Differential Centimeter PCT a A even hundreds of kilometers, a network
accuracy Code Solution accuracy .
of reference stations becomes mandatory.
À All data is first sent to a master station,
where an integrity check is performed.
Figure 3. On-line GPS operation modes. At the same time an on-line monitoring
of the ionospheric delay and an orbit
improvement becomes possible. This information is then broadcast to the users. Such systems are under development
and may be available in some years in different countries. The expense of such a system are quite considerably.
At the moment, however, for many high precision, especially large scale airborne applications the off-line mode is the
only practicable way. The data are collected by the individual receivers, downloaded and treated in a post processing.
The advantage of the off-line mode is the far simpler set up. Also a greater flexibility in the data processing increases
the quality of the result. Precise instead of broadcast ephemeris may be used and the whole data set allows a more reli-
able fixing of the ambiguities. Especially in the case where it is not possible to solve the ambiguities processing in the
off-line. mode has the advantage that the whole data set may be used in order to get the best real valued estimate of the
ambiguities.
Today, a possible pragmatic solution for airborne applications may consist in the following approach : using range cor-
rection values from a reference station the on-line solution on the meter level is based on differential code measure-
ments and is used for the flight guidance. Simultaneously the raw measurements (phase and code) are collected, allow-
ing a recovery of a high precision trajectory in the post processing.
8. DETERMINATION OF ATTITUDE
The main task GPS is used for is the determination of the position and its derivatives, velocity and acceleration, of a
moving platform. The attitude angles are normally measured by an inertial system. There is, however, also a possibility
to use GPS for this task. Dedicated receivers for attitude determination are commercially available. The main difference
to the conventional receivers is the possibility to connect several antennas (normally four), which have to be fixed in
different places on the moving platform i.e. on the airplane in order to put up a reference frame. Normally these receiv-
ers are only single frequency receivers. By putting one antenna as reference, the differential positions of the remaining
antennas can be derived. The advantage of using only one receiver with multiple antennas is an increase in accuracy.
All receiver dependent synchronization errors are derived from the same oscillator and therefore cancel in the differen-
tial mode. This leads to a decrease of the number of unknowns and to a better geometrical condition of the solution.
[Cohen et al. 1992] have shown that by taking advantage of the fact that the distances between the antennas remain
constant the fixing of the ambiguities does not cause any real problem,
The resulting accuracy of the attitude angles depends on the distance of the antenna. On a small aircraft an angular
resolution of 0.2 - 2 mrad can typically be achieved. By putting two antennas on each wing, the main problem is the
deformation of the aircraft during the flight. An interesting comparison of GPS derived attitude data with INS is given
in [Cohen et al. 1994].
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995
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