1-2-5 
Figure 4: The Navigation Module 
After the processing of the GPS/INS data the actual survey 
trajectory along with accuracy estimates are stored in a 
georeferencing file. The planning module then uses the 
georeferencing file to highlight those segments which have 
been surveyed using map matching techniques such as 
slope-intercept and nearest future search methods, for more 
details see Bullock, 1995. 
3.3 The Expert Knowledge in the Quality Control 
Quality control usually has a real-time and a post-mission 
component. In the real time component, expert knowledge 
decide whether a specific set of data is sufficient to provide 
the required accuracy with a certain level of probability. In 
the post mission component, knowledge decide analyze the 
result and performance achieved in different environment 
to increase the knowledge base of the system. If the results 
of the real-time prediction differ considerably from the 
results of post-mission analysis, the real-time model needs 
improvement. This can only come from the analysis of 
large discrepancies between prediction and post-mission 
results. Thus, each real-time model includes a certain 
amount of expert knowledge that has been gained in post 
mission analysis. It is the art of real-time quality control to 
combine this expert knowledge with the minimum 
information on the measurement process and still to arrive 
at reliable predictions. Such predictions would provide a 
systematic operation procedure that guaranty the quality of 
the results required for the mission. 
Real-time Quality Control: The real-time quality control 
component gives alerts to the operator in situations where 
the specified trajectory quality can most likely not be 
achieved.. These alerts are determined by the computer 
which monitors data accuracy of GPS and INS, tracks 
signal blockage for each satellite, and introduces expert 
knowledge on INS bridging (smoothing) and the 
convergence time of OTF ambiguity resolution techniques 
into the process. In those cases, where an alert is given, the 
vehicle is stopped to either allow an additional ZUPT or, in 
more critical cases, an independent ambiguity resolution. 
Using these additional measurements periods, the required 
post-mission trajectory accuracy can be obtained in the 
specified number of cases and re-surveys can be avoided. 
Examples of expert knowledge rules used in the real-time 
quality are: 
1. Switching to the INS stand-alone mode of operation is 
the number of tracked satellites is less than 4. 
2. Alerting the survey crew to the start/end of the ZUPTs 
by monitoring the time since the start of INS stand 
alone mode of operations. 
3. Monitoring the INS gyro rates during ZUPTs such that 
they do not exceeds certain threshold, In those cases 
where the gyro rates exceeds the threshold, due to 
passing cars or movement inside the van, an alert is 
issued to extend the ZUPT time. See Figure 5, for an 
example of ZUPT splitting due to movement during 
ZUPT. 
4. Monitoring the GPS data such that if at least 4 
satellites have been continuously tracked for certain 
time period, defined in the expert system database as 
the OTF convergence time, the expert system switch to 
GSP/INS navigation mode. 
5. Switch off the cameras if the van is outside the 
planned survey area. 
Post Mission Quality Control: In post-mission quality 
control, all available resources are used to obtain the best 
possible trajectory. This includes a suite of programs for 
cycle slip detection and correction, INS bridging and 
backward smoothing in case of GPS outages. The major 
task of this segment is quality assurance, i.e. the certainty 
that a specified percentage of the stored INS and GPS data 
are of sufficient quality to allow a continuous computation 
of the vehicle trajectory within the specified accuracy 
limits. For example, the requirement to achieve a standard 
deviation of 30cm in position for 98% of the post-mission 
trajectory computation would be such a specification. The 
post mission component should therefore provide all the 
tools to provide necessary information to the expert 
knowledge database. This information includes: 
1. Satellite availability, start/end of GPS static mode and 
of INS ZUPTs 
2. OTF convergence time 
3. INS bridging accuracy for both LI and wide-lane 
4. Difference between INS and GPS solution