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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi. XXXVII. Part B5. Beijing 2008 
immediately after the outage. For airborne applications this 
means there is no longer any need to fly flat turns to avoid 
signal outage. Flying a turn at 15 deg bank angle at a typical 
survey speed of 150 knots, gives a turn rate of about 2 degs/sec 
and a turn radius of about 2.3 km. The time to compete a 180 
deg turn will take minimum of about 1.5 minutes. In contrast, if 
the aircraft bank angle is not restricted, the same turn can be 
made in approximately half the time and radius, simply by 
banking at 30 degs instead of 15 degs. This savings means the 
mission can be flown in less time (reducing fuel costs), or more 
lines can be flown per mission. Furthermore, the smaller radius 
of the turns also allows more flexibility for flying missions in 
restricted airspace. 
1.2 Applanix SmartBase 
At distances greater than 20 to 30 km from a reference station, 
the residual ppm error caused by the atmosphere delaying the 
GNSS signals reaches a magnitude such that the correct carrier 
ambiguities can no longer reliably be estimated. Hence with 
traditional KAR differential GNSS processing, it is always 
necessary to be within 30 km of a reference station sometime 
during the mission in order to resolve the ambigutities. Once the 
correct ambiguties are resolved, the aircraft can fly up to about 
75 km from the nearest reference station before the magnitude 
of the ppm error exceeds level required for high-accuracy 
applications. For land-based applications a significant 
productivity improvement in Real-Time Kinematic (RTK) 
positioning has been achieved using the concept of a “Virtual 
Reference Station” or VRS (Landau H., 2002), illustrated in 
Figure 2. Here observables from a dedicated network of GNSS 
reference stations are processed to compute the atmospheric and 
other errors within the network. These are then interpolated to 
generate a complete set of GNSS observations as if a reference 
station was located at the rover. 
Figure 2. Virtual Reference Station (VRS) Concept 
There are a number of significant benefits to a VRS approach: 
• the distance to the nearest reference station can be 
extended well beyond 30 km 
• the time to fix integer ambiguities is significantly 
reduced 
• the overall reliability of fixing integer ambiguities in 
increased 
• the cost of doing a survey is reduced by eliminating 
the need to set up dedicated base stations. 
• no special processing is required in the RTK engine, 
as it is the case for a centralized multi-base approach 
Real-time positional accuracies using a VRS approach are at the 
cm RMS level anywhere within the network (Hakli P., 2004). 
With the POSPac MMS software, Applanix has introduced a 
post-processed version of VRS called the Applanix 
SmartBase™. Based upon the industry leading Trimble® 
VRS™ technology, the Applanix SmartBase software has been 
optimized for large changes in altitude by the rover, and 
extended to work with reference stations separated over very 
large distances. With this approach it is only necessary to be 
within the network and at least 70 km to the nearest reference 
station to initially resolve the correct ambiguities. Once 
resolved, the aircraft can then fly up to 100 km away from the 
nearest station within the network, while still achieving 
positioning accuracy at the 10 - 15 cm RMS level (Figure 3). 
Figure 3. Applanix SmartBase Concept 
The ability to accurately correct the atmospheric errors within 
the network will of course depend upon the amount of 
atmospheric activity during the survey, and the density of the 
reference stations. Tests conducted by Applanix have shown 
that it is possible to achieve better than decimeter RMS 
accuracies with a sparse network of only 4 reference stations 
separated by over 100 km, but the results are highly dependent 
upon the particular data set. However for existing dense 
networks such as the CORS network in Ohio State, the GSI 
network in Japan, or in the SAPOS™ network in Germany, 
where the there are literally 10’s to 100’s of stations separated 
by distances of typically 50 - 70 km, the robustness improves 
tremendously and the area that can be flown is virtually 
limitless. The Applanix SmartBase includes a rigorous 
adjustment of all the reference station antenna positions within 
the selected network over an 18-24 hour period. This quality 
control function ensures that all the reference station data and 
coordinates are correct and consistent before the rover data is 
processed. Such a concept is done routinely in land survey as 
part of best practices, but has been a weak point in the aerial 
mapping and survey industry. Too often data from a single 
reference station or a CORS network are used without proper 
quality control. Quality failures can include incorrect published 
antenna coordinates, incorrect datum or poor observables, any 
of which can result in accuracy and reliability failures in the 
final product. 
The Applanix SmartBase module together with the Applanix 
IN-Fusion technology enables missions to be flown with bank 
angles above 20 degs, with the only restriction that the turns be