hniques 
  
res 
ing 
Iccuracy 
1 (30-60 min) 
  
ing 
ub-metre 
n (5-15 min) 
receivers 
s < 15 km 
ing 
vel 
4 SVs 
s < 15 km 
d processing 
several m 
in real-time 
ng 
equired 
al cm level 
s < 50 km 
4 SVs 
ng 
1 required 
al cm level | 
s « 50 km | 
less units 
is usually 
quirements, 
sults during 
cial. Of the 
e kinematic 
> easiest to 
0 the use of 
plifies the 
s as data 
itor receiver 
0 bps using 
format, i.e. 
me mode of 
achieved in 
reaks in the 
t Guard has 
correction 
adiobeacons 
't of the US 
dco. Future 
  
plans are to expand the service to the entire US 
coastline (Alsip,1993). The Canadian Coast Guard 
is also in the process of deploying a similar service 
in Canada. 
To achieve cm-level accuracies in real-time, the 
requirements for data transmission may exceed 
2000 bps since the raw carrier phase data must be 
transmitted. As with post-mission processing, the 
algorithms are more complex than for code 
differential processing and thus the development 
of real-time, cm-level accuracy systems has lagged 
code differential systems. However, recently 
systems have been developed which show the 
feasibility of using real-time kinematic systems 
for high precision surveys (e.g. Frodge et al., 1994). 
4. LAND-BASED APPLICATIONS 
41 Sub-Metre Positioning in Static Mode 
Many GIS applications traditionally required a 
few metre accuracy for georeferencing static points 
occupied during a survey. The current emphasis is 
on sub-metre accuracy which can place particular 
constraints on the type of GPS receiver technology 
that must be used as well on the time required to 
occupy a point. Generally, the GIS community uses 
standard C/A code technology which delivers 1-3 
m accuracy. In order to improve the accuracy below 
a metre with these receivers, the carrier phase 
observable must be used and the point occupied for 
a longer period of time in order to acquire sufficient 
satellite geometry. 
A study to determine the time required to reach 
the sub-metre level, a test was conducted with the 
Motorola LGT10007M, a GPS/GIS terminal which 
can track six satellites simultaneously and can also 
output the carrier phase measurement. Data was 
collected on baselines of 500 m and 10 km. About 2 
hours of data were recorded for each baseline, and 
post-processed using The University of Calgary's 
SEMIKIN™ program (Cannon,1990). The data was 
processed in subsets and the resulting coordinates 
were compared to the known baseline coordinates 
in order to determine the achievable accuracy. 
Figure 3 shows the relationship between site 
occupation time and 3-D accuracy for the 100 m 
baseline. Figure 4 shows the same for the 10 km 
baseline. 
The two figures clearly show that the achievable 
accuracy improves as a function of the site 
Occupation time. Although the results are slightly 
different for the two baselines due to the increased 
errors on the 10 km line, they both show that 
within 5 minutes of data acquisition, the sub-metre 
level can be met. 
See Cannon et al. (1993) for further details on the 
above test. 
70 
  
3-D Error (cm) 
es) 
S 
À 
   
   
  
  
0 Li i T 
1:52 
th Lori TI 
3/54. 5.6.7 8 9 10 
Occupation Time (min) 
Fig. 3: Position Accuracy as a Function of Station 
Occupation Time - 500 m Baseline 
  
70 
60 - 
50 - 
40 - 
30 - 
3-D Error (cm) 
20 - 
  
10 4 
  
  
  
a 4°°5 60 172,8. 9,10 
Occupation Time (min) 
Fig. 4: Position Accuracy as a Function of Station 
Occupation Time - 10 km Baseline 
4.2 Use of GPS in an Urban Environment 
One of the major limitations of using GPS for GIS 
applications is the shading problem that is 
experienced under foliage and near buildings. 
Although it is an important concern for static 
applications, it is magnified for kinematic 
applications when a continuous trajectory is 
usually required. The susceptibility of GPS to 
shading is partially a function of the receiver that 
is used. For example, it is expected that a 12 
channel receiver will generally be less susceptible 
167