2-4-5 
that 10' 7 deg is approximately equal to 1 cm for the 
altitude of about 300 m, the average differences were 
about 4 cm (max 6 cm) in latitude, and about 2 cm in 
longitude, respectively. The altitude difference between 
the trajectories was on average equal to 12 cm. The 
observed differences between the trajectories are caused 
primarily by the separation between the base stations, 
and the fact that for most of the presented flight the 
Norwood base station tracked LI and L2 phases, 
whereas MIT base station tracked only LI (L2 was not 
accessible most probably due to signal interference). 
East 
North 
Height 
Total error [m] 
0.19 
0.40 
0.58 
Gap duration [sec] 
84 
INS calibration [sec] 
236 
Total error [m] 
0.12 
0.06 
0.18 
Gap duration [sec] 
140 
INS calibration [sec] 
723 
Total error [m] 
19.0 
33.4 
7.2 
Gap duration [sec] 
320 
INS calibration [sec] 
860 
Total error [m] 
131.0 
209.2 
162.4 
Gap duration [sec] 
1200 
INS calibration [sec] 
1050 
Table 1. Positioning error growth during the GPS 
losses of lock. 
■71.06 
■71.08 - 
-71.1 - 
■71.12 
•71.14 
-71.16 
■71.18' 
42.15 
42.2 
42.25 42.3 
latitude [deg] 
42.35 
42.4 
Figure 7. Flight trajectory December 12, 1998, MIT 
campus, Boston. 
The first peak in longitude that can be observed in 
Figure 8 is caused by the loss of GPS lock by the 
Norwood base at epoch 568400 that lasted for about 
150 seconds, reaching its maximum at epoch 568555. 
Thus the difference for this part of the trajectory is 
primarily driven by the error growth during the free 
navigation mode from the Norwood base. At epoch 
568555 the lock was regained and the difference 
between trajectories dropped down to about 1cm in 
longitude and 4 cm in latitude, as shown in Figure 9 
(this plot shows enlarged portion of Figure 8 for 
clarity). The second peak in differences plotted in 
Figure 8 resulted from a partial loss of GPS lock again 
at Norwood base around epoch 569740. For about 1.5 
min the MIT station was collecting data from 9 
satellites while Norwood observed only 6. Both events 
seem to affect longitude more than latitude. 
Nevertheless, the repeatability and consistency of the 
trajectory estimation from two independent base 
stations provide an indirect quality test that indicates the 
high accuracy achievable by the system when GPS is 
observable. 
Figure 8. Latitude and longitude difference between 
trajectories estimated from MIT and Norwood 
base stations (10' 7 deg approximately equals to 1 
cm). 
x 10 
-2 
- 
-4 
- 
-6 
- 
-8 
- 
10 
- 
12 
5.684 
latitude 
longitude 
5.6845 5.685 5.6855 
GPS time of week [sec] 
5.686 
5.6865 
x 10 s 
Figure 9. Latitude and longitude difference between 
trajectories estimated from MIT and Norwood 
base stations during Norwood loss of GPS lock.