Aerospace and Electronic Systems - October 2018 - 19

Rodriguez et al.

Figure 6.

Percent coincidence of predicted viewshed area from selected GNSS receivers' data with those predicted by Geo 7 data for fuel cycle 1 and 2, where
black indicates overlapping viewsheds areas, grey indicates areas not in the Geo 7 viewshed, and white indicates areas not in the viewshed of the GNSS
receiver of interest. The lowest amount of overlap (F4-80% and N7-83%) occurred during the first cycle in which the Geo 7 experienced losses of satellite signal. The lowest amount of overlap for all other cycles was 82%.

standard deviation is relatively low and becomes larger at lower
percentages. This is likely due to the directional nature of obstruction by landscape. Models of satellite visibility based on terrain
and the rotor disc are depicted in Figure 5. The rotor disc is the
largest obstruction with 130° of interference, leaving only a small
portion of low-angle sky on the horizon with direct line of sight to
a fraction of the satellites in the hemisphere. However, modeling
only this area of open sky resulted in a low correlation with PDOP
(Figure 4b). We speculate that the rotor blades may be the biggest
impediment to GNSS signal acquisition, but interference is intermittent with occasional signal loss caused by obstruction of line of
sight. Improvements could be achieved by mounting the receiver's
antenna to the aircraft beyond the influence of the main rotor, but
this would require compliance with airworthiness standards. While
there are antenna mounts with supplemental type certificates for
other aircraft, such as the Bell 206 series [36], there are currently
none with FAA approval for a Hughes 500D model helicopter,
which is the preferred aircraft under these conditions [37].
The viewshed analyses performed for each of the GNSS data
from the first two operations translated "search areas" from the
three-person FOV conducting low-altitude flights (Figure 6). The
average overlap for all of the GNSS units search area depictions
with the G7 baseline depiction was over 85%. The least amount
of overlap was measured in the first operation, corresponding to
the greatest positional deviations (see above), with signal losses
experienced by the G7 as the likely culprit.

CONCLUSION

REFERENCES
[1]

[2]

[3]

The standard accuracy achieved by survey-grade GNSS was expectedly compromised in this highly confounding, dynamic environment, where the computational demands to achieve highly
accurate positions require static conditions in open sky. Based on
the sky visibility, obstruction by aircraft surfaces and mountainous
OCTOBER 2018

terrain appear to be the primary factors influencing accurate GNSS
positioning. Overall, differential postprocessing offers the most accurate positional data with four times less deviation measured in
this trial than the consumer-grade GNSS. In these aerial surveillance operations, the vertical z coordinate is just as important as the
horizontal x and y coordinates, yet compounds the deviation twofold, regardless of receiver. Independent altimetry (e.g., laser and
radar) and better real-time correction sources (e.g., Trimble RTX™
or Trimble Real-time Network) could improve position accuracy.
We also acknowledge the error associated with datum transformations and the DEM used as the current topographical reference.
The error associated with consumer-grade GNSS is within the
field of view of manned surveillance operations (i.e., within 20 m)
which is considered an acceptable threshold in order to return to
a recorded point. With all of these compounding effects, the consumer-grade GNSS performed reasonably well against the higher
grade GNSS differential receivers and ultimately demonstrated
high reliability in data acquisition, a necessary, practical condition
to recording these surveillance operations.

[4]

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