Test Course—Puerto Rico
To evaluate GPS receivers under a tropical canopy a Test course
was established on the Caribbean National Forest (CNF) in January
of 2004 under the direction of Carlos D. Rodríguez, ecologist
with the USFS, International Institute of Tropical Forestry. The
local Surveyor’s, Hector M. Sanabria Valentín, Luis
Berrios Montes, and Hector Cruz performed all the surveying work.
The Network was established from a baseline from two existing towers
in the CNF, Youkahu and El Yunque. Mt. Britton, a third tower,
has a control point with a horizontal order B (NGS Station AB9835).
A new control point was established at the Yokahu Tower using three
horizontal order B control points (Mt. Britton-AB9835; Fajardo-AB9834,
and Humacao 2-AB9846). El Yunque tower has a second order control
point (TV0917) that was reobserved along with the Yokahu Tower.
The first two monuments of the test course have direct line of
sight to the Yakahu Tower.
The CNF GPS Test Network is located in one of the most popular
trails of the forest. The Big Tree trail originates at PR 191,
km 10.4 in the CNF El Yunque Recreational Area. The asphalt paved
trail is 1.4 km long and is a self-guided interpretive trail (signs
in both Spanish and English are available along the trail), descending
from 1,804 feet (550 meters) to 1,640 feet (500 meters). Hiking
time is approximately 40 minutes, one-way. The trail is rated as
moderate in difficulty, and is steep in some places. This trail
is within the Tabonuco forest, the largest and most abundant of
the four forest types found in the CNF.
Figure 1. The GPS monuments are located on the Big tree trail,
road PR 191 and La Mima Falls.
The open site
is located at the
top of Yakahu Tower.
Figure 2. The Caribbean National Forest
The Tabonuco forest type is characterize by tall trees and low
light intensities at ground level. Canopy trees tend to be very
straight trunked with their first branches high above the ground.
This forest type contains the richest flora on the Forest with
175 tree species. This zone appears on the foothills and slopes
below 2,000 feet (610 meters) in elevation, which covers some 5,430
hectares (13,417 acres) of the Caribbean National Forest (USFS,
The mean annual rainfall range about 3470 mm for the past 30 years
in the Tabonuco Forest (Neuman, 1994).
Figure 3. Typical vegetation around GPS monuments.
There are 7 monuments located along or near the trail that have
been surveyed in. This site has a topographic obstruction to the
west, a ridge, blocking satellites below 30 degrees at most of
the monuments except monument number 1 and 2.
Figure 4. GPS Monuments located along the Big Tree Trail
control and position brought in by traditional survey methods.
The base station used
to post-process the GPS data was the NGS CORS Base Station located
at Isabela, PR, (PUR3) located in the
NW corner of Puerto Rico about 137 km 277 degrees from the test
course. On one of the days this site was non operational, so
the Christiansted, St Croix NGS CORS site was used to post-process
the data. It is located 139 km at a direction of 116 degrees
the test site.
Real-Time Beacon Station
For the real time beacon station, the
Isabela PR site was used. It transmits on a frequency of 295
Mhz and at a transmit rate of 100 bps.
The WAAS satellites are geosynchronous satellites
in orbit near the equator. This satellite requires that the receiver
line of sight. At the latitude of Puerto Rico these satellites
are higher on the horizon and the correct signal more available
that at latitudes of the northern US. At this location WAAS
satellite PRN 122 was used to broadcast correction signals to the
The GPS receivers were set up on the monuments
and 5 point and 60 point averages were taken for the different
configurations. 3 repetitions were conducted for each receiver/configuration
and then the average error calculated for the course and the
RMS and NSSDA (95% confidence limit) values calculated.
TeraSync 3.31 software was used to collect data from the Trimble
Geo-XM and Geo-XT receivers. Trimble Asset Surveyor software was
used to collect data from the Trimble Pro-XR receiver. ArcPad 6.0.3
and SoloField 3.1.1 was used to collect data from the other GPS
receivers tested. This data collection software was used to filter
the positions collected so that only 3-D positions were collected
with a PDOP less than 10. However the elevation mask (elimination
of signals received from near the horizon as they are more susceptible
to multipath or signal reflection), and SNR values (If signal strength
is weak—more chance for error) are not filtered by these
software packages and are subject to greater error possibility.
Due to the heavy canopy and the topographic obstruction
to the west, sometimes long occupation times occurred before
number of points was collected. This was especially true when
only the internal GPS antenna was used. The external antenna
increased the efficiency of the data collected. The less expensive
receivers would use any signal received that met PDOP requirements
but were weak or low on the horizon and a 60 position fix was
obtained sooner but usually was not as accurate as the others
that had other filter settings.
The results of the GPS receivers tested are shown in table
in Appendix B. A graph shown in figure 5 illustrates typical
can be expected with the tested receivers. This shows results
with external antennas and 60 points averaged for these receivers.
In field situations such as this, with topographical obstructions
at certain horizontal elevations in known directions, mission
planning can improve data collection efficiency by indicating
times when satellites are visible in the open unobstructed
skies and not when they are masked out by physical obstructions.
GPS Coordinates of Monuments
Receiver Results on PR Site
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