Agriculture Reference
In-Depth Information
Observations from the real reference stations are used to generate models of
the distance-dependent biases. Individual corrections for the network of VRS are
predicted from the model parameters and the user's position. This kind of network
applied to DGPS and RTK systems is known as wide-area DGPS and network RTK,
respectively. An example of a commercially available network RTK is Trimble's
VRS that provides high-accuracy RTK positioning for wider areas. A typical VRS
network setup consists of GNSS hardware, communications interfacing, and mod-
eling and networking software. In the United States, the primary network is the
CORS network managed by the National Geodetic Survey (NGS). This 1800-station
network provides carrier phase and code range measurements for GNSS. CORS is
a partnership including more than 200 government, private organizations, and aca-
demia. These agencies own and operate the CORS network and share their data with
NGS. Figure 5.1 illustrates the level of densification of the CORS systems in the
United States as of mid-2012.
5.5.2 W
IRELESS
C
OMMUNICATIONS
For large-scale high-technology agricultural operations, establishing vehicle-to-
vehicle and vehicle-to-office communication is becoming imperative to manage the
logistics of the tasks and to ensure the safety of the machines working in the field.
The capability to transfer data wirelessly can help monitor the working statuses of
these machines and allow dynamic reallocation of tasks in the event of malfunc-
tions. Point-to-point and point-to-multipoint communication can specifically be used
for leader-follower systems. Cell GSM, Wi-Fi, WLAN, and wireless stand-alone
modems are typically used for vehicle-to-vehicle and vehicle-to-office communica-
tions. These technologies compete with each other with regard to bit rate, mobility of
terminals, signal quality, coverage area, cost, and the power requirements. WLANs
are used for high bit rate transfers, whereas cellular GSM networks are used for large
coverage areas. From a cost and power requirement perspective, cellular networks
are far more expensive to establish and maintain than WLAN access points. The
power requirement for a cell phone to transmit can be as high as several hundred mil-
liwatts, whereas WLAN requires a maximum of 100 mW (Wireless Center, 2010).
In terms of mobility and controlled signal quality, cellular GSM are superior to
WLANs. WLANs suffer from low mobility, isolated coverage, and vulnerability to
interference. Each technology is strong where the other is weak, and hence WLAN
and cell GSM networks are complementary.
WLANs operate in the 2.4-GHz unlicensed frequency band. The signaling rate
is 11 Mbps, and the terminals use CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance) to share the available radio spectrum. The distance between
the transmitter and the receiver has the greatest influence on the signal quality, and
thus the quality worsens as distance increases. For a 2.4-GHz spectrum band, if the
distance is within 28 m the data transfer rate can be up to 11 Mbps, whereas for dis-
tances greater than 55 m the transmission cannot be more than 1 Mbps. A GSM signal
occupies a bandwidth of 200 kHz and can have channel rates of up to 271 Kbps. The
strengths of both cell GSM and WLANs are provided by wireless internet (Wi-Fi).
