GPS
Basics
How It Works
How Accurate is GPS?
RTK Glossary
Radios
Topcon
The GPS
usage within agriculture has been building steadily since the
mid 1990's, and 10 years on the adoption rate of farmers using
GPS to enable cost savings on their farms is increasing rapidly.
It is thought
that, over the next 5 years, every broadacre and row crop farmer
will have and use at least three pieces of GPS equipment, of
varying degrees of accuracy, on their farms. With this in mind,
and the release of the RTK precision positioning devices, an
easy to read booklet, with easy explanation of GPS and its application
for the modern farmer has been produced. This
is also for the dealer, who requires some basic knowledge of
what is available, and what to advise his customers when they
are purchasing equipment.
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GPS
Basics
The Global
Positioning System (GPS) is a satellite-based navigation system
made up of a network of 24 satellites placed into orbit by the
U.S. Department of Defence known as NAVSAT. The Russian government
also put up a similar system called GLONASS*. GPS was originally
intended for military applications, but in the 1980s, the USA
government made the system available for civilian use. GPS works
in any weather conditions, anywhere in the world, 24 hours a
day. There are no subscription fees or setup charges to use
GPS.
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How it works
GPS satellites
circle the earth twice a day in a very precise orbit and transmit
signal information to earth. GPS receivers take this information
and use triangulation to calculate the user's exact location.
Essentially, the GPS receiver compares the time a signal was
transmitted by a satellite with the time it was received. The
time difference tells the GPS receiver how far away the satellite
is. Now, with distance measurements from a few more satellites,
the receiver can determine the user's position and display it
on the unit's electronic map
A GPS receiver
must be locked on to the signal of at least three satellites
to calculate a 2D position (latitude and longitude) and track
movement. With four or more satellites in view, the receiver
can determine the user's 3D position (latitude, longitude and
altitude). Once the user's position has been determined, the
GPS unit can calculate other information, such as speed, bearing,
track, trip distance, distance to destination, sunrise and sunset
time and more.
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How accurate is GPS?
Today's
GPS receivers are extremely accurate, thanks to their parallel
multichannel design. 12 channel receivers are quick to lock
onto satellites when first turned on and they maintain strong
locks, even in dense foliage or urban settings with tall buildings.
Certain atmospheric factors and other sources of error can affect
the accuracy of GPS receivers.
GPS receivers
are accurate to within 15 meters on average, and this is not
accurate enough for agricultural applications, so the signals
from the satellites must be "corrected". This is known
as differential correction.
- Sub Metre:
Sub Metre correction is derived from five possible sources.
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- OmniSTAR
VBS: recognised as accurate to less than one metre 95% of
the time. This is a signal broadcast from base stations via strategically
placed satellites, which corrects the raw GPS satellites. Available
worldwide, it is recognised as the benchmark in sub metre corrections.
This is a subscription based signal, paid yearly to OMNISTAR.
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- Marine
Beacon: commonly regarded as accurate to less than two metres
95% of the time. This signal is derived from Marine Base stations
(BEACONS) that transmit the correction signal to Marine Beacon
capable receivers. It is restrictive in distance from the base
transmitter, although there have been reports of acceptable signal
receivers of up to 500kms from the transmitting base. The signal
will deteriorate and be less accurate the further away from the
transmitting base station that you travel. This signal is free
to the end user. -top
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WAAS:
WAAS is only available in North America and parts of Europe.
This is a signal that uses numerous land based transmitters
and satellites to transmit corrections. It is regarded as
sub metre, with the accuracy closer to the Marine Beacon status.
This is a free signal, and compatible WAAS receivers must
be used. -top
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- HP:
HP is an industry standard terminology for the "HIGH
PRECISION" signal derived from the OmniSTAR service.
The GPS engine used is a dual frequency, rather than the standard
sub metre, which is single frequency, and is therefore retailed
at a greater cost.
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The
HP signal is a sub 10cm signal, which means that the scatter
plot will be within 10cm for a minimum of 90% of the time.
In trials conducted over the past year a "pass to
pass" accuracy of less than 5cm (2 inches) was experienced,
which seems more than acceptable for the majority of broadacre
applications. This accuracy gives the operator the ability
to come back to the same guidance lines (within 10cm)
year after year.
The
"nudge" feature within PRO Steer ensures
accurate repeatability without the problems that base
stations sometimes incur i.e. range, power demands,
damage etc.
The
HP signal is not confined to a small
area, so large farmers are able to
roam the farm from one end to the
other without signal failure. The
HP signal covers the same area as
the VBS signal. It is a yearly subscription,
and farm packages are available to
allow more than one unit to be subscribed
under the one license.
(OmniSTAR
is to be contacted for more exact
costing).
John Deere also has a
10cm repeatable signal that is
referred to as Starfire 2(SF2)
receiver. On the JD web site it
is stated that "Pass to pass
accuracy of this signal option
is +/- 4 inches.
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RTK:
RTK
is generally referred to as the sub 2cm, or sub 1inch accuracy
signal. This system works by having two DGPS units operating
together. One being the "Base" unit and the other
being the "Rover" unit. The Base unit has the co-ordinates
"locked" and receives the normal satellite signal,
measures the difference, and sends the corrections to the
Rover via an external radio.
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Both of
the DGPS units have radios and antennas for the transmission
of corrections. This system is widely regarded as giving the
most accurate repeatable signal due to the Base and the Rover
being in close proximity to one another, thus reducing the atmospheric
changes that can distort satellite signals.
The Base
station can be either be fixed, or be moved around from one
spot to another. Each time it is moved, the DGPS has to be configured,
and "locked" into position. It is important to have
the Base DGPS antenna placed in exactly the same position each
time you return to a spot, otherwise the repeatability will
be lost. The radios are the limiting factor to the RTK system,
as typical reception is 5-10 km's (3-6 miles), due to the "line
of sight" capabilities of the radios. This is why the Base
station must be able to be moved.
There are
two types of RTK depending on the type of DGPS receivers used.
Single Frequency (SF) RTK, uses only the L1 band of satellite
information is cheaper, but signal can drop out easily from
trees etc. Dual Frequency (DF) RTK uses L1 and also L2 bands.
This gives greater accuracy, and also has less chance of the
signal dropping. If the signal does drop out, the reacquisition
time is quicker than the SF RTK system.
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RTK Glossary
- Range:
RTK range is the distance from the base station to the rover.
It is generally accepted the nominal distance is around the 6mile/10km
mark. After this the accuracy of the GPS progressively gets less
acceptable.
The
radio signal is possible to be received at 12mile/20km range,
but the accuracy is no longer recognised as 1"/2cm, and
dropouts of the radios will occur more often. Reacquisition
of the RTK accuracy after a dropout will also take longer.
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Base
Station: This is the reference station and must not be
moved whilst the rover is active. GPS unit and antenna that
is "locked" in position. The GPS looks at all of
the satellites, and works out the difference in measurement
from their signals, to that of where the base GPS is "locked"
at. The correction of the raw GPS signal is then broadcast
from a radio to a ROVER Radio/GPS.
It
is imperative that the base station GPS antenna is placed
in "exactly" the same position each time the
base is moved(if it is a mobile base station) otherwise
repeatability will not be achieved. -top
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Rover:
This
is the vehicle mounted unit (in the tractor/combine/sprayer.)
The corrected signal sent from the "base" GPS/radio,
is received by the "rover" radio. The corrections
are then sent to the "rover" GPS, thus correcting
the raw GPS data being received by the GPS antenna.
The rover GPS can operate within the distances of the
radio transmitter, and gain a high accuracy of differential
correction positioning. -top
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- Repeater:
Radio
repeaters are available, that can help send the signal from the
base station to the rover, if the rover is in a "black spot"
unable to receive the base signal. The repeater receives the base
signal, and then "on sends" it to the rover, in a gully
for example.
The
repeater is not to be used to "extend" the distance
of the signal from the base, but merely to fill in areas where
the base signal is shielded from the rover.
The
repeater comes with a small "whip" antenna,
and can be permanently mounted to a vehicle parked on
a high vista, to enable the forward projection of the
signal. The repeater should be in "line of sight"
of the base and the rover. It does not matter if the repeater
moves, as it is only a transmitter. -top
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Reacquisition:
This is the time that the GPS and radios take to re-establish
contact and position after a dropout of signal. This time
can vary from, almost instant (if close to the base) up to
20-30 minutes if there are large distances involved, particularly
with Single frequency. -top
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FREE
WAVE: Free
wave radios transmit the correction data from the Base Station
GPS to the Rover (vehicle GPS). As the name suggests, there
is no licence fee, so there are no ongoing costs. -top
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The
free wave radios transmit on a frequency of 900 MHz, or
thereabouts. The downside of this type of radio is that
they generally only output 1W, so therefore the transmitting
range (distance) is usually only about 2-5kms. This distance
is usually not considered sufficient in normal farming
practices, but is fine if the user continually wishes
to move the base station from one position to the next,
when changing locations (only used in North America).
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UHF:
The UHF radio is a known radio to most farmers as it transmits
in the 450 MHz range, as does their normal "CB"
type radios. This type of radio gives a greater range than
the Free wave radio, and the output wattage is able to be
increased to 2 watts or more. With a slightly better range,
(6-15km), this radio can also be used as a "mobile"
base station, or can equally be used as a "fixed"
base station.
The
fixed base station refers to the Base GPS being operated
from one position, and not moved. The fixed base usually
has a high gain antenna and low loss antenna cable, so
the antenna can be positioned at a high point to get the
upmost transmission distance.
The
UHF system can drop out around trees, and in damp
conditions may find the wet leaves and grass to be
a type of reflector, thus reducing the range. -top
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TOPCON:
The Topcon range of GPS units offers some real benefits to the end
user. The Rover GPS units come with an internal battery, so
they are able to be operated without being connected to vehicle
power. The Fence Post base station is extremely convenient to
use, as it also has an inbuilt battery, and also has a UHF radio
and antenna built in. The battery will last up to 6-7 hours,
so this makes for an extremely mobile unit. The fixed base station
is not as compact as the mobile "fence post" unit,
as the GPS, antenna, UHF antenna and GPS receiver are separate
items. The Topcon RTK uses UHF radios. The other main
benefit for using the Topcon RTK units, is that they also derive
information from the GLONASS satellites, as well as from the NAVSAT satellites (USA). This means the GPS receivers are seeing
more satellites, and therefore will get better positioning,
with fewer hold-ups due to low satellite numbers. -top
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Fence
Post Base Station: This is the easiest receiver to be
used for a mobile base station. It is compact with the 2w
UHF radio and GPS antenna built in. This unit also has a inbuilt
battery, which means that an external battery source is not
required for jobs lasting less than 6 hours. -top
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Fixed
Base Station:
This is the UHF fixed base station. Complete with an external
GPS antenna. The 35w UHF antenna and radio are also separate,
enabling the radio antenna to be positioned as high as possible
to gain maximum range. -top
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HP
/ Rover: The GPS and antenna are separate items, but the
GPS has provision for a UHF antenna to be fitted. The GPS
has the UHF receiver built into it. This makes the upgrade
from HP to RTK an easy physical option, by simply adding a
UHF antenna. NOTE: The software has to be upgraded to accept
the RTK inputs. This unit also has an internal battery with
option for an external power source. -top
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*GLONASS
support is available as a separately priced option
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