RTK Set Up
RTK is generally referred to as the sub 2cm, or sub inch accuracy signal.
This system works by having two GPS units operating together. One Unit
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,
and measures the difference, and sends the corrections to the Rover via an external radio.
Both of the GPS units have either an external or internal radio 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 GPS has to be configured,
and “locked” into position. It is important to have the Base GPS
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.
The transmitting radio antenna must be mounted as high as possible.
HEIGHT IS EVERYTHING.
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L1 and L1/L2
There are two types of RTK depending on the type of GPS 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.
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.
This is the reference station and must not be moved while 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.
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.
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Radio 2w PDL Repeaters
Radio 2W repeaters are available, that can help send the signal from the Base station to Rover.
If the Rover is in a “black spot” unable to receive the Base signal.
The repeater receives the Base Signal and then “Re-Transmits the Signal to the Rover”.
This helps with gully's were the signal can not reach the Rover.
The repeater should not always 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, 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 transmits the Signal from the Base.
Radio 2W repeaters are available, that can help send the signal from the Base station to Rover.
If the rover is in a “black spot” unable to receive the base signal. The repeater receives the Base Signal and then "Re-Transmits the signal to the Rover.
This helps with gully's were the signal can not reach the Rover.
The repeater should not always 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, 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 transmits the Signal from the Base.
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Re-acquisition of Signal
This is the time that the GPS and Radios take to re-establish Base position contact after the
dropout of the Radio signal To the Rover.
This time can vary from,almost instantly (if closer to the base) Or much longer if there are large
distances between the Base and The Rover.
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UHF
The UHF radio is a known radio to most Surveyors as it transmits in the frequency of either
430-450Mhz (In Canada) and 450-470 Mhz range which is used mostly in the USA
This type of radio gives a greater range than the Free wave radio, and the output wattage is able
to be increased from 2 watts to 35 watts on the Base Radio
With a slightly better range, (6-15 kms), this radio can also be used as a "Mobile Base Station"
or can equally be used as a “fixed” Base Reference 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 up most transmission distance.
The UHF system can drop out around trees, and in damp conditions you may find the wet leaves
and grass tend to be a of reflector, thus reducing the Radio Range.
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Pacific Crest Base and Rover Modems
How can I improve the performance of my Radio modems?
To get the best performance from your radio modems, keep the following basic rules in mind:
Rule #1 - Antenna positioning is critical to good performance. Radio waves cannot penetrate through the earth, so place your antennas as high as possible to provide the best line-of-site condition. A telescoping tripod antenna mast is useful in positioning your antenna for best performance. Also, make sure that your antenna is properly tuned for the frequency you are using.
Rule #2 - Maintain your equipment in good condition. Field failures are most often caused by broken cables and connectors. Inspect cables and connectors frequently. Replace worn cables and damaged antennas before they shut you down in the field. It is always a good idea to have spares available.
Rule #3 - Avoid interference through monitoring the available frequencies prior to using your radio modems. For the RFM96 product family, we recommend you use a radio scanner, available from Radio Shack or others, to monitor the frequencies. With the PDL product family, use the patent pending AutoBaseTM technology to select the best channel. Press the channel button until the small "b" is displayed. The PDL will scan all available channels, and select the channel with the lowest interference level. Please note that the AutoBaseTM algorithm takes one minute per channel analyzed.
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GPS Fundementals
Take a look at this Link
How to Understand GPS Booklet
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