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How does RTK GPS/GNSS work?

Real Time Kinematics (RTK) is a technique used to enhance the precision of position data derived from satellite-based positioning systems. Traditional GPS measures the satellite code stream, which can only be measured to several meters. Additionally, traditional GPS loses precision as a result of ionospheric delay. The resulting position is only accurate to several meters. RTK provides centimeter-level position by using a different signal measurement and by correcting for ionospheric delay. Instead of measuring the code stream, an RTK receiver measures the carrier phase to under a centimeter. Next, RTK systems use two receivers, broadcasting corrections to each other to cancel out the ionospheric delay and output a relative position between the receivers. For a deeper understanding, read our write up on Understanding GPS RTK Technology.

What does RTK positioning give you?

RTK gives you centimeter-accurate relative positions between the two receivers. To get a position that is centimeter accurate with respect to the Earth, one unit must be used as a stationary base station. To get an absolute position (in a coordinate system such as ECEF), a CORS base station must be used or the base station must be placed at a known geodetic location. A list of these locations in the US can be found here. If both receivers are in motion, RTK will give you the relative position between the two but not fixed position relative to the environment.

What constellations and bands does Piksi support?

The Piksi® Multi is capable of receiving, processing and utilizing GPS L1 and L2C signals in the position solution. Piksi Multi is hardware-ready for GLONASS G1/G2, BeiDou B1/B2, Galileo E1/E5b, QZSS L1/L2 and SBAS. "Hardware-ready" implies that the Piksi Multi GNSS Module has the RF front end to receive these signals but not process and utilize them in the position solution. Swift has plans to roll out support for these constellations in the future, starting late 2017. These are anticipated to be provided as no cost firmware updates.  The Original Piksi (v2.3.1) and Piksi Space are capable of receiving only GPS L1 signals.

What is GPS L2C and how is it different from L2P?

Piksi Multi receives the L2 civilian signal broadcast from the GPS satellites which are called the "L2C" signals.  This is different from other dual frequency GNSS receivers which often track the military "L2P" signals.  Both L2C and L2P signals are in the same frequency band but the L2C signals are designed for use by civilian equipment and supported for that purpose.

It is important to note that only 19 out of 31 GPS satellites are capable of transmitting GPS L2C signals as of May 2017.  Eventually, the entire GPS constellation will be capable of transmitting L2C signals.  This means that Piksi is unable to create L2C measurements for some of the GPS satellites in the sky at any given time.  In detail, the Block IIR-M and IIF satellites are fully operational on L2C signals, but the older block IIR satellites do not transmit L2C.  Refer to navcen website for information 
for information about the current GPS satellite constellation.

How long can I expect a GPS satellite to remain visible?

A GPS satellite, orbiting at a nominal height of 26,440 km above Earth center, completes a full orbit in 11 hours and 58 minutes. For an observer on the Earth's surface, nominally 6,300 km above Earth center, a satellite transiting through zenith (directly above the observer) will take 5 hours and 3 minutes.

Does RTK work indoors?

No, it is not possible to do RTK positioning indoors. Not only are GPS signals are extremely weak indoors, but you can no longer assume they have traveled in a straight line to the receiver. The signal will usually have reflected several times off of walls and surfaces before it makes it to an indoor receiver, destroying one of the primary assumptions the receiver makes in calculating the position/velocity/time (PVT) solution - that the signal has traveled in a straight line from the satellite to the receiver's antenna.

What is the accuracy of Piksi?

Piksi is accurate to 2 cm horizontally on short baselines with a good sky view. Vertical precision is typically 2-3 times worse than horizontal precision for GPS receivers, so we expect 6 cm accuracy on short baselines. Due to the nature of RTK, accuracy degrades at a rate of 1 mm horizontal and 3 mm vertical for each km between the base and rover.

What is the maximum distance between the receivers?

The theoretical maximum distance is about 10 km. Practically, the maximum distance is limited by the quality of the radio. The radios included in the Original Piksi kit have 200 meter range with line of sight, but other radios are available for ranges up to 10 km. There is no minimum distance between the receivers.

How does the receiver perform under poor signal or multipath environments?

Like all GPS/GNSS receivers, the receiver must connect with at least four satellites. If these connections are lost, then the unit must reinitialize. Lost connection can occur due to dense vegetation coverage or buildings obstructing sky view (urban canyons).

How long does initialization take?

Time to first RTK fix is not deterministic, so it will take different amounts of time, depending on your environment. With Original Piksi it takes between 5 and 10 minutes. With Piksi Multi it takes about a minute.

What accuracy can I expect with only one receiver?

Using a single receiver will not allow you to perform RTK for centimeter accuracy. The typical accuracy we observe is 3-5 meters.

Can one base station service multiple rovers?

Yes, one base can provide corrections to multiple receivers simultaneously. For such configuration, radios need to be set in Point-to-Multipoint configuration.

Can two receivers go on the same vehicle?

Yes (see also "What does RTK positioning give you?"). However, in this case, the relative position  will give you information about the attitude of the vehicle, and not centimeter accurate relative positioning relative to an external coordinate frame (such as NED or ECEF).

Last Updated: Nov 27, 2017 12:00PM PST
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