Here we try to answer typical questions about our receivers and GNSS in general.
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, SBAS, and Glonass G1/G2, BeiDou B1/B2, and Galileo E1/E5b signals in the position solution. Piksi Multi is hardware-ready for QZSS L1/L2. "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 a proven track record at enabling new constellations through firmware update and plans to roll out support for these constellations 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 Multi is accurate to 1 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 2-3 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?
There is no software limit on the baseline length in Piksi Multi, but at distances beyond 30 km assumptions in RTK begin to break down and performance can begin to drop off. Additionally, accuracy is degraded by about 1 part per million, which means 1 additional millimeter of expected error per kilometer of distance from base station. Practically, the maximum distance is limited by the quality of the radio. The radios included with the PIksi Multi evaluation kit can have multiple kilometers of range given good line-of-site view and even more range with improved or directional antennas. Using different radios or the cell network to deliver corrections can increase range. 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 Piksi Multi it takes about a minute from power on, or 5 to 10 seconds after an obstruction that prevents the view of the sky. With Piksi v2.3.1 it took between 5 and 10 minutes.
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. As of Piksi Multi firmware version 1.5, Satellite Based Augmentation System support is included which increases accuracy for a single receiver. In many cases this accuracy is below a meter. Note that the SBAS accuracy depends upon the system. The US system called WAAS is designed to provide less than 5 meters of 3D error RMS error to support aircraft landing systems with a very high degree of reliability.
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. If a LAN network is in use, the TCP/IP server on the base station can be configured to serve multiple rovers.
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). Multiple receivers on the same vehicle can allow determination of heading. See the heading and moving baseline articles on the Swift Support portal for more information.
In what datum do Piksi operate?
For SPP positioning and SBAS positioning, we output in the datum provided by the GPS system (which people usually refer to as WGS84).
For RTK, Swift receivers output a differential position relative to a base station and the solution will be in whatever datum the coordinates for the base station were determined. If a third party base station or CORS station is used, consult the provider for the datum in which the base station position is provided.
Skylark (Swift’s correction service) outputs position in ITRF2020 reference frame.
In all cases, the altitude measurement is an ellipsoid altitude, not a Geodetic or MSL altitude.
We have no implicit datum conversion features onboard multi.
Does Piksi Multi provide a PPS signal?
Piksi Multi has a configurable PPS signal output which is available as a digital signal on the Piksi Multi hardware interface and is broken out into an SMA connector on the Piksi Multi evaluation board. Please refer to the Hardware Specification for information about the electrical signal and the Software Settings Manual for information about how to configure the parameters of the PPS including its propagation mode, its width, and its polarity. Both documents are available on the Piksi Multi Specifications support page.