GNSS Positioning Techniques
Global Navigation Satellite Systems Positioning Concepts
Global Navigation Satellite Systems (GNSS) are at their core, a timing system where all satellites clocks are closely synchronized. Satellites broadcast coded signals at exact times and the user’s receiver receives the coded messages and can estimate the time it took for each signal to travel from the GNSS satellite antenna to the user’s antenna. Once the time of flight is estimated the distance can be approximately computed by multiplying the time time of flight by the speed of light to arrive at a distance measurement in metres for each satellite.
GNSS satellites also broadcast messages that enable the user’s GNSS receiver to determine the satellites antenna position at the time the signal was broadcast (cartesian X,Y,Z coordinates). To estimate the ground antenna position the receiver must measure the time delay from at least four satellites, as four unknowns have to be estimated (X,Y,Z and T (receiver clock time)). Figure 1 shows the intersection of the four measured ranges.
In the conceptual description of GNSS systems above, only the code measurements have been considered. In practice, both the code and carrier phase measurements are used to enable high precision GNSS positioning. Code and phase measurement accuracy is affected by the following: local multipath environment, antenna and GNSS receiver quality. A typical cell phone GNSS antenna and receiver pair enables range measurement accuracy at the 3-5 metre level. On the other hand, geodetic and survey grade antennas (VeraChoke, VeraPhase, VeroStar and Accutena) and receiver pairs can produce code measurements with an accuracy of approximately 1.0 – 0.3 metres. Phase measurements can be very accurately measured (100th of a wave length) in the case of GPS L1, phase measurements have a wave length of approximately 19cm and can be measured with a precision of approximately 1 mm. However, the exact number of wave lengths from the satellite antenna to the user’s antenna (N) is unknown and must be estimated. Figure 2 shows phase measurement over time and the unknown parameter N that must be estimated.
To estimate high precision GNSS positions carrier phase measurements must be used, and atmospheric effects must be eliminated or minimized. Currently, two techniques are commonly used: one uses a local differential method: Real-Time Kinematic (RTK) and the other uses a wide area correction approach commonly called Precise Point Positioning (PPP).
Real-Time Kinematic Positioning
Local differential GNSS or Real-time Kinematic GNSS techniques use differencing techniques to eliminate or minimize systematic errors such as satellite clock and orbit errors, troposphere and ionosphere effects. Local differential techniques require a base station that has known coordinates and a rover for which the coordinates are estimated. The base station broadcasts its coordinates and the GNSS observations. The rover receives the base station coordinates and satellite observations (ranges and phase measurements) and estimates its position. This technique can provide very accurate positions (2-3cm) for short baselines. Figure 3 shows the typical RTK architecture.
Precise Point Positioning
Precise Point Positioning (PPP) is a technique that employs corrections to the satellites broadcast orbit and satellite clock. In addition to the satellite errors the GNSS signal is delayed or advances as it travels through the atmosphere and the signal must be corrected. Typically, a PPP correction service broadcasts information that enables the computation of an ionospheric correction. A tropospheric correction is estimated using a reference model. High precision (cm to dm) PPP requires a high precision multi-frequency GNSS receiver and antenna. High accuracy PPP typically estimates the ionospheric and tropospheric delays. A PPP correction service consists of the following components: ground-based tracking stations, analysis software to estimate orbit, clock and ionospheric corrections and a method of distributing the corrections to users. Figure 4 shows the typical architecture of a PPP correction service.
All GNSS constellations and signals require a high quality GNSS antenna. Without an accurate and precise antenna even the best GNSS receiver cannot produce good results.
For more information See: https://www.meted.ucar.edu/training_module.php?id=1216