Lecturer: Jinglin Wang Student name: Hao Li Student ID:

Generally, there are three types of GPS receivers nowadays. Each of the three types offers different levels of accuracy, and has different requirements in positioning procedures. (1)C/A code receivers C/A code (1.023MHZ), Absolute positioning: meters level accuracy (2) Carrier phase receivers Carrier L1 (1575MHZ) or Carrier L2(1228MHZ), Relative positioning: millimeters level (3) Dual frequency receivers Carrier signal L1 and Carrier signal L2, centimeters level or better

“Real-Time Kinematic (RTK) positioning is performed using two or more GPS receivers — a reference receiver and one or more roving receivers. The reference receiver takes measurements from the satellites in view and transmits those measurements, along with the position of the reference receiver, in real-time to the roving receiver. The roving receivers also take measurements from the satellites in view, and process them in real-time with the measurement data and location from the reference receiver. The result is measurement vectors in the WGS84 datum from the reference receiver to the rover receiver. Using these measurement vectors, coordinates for the points occupied by the rover can be computed.” [1]

Standard RTK Setup With Base/Rover

It can be difficult to distinguish between RTK and DGPS. Here's a quick review the differences: (1) To get initialized, RTK needs a minimum of five satellites. After that it can operate with four. DGPS needs a minimum of three, though at least four are required for sub meter accuracy. (2) For RTK, you need a dual frequency GPS receiver. Single frequency receivers are sufficient for DGPS. (3) For RTK, your GPS receiver must be capable of On-the-Fly initialization (obtaining centimeter accuracy while moving). For DGPS, this isn't necessary. (4) With RTK, it takes one minute to initialize. DGPS receivers initialize immediately. (5 )You can expect accuracy of a few centimeters in all three dimensions using RTK. With DGPS, you can achieve sub meter accuracy in horizontal position only. (6) To obtain GPS corrections for RTK, you need your own base station that is no more than ten kilometers from the field you are working in. For DGPS, you can use your own base station, a correction service provider, or make use of the free radio beacon broadcasts in many regions.

The ionosphere structure consists of various layers including D, E and F layers. The ionospheric processes and effects have different characteristics between these regions. The free electrons are considered to be the most important parameter affecting GPS applications, and the vertical electron density profile is shown in the following figure.

RTK Dual-frequency Ionospheric free model “Since the ionospheric delay is dispersive, it depends on the frequency. The density of free electrons varies with the time of the day and the latitude. The variations from the solar cycles and seasons short-terms effects are strong but less predictable. If the delay were not accounted for at all, the ranging errors on L1 frequency in the zenith direction could reach 30 meters. So estimating the ionospheric delay, the dual-frequency GPS users can use the pseudo ranges P1 and P2 measurements on L1 and L2 to estimate the ionospheric delay”[2]

Main differences: (1)Two receivers (roving and reference) are used in RTK (2) Dual frequency is used in RTK to remove ionospheric effects in real time

[1] Gao Y, Liu Z.Z (2002). Precise Ionosphere Modeling Using Regional GPS Network Data. Journal of GPS positioning Volume 1, Number 1, [2] Iyiade A and Owusu-Nkasah K, (2002). Real-time Kinematic GPS. Institute of Electronics Systems (IES), Aalborg University. Denmark. [3] Richert T, (2003). Kinematic Positioning Inside the Ionosphere. Waypoint Consulting Inc. U.S.A [4] Rueger J.M (2002). Refractive Index Formulae for Radio Waves. International Federation of Suveryors 22 nd International Congress, 19th-22nd April, 2002.