The differences between normal and RTK GPS

Lecturer: Jinglin WangStudent name: Hao LiStudent ID: 3309990

Outline:· Introduction 1 Standard GPS receivers 2 GPS receivers for RTK

· Comparison of normal receivers and RTK receivers 1 Working principles of normal receivers 2 Working principles of RTK receivers 3 Differences between RTK and DGPS 4 Ionospheric effects

· Conclusion

· References

Introduction: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

Standard GPS receivers= C/A code receivers + Carrier Phase L1 receivers

(Single frequency receivers)

Garmin eTrex series Sony GPS-CS3KA Topcon Hiper L1

RTK GPS receivers

Leica 1200 RTK GPS Trimble 5700 RTK GPS

= Dual frequency receivers

Comparison of normal receivers and RTK receivers1 Working principles: Standard GPS receivers

SPP

Differencing

DGPS

“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]

Comparison of normal receivers and RTK receivers2 Working principles: RTK GPS receivers

Comparison of normal receivers and RTK receivers2 Working principles: RTK GPS receivers

Standard RTK Setup With Base/Rover

Comparison of normal receivers and RTK receivers3 RTK & DGPSIt 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.

Comparison of normal receivers and RTK receivers4 Ionospheric effects

The ionosphere structure consists of various layersincluding D, E and F layers. The ionospheric processes andeffects have different characteristics between theseregions. The free electrons are considered to be the mostimportant parameter affecting GPS applications, and thevertical electron density profile is shown in the following figure.

Comparison of normal receivers and RTK receivers4 Ionospheric effects

Comparison of normal receivers and RTK receivers4 Ionospheric effects

RTK Dual-frequency Ionospheric free model “Since the ionospheric delay is dispersive, it

depends on thefrequency. The density of free electrons varies with thetime of the day and the latitude. The variations from thesolar cycles and seasons short-terms effects are strong butless predictable. If the delay were not accounted for at all,the ranging errors on L1 frequency in the zenith directioncould 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]

Conclusion

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

References

[1] Gao Y, Liu Z.Z (2002). Precise Ionosphere Modeling Using Regional GPS Network Data. Journal of GPS positioning Volume 1, Number 1, 18-24.

[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 22nd International Congress, 19th-22nd April, 2002.