GMAT9205 – GeopositioningLecturer : Dr Jinling Wang

RTK vs SPP

Student: Boata IABETA (z3316370)

SCHOOL OF SURVEYING & SPATIAL INFORMATION SYSTEMS

Outline:1. RTK GPS technique2. Performance of RTK3. Suitability of RTK4. Application5. SPP6. Conclusion

• “Carrier phase based” relative positioning• Employs 2 receivers or more - simultaneously tracked the same satellites (at least 4)• 1 receiver as Base station – remain stationary

over known point , radio transmitter, • Another receiver over unknown point• Position of unknown point – built-in software

in receiver.

RTK GPS technique…..

RTK performance…It is different from Conventional DGPS in 3 majors aspects;1. Typically accuracy – “hundreds times as good as DGPS”2. Initialization period – “ There is an initialization following power on. The initialization calculates the integer number of carrier phase measurement – known as fixing integer”3. There is no-zero probability that the initialization will be wrong

source: Frank (1997)

RTK performance…

To assess the RTK performance, 3 main issues: Reliability, Speed and Accuracy.

• Reliability. - “The best RTK receivers offer greater than 99.9% reliability”

eg. and receivers provide greater than 99.9% reliability, as well as providing the user with control of the reliability.

e

source: Frank (1997) 1 - Single frequency receiver 2-Dual frequency receiver

RTK performance…Speed – “RTK initialization is split into two stages”i. the acquisition phase (when the satellites signal is acquired) ii. the integer fixing stage (when the integer numbers of wavelengths are

computed.

E.g. receiver is always faster than a dual-frequency receiver for the acquisition phase

(because it only needs to acquire the single-frequency C/A code, which is easy).

Short baselines (<1km) the GG-RTK receiver is also faster than dual-frequency receivers for the integer-fixing stage.

On medium and long baselines the receiver is faster than GG-RTK for the integer fixing stage.

Typical times to fix integers are 30” through 2’

source: Frank (1997) 1 - Single frequency receiver 2-Dual frequency receiver

RTK performance…

ACCURACY – “Once the integers are fixed correctly, RTK accuracy is at the centimetre-level.”

“During the integer fixing phase, while the integers are being fixed they are modelled as real numbers (or floating-point numbers), and the position is referred to as ‘float’ solution.”

source: Frank (1997)

RTK-GPS suitable…. Survey involves a large number of unknown

points located in the vicinity of the known points

The coordinates of the unknown points are required in real time

Line of sight, the propagation path is relatively unobstructed.

source: El-Rabbany (2002)

RTK application.MONITORING THE STABILITY AND DISPLACEMENT OF TV Tower (Stuttgard, Germany)

GPS UNIT ATTACHED TO THE TV TOWER (155m above ground level)

REFERENCE STATION - 450m (away from the structure)

MEASUREMENT – commences for a (duration of 2mins)

source: Breneur et al (2002)

RTK applicationResult :

2min measurements with wind velocity of 7m/s

- elliptical movement

- cross-wind displacement - approx 4cm

- along wind displacement - 2.5cm

SPP technique….

• Standalone or autonomous positioning • Involves one GPS receivers• Based on “pseudo-range measurement”• Receiver position based on the satellites

coordinates as well as a minimum of 4 ranges to four satellites are required.

Conclusion…• Both GPS system – produce the position of unknown

right in the field – that is in real time with no post-processing is required.

• RTK provide quality data – cm level, more accurate GPS technique than SPP technique.

References:- Breneur P & Chmielewski T & Gorski P & Konopka E, 2002 “ Application of GPS technology to measurements of displacement of high-rise structure due to weak winds” Elsevier Science, Germany- El-Rabbany, A. 2002, “ Introduction to GPS – The Global

Positioning System” Artech House, USA- Frank. V D, 1997 “ GPS and GPS+GLONASS RTK” Ashtech Inc.

Sunnyvale, California, USA

RTK result….

SPP result….