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What is Carrier Phase?

What is Carrier Phase ?

Ashtech offers the possibility to use carrier-phase techniques to achieve higher accuracy (centimetre level). This method counts the exact number of cycles of the carrying frequency between the GPS satellites and the receiver’s antenna.

The process requires an initial observation time to resolve the carrier-phase ambiguity. This period varies depending on the number of satellites available, the quality of the constellation, the baseline length (distance between base station and rover) and the technique used (On-the-fly Realtime Kinematics, Post-processed Static or Semi-kinematics, etc).

Below are the various requirements for proper carrier phase operation:

  1. Carrier phase work requires a base station and a rover.
  2. For single frequency, a baseline length of less than 10km is preferable; The longer the baseline length, the longer the initialization period required.
  3. There can be no canopy obstruction neither at the base station nor at the rover and the GPS antennae have to remain in clear view of the sky at all times, otherwise, cycle slips will occur. In simple words, the count of the carrier cycles will be lost and the initialization process must be repeated.

Carrier-Phase Post-processing

With Post-processing, data is collected in the field by both the base station and the rover then transferred to a PC. A post-processing software is used on the PC to compute the solutions and perform various other tasks. Depending on the occupation time and the environment, the results will be either fixed (cm-level) or float solutions.

  1. Static. This is the most accurate method to achieve cm or better. The rover occupies each location for a certain period of time (usually a minimum of 15-20 minutes for single frequency for 10km baselines or less).
  2. Semi-Kinematics. This method requires an initialization of the rover, by either
  • occupying an unknown point (usually 20 minutes) or
  • occupying a known point or
  • using an initialization bar at the base station (around 2 minutes). Then the surveying data collection can begin. If carrier phase lock is lost, one can re-initialize on a known point or one of the last recorded points. This method yields cm-level.
  1. Kinematics. Usually gives 1-3cm accuracy once a long enough observation time has been reached. This method is similar to the On-The-Fly (OTF) of Realtime Kinematics. The receiver can be in motion.

Realtime Kinematics

Realtime Kinematics (RTK) is a very efficient and productive way of achieving 1-3cm accuracy in the field. The coordinates of the base location are entered in the base receiver and a communication link is used to transfer corrections from the base station to the rover.

The initial investment might be a bit more than post-processing depending on the type of radio used (Spread Spectrum vs. higher power UHF vs. cellular communication), but productivity is increased. Also, RTK is the only option when stake-out is needed (transferring points or polylines from the GIS/CAD to the field).