Distance measurement with carrier

Carrier measurement is more complicated than code measurement. This is because the signal carrier has no time markers.

The principle of phase measurement

Solving period acquaintances, i.e. finding out how many whole wavelengths the distance between satellite and receiver consists of, then becomes critical.

The carrier on L1 has a much higher frequency than C/A- code, 1575 MHz, which corresponds to a wavelength of 19 centimeters. Based on the rule of thumb that the signal resolution is about one percent of the wavelength, an individual carrier measurement can in principle be made with an accuracy of about two millimeters!

Why does not all measurement with carrier take place if it is so much more accurate than code measurement? The simple answer is that carrier measurement is more complicated, both in terms of equipment and method of measurement.

 Illustration of the principle of carrier measurement. The satellite signal carrier and the GNSS receiver's copy of the signal can not be easily matched because the carrier is not timed.

The coded signal, such as the C/A code for GPS, is intentionally designed to facilitate signal matching and timing. however, has no time markers at all, because each full wavelength or period in the carrier is the same (see picture above; in reality, however, the satellite signal is Doppler shifted).

Determination of phase ambiguities

and receivers in carrier measurement can in principle be expressed as a number of whole periods plus part of period (see picture below). Part of period is carefully determined by phase measurement, but the number of whole periods will initially be unknown. ambiguity in English.The determination of period acquaintances is therefore very critical for accurate positioning. too long or too short!

Illustration of the term period unknown, which is the number of whole wavelengths of the carrier between satellite and GNSS receiver, when the measurement begins.

The determination of period acquaintances is facilitated by

  • relative positioning, which reduces sources of error
  • multi-frequency measurement
  • good mathematical models
  • combination with code measurement, which provides the opportunity to limit or "circle" the number of possible period acquaintances. The process is called initialization and requires the GNSS receiver to keep the signal locked from the start of the phase measurement. Temporary interruptions result in an unknown number of periods being lost in the measurement, so-called period cancellations. with the appropriate method, the phase measurement must be restarted.

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