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2.1.3.1 Shift and drift parameters

When during the bundle triangulation emerges that the used GPS-coordinates are systematically falsified, or when a reliable solution of the phase ambiguities was impossible, so that arbitrary determinations had to be fixed, shift and drift parameters can be used for correction, i.e.:

Equation preview
(2.1.3/2)

The antenna coordinates are shifted by a fix vector XS (shift) and by a parameter-depending vector XD (drift). The time is used as parameter t. XS and XD are unknowns in the adjustment.

When the strips are not too long (only a few flight minutes) and the connection to none of the satellite was lost during the flight track even shortly (cycle slip), and when the differences from the correct position are small, the shift and drift parameters will correct the falsified antenna coordinates reasonable.

The choice, which parameters are applied for which block parts, strips or strip parts, is decided by the user. Judgement criteria in this case are mainly the remaining systematic effects of the GPS-coordinate residuals. The programs REPLO, 3dView and especially GpsView (BINGO tools) allows a graphic demonstration of these residuals immediately after the adjustment.

Using these shift and drift parameters lead in every case to a weakening of the bundle block. When only four corner control points shall be used, in every case cross strips at the block ends are required.

The disadvantages of using shift and drift parameters are obvious:

  • The mathematic model is not rigorous. Therefore several different shift and drift parameters are necessary in case of very long flight strips.
  • Every time when the connection to a satellite is lost in one strip, in most cases a new set of shift and drift parameters is necessary at this position; otherwise this satellite cannot be used for this strip.
  • The number of unknowns gets considerably higher.
  • Cross strips are necessary for blocks with only four corner control points. At least these cross strips have to be flown provisionally.
  • This mathematical model is not rigorous. For long flight lines it is therefore sometimes necessary to apply several different sets of shift and drift parameters for one flight line.