I've read that to get a higher accuracies in GPS, we use differential positioning techniques which compute for satellites errors, using well-known position base station.

I want to know if the Satellite Based Augmentation System uses the same techniques ?


1 Answer 1


No. SBAS is an augmentation system, but it's not identical.

Differential positioning is pretty simple in concept. If you have a single receiver at a known point and a nearby receiver, the first can record how it is receiving the signals and compute an offset between when the signal is expected and how it actually arrives. This offset is then sent to the nearby receiver and is applied to the incoming satellite signals.

Any source of delay that is common to the two receivers is therefore removed. But you need that nearby receiver, so it doesn't scale up to cover a large area easily.

The SBAS in use in the US (WAAS) uses a different technique. Although there are multiple error sources in the signal, the largest one is usually the uncertainty in the delay due to propagation through the ionosphere. The depth and density of the ionosphere changes on somewhat short time periods. At the same time, just like a large weather system the state of the ionosphere is correlated between nearby locations.

WAAS uses a network of reference stations throughout the US. Each one records the current offset of the received signals and uses that to estimate the ionosphere delay. Then all the information is combined together to form a model of the delay everywhere in the US. This model can then be compressed and sent via another satellite. Any receiver in the target area can now get increased precision and also updates about any particular satellites that are sending unusable or incorrect data.

Similar SBAS systems are available in other areas of the globe (like European EGNOS.

  • $\begingroup$ If the SBAS uses a reference station that records the offset, then it must know its location , which means this is differential technique, isn't it ? I didn't understand what is the difference between this and differential technique ? $\endgroup$ Commented Feb 16, 2022 at 8:02
  • $\begingroup$ so you mean that these stations don't know their exact locations? if so, how can they compute a model ? and I want to exactly what is a model ? is it an AI model ? $\endgroup$ Commented Feb 16, 2022 at 8:31
  • $\begingroup$ Differential GPS is usually used to describe a 1:1 offset subtraction between a station and a nearby reference station. WAAS doesn't provide the offsets directly. It uses the offset to compute a model and the receiver interprets the model at its location. Offsets are used to compute the model but are not sent as data. $\endgroup$
    – BowlOfRed
    Commented Feb 16, 2022 at 8:32
  • $\begingroup$ So, the SBAS computes the offset to make a model that estimates the corrections for the whole area formed by the reference stations ? $\endgroup$ Commented Feb 16, 2022 at 8:37
  • $\begingroup$ Yes. A receiver that is too far outside the model area will regard it as not usable and ignore the corrections (but it might still apply any valid integrity warnings). $\endgroup$
    – BowlOfRed
    Commented Feb 16, 2022 at 8:55

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