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A
Signals from satellites are received by 2 different antennas which are located a fixed distance apart. This enables a suitable receiver on the aircraft to recognise and correct for multipath errors.
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B
Fixed ground stations compute position errors and transmit correction data to a suitable receiver on the aircraft.
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C
The difference between signals transmitted on the L1 and L2 frequencies are processed by the receiver to determine an error correction.
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D
Receivers from various manufacturers are operated in parallel to reduce the characteristic receiver noise error.
Refer to figure.
DIFFERENTIAL GPS (dGPS)
dGPS (differential GPS) is a method of augmenting GNSS signals to remove measurable errors such as satellite clock error, ephemeris and ionospheric propogation effects. This is done by placing multiple reference antennas on the ground in the region that is to be used, then measuring their GNSS computed position against their actual known position, then calculating the corrections to be made to each satellite's pseudo-range. This allows accuracies of around 1 m in many cases, and is the technique used by GBAS (Ground Based Augmentation Systems) for small area aviation use cases, as well as SBAS (Satellite Based Augmentation Systems) for large area dGPS uses.
Receiver errors, multipath signals and some small atmospheric propogation errors cannot be accounted for, but they do not affect the accuracy too much. dGPS systems like GBAS and SBAS can also be very useful for monitoring the integrity of GNSS satellites, as they are constantly monitoring the signals, and can give loss of integrity alerts in 6 seconds rather than a potential 3 hours for standard GPS.
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