An aircraft landing system comprising: at least two mobile base station GNSS antennae (112,114)having a known fixed distance therebetween for receiving signals from a GNSS satellite constellation; a mobile base station module comprising a processor and connected to receive GNSS signals from the at least two GNSS antennae (112,114), the module configured to: process the GNSS signals to extract a relative position of the at least two mobile base station GNSS antennae; calculate a distance between the at least two mobile base station GNSS antennae using the measured relative position; compare a calculated distance to the known fixed distance; and produce integrity data indicating whether or not mobile base station integrity is confirmed, wherein mobile base station integrity is confirmed if the calculated distance is within a predetermined threshold of the known fixed distance; a data transmitter for transmitting to an aircraft the mobile base station integrity data, approach path data, and GNSS satellite measurement data for at least one of the at least two mobile base station antennae (112,114); an air GNSS antenna for locating in an aircraft to receive signals from the GNSS satellite constellation; an air data receiver for receiving the mobile base station integrity data, the approach path data, and the GNSS satellite measurement data; and an air module connected to the data receiver and to the air GNSS antenna and adapted to extract satellite data from the GNSS satellite constellation signals, determine a relative position of the air GNSS antenna using the extracted satellite data, the mobile base station GNSS satellite measurement data, and the mobile base station integrity data, and determine approach guidance for the aircraft using the relative position of the air GNSS antenna and the approach path data. at least two mobile base station GNSS antennae (112,114)having a known fixed distance therebetween for receiving signals from a GNSS satellite constellation; a mobile base station module comprising a processor and connected to receive GNSS signals from the at least two GNSS antennae (112,114), the module configured to: process the GNSS signals to extract a relative position of the at least two mobile base station GNSS antennae; calculate a distance between the at least two mobile base station GNSS antennae using the measured relative position; compare a calculated distance to the known fixed distance; and produce integrity data indicating whether or not mobile base station integrity is confirmed, wherein mobile base station integrity is confirmed if the calculated distance is within a predetermined threshold of the known fixed distance; a data transmitter for transmitting to an aircraft the mobile base station integrity data, approach path data, and GNSS satellite measurement data for at least one of the at least two mobile base station antennae (112,114); an air GNSS antenna for locating in an aircraft to receive signals from the GNSS satellite constellation; an air data receiver for receiving the mobile base station integrity data, the approach path data, and the GNSS satellite measurement data; and an air module connected to the data receiver and to the air GNSS antenna and adapted to extract satellite data from the GNSS satellite constellation signals, determine a relative position of the air GNSS antenna using the extracted satellite data, the mobile base station GNSS satellite measurement data, and the mobile base station integrity data, and determine approach guidance for the aircraft using the relative position of the air GNSS antenna and the approach path data. process the GNSS signals to extract a relative position of the at least two mobile base station GNSS antennae; calculate a distance between the at least two mobile base station GNSS antennae using the measured relative position; compare a calculated distance to the known fixed distance; and produce integrity data indicating whether or not mobile base station integrity is confirmed, wherein mobile base station integrity is confirmed if the calculated distance is within a predetermined threshold of the known fixed distance; a data transmitter for transmitting to an aircraft the mobile base station integrity data, approach path data, and GNSS satellite measurement data for at least one of the at least two mobile base station antennae (112,114); an air GNSS antenna for locating in an aircraft to receive signals from the GNSS satellite constellation; an air data receiver for receiving the mobile base station integrity data, the approach path data, and the GNSS satellite measurement data; and an air module connected to the data receiver and to the air GNSS antenna and adapted to extract satellite data from the GNSS satellite constellation signals, determine a relative position of the air GNSS antenna using the extracted satellite data, the mobile base station GNSS satellite measurement data, and the mobile base station integrity data, and determine approach guidance for the aircraft using the relative position of the air GNSS antenna and the approach path data.