A ground-based system comprising: a plurality of reference receivers configured to receive satellite signals; and a processor coupled to a memory, wherein the processor is communicatively coupled to the plurality of reference receivers and is configured to: determine whether the line of sight of a newly available satellite is within an ionosphere gradient; when it is determined that the line of sight of the newly available satellite is not within an ionosphere gradient, determine a monitor measurement using accumulated carrier data for each pair of reference receivers of a plurality of reference receiver pairs, wherein accumulated carrier data is measurement data received from a reference receiver/satellite pair which is comprised of the accumulation of the carrier phase cycles, converted to meters, which have been received since first tracking the satellite signal; combine monitor measurements for a subset of the plurality of reference receiver pairs into a monitor discriminator; and output an alert when the monitor discriminator exceeds a first threshold, characterized in that: when it is determined that the line of sight of the newly available satellite is within an ionosphere gradient for a pair of reference receivers, repeat the determination of whether the line of sight of the newly available satellite is within an ionosphere gradient for the pair of reference receivers until it is determined that the line of sight of the newly available satellite is not within an ionosphere gradient for the pair of reference receivers. a plurality of reference receivers configured to receive satellite signals; and a processor coupled to a memory, wherein the processor is communicatively coupled to the plurality of reference receivers and is configured to: determine whether the line of sight of a newly available satellite is within an ionosphere gradient; when it is determined that the line of sight of the newly available satellite is not within an ionosphere gradient, determine a monitor measurement using accumulated carrier data for each pair of reference receivers of a plurality of reference receiver pairs, wherein accumulated carrier data is measurement data received from a reference receiver/satellite pair which is comprised of the accumulation of the carrier phase cycles, converted to meters, which have been received since first tracking the satellite signal; combine monitor measurements for a subset of the plurality of reference receiver pairs into a monitor discriminator; and output an alert when the monitor discriminator exceeds a first threshold, characterized in that: when it is determined that the line of sight of the newly available satellite is within an ionosphere gradient for a pair of reference receivers, repeat the determination of whether the line of sight of the newly available satellite is within an ionosphere gradient for the pair of reference receivers until it is determined that the line of sight of the newly available satellite is not within an ionosphere gradient for the pair of reference receivers. determine whether the line of sight of a newly available satellite is within an ionosphere gradient; when it is determined that the line of sight of the newly available satellite is not within an ionosphere gradient, determine a monitor measurement using accumulated carrier data for each pair of reference receivers of a plurality of reference receiver pairs, wherein accumulated carrier data is measurement data received from a reference receiver/satellite pair which is comprised of the accumulation of the carrier phase cycles, converted to meters, which have been received since first tracking the satellite signal; combine monitor measurements for a subset of the plurality of reference receiver pairs into a monitor discriminator; and output an alert when the monitor discriminator exceeds a first threshold, characterized in that: when it is determined that the line of sight of the newly available satellite is within an ionosphere gradient for a pair of reference receivers, repeat the determination of whether the line of sight of the newly available satellite is within an ionosphere gradient for the pair of reference receivers until it is determined that the line of sight of the newly available satellite is not within an ionosphere gradient for the pair of reference receivers. determine a monitor measurement using accumulated carrier data for each pair of reference receivers of a plurality of reference receiver pairs, wherein accumulated carrier data is measurement data received from a reference receiver/satellite pair which is comprised of the accumulation of the carrier phase cycles, converted to meters, which have been received since first tracking the satellite signal; combine monitor measurements for a subset of the plurality of reference receiver pairs into a monitor discriminator; and output an alert when the monitor discriminator exceeds a first threshold, characterized in that: when it is determined that the line of sight of the newly available satellite is within an ionosphere gradient for a pair of reference receivers, repeat the determination of whether the line of sight of the newly available satellite is within an ionosphere gradient for the pair of reference receivers until it is determined that the line of sight of the newly available satellite is not within an ionosphere gradient for the pair of reference receivers.