A navigation device comprising: a receiver configured to receive a plurality of signals transmitted from a plurality of space-based satellites in each of a plurality of Navigation Satellite System constellations; an inertial measurement unit; a processing unit operatively coupled to the receiver and configured to calculate a respective sub-solution for each of a first plurality of sub-sets of the plurality of signals in a first domain, the first domain defining a satellite domain wherein each respective sub-solution of the first plurality of sub-sets in the first domain is determined from signals received from all of the plurality of space-based satellites from all NSS constellations except at least one space-based satellite, wherein the inertial measurement unit is configured to provide inertial measurements to the processing unit; wherein the processing unit is further configured to calculate a respective sub-solution for each of a second plurality of sub-sets of the plurality of signals in a second domain, the second domain defining a constellation domain wherein each respective sub-solution for each of the second plurality of sub-sets in the second domain is determined from signals from space-based satellites from all NSS constellations except at least one NSS constellation; wherein the processing unit is further configured to identify an error in a navigation system based on a comparison of the second plurality of sub-solutions in the second domain with a full-solution for the navigation system based on all of the plurality of signals from all of the NSS constellations, and on a comparison of each respective sub-solution in the first domain with a corresponding sub-solution in the second domain or the full-solution; and wherein the sub-solutions and the full-solution are navigation solutions. a receiver configured to receive a plurality of signals transmitted from a plurality of space-based satellites in each of a plurality of Navigation Satellite System constellations; an inertial measurement unit; a processing unit operatively coupled to the receiver and configured to calculate a respective sub-solution for each of a first plurality of sub-sets of the plurality of signals in a first domain, the first domain defining a satellite domain wherein each respective sub-solution of the first plurality of sub-sets in the first domain is determined from signals received from all of the plurality of space-based satellites from all NSS constellations except at least one space-based satellite, wherein the inertial measurement unit is configured to provide inertial measurements to the processing unit; wherein the processing unit is further configured to calculate a respective sub-solution for each of a second plurality of sub-sets of the plurality of signals in a second domain, the second domain defining a constellation domain wherein each respective sub-solution for each of the second plurality of sub-sets in the second domain is determined from signals from space-based satellites from all NSS constellations except at least one NSS constellation; wherein the processing unit is further configured to identify an error in a navigation system based on a comparison of the second plurality of sub-solutions in the second domain with a full-solution for the navigation system based on all of the plurality of signals from all of the NSS constellations, and on a comparison of each respective sub-solution in the first domain with a corresponding sub-solution in the second domain or the full-solution; and