WHAT IS FOM? A FOM is an indicator of receiver positioning or time accuracy which may be displayed to the operator or communicated to an integrated system. A FOM may be either a qualitative or quantitative measure, depending on the accuracy and integrity of the data used to calculate the FOM. In general, a FOM is not suitable for making integrity decisions where safety of life is concerned. However, a qualitative FOM may be perfectly suitable for integrity decisions regarding unmanned missions. (Refer to Chapter 12 for additional discussion of integrity.) A FOM is typically calculated as the root-sum-square of the estimated errors contributing to the solution accuracy. Example criteria include: a. GPS receiver state (e.g., carrier tracking, code tracking, acquisition) b. Carrier to noise ratio c. Satellite geometry (DOP value) d. Satellite range accuracy (URA value) e. lonospheric measurement or modelling error f. Receiver aiding used g. Kalman filter error estimates. The resultant FOM can be presented as a numerical value, for example from 1 to 9, where 1 indicates the best navigation performance. It can also be presented directly as an error estimate in metres, at a specified probability level, or even as a simple pass/fail indication. A time figure of merit (TFOM) can also be calculated to indicate the quality of the precise time information available from the GPS receiver via the PTTI interface (see paragraph 4.3.3). Table 6-10 gives the FOM and TFOM numerical assignments and equivalent estimated errors for the Rockwell-Collins family of receivers developed for the GPS JPO. ___________________________________________________________________________ Sam Wormley - http://www.cnde.iastate.edu/staff/swormley/gps/gps_books.html