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.
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Sam Wormley - http://www.cnde.iastate.edu/staff/swormley/gps/gps_books.html