aiding should be treated with extra caution.  Barometric altimeters are excellent devices

for measuring pressure altitude, but pressure altitude can vary widely and non linearly

from geometric altitude.  The resulting vertical errors should be modeled carefully since

the errors can depend on meteorological conditions and vehicle dynamics.  For

additional discussion of GPS aids, refer to Chapter 7.

6.3  FIGURE OF MERIT (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. Ionospheric 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.

6 17