An atomic clock with a stability  of  10

 13

 s/s will  keep  the  GPS receiver within a 100  m

(95%) position error for several minutes.

8.3.2  GPS/INS Integrations

The GPS INS aided configuration (see Figure 8 2) is useful for medium to high dynamics

applications.  GPS receives INS sensor data from either a serial (e.g., ARINC 429) or

multiplex (e.g., MIL STD 1553) data port. Output is provided through the ARINC 461 and

429 databus port to the CDU and instruments.   The GPS receiver may also provide

position and velocity data to the INS.  For older INS s in particular, there may not be an

existing interface that will take data from a GPS receiver.  This situation is often

encountered when retrofitting GPS to vehicles that have been in service for a number of

years.  For integrating these systems with GPS, either the INS firmware must be modified to

accommodate GPS data, or the GPS receiver must accept the INS data, or the GPS/INS

integration must occur in a separate processor that accepts data from both the INS and

GPS.  In the last two cases, the INS continues to run free inertial, open loop with no

feedback from the navigation processor if one is used.

ANTENNA

ARINC 461

ALTIMETER

CDU

CLOCK

ARINC 429

GPS

RECEIVER

INSTRUMENTS

ARINC 575

INS

OR ARINC 429

Figure 8 2.  GPS INS aided Configuration

If the INS solution can be updated by GPS, and is taken as the system navigation solution,

it will have an accuracy similar to GPS whenever GPS is available. If the GPS data is used

to estimate the major error sources that affect INS position, then if GPS becomes

unavailable, the INS accuracy will degrade at a reduced rate when compared to the free

inertial INS position error growth rate. Baro metric altimeter and clock aiding may also be

available but is not necessary in the INS aided mode.

As discussed in paragraph 9.4.2.1, these are different levels of GPS/INS integration. 

The simplest is referred to as "loosely coupled", where the GPS receiver processes

pseudorange and deltarange to produce geodetic position and velocity, which is then

used by the INS.  In this case, the INS is unaffected by the satellite geometry implicit in

the GPS solution.  However, if the receiver tracks less than four satellites, it may not be

able to produce a geodetic solution, causing the INS to lose aiding by the GPS.  A

more complex GPS/INS integration is called "tightly coupled" where the INS uses

pseudorange and/or deltarange