BY ORDER OF THE SECRETARY OF THE AIR FORCE AIR FORCE ...

AFMAN 11-217 V1 3 JANUARY 2005 103
of the aircraft. GPS can greatly enhance the performance of an INS. The INS in turn
increases the usefulness of GPS equipment. INS has the ability to accurately measure
changes in position and velocity over short periods of time using no external signal;
however, errors are cumulative and increase with time. GPS can provide a continual
position update that allows the INS to calculate error trends and improve its accuracy as
time increases. The INS aids the GPS receiver by improving GPS anti-jam performance.
When GPS is not available (due to mountain shadowing of satellites, jamming, or high
dynamic maneuvers), this improved INS will provide the integrated navigation system
with accurate position information until the satellites are in view or the jamming is over.
An added advantage is that GPS provides an in-flight alignment capability for the INS.
6.12.4.1. Wide Area Augmentation System (WAAS). WAAS augments the basic
GPS signal for IFR use from takeoff through Category I precision approach. This
system improves the accuracy, availability, and integrity provided by GPS, thereby
improving capacity and safety.
6.12.4.1.1. System Description. Unlike traditional ground-based navigation aids,
the WAAS covers a more extensive service area. Wide-area ground reference
stations (WRS) are linked to form a U.S. WAAS network. These precisely
surveyed ground reference stations receive signals from GPS satellites and any
errors in the signals are then determined. Each station in the network relays the
data to a wide-area master station (WMS) where correction information for
specific geographical areas is computed. A correction message is prepared and
uplinked to a geostationary satellite (GEO) via a ground uplink station (GUS).
The current WAAS site installation consists of 25 WRSs, 2 WMSs, 4 GUSs, and
the required terrestrial communications to support the WAAS network. The
message is then broadcast on the same frequency as GPS (L1, 1575.42 MHz) to
WAAS receivers within the broadcast coverage area of the WAAS. The WAAS
broadcast message improves the GPS 95 percent signal accuracy from 100 meters
to approximately 7 meters.
6.12.4.1.2. Planned Expansion. Planned expansion of the U.S. ground-station
network will include Canada, Iceland, Mexico, and Panama, and has the potential
to expand to other countries as well. Additionally, Japan and Europe are building
similar systems that are planned to be interoperable with the U.S. WAAS. The
merging of these systems will create a worldwide seamless navigation capability
similar to GPS, but with greater accuracy, availability and integrity.
6.13. Inertial Navigation System (INS).
6.13.1. Description. The INS is a primary source of groundspeed, attitude, heading, and
navigation information. A basic system consists of acceleration sensors mounted on a
gyro stabilized, gimbaled platform, a computer unit to process raw data and maintain
present position, and a control display unit (CDU) for data input and monitoring. It
allows the aircrew to selectively monitor a wide range of data, define a series of courses,
and update present position. The INS operates solely by sensing the movement of the
aircraft. Its accuracy is theoretically unlimited and affected only by technology and
manufacturing precision. Since it neither transmits nor receives any signal, it is
unaffected by electronic countermeasures or weather conditions. The INS can also