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AIS/MET System - User Documentation (GPS RAIM)

Approvals
GPS RAIM Prediction Overview
RAIM Outage Information
Message Format via the AFTN and Internet
Pilot Request for GPS RAIM Availability
Incorporation of Aerodrome Data
Further Information

Approvals

The Global Positioning System (GPS) is a world-wide satellite radio navigation system used extensively for air navigation. In Australia, GPS is approved for IFR operations as follows:

"Supplemental means" en route (implemented August 1994 by SUP H18/94);
"Primary Means" en route and GPS arrivals (implemented December 1995 by SUP H50/95);
"Primary Means" oceanic and remote areas (implemented 1 January 1998 by SUP H2/98);
Runway aligned, stand-alone "supplemental means" non-precision approaches (NPAs) (implemented 29 January 1998 by SUP H1/98); and
“Primary Means” en route, terminal and non-precision approach, TSO-C145a/146a avionics (implemented by AIP on 16 March 2006).

GPS - Basic Operation

GPS comprises 24 to 30 satellites orbiting the earth every 12 hours. By signals received from multiple satellites simultaneously, a GPS Receiver/Navigator can determine its position with sufficient accuracy to support oceanic, en route, terminal and non precision approach operations.

GPS - Integrity

An essential requirement for aircraft navigation system is a very high level of integrity; the position given by the avionics must be trust worthy. Approved GPS avionics calculates multiple position solutions using different selections of GPS satellites. If all is well, these multiple solutions will all be very similar; otherwise there is something wrong and the position should not be trusted. This technique is called Receiver Autonomous Integrity Monitoring (RAIM) – Fault Detection (FD).
If there is an additional satellite in view, the faulty signal can be identified and excluded from the position solution. This technique is RAIM – Fault Detection & Exclusion (FDE). Some GPS Approvals require RAIM – FDE.
For these two RAIM techniques to work, there must be a sufficient number of satellites visible and in a suitable geometry relative to the avionics; this is known as having adequate constellation.
The GPS Approvals required RAIM – FD for most operations and RAIM – FDE for some operations.

GPS RAIM Prediction

When planning a flight, it would be useful to know if RAIM will be available at the destination at the planned arrival time. Given the place and time, the availability of RAIM is highly predicable.

Airservices Australia, as part of the Briefing Service, provides a RAIM Prediction Service (RPS). RPS provides RAIM Outage Prediction for all locations which have a published GPS Arrival, Approach or Departure Procedure. The predictions cover a 72 hour period. RPS uses the current satellite almanac and schedules of satellite (maintenance) outage in determining the satellites which will be available.

RAIM Outage Information

Airservices Australia RAIM Prediction Service provides RAIM Outage Prediction for aerodromes with an approved GPS Arrival, Approach or Departure Procedure.

When planning a flight which will use GPS navigation, pilots should obtain a RAIM Outage Prediction and ensure that RAIM (FD and/or FDE) required by the navigation Approval will be available.

The GPS RAIM Prediction System is located in the Australian Flight Information Centre (AusFIC) in Brisbane. Prediction information is made available to pilots via the National Aeronautical Information Processing System (NAIPS). Airservices briefing staff from AusFIC and air traffic control staff in the Air Traffic Services Centres also use this information.

GPS RAIM predictions for NPA are also transmitted to ATS Centres, as required, via the AFTN, and dispatched to the Airservices Internet web server 'http://www.airservicesaustralia.com'.

RAIM outage data for all aerodromes in the database is computed once per day and also when a satellite outage NOTAM is received. This computation is for the following 72 hours period. Once the computation is finished, two things occur:

Outage data are made available through request/replies via NAIPS; and
Outage data are disseminated via the AFTN.

In the case of NAIPS, when the RAIM outage data are received from the GPS RAIM Prediction System, NAIPS formats the next 48 hours of data from the time of the request for distribution to the pilot. This information is presented as::

YBBN


TSO-C129(a) (and equivalent)
Fault Detection
No GPS RAIM FD Outages for NPA


TSO-C146A (and equivalent)
Fault Detection Only
No GPS RAIM FD Outages for NPA


TSO-C146A (and equivalent)
Fault Detection and Exclusion
0610181803 TIL 0610181808
0610190653 TIL 0610190710
0610191759 TIL 0610191804
0610200649 TIL 0610200706
GPS RAIM FDE Unavbl for NPA

Message Format via the AFTN and Internet

RAIM availability information is distributed using the AFTN. RAIM messages are sent once per day at 1400 UTC, or when there is a change to the constellation status. These messages are distributed using a Pre-Determined Addressee Indicator (PDAI) which includes the Internet briefing server.

The message format for notification of RAIM outages via the AFTN is:

GG YBBBMONX
171400 YBBBRAIM
GPS RAIM PREDICTION 171400
YBBN

TSO-C129  (AND EQUIVALENT)
FAULT DETECTION
NO GPS RAIM FD OUTAGES FOR NPA

TSO-C146A (AND EQUIVALENT)
FAULT DETECTION
NO GPS RAIM FD OUTAGES FOR EN-ROUTE
NO GPS RAIM FD OUTAGES FOR TERMINAL
NO GPS RAIM FD OUTAGES FOR NPA

FAULT DETECTION AND EXCLUSION
NO GPS RAIM FD OUTAGES FOR EN-ROUTE
NO GPS RAIM FD OUTAGES FOR TERMINAL
NO GPS RAIM FD OUTAGES FOR NPA

The third line of the message contains the UTC time of the last RAIM prediction and this is followed by a list of the location’s outages for the 72 hour period from this time.

GPS RAIM information may also be requested by sending a correctly formatted AFTN request message to NAIPS (YBBBZEZX) from an authorised AFTN address. Authorisation to access NAIPS via AFTN can be obtained by contacting the Australian NOTAM Office (AFTN: YBBBYNYX, Phone: +617 3866 3648, Fax: +617 3866 3685).

An example of the format for an AFTN request message is:

GG YBBBZEZX
230410 (originator indicator)
REQ RAIM YSCB,YTEM,YPAD

Up to twenty locations may be included in one request message. Each location must be separated by a comma.

Pilot Request for GPS RAIM Availability

Pilots may request the availability of GPS RAIM during the preflight stage of the operation by:

Obtaining an AvFax briefing, using the product identifier 7 followed by the code for the location. e.g. 74001 for Brisbane; or
Direct access to NAIPS via Direct Dial-in 019 8304767; or
Direct access to NAIPS via the Internet (http://www.airservicesaustralia.com); or
Contacting the briefing office.

Incorporation of Aerodrome Data into the GPS RAIM Prediction System

The GPS RAIM Prediction System requires a database of aerodromes. The information for each aerodrome consists of the airport identifier and the latitude and longitude for the aerodrome. This database only contains a record of those aerodromes with approved GPS approaches. The aerodrome database will be updated as new GPS approaches are added.

A number of locations in New Zealand, and the Pacific have also been included in the database.

Further information about this service is available from:

Last updated: 18 October 2006 (GAK)