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Positioning Application: What is Galileo Search and Rescue and how can I test it?
Positioning Application (PosApp)
What does GNSS have to do with Search & Rescue?
Since the 1980s, satellites have been used to detect the signals from emergency beacons and relay them to the appropriate Search & Rescue (SAR) authorities.
This is coordinated through the International COSPAS/SARSAT Programme.
Initially an agreement between the USA, Canada, France and the USSR, the programme now involves more than 40 countries who use the service, provide space technology or run satellite monitoring sites on the ground.
How does Galileo fit in?
Originally, distress beacon signals were detected and located using satellites in Low Earth Orbit (LEO).
Geostationary (GEO) satellites were later added to improve coverage and reduce detection times but these could not be used to calculate the beacon’s position.
Medium-earth orbits (MEO) were identified as providing the best solution: still able to calculate the beacon position using Doppler shift measurements, a MEO constellation could provide global coverage and better visibility for beacons in obscured environments.
The latest GNSS satellites are fitted with 406 MHz relays to transfer emergency beacon signals to dedicated ground stations via an L-band link.
SAR is a key requirement of the Galileo constellation, whose focus is on civilian rather than military uses, and relays are also fitted to some GPS and GLONASS satellites.
An additional feature, unique to Galileo, is the Return Link Message (RLM) which informs the emergency beacon that assistance is on the way. This message is included in the E1-B NAV message and is due to be enabled in mid-2018.
The following Return Link Messages have been defined to date (Feb 2019):
• Acknowledgement Type-1 RLM: An automated response that acknowledges receipt of the RLS request,
• Test Return Link Message: An RLM dedicated for testing, including automatic response that acknowledges receipt of the RLS request from an RLS beacon coded with an RLS Location Test Protocol.
For more information on the technical details refer to:
Galileo OS SIS ICD, Issue 1.3, December 2016
COSPAS-SARSAT 406 MHz Distress Beacons, C/S T.001 Issue 4 – Revision 4 February 2019
How does this affect my simulation?
When creating a SimGEN scenario (or requesting a SimREPLAY/SimREPLAY
one) you can choose to have RLMs broadcast by one or more Galileo satellites.
By default, dummy RLM data will be broadcast (alternating ones and zeroes, which will be ignored by a beacon).
How do I include an RLM in my SimGEN scenario?
RLM message broadcasts and their content are defined in the ‘Search and rescue RLM’ section of the Galileo signal sources file. Here you can specify:
The time at which you want the message to be broadcast
Which satellites you want to broadcast the message
The ID of the beacon to which the message relates
The RLM message code
Whether the message is of the short or long type
The message content
Each command causes a single RLM message to be broadcast (with duration 8 seconds for a short message, 16 seconds for a long message). If you want the message to be repeated, you will need to create a command for each instance of the broadcast.
Below is an example screenshot showing how to enter and schedule RLM messages in the Galileo signal sources file for:
Beacon ID = 1
Message code = 1 (Acknowledgment Service - Type 1)
First to bits of RLMN parameters field are "10"
the second example violates the parity conditions of message data according to the ICD:
The parity bit (last bit of the parameter field) shall ensure that the total number of ones (1) in the fields "Beacon ID", "Message Code" and "Parameters" (including spare bits) is even.
The Positioning Application software does not check or correct for parity in these messages.
Find out more
The History and Experience of the International COSPAS-SARSAT Programme for Satellite-Aided Search and Rescue
(available at https://www.iafastro.org/wp-content/uploads/2017/01/Cospas-Sarsat-Report-1.pdf) provides a fascinating history of the programme, its technology and its politics. There are many, more concise, resources available online which describe the COSPAS-SARSAT programme and the Galileo constellation’s part in it.
For more details about including RLMs in SimGEN scenarios, see section 12.9 of the SimGEN Software User Manual.
Product : Positioning,Position Apps,SimREPLAYplus,SimREPLAY,SimGEN,PosApp,Scenario,I/NAV,E1,Galileo