CONTINUOUSWAVE

The GALILEO FOC-series space vehicles also contain a 406-MHz SARSAT receiver. The antenna for the 406-MHz band is not a little added afterthought; it is a six-element array of helical antennas. The antenna is shown in detail below. It occupies considerable space of the nadir deck on the spacecraft; compare it to the main navigation antenna, seen in Figure 2, below.



In the center of the 406-MHz array is the L-band downlink antenna. The SARSAT signals are retransmitted to ground stations using this antenna.

Considering the cost-per-kilogram to put something into orbit, the inclusion of the two antenna systems and the bent-pipe transponder for SARSAT must represent quite an added expense for the GALILEO program just in terms of the launch payload weight, as well as the power requirements to operate all the electronics for SAR, room on the space vehicle for the devices, and overall engineering of the satellite system.

Regarding the 406-MHz antenna array, I initially thought must have some considerable antenna gain. The GALILEO satellites are in orbit with an altitude of 23,222- km (14429.5-miles). Even assuming free space (best case) conditions, the path loss at 406-MHz at that distance is staggering. Transmission through the atmosphere will add more loss.

The transmitter power of a typical EPRIB or PLB beacon is 5-Watts or +37 dBm. Assuming no antenna gain at the transmitter, the signal at the satellite would then be arriving at a level as low as -135 dBm, which is a very low signal level for UHF reception. A typical receiver might have a sensitivity of about -120 dBm.

On the other hand, because the receiver is in space, the local noise level should be extremely low, most likely limited only by any locally-generated radio noise from the spacecraft itself. The temperature of the antenna should also be extremely low, reducing any thermal noise contribution. And the antenna will have gain.

In searching for some details about the 406-MHz array and its antenna gain, I found:

RECOMMENDATION ITU-R M.1478
PROTECTION CRITERIA FOR COSPAS-SARSAT
SEARCH AND RESCUE PROCESSORS
IN THE BAND 406-406.1 MHz
https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.1478-0-200005-S!!PDF-E.pdf

In that publication, a specification of

−161 dBW (minimum level of the received signal) at the input of the receiver

is mentioned. Converting -161 dBW to dBm gives -131 dBm. The publication also assumes 1.6 dB of transmission line loss, so the signal at the antenna would be -128 dBm. That is not too far off from rough estimate (above). Also signals arriving at angles other than a 0-degree nadir angle will have a longer path, and thus more path loss.

[For more about antenna gain, see the follow-on post below.]

The mentioned specification is mostly concerned about local radio frequency noise density generated on the spacecraft. The local noise must be kept extremely low so as not to interfere with SAR reception at 406-MHz, as the received signal levels are close to the minimum level to produce an acceptable bit-error-rate of the digital data contents.

I found an interesting publication that gives more detail about SARSAT 406-MHz System on GALILEO FOC Satellites:

DESCRIPTION OF THE
406 MHz PAYLOADS USED IN
THE COSPAS-SARSAT MEOSAR SYSTEM
https://www.cospas-sarsat.int/images/stories/SystemDocs/Current/CS-T016-DEC-2016-1.pdf

In Table 4.3, a value is given for antenna gain at the "edge-of-coverage" which is explained in a footnote to be for signals with a 5-degree elevation angle to the satellite from Earth. The array gain at that receive angle is 11.6 dB.

The antenna gain is specified for right-hand circularly polarized (RHCP) signals. I presume the typical EPIRB transmitter is also using RHCP. The cross-polarity response of the antenna is not very encouraging; LHCP response is typically lower by 9 to 15 dB.

The cited publication also includes this graph of antenna gain for the 406-MHz receive antenna on an earlier GALILEO satellite:



The above graph shows the 406-MHz SAR receive antenna has considerable gain, peaking at over 13.4 dB at 0-degree nadir angle. That is more like what I was expecting to find. Assuming a signal arrives at -135 dB, with 13.4 dB antenna gain the receiver could be getting a signal at about -122 dBm, which is generally strong enough for good reception. As discussed in another article, the antenna on a MEO satellite would only have to look for signals at a maximum of 13-degrees off nadir.

So far we have been discussing the capability of the GALILEO GNSS constellation to receive signals from 406-MHz distress beacons and relay those signals to ground stations for further processing and action. This is called the FORWARD LINK ALERT SERVICE. With the GALILEO Full-Operational-Capability (FOC) series satellites, the satellite has the capability of adding a new feature to the SARSAT system: RETURN LINK SERVICE. A return link service will be able to send a message back to the beacon device, with the primary function likely to be to acknowledge reception of the distress signal. We look into this in more detail.

In order for the return link service to work, the satellite has to be able to send a message and the the beacon device must be able to receive it. In the case of the GALILEO (and other GNSS) satellites, those satellites are already transmitting a signal that the distress beacon device may already be receiving: the satellite's navigation message signal on the L-band carrier. If the distress beacon has a GNSS receiver that works with GALILEO, it will likely be able to receive the navigation message. To get a return link service, the satellite just needs to be able to add the return message to the navigation message it is already sending, and the distress beacon GNSS receiver needs to know how to find that message in the navigation message and communicate it to the user of the distress beacon.

In the SAR-GALILEO Service Definition Document, Version 1.0, December 2016, the roadmap for this feature is described as follows:

7.2 Return Link Service
The SAR/Galileo Service roadmap foresees the introduction of an additional service called "Return Link Service" to the currently provided Forward Link Alert Service. This provides an acknowledgment capability to the distress beacons confirming that the alert has been detected and located by the system. This service, which will be provided worldwide, will rely on the Galileo E1-B navigation signal (see Annex A[9]) to transmit the acknowledgment messages to the distress beacon and will therefore only be available for RLS enabled beacons (refer to C/S documentation Annex A [2], [3} and [4]). This new service is planned to be introduced by mid-2018.

In order for a distress beacon to work with the Return Link Service planned to be provided on GALILEO, the distress beacon device will have to contain three important components:

• first, a GNSS receiver that can work with the GALILEO system;
• second, that GALILEO receiver must be able to demodulate the E1-B navigation message to find the return link message embedded in that data; and
• third, provide some electronic interface between the GALILEO receiver and the user-interface on the beacon so the user can read the message.

As far as I can tell, there is no EPIRB distress beacon that posses all those components available right now on the market.

In some further reports about the plans for the Return Link Service, the following features were highlighted in a paper presented to the Institute of Navigation (ION) conference in 2017 in Portland, Oregon:

...the SAR/Galileo Service will also introduce a new Search and Rescue function, called the Return Link Service (RLS), which provides acknowledgment messages to distress beacons equipped with a Galileo receiver, through the Galileo E1 signal. Galileo is currently the only GNSS system planning to provide this service worldwide...

Although not included in the current definition, in the future the Return Link Service could be extended to provide many other features in addition to the acknowledgment services such as:

• Remote activation of beacons on boats and aircraft that have been reported missing;
• Turning off beacon transmission when the SAR mission has been completed, but where it was not possible or practical to recover and turn off the beacon manually;
• Changing the repetition rate of the beacon transmissions after the alert has been received and location established without ambiguity, with a view to saving battery power or reducing the beacon message traffic load on the satellite system.
• Message service using the capacity offered by the Return Link Service to transmit information back to the radio beacons.

...The Galileo Programme currently expects to have the Return Link Service start of operations in the timeframe end 2018 or beginning 2019....

I have read some rather optimistic descriptions which suggest that the Return Link Service will give the beacon's user a message "that help is on the way." According to the plans in place at the moment, the Return Link Service is going to only be able to generate a return message that will just confirm that the beacon's distress alert was received by the satellite, and without any further information or knowledge that any rescue action has been initiated or is underway. It is important to differentiate the two types of return link service messages. In a workshop for beacon manufacturers, the two types of return link service messages were explained:

During the development process two types of acknowledgments were considered in the Return Link Service:
--Acknowledgment Type 1 (also called System acknowledgment): in this case, the Galileo system is only responsible for the automatic transmission of Return Link Message (RLM) to the emitting beacon once the alert has been detected and located.
--Acknowledgement Type 2 (also called RCC acknowledgment): in this case the Galileo system would send the RLM to the emitting beacon once it has received authorization from the RCC (Rescue Coordination Center). This acknowledgment would inform the user that the distress has been received by the RCC.

Also at this same workshop, manufacturers were informed of a conclusion made in 2012 by a Joint Working Group (JWG) of the International Civil Aviation Organizations (ICAO) and International Maritime Organization (IMO) "...that the RLS Type-2 and further applications of it (e.g. two-way messaging) should not be pursued for the time being."

I mentioned in a separate article about the Air Force GPS III-series satellites that there is a plan to add 406-MHz receivers for SARSAT to those satellites, but not until the second phase of the deployment, which will not begin until c.2025, or about six years from now. I have not seen any information regarding the provision for having a Return Link Service in the GPS-III satellites. I will try to find more information on that aspect of the GPS-III series plans.

In the two years since I wrote the initial article in this thread, at least one manufacturer of SARSAT 406-MHz beacon devices has tentatively announced a new product that will support Return Link Service and provide a Return LInk Message to the SARSAT 406-MHz beacon device that has initiated a distress alert.

Orolia Maritime announced at a recent trade show (METS 2019) that it plans to have a new device on the market (subject to regulatory approvals) that will support the Return Link System capabilities. The device will be branded FAST FIND RETURN LINK. In a press release, Orolia said:

Orolia wrote:The PLB displays a blue light to inform the user that search and rescue professionals are aware of their situation and location and that they are not alone.

Whether this device is reporting reception of the Acknowledgment Type I message sent autonomously by the GALILEO FOC satellite that received the 406-MHz transmission or is reporting reception of an Acknowledgment Type II message sent by a SARSAT RCC is not made clear in the Orolia press release.

From their statement that their product is "...the first of a new range of SAR beacons that receive a reassurance signal confirming distress alert and location have been received by the search and rescue authorities," (emphasis added) a reasonable inference to make is the acknowledgment message would be the Type II message initiated by the Rescue Coordination Center of SARSAT. However, a diagram in an Orolia document suggests the RLM will come from the Mission Control Center, which is upstream of the Rescue Coordination Center.



Further, Orolia says:

The fashing blue light [acknowledgment signal] is sent around 15-minutes after confirmation that the distress signal has been detected and the beacon[']s location independently located.

The delay of 15-minutes implies that the acknowledgement is not autonomously generated by the GALIELO system, but originates in the SARSAT system.

Actual availability, delivery, and use of the new FAST FIND RETURN LINK PLB will have to wait for the GALILEO system to bring the Return Link System to full operation status; this is anticipated to happen in early 2020, according to Orolia.

PANBO writer Ben Elison reports that also at METS 2019 he and Ben Stein were given a private preview of a similar PLB with Return Link Service manufactured by ACR. More at

https://www.panbo.com/return-link-service-a-major-plb-epirb-improvement/

jimh wrote:... I presume the typical EPIRB transmitter is also using RHCP [Right Hand Circular Polarization]...

My presumption about "the typical EPIRB transmitter" antenna polarization was probably too optimistic. Most of the devices appear to be using a simple linearly polarized antenna, usually just a whip extending from the body of the device.

Earlier I mentioned that Orolia comments on a 15-minute delay:

jimh wrote:

Orolia wrote:The fashing blue light [acknowledgment signal] is sent around 15-minutes after confirmation that the distress signal has been detected and the beacon[']s location independently located.

The delay of 15-minutes implies that the acknowledgement is not autonomously generated by the GALIELO system, but originates in the SARSAT system.

Upon further (and perhaps clearer) thinking on this topic, I would like to alter my opinion. The message going back to the EPIRB or PLB device that initiated a distress alert broadcast will be sent by the satellite system (in this case the GALILEO FOC satellites) as part of their normal navigation message. At this moment I am not expert on GALILEO signals, but if they are akin to the GPS signals, the navigation message is a long message, sent in segments, and encoded at a very low bit-rate, about 50-bps. For a GLS L1 C/A signal, the complete navigation message takes about 15-minutes to send. The 15-minute "delay" that Orolia mentions might be in reference to the possible delay that would occur due to the EPIRB's or PLB's GALILEO receiver having to receive the entire navigation message over again, in order to get the segment of the message that contains the encoded acknowledgement message.

A further question to be answered is how other satellites in the constellation get instructions to also send the acknowledgement message. The message and instructions to send it would have to be uplinked to them from a GALILEO ground control station.

When (and if) I find out more, I will append to this thread.

The Return Link Message for SARSAT is sent on the E1-B signal from GALILEO and contained in the I/NAV message. The I/NAV message frame takes 720-seconds to send (or 12-minutes).

In order not to waste time sending acknowledgment messages to every EPIRB or PLB that initiates a SARSAT 406-MHz distress signal, the EPIRB or PLB must encode in its distress signal an indicator that it wants and can receive a RLM acknowledgment.

A further requirement for the EPIRB or PLB that is expecting a RLM message is for it to keep its GNSS receiver operating for periods of 20-minutes at intervals in order to be listening for the message.

For more details see:
Wikipedia: Galileo Navigation Message
https://gssc.esa.int/navipedia/index.php/Galileo_Navigation_Message#I.2FNAV_Structure

SAR/Galileo Service Definition Document
https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-SAR-SDD.pdf

European GNSS (Galileo) Open Service: Signal-in-space Interface Control Document
https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-OS-SIS-ICD.pdf

SPECIFICATION FOR COSPAS-SARSAT 406 MHz DISTRESS BEACONS
C/S T.001 concern 4 – Revision 1 May 2017
https://www.cospas-sarsat.int/images/stories/SystemDocs/Current/CS-T001-MAY-2017.pdf

In particular see page 4-11 and section 4.5.7 RLS Beacon Requirements

UPDATE
More on signal levels and path loss for 406-MHz SARSAT.

Transmitter Power Specification
The typical personal locator beacon (PLB) will have a 5-Watt transmitter at 406-MHz. The antenna is just a simple linearly polarized vertical, so for purposes of estimating the effective power, I will use 0-dB gain for the antenna. There is no transmission line loss as the antenna is integral with the transmitter.

Receiver Sensitivity Specification
In the design specification mentioned above in the second article in this thread, the anticipated signal at the receiver input is mentioned as -161 dBW, which is a reference to 1-watt. Converting that specification to dBm or reference to milliWatts, shifts the reference by 30-dB (the difference between 1-Watt and 0.001-Watt) so the signal level in dBm expected at the receiver is -131 dBm.

There is also a mention of a loss of -1.6 dB in the receiver feed line. And from the graphic showing antenna gain as a function of the angle the signal arrives at the antenna, the worst case is a gain of 11.6 dB (at ±13-degrees off nadir). Another consideration is the polarization of the receiving antenna; it appears to be a circularly polarized antenna, but the transmitted signal is coming from a linearly polarized antenna. What this means, however, is that no matter the orientation of the transmitting antenna there won't be any loss due to cross-polarization as the linear antenna changes position.

One additional element to add to the calculations is the desired signal margin, that is, how much loss of signal can the specification tolerate. This parameter might be specified in the design goal, but a reasonable estimate to improve reliability would be to add 6 dB to the minimum possible signal level to allow for some fading, which is generally referred to as the fade margin. This increases the minimum signal at the receiver to -125 dBm from -131 dBm.

Putting all these figures into a calculation can help determine how much distance in the path between transmitter and receiver can be tolerated, assuming you know something about the anticipated path loss. Here I assume the path is a line-of-sight (no obscuring terrain or objects in the path from sea to sky), and the attenuation will be the "free space" rate of inverse-square law decrease.

When a GALILEO satellite happens to be directly overhead, the path length will be the shortest, that is, the orbit height of the satellite above the Earth (14429-miles or 23,222-km path length) and nadir angle will be boresight resulting in the highest antenna gain (13.4 dBi), giving 7.75 dB fade margin instead of the estimated 6-dB.

When the satellite is not directly overhead, the path will be longer. In a separate article I have calculated the path length when the GALILEO satellite appears at an elevation angle of only 15-degrees to the observer to be 16,959-miles or 27,293-km. With this path the antenna gain reduces to 11.5-dB due the signal arriving off boresight and the path loss increases due to greater distance. At this range and nadir angle the fade margin is reduced to 4.6-dB.

The receive antenna pattern is only shown for ±13-degrees off nadir, but because of the high orbital height this covers a huge percentage of the Earth that is in view. (See separate article for analysis.)

Regarding the satellite orbits themselves, the GALILEO orbit is inclined 56-degrees. This means that the highest latitude possible in their orbit for being directly overhead is 56-degrees North or South. The SARSAT organization doesn't have any say in the GALILEO orbit geometry, so they just have to accept that the GALILEO satellites have some limitation in higher latitudes.