The Galileo Satellite constellation and 1296 EME operation

When I'm operating on 1296 EME I run two receive systems. One is a 144-1296 Mhz transverter coupled to a Yaesu FT-897 transceiver. This is my main WSJT(x) transmit and receive system. The other is a FunCube Pro + SDR which couples to a laptop running MAP65, which shows me a 90KHz wide region of the spectrum (typically from around 1296.005 to 1296.095MHz. This allows me to see (and sometimes decode) activity on 1296 EME. Since I have the 90kHz wide baseband audio available, I also couple it into SpectraVue and, using the continuum display mode, show the average noise power in that region of the spectrum. Typically any EME signals are too weak to show up as a change in the noise power over cold sky, so it's normally a measure of my background noise.

It can be used to measure cold sky, sun noise, ground noise or noise when the LNA is connected to a 50 ohm load. All these things are useful in testing the system. It can also be used to measure moon noise when I'm listening on 10GHz EME since the moon noise (with my 85cm dish) is about 0.2dB over cosmic background noise. With a 3m dish on 1296 I do not see moon noise. On 10Ghz with larger dishes, many stations monitor moon noise in order to peak their narrow beamwidth dishes on the moon. It's rarely, if ever, used for this purpose on 1296 because (a) you'd need a big dish to see it and (b) conventional tracking systems are good enough to track the moon without the need to peak them up on moon noise. A 3m dish on 10 Ghz has a 1dB beamwidth of around +/- 0.2 degrees, while a 6m dish on 1296 has a 1dB beamwidth around +/- 0.75 degrees.

So I maybe one of the few stations that routinely monitors background noise (or the lack of moon noise!) while doing EME on 1296. I don't know how many other do, but it's certainly true that the majority of stations don't. So I may be more ware of changes in background noise than most.

Over a number of months I noticed times when I saw a rise and fall in background noise over periods of 10-20 minutes. The noise level might go up by as much as 3dB. At first I assumed that it was just antenna sidelobes picking up some local broadband noise source, or pointing at trees or other structures and I was seeing ambient temperature thermal noise. However, if I noted the AZ and EL of the antennas when I saw the noise and went back later to look for the noise, it was gone. Also, if I stopped tracking the moon and just left the dish pointing where the noise peaked, it would slow drop back down to it's normal "galactic background" level over the course of ~5-10 minutes. Here's something of a typical event. This time the noise background rises and falls by about 1.5dB over a time interval of around 15 minutes.

After thinking about this for a while, asking questions on Moon-net and doing some tracking of the noise signal, it gradually became apparent that the noise source was space based and wasn't cosmic. It appeared to be due to a satellite (or satellites) in relatively high orbit. The time for the noise to rise and fall (10-20 minutes) couldn't be from an LEO (Low Earth orbit) satellite, nor could it be from a geostationary satellite. Its motion was consistent with something in an orbit about 25000km high. That's an MEO and the sort of orbit GPS satellites use. Galileo navigation satellites use are also in MEO (medium earth orbit).

The Galileo Satellites

We all know about GPS satellites, but there is a similar system under European control based on the Galileo constellation of satellites. I believe that at the beginning of 2020 there are 26 satellites in orbit (though not all are active). The plan is for 30 satellites of which 24 will be operational (the others are spares in case of active satellites failure). I think currently there are 31 operational GPS satellites. These Galileo satellites are in different (though similar) orbits from the GPS satellites and operate on slightly L-band different frequencies, but they provide similar navigation and time services. So why are they of interest to 1296 EME operators?

The reason is that their E6 transmissions can extend into the 1296 MHz region as shown below:

(above info from https://galileognss.eu/)

GALILEO E6B/C NORMALIZED PSD

So it's not surprising that from time to time a Galileo constellation satellite active in mode E6 occasionally passes close enough to the moon that it's effect on the noise background can be seen. Though the Galileo downlink is RHCP and EME receive is on LHCP, in neither case is the axial ratio of the signal likely to be 0dB. Both the Tx and Rx antennas are likely to be elliptically polarized to some extent, so some of the signal will be seen. Since it's a 40Mhz wide broadband signal, it will appear as rise of the noise floor rather than at any discrete frequency. The rise and fall time and the magnitude of the rise will depends on the a few things, including:(1) How close it comes to the moon when their tracks cross. (2) The orbital position of the satellite. (3) Assuming a non-zero axial ratio, the relative alignment of the (mostly RHCP) elliptically polarized signal and the (mostly LHCP) elliptically polarized receive antenna. The satellite will be (slightly) further away and appear to move slower when the elevation is low. It will be closest and appear to move fastest when it is directly overhead.

Further confirmation of the source of the noise is looking at the signal with both RHCP (right hand circular polarization) and LHCP (left hand circular polarization). Normally EME stations are setup for RHCP transmit and LHCP for receive, but I can connect my normal Tx port to a second LNA and bring the RHCP received signal into the shack. I set up my dish to find one of the Galileo satellites and took a look that the difference in signal strength with the dish pointed at the satellite and this dish pointed at the adjacent sky.

As you can see, the signal received with RHCP is significantly stronger than that received using the EME normal LHCP, which is consistent withe the Galileo system specifications of an RHCP transmitted signal. Converting the observed (S+N)/N values to true S/N, the difference in actual signal strength between LHCP and RHCP reception is approximately 9.3dB.

This implies a fairly high axial ratio of either the Galileo TX signal or my receiving system or both. I normally receive EME signals at least as well as (and sometimes better) than others using similarly sized dishes. This suggests that what ever my axial ratio is, it's not causing any appreciable loss of sensitivity on 1296 EME signals. An OK1DFC septum feed typically shows and axial ratio in the 1 to 2dB range and that should null out an incoming perfect RHCP signal by more than 9dB. I'm using a modified feed with a square to circular flare, which should, if anything decrease the axial ratio. You'd expect maybe 9 dB or more if both the RHCP Tx and LHCP Rx had axial ratios of 3dB (or some other combination adding up to about 6dB). So the relatively small difference between LHCP and RHCP signal strength is, as yet, not fully understood. The L band downlink antenna on the Galileo satellites is a phased array, but I have found no information on the axial ratio of the transmitted RHCP signal.

As a point of reference, on a recent set of RHCP vs LHCP measurements of the ON0EME beacon signal, I saw a difference in average signal strength of around 10.5dB between the two polarizations. This is not greatly different from the observed 9.3dB on the Galileo signal.

Note that on signals of the same CP, i.e. receiving a normal EME signal, if both Tx and Rx had an axial ratio of 3db, then the signal loss could range from a minimum of 0dB (no loss at all, depending on the orientation of the two elliptical polarizations) to a maximum of 0.5dB. So the affect on normal EME reception would be small.

An earlier theoretical look at potential interference to 1296 EME from Galileo (see Ref [1]) had suggested that it should not be visible with a 3m dish, and only just visible with a 10m dish. This appears not to be the case in practice (at least with my dish and feed system!).

Is it a problem and what can you do about it?

The answers are not often and not much. The signal is there in the band so no amount of filtering will change the strength. It's there legally and 1296 is a shared frequency. With the right adaptive polarization setup you could probably minimize the signal.

However the added noise will only affect the decoding of weak signals (maybe <-23dB using JT65C) - and it typically doesn't last very long (~10 minutes). It also doesn't happen very often (though often enough for me to have become aware of it).

I have, on at least one occasion (and possibly more I didn't identity) had problems decoding a weak 1296 EME signal in the presence of the Galileo background signal. It's not a common situation, but it happens if you are active often enough. If you are trying to work a JT65C EME station that's around -23db or less, 3dB of added noise will probably prevent decoding, and I've seen that happen.

I'm writing this on 1/19/2020 and I took a quick look at some orbital data for the Galileo satellites and the moon. I noticed that from my location at 05:55 local time, Galileo satellite E26 passed very close to the moon (see image below). Finding a close pass is something of a laborious task at present, so I didn't look for other close passes.

There are issues with 1296 and Galileo in europe for terrestrial (and presumably EME) operation and if you want to explode your brain reading about them, here's a link. https://ukamsat.files.wordpress.com/2019/05/2015-ec-jrc_compatibility-between-amateur-and-galileo.pdf - Compatibility between Amateur Radio Services and Galileo in the 1260-1300 MHz Radio Frequency Band. This is a 72 page report from the European Commission, so it's not exactly light reading. There's some lighter reading on the ARRL website detailingWRC-23 concerns. The basic worry in Europe is that terrestrial 23cm transmissions (especially ATV) might interfere with ground reception of Galileo signals. Basically a concern that a 23cm transmission could result in Galileo based "GPS" type devices losing the Galileo signal. See also reference[1] below.

Resources and References

Acknowledgments

Thanks to Peter, G3LTF, for discussions, suggestions and comments on this topic.