GRAVES beacon too strong in meteor echoes detection

[morinmau]

Hello guys,

I recently set up a meteor echoes detection station using GRAVES radar as transmitter. I have a problem with the signal from GRAVES, i'd like to know your opinion.

I live in Northern Italy, about 80km south of Milan (44.988211N, 9.124843E). Initial antenna (yagi 5 elements) bearing was toward GRAVES radar, so azimuth 314° and about 25° elevation (in order to cross GRAVES beam at 100km height), horizontal polarisation. Distance from GRAVES is 384km. I use HDSDR going to SpectrumLab through virtual cable. Receiver is Noolec NESDR SMArTee v2 dongle.

GRAVES beacon is always present, I would say 70-80% of the time on 24h basis, from strong to medium weak. Furthermore, the signal drifts in frequency from its main location, and often doubles. Some waterfall examples below.

 

As you can imagine, when the signal is strong enough, i can get as much as 50% false positives on the total hourly captures. I implemented several test in SpecLab CA to filter the beacon out, but essentially without success. I even set up two frequency bands for trigger (each with different thresholds), but since the signal is drifting sometimes, the 'rejection' band spell should be too large to be effective, so to remove GRAVES beacon but I also miss genuine captures.

So, I moved antenna orientation toward west, azimuth about 270° and lower elevation, around 10°. In the map below you see original antenna setting (in green) and this 'west' position (yellow line). I thought that is the most extreme position you can pick up in order to be still in GRAVES radiation area.

The situation slightly improved (GRAVES a bit weaker in average) but nothing changed in term of false positives.

My impression is that I'm too 'close' to the radar, but honestly i don't understand (technically) how the signal (that comes from tropo reflection) could be sometimes so strong and persistent, since I don't see the TX directly, and tropospheric conditions are far from ideal this period.

Any suggestion is welcomed.
Maurizio IU2JWG

[wxsatuser]

The double traces and diverging ones are more than likely aircraft returns.

[Geminids]

Your reasoning for elevation angle is sound, but if you are still getting the main beam signal directly as it seems you are, then I would suggest that you are seeing the main beam through one of the antenna side lobes.  Try higher angles of elevation, changing antenna polarisation or pointing away from GRAVES.  From your location I would think you get very strong pings from meteors wherever you point.

By the way where did you get the map that you have drawn your directions on?

Mike

Edited November 6, 2020 by Geminids

[morinmau]

On 06/11/2020 at 01:21, wxsatuser said:

The double traces and diverging ones are more than likely aircraft returns.

Hello wxsatuser,

Ok but why i don't get the 'usual' streaks at low speed/doppler shift that means a reflection from a plane (as I see in waterfalls from other radio stations) ?

[morinmau]

20 hours ago, Geminids said:

Your reasoning for elevation angle is sound, but if you are still getting the main beam signal directly as it seems you are, then I would suggest that you are seeing the main beam through one of the antenna side lobes.  Try higher angles of elevation, changing antenna polarisation or pointing away from GRAVES.  From your location I would think you get very strong pings from meteors wherever you point.

By the way where did you get the map that you have drawn your directions on?

Mike

Hello Mike,

i'll try all alternatives bearings and see what happens.
I'm sure I dowloaded the map from some forum o blog, i just added the two coloured lines. I can attach the original one, but i don't remember the source.
Cheers

 

[Geminids]

Hi Morinmau,  a pretty basic question but are you sure of the frequency of that signal and that it is GRAVES?  I do not know the Noolec NESDR SMArTee v2 dongle but the ubiquitous FCDP+ can be a few hundred Hz off frequency - having said that, I have just looked at your last spectrumlab plot and the meteor ping is around 900 Hz with the "GRAVES" signal is just below.

[morinmau]

On 07/11/2020 at 18:07, Geminids said:

Hi Morinmau,  a pretty basic question but are you sure of the frequency of that signal and that it is GRAVES?  I do not know the Noolec NESDR SMArTee v2 dongle but the ubiquitous FCDP+ can be a few hundred Hz off frequency - having said that, I have just looked at your last spectrumlab plot and the meteor ping is around 900 Hz with the "GRAVES" signal is just below.

Hi Mike, the Noolec was already frequency calibrated, so the signal is indeed GRAVES. Below you'll find two examples of genuine captures along with a spell of GRAVES beacon.

Cheers

Maurizio

 

 

[wxsatuser]

On 07/11/2020 at 08:41, morinmau said:

Hello wxsatuser,

Ok but why i don't get the 'usual' streaks at low speed/doppler shift that means a reflection from a plane (as I see in waterfalls from other radio stations) ?

Hi Maurizio, I suppose it's where you are in relation to the aircraft.

I can receive GRAVES direct but not all the time and I get the diverging/parallel lines.
BTW I am located in IO90uv UK south coast my call is G1HWY.

[johnb]

Hi im sure its just settings etc, there is no reason why you should not get get GRAVES all the time its transmitting, I collaborate with others and some are on the south coast like yourself and they have no issues

 

 

John B

 

[wxsatuser]

27 minutes ago, johnb said:

Hi im sure its just settings etc, there is no reason why you should not get get GRAVES all the time its transmitting, I collaborate with others and some are on the south coast like yourself and they have no issues

John B

 

I do receive it direct most of the time, which is surprising with 400feet of chalk in the way.
Yesterday afternoon it was completely missing apart from a few seconds here and there.
Most of the time as well as direct signal I get many pings and a few short bursts, occasionally some nice long bursts.
Have had many satellites like ISS, and Moon reflections a few times.

[johnb]

interesting - do you run a 24x7 setup ?

 

John

[wxsatuser]

3 hours ago, johnb said:

interesting - do you run a 24x7 setup ?

 

John

Not at the moment, just weekends.

Most of my time and the computers time is spent running hamradio digital modes on HF and VHF.
When nothing is doing on VHF I put the receiver on GRAVES.

I do have two spare SDRplay RSP1As and I will try and get one doing 24/7 GRAVES shortly.