1296MHz Reflections
VK7DC 10th August 2021


I have been using the WSJTX software on 2m WSPR for some time and I have occasionally been seeing reflected signals on the WSPR trace.

It has been more noticable on stronger received signals from a station that was close to me. His signals are normally received at around 0dB and I get reflections when there was an aircraft flying in the general area between us.

 
I have also received signals from VK3's across Bass Strait where there was no apparent originating signal visible and getting the sloping doppler shifted signal that does not decode due to too much frequency drift over the 2 minute receive cycle.

I thought about monitoring the Gippsland VK3RGI beacon on 1296MHZ to see what happened, hoping to get better reflections.

Hooking up the IC-9700 tranceiver remotely to my PC inside the house allowed me to continuously monitor the beacons on WSPR mode using WSJTX software when I am at home of an evening. The VK3RGI beacon is always present here on WSPR trace as I use a 1.8m dish at around 26dB gain and the beacon is relatively close at approximately 280km. I have also used my 15 element 1296mhz yagi with similar results with reflections.

If you wanted to try this but did not have a 1296mhz radio it would be worth trying this using a cheap SDR dongle.


Monitoring aircraft movents using flightradar24.com on the net I was able to sometimes observe very minor reflections as air traffic crossed the path of the radio signal.  The doppler shifted signal always begins on the high frequency side of the original signal as it appproaches the signal path and when it crosses over it the reflection frequency goes lower as it leaves the signal path.

Using VK3RGI beacon I was somewhat disappointed with the results and relatively short duration of signals.


My next step was to try the local VK7RAE 1296MHz beacon in hope of getting stronger reflections from aircraft.

I monitored VK7RAE 1296.474MHz carrier by tuning to 1296.473 USB to give a 1KHz beat note. The beacon is at bearing 108deg and approx 37km away. The path is not too good to me as it has a few coastal hills to get over but is reasonably strong and well audible whichever direction I point.

Unless mentioned otherwise my dish is pointed bearing 10deg at VK3RGI, Gippsland. I am at 130mASL and the VK7RAE beacon is at about 80mASL with hills in between reaching about 173mASL. Signal level is just enough to hear the beacon but not overload my receiver so as to hear weak reflections.


What I saw surprised me to say the least. A little bit of aircraft activity but mainly shipping.

My home is about 3km inland and a has small amount of view to sea with and my worst general northward horizon at just below 0deg. VK7RAE looks out to sea and along the coast but the Burnie port is shielded by some steep hills. Ships become line of sight to VK7RAE once clear of Burnie's port in Emu Bay, less than a kilometre out from port. 


The following images are screenprints of the WSPR waterfall




Picture 1: Normal signal from VK7RAE
Not sure about the random squiggley bits but I think might be reflections from moving vehicles nearby. The regular 1 minute apart spreading bursts are the beacon identifying in CW. As CW is a form of amplitude modulation so it has width when keyed sharply, (key clicks). The spectum scope on left is part of the ICOM remote software package of the IC-9700. The green line at the bottom of the WSPR trace is the received spectrum energy level for the current 2 minute segment starting on every even minute.





Picture 2: Tasmanian Achiever (SHIP) on left side trace (lower frequency)
It is about 15km out of Burnie port heading away to Melbourne.
 The speed of ship is constant but the decrease in angles between beacon and me from the ship over time cause a bigger frequency shift. On the bottom graph it is 10dB stronger than the direct signal from beacon. On the right side (Higher frequency) is a smaller ship coming into port. I have a bad path to the port as it is tucked in behind a steep hill so signals are weak from that area.





Picture 3: Spirit of Tasmania heading to Melbourne from Devonport.
This is at about 50k out and seems to be the practical limit I can get echos from. Guess it is getting beyond VK7RAE's radio horizon and not being illuminated sufficiently. It travels faster than the ship in previous image so has a larger frequency deviation.

 

 
Picture 4: A ship heading towards Burnie port at about 5 knots.
The seas are rough. I can see the echo modulating with the ship rocking/pitching
.
There is also an aircraft reflection starting at about 05:28
 


Picture 5: Ship, STI Dama, approaching the Burnie port.
It does a U turn and heads back out to the anchor area.

The trace gets very wide during the turn as the opposite ends of the ship are traveling towards me at different velocities, giving a broad spectrum of doppler shift.
Notice it is also rocking as it turns. The weather was very poor at this time.




Picture 6: Later the same ship aproaching very slowly again.
 
Perhaps it is somewhat sideways. The severe spikes are about 10 seconds apart. Presume rocking causing this.

Could be reflecting off the ship from different distance areas as it rocks.?? Not totally sure about this.




  Picture 7: Another ship travelling away with similar effect as previous image.
We have had some bad weather lately.


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Change of subject - We have had lots of rain.

I moved the dish to looking north west into prevailing wind (westerly) at the time.



Picture 8: Reflection from heavy rainstorm.
Note the doppler shift is due to the combined angles to TX and RX and the speed and direction the reflection area is moving, not the distance away.

The storm front area was initially light rain and got much heavier as it approached the dish's beam area. You can see the intensity increased to bright red.




Picture 9: Image from WX radar for previous WSPR image.
The area of interest is shown as bright yellow area out to sea to the northwest of Burnie and is heading easterly.


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The next images are a series of another storm passing by. Still beaming northwest. Remember the newest info is at the top of the waterfall.

 

Picture 10: Rain storm at 10k away.
Notice the extra line appearing on the condensed IC-9700 Spectrum Scope at left of image.





Picture 11: Just started raining at home and getting heavy.




Picture 12: After 10 minutes of rain.
An amount of the rain is now between me and the beacon and produces very little frequency shift.





Picture 13: After 20 minutes.
More rain activity aproaching.





Picture 14: After 30 minutes.
The rain has passed.




Picture 15: After 40 minutes and storm is passing away
 There is some weak lower frequency doppler shift as it is moving away from the signal path and out of antenna main lobe.

Notice another storm front coming into view at near 150hz shift





Air traffic.
These are two good examples
.


 Picture 16: Air ambulance leaves Wynyard airport heading east.
 It then turns south before it gets near me and does a few other maneuvers to give result seen at top of the screen.




Picture 17: Rescue helicopter came from Wynyard airport and circled around Burnie hospital and landed.
Final approach to the hospital is about 1k north from me and right in my normal beam path. The solid sweeping line is the chopper body reflection
and the multiple short lines seem to be a mixture of the main blade rotation and digital sample rates of the WSPR software.
Presume it would have actually been a big smear of frequency shift across the screen if one had means to detect it.



How much frequency shift should I get from a moving object???


Download the XLSX calculator that I have written.
Calculator

Get object velocities and bearings from Flightradar24.com or the Marine Tracker app. The marine tracker app on the phone seems to work better for me than the web interface.



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