Aircraft Scatter

 

Scattering or reflecting radio signals off of objects in the sky is used by radio amateurs to increase communications range whether they are doing Meteor Scatter, EME, Aurora, Sporadic E, or other modes of communication.

Aircraft can also be used to increase communications range, by using Aircraft Scatter.  Recently I became much more interested in Aircraft Scatter when I learned that Rex Moncur VK7MO and David Smith VK3HZ have completed Aircraft Scatter contacts of up to 842 km on 10 GHz and 462 km on 24 GHz.  Their paper describing these achievements is here.

If you are looking for AircraftScatter Sharp, you can download a zip file of the program here. More information on the program is here

This page is divided into the following sections:

Software to Assist with Aircraft Scatter Operations
Aircraft Scatter Theory: The bistatic radar equation, and more
How to Get Started
Receiving Local Aircraft Over the Air
Information Sources Regarding Direct Reception of ADS-B Signals
Excellent Aircraft Scatter Information Sources on the Web

An older version of this page is here.
 
Software to Assist with Aircraft Scatter Operations:

To do aircraft scatter work, you need to know where the airplanes are. 

There are two ways of determining where aircraft are at any moment. The first is to receive that information yourself over the air, and the second way is to get that information from internet servers. Both of these methods rely on the fact that virtually all aircraft now regularly transmit information regarding their identity and position.

The two technologies in common use are Mode S and ADS-B. Aircraft carrying mode S transponders will reply to an interrogation from a ground station by sending back a packet of information. Mode S aircraft can also send out unsolicited packets called squitters. The ADS-B or Automatic Dependent Surveillance-Broadcast system sends out unsolicited packets. With both of these systems, aircraft send out their data signals on 1090 MHz. Whether you receive your information over the air or via the internet, it originated with one of these systems. 

There are several servers that you can use. I initially used PlanePlotter, because the first generation aircraft scatter software that I wrote several years ago worked through PlanePlotter to get its data. There is a description of this older program of mine on one of my webpages. Don't spend any time there learning the messy details of how to set up that program with PlanePlotter, however, because I have since written a newer program that connects directly to the servers on the web and does not require PlanePlotter or any other program to function.

The newer program is called Aircraft Scatter Sharp and it has several important capabilities:

1. Real-time plane position information capture and display for planes captured either by internet servers, or by a local RTL1090 Mode S receiver/server which you have running on your local network, or both at the same time.

2. Display of the direct path line between two stations, along with skew lines to allow a quick assessment of the angular deviation of an aircraft position from the direct path line for both stations, and a midpoint circle to show when an aircraft is within a specified distance from the midpoint of the path. Path altitude and elevation/obstruction profiles are also shown. 

3. Highlighting of aircraft near the ideal position for reflection, based both on distance from the midpoint of the path as well as angular deviation from the path.

4. Real-time calculation of path loss/received signal strength/signal margin for aircraft scatter communications using the selected plane, based on plane location and user-adjustable station parameters for the stations at both ends of the direct path (frequency, transmit power, receiver noise figure, receive bandwidth, and antenna gain).

5. An integrated SQLite database that allows you to save information on all planes appearing on your screen for however long you want [minutes, hours, days, weeks, months] and to then analyze that data to determine when aircraft scatter opportunities will most likely appear in the future. You can analyze the data without interrupting its collection, and powerful SQL search functions are automatically included and easily selectable using only mouseclicks to generate the SQL query statements.

The last piece, the SQL database, provides what has been missing from previous aircraft-scatter software (except for my earlier program mentioned above, which also included this feature). For EME we have software predictors of when the moon will be “available” to us for use as a reflector. For rain-scatter we have RainScatter, by Andy Flowers, K0SM to give us this information. But there has been nothing similar for aircraft scatter until now, and to my knowledge is nothing else except for this program.

An article which discusses my program, Aircraft Scatter Sharp, is here  It will give much more detail on Aircraft Scatter Sharp and help you to get started.  You must read this.  There are far too many features of this program to list here on this webpage.

At the very top of this webpage is a screen grab of  Aircraft Scatter Sharp.  You can download a zip file of the program here.  Just double-click on "Setup.exe" to install.  The program has been tested and works with Windows XP 32 bit and both the 32 and 64 bit versions of Windows 7.  It has not been tested with other operating systems.  More information on the program is here  A screen grab of the program displaying plane data from 60 planes obtained from my own RTL1090-based Mode S receiver is below:

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Aircraft Scatter Theory: The bistatic radar equation, and more

Aircraft scatter is NOT magic.  You still need to consider the path-loss equations, and in addition the less than perfect reflective characteristics of the aircraft.  The aircraft gives you an (imperfect) reflective surface that is positioned high in the sky, allowing you to extend your horizon.  But if you have insufficient signal to make up for the path losses, including the losses sustained at the imperfect reflection from the aircraft, you will not be able to hear the other station or make a contact.  The radar equation can be used to figure out whether or not a contact can likely be completed; it relates power at the receiver to power at the transmitter, taking into account path losses and system characteristics.  Emil Pocock W3EP wrote a nice discussion of this applied to amateur radio aircraft scatter in The ARRL UHF/Microwave Experimenter's Manual, pages 3-28 and 3-29.  He reformed the equation in terms of path loss:

total loss (in dB) L = 10 log (( lambda**2) * S / (((Rt)**2) * (Rr)**2))) - 153 where
Rt = distance from transmitter to reflector in km
Rr = distance from receiver to reflector in km
lambda = wavelength in meters
S = radar cross section of the aircraft

"S", the radar cross section of the aircraft, will obviously depend upon the aspect the aircraft presents to the observer, and it may also have a frequency dependence if certain materials are used.  Emil gave estimated values of 2 M**2 for a Lear Jet, 8 for a Douglas DC-9, 16 for a Boeing 707, and 63 for a Boeing 747.

If we use this equation to evaluate the path loss for a Boeing 707-sized plane for 10  GHz work with the plane at the midpoint between two stations 500 km apart, we get a path loss of 267 dB.  By comparison, the EME path loss for 10 GHz at perigee would be 287 dB.  The path loss over 500 meters in free space would be 167 dB.  So aircraft scatter loses an extra 100 dB relative to free space loss, and over a 500 kM path is only 20 dB better than moon bounce!  If you were using a 30 inch dish and 4 watt transmitter and had a receive sensitivity of -130 dBm, then over a 500 km free space path you would have a fade margin of 71.2 dB.  With aircraft scatter you would be 29 dB BELOW the noise.  So it would seem that aircraft scatter is useful only if you have a high ERP as well as a sensitive receiver. With the same receiver characteristics but with 100 watts transmit power and a 3 meter dish, your fade margin for a 500 km path in free space would be 109 dB.  So for aircraft scatter you would be 9dB out of the noise and could make a CW contact.  Note that VK7MO and VK3HZ were able to make contacts using only 10 watts and a 34 dBi dish at one end and 8 watts and a 32 dBi dish at the other end of a 628 km path, but they used JT65C to give additional receive gain.  For 5 GHz, the path loss for the same aircraft scatter path would be 261 dB rather than 267 dB.  You can check the math if you wish by using 6.25E11 meters for the radar cross section of the moon and 356,000 km as perigee, and solving the radar equation for the path loss with the moon as the reflector.  You will get 281 dB.  As noted above, my program will do path loss calculations for the selected DX station and aircraft of your choice.  More information on that is in a companion pdf file, starting at page 13.  Here is a screen shot of the EN90gg <>FN20ag path I use for illustration: with the parameters chosen, the signals are about 11 dB out of the noise.  You can see the path and the designated aircraft on the Aircraft Scatter Sharp map, and the signal strength parameters on the Path Loss Calculator portion of the form. 

However, there is some potential MAGIC that may crop up: 20-30 dB or more of signal enhancement may occur when the aircraft is located on or very near to the direct path line between the two stations.  This enhancement is called "forward scattering" enhancement and falls off extremely rapidly if the aircraft is positioned even slightly off of this line.  Rex Moncur VK7MO has an excellent paper discussing this here, and VK2KU Guy Fletcher discussed it as well in the paper mentioned immediately above.  This enhancement is also frequency-dependent, as noted in the above papers, and likely of reduced utility on 10 GHz and higher, due to the very narrow lobes of the scattering produced at this higher frequency.  VK7MO and David Smith, VK3HZ discuss this here.  Note that Rex and David were able to make use of this phenomenon on both 10 and 24 GHz, in spite of the frequency constraints noted above.  Using a "new" WSJT mode called ISCAT-A for 10 and 24 GHz Aircraft Scatter, Rex and David have completed Aircraft Scatter contacts of up to 842 km on 10 GHz and 462 km on 24 GHz.  Their paper describing these achievements is here.  Rex describes this new mode like this: 

"This mode is around 15 dB more sensitive than FSK441 and is a further development of ISCAT which is included in the publicly released version of WSJT9. The new version includes both ISCAT-A and ISCAT-B with ISCAT-B being the old ISCAT included in the publicly available version. Joe Taylor kindly developed ISCAT-A to meet our requirements for aircraft scatter at 10 GHz. It can be run in 15 second periods and can cope with very rapid Doppler variations of up to 1000 Hz/min as occur at 10 GHz when an aircraft crosses the path at right angles. It also copes well with the short bursts of a few seconds that we receive at 10 GHz and has an averaging feature that allows it to take advantage of longer but weaker bursts".

Rex was kind enough to send me some plots of forward scatter enhancement as it occurred on the 561 km path between his QTH and that of VK3GHZ.  The top plot is a waterfall plot of frequency vs time and the bottom graph shows signal strength vs time.  You can see that the Aircraft Scatter signal peaks at 30 dB above the noise on this 561 km path!

Rex has further description of these plots here.

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How to Get Started:

1.  Read my pdf article here

2.  Download and install the zip file containing Aircraft Scatter Sharp.  Just double-click setup.exe to start the installation.  The program has been tested and works with Windows XP 32 bit and both the 32 and 64 bit versions of Windows 7.  It has not been tested with other operating systems.  More information on the program is here

3.  Get WSJT 9 so you can run ISCAT-A

4.  Find a DX station who is also interested

5.  Use my program (Aircraft Scatter Sharp, see above) or a similar one to build a database of when aircraft are actually in suitable positions, or cull the flight schedules or use a site like http://flightaware.com/live/ where you can type in destination and origin cities and get a list of all flights, and determine favorable times for attempted contacts.  I use both.  The FlightAware data gives you an idea of when planes "might" be where you want them to be.  Information gathered over a period of days or weeks using my Aircraft Scatter Sharp program gives you actual data on when there were actually planes in the locations you require.  Believe it or not, airline schedules are not actually consistent from day to day!  The scheduled times may be consistent, but the actual flight times are not.

6.  Use the Path Loss Calculator form from my program (or do the math) to figure out your link budget, and thus determine the probability of success with and without forward scatter enhancement, using appropriate values for transmit power, antenna gain, and receiver noise figure.

7.  Give it a try using Aircraft Scatter Sharp and ISCAT-A, at times determined as above.  With Aircraft Scatter Sharp running, you can see how things look second to second, and also see if new aircraft come into favorable positions for scatter.

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Receiving Local Aircraft Over the Air

There are a number of receivers available to enable you to receive these signals directly over the air. These include the Kinetic-Avionic SBS3, the AirNav Systems Radar Box, the microADSB receiver, the Aurora Virtual Radar Mode S Receiver, and the Mode-S Beast18, which was designed by DL4MEA and is considered to be the best 1090 MHz receiver available. Each of these has a web site easily found and full of information. Or, you can save some money and use as your receiver a cheap RTL2382 Dongle obtainable for $20 or less. That is what I did, getting a “NooElec TV28T v2 USB DVB-T & RTL-SDR Receiver” from Amazon for $19.95 with free shipping. I then went to the website http://rtl1090.web99.de/ where I downloaded the RTL1090 software and drivers (do NOT install the drivers that come with this device if you want to use it for this purpose!). This site also has an excellent instruction manual on getting up and running with an RTL2382 dongle. You want to run the RTL1090 software in “Mode S” mode.

I think the easiest and cheapest way to start receiving local aircraft, if you want to do that rather than (or in addition to) using the internet data is to get one of the cheap RTL 2832 Dongles from Amazon, as described above.

The best URL for getting software and guidance to get going on Windows is http://rtl1090.web99.de/

There you can download the RTL1090 software that you can use to feed Aircraft Scatter Sharp,  PlanePlotter or FlightRadar24.  The site also has an excellent MANUAL for getting started.  Look at the yellow-orange buttons at the top left of the webpage, and click on the one that says "Manual".  Then click on the blue letters that spell "Manual" that appear beneath the button, and you will have the manual on your screen.  All should be self-evident.  As they say, make SURE you do NOT install any of the software [drivers or otherwise] that came with the dongle!  I got good results just using the little 4 inch-or-so long indoor antenna that came with the dongle, but much better results with an outside antenna and preamp mounted at the antenna.  I currently have a WIMO GP-1090 antenna and a Kuhne 1090 MHz mast-mount preamp and get excellent performance from them, as you can see here.  Here in FN20ag I have significant obstructions, and find that my typical range with the NooElec dongle is as follows:

NW 410 km
N     310 km
E     180 km
S     370 km
SW 200 km

You cannot expect optimal performance at 1.09 GHz without a mast-mounted preamp and low loss coax, unless your dongle is right at the antenna.  Furthermore, the dongles are extremely wideband by design, and have very poor strong signal tolerance.  Therefore, using an antenna with some frequency selectivity and a preamp with helical or other filters to remove signal outside the desired passband will greatly enhance performance, by preventing performance degradation from out-of-band strong signals.  So even if you have the dongle right at the antenna, adding a preamp with frequency selectivity will improve performance by attenuating out-of-band signals before they reach the dongle.

There is further information available at:  http://www.rtl-sdr.com/adsb-aircraft-radar-with-rtl-sdr/

You can also play with things on Linux.  I did my original installation of the E4000 on Linux, but I haven't played with things on Linux since.  The following URL walks you through getting set up for Linux step by step.  It is simple as long as you follow the directions, and I had no problems with it.  http://www.hamradioscience.com/the-rtl-2832u-sdr-and-ads-b/ .  I have enjoyed playing with this, but if you have internet access, actual received "through-the-antenna" signals are redundant to the internet-received data, because the internet data is all time-stamped.  I don't use this at all to plan aircraft scatter operations or tests.  But its fun to play with, and I do upload my real-time aircraft data to the web for others to use.

You plug the RTL Dongle into the USB port on your Linux computer, and hook it up to the indoor stock 4 inch antenna that came with it.  You install gnuradio, the gr-air-modes application, and the VirtualRadar server program on your Linux box and launch gr-air-modes and then Virtual Radar, and the VirtualRadar server will start sending data to PlanePlotter that is running on your windows box.  This is a painless and very inexpensive way to see where the aircraft you might use for scatter are located.  Below is a screen shot of all of this running on my Linux box.  Again, you can see a larger version by right-clicking on the image and then clicking on "View Image":

 

Below is a graph of the elevation angles vs distance for planes that I have tracked here with PlanePlotter.  Only really close-in planes will have an elevation of above 2 degrees or so, so you don't need to have elevation control unless your antenna beamwidth is very narrow:

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Information Sources Regarding Direct Reception of ADS-B Signals

DL4MEA has a webpage describing a much more sophisticated receiver / decoder that he designed at: http://www.qsl.net/dl4mea/fpgaadsb/fpgaadsb.htm .  This device known as the "Mode-S Beast" and is described further and is for sale at: http://www.modesbeast.com/ .

A technical description of the ADS-B system is at:

www.ll.mit.edu/mission/aviation/publications/publication-files/journal-articles/Orlando_1989_JA-6373.pdf  Additional references are in the references section at the end of my article mentioned above.

The ADS-B transmissions use Manchester coding, which is described at:
http://en.wikipedia.org/wiki/Manchester_code

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Excellent Aircraft Scatter Information Sources on the Web:

There are some excellent pages discussing various aspects of using Aircraft Scatter for Amateur Radio purposes:

VK3HZ  and VK7MO have some great web pages and articles on Aircraft Scatter work that they have done.  They have done extensive investigation and have made aircraft scatter contacts on microwave frequencies up to 24 GHz, using JT65C and ISCAT (a new WSJT mode).  They have completed Aircraft Scatter contacts of up to 842 km on 10 GHz and 462 km on 24 GHz.  They make extensive use of the ADS-B system and the PlanePlotter software.  I was pleased to see of their great success and accomplishments with these techniques...it validates my plans and tells me that I am on the right track with my beginning efforts here.  You especially want to check out the sections of their web pages labeled "24 GHz Propagation Tests", "Radio Site Display", and "Aircraft Enhanced Propagation"! :
http://www.vk3hz.net/

 The exact Doppler shift seen with aircraft scatter for a given situation will depend on the geometry.  Ron Cook VK3AFW, Rex Moncur VK7MO, and David Smith VK3HZ have a nice paper on this at: http://www.vk3hz.net/microwave/Doppler-Shift-Estimation-10GHz-AE.pdf

This page by VK3HZ gives links to multiple other Aircraft Scatter pages from Down Under:
http://www.vk3hz.net/ae.htm

SM6FHZ's aircraft scatter page:
http://www.2ingandlin.se/ACS.htm  He has an aircraft scatter path-loss Excel Spreadsheet here.

SM0DFP has a page describing his use of ADS-B signals to assist with airplane scatter contacts:
http://sk0ct.se/propagation/flygscatter1.htm

Here is an nice page on airplane scatter by GM4XCM:
http://www.rayjames.biz/gm4cxm/id28.html

Here is an interesting page on airplane scatter by G0ISW:
http://www.qsl.net/g0isw/g0iswair.htm

SM0LCB has an overlay for GoogleMaps to help you plan contacts:
http://sk3w.se/sm7lcb/www-sm7lcb/maps/qso_map/pathmap2.htm .   See also the bottom of this webpage for more:  http://sk3w.se/sm7lcb/www-sm7lcb/maps/qso_map/index.htm  Below is a picture of his GoogleMaps page showing a possible aircraft scatter path between my location and EN90gg.  This direct path information is included in the information given by my Aircraft Scatter Sharp program described above. 

A PowerPoint talk by DF9IC on airplane scatter:
http://www.df9ic.de/doc/2006/sletten_2006/sletten06_airplane_reflection.ppt

The ADS-B system (Automatic Dependence Surveillance-Broadcast mode" is described by AE5X: 
http://www.ae5x.com/blog/2010/01/18/ads-b-aviations-aprs/

There is a nice Aircraft Scatter program called AirScout by Frank, DL2ALF which uses airplane data which is available from the web.  It runs on Windows.  You can read about this here, and download it from here.  Here is a screen grab of his program running here at W3SZ:

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