SBS-1, a new tool to assist in airplane scatter

By SM0DFP Per Green October 2006

Recently (2005)Kinetic Avionichas released a mode S transponder RX that detects and displays some info about the airplane. The transponder broadcasts info about airplane such as long/lat, altitude, ground speed, callsign etc. The transponder in the airplane transmit with a pulse power of 23 to 27 dBW ( 200-500W) on 1.090GHz. There is a lot of information on how the system works on internet. Use “mode S” and “ADS-B” with your search engine for more info.

In the 2006 VHF meeting in Sletten, Denmark, DF9IC, Henning presented a very interesting paper on the SBS-1. Inspired by Henning’s enthusiasm over the SBS-1 one such device was purchased. I had it installed and upgraded with the latest SW downloadable from the Kinetic Avionic site. Within seconds after starting device the first airplanes were detected and displayed on the radar screen. The antenna was at the time still indoors. Once the antenna was placed outdoors on the roof top of my radio shack, the range increased to about 250km. Later I installed a GP antenna with a LNA on the 18m level in my mast. The range increased again to a staggering 400km average. Some airplane DX-ing yielded distances of up to 500+ km, however with QSB coverage.

A new kind of conds indicator

After having had the SBS-1 in use for a month, several interesting phenomena were noted. In normal vs. enhanced conditions the range of the radar did surprisingly not significantly change.BUT, the detection height did noticeably decrease i.e. airplanes could be seen at lower altitudes with good conditions.The same tendency was valid for all airports that I could see.

Reflection mechanism for extended microwave coverage
The hypothesis used is that a microwave signal is reflected by the airplane body and wings according to the famous formula “angle of incidence is equal of angle of reflection”. As the airplanes form is not flat, but curved, there is always some part of the airplane that produce a usable point to bounce of for any two radio stations as long as both radio stations has LOS (line of sight) to the airplane, i.e. the airplane is above the radio horizon. The magnitude of the reflected signal is proportional to the size of the reflecting area. Read more about the physics involved in airscatter in the ARRL UHF/Microwave handbook, an article called “Airplane Reflections” deal extensively with it. The larger the airplane the better reflection, read signal strength, there will be. Another interesting effect is that the signal strength is not frequency dependent, given that all other factors are the same, e.g. ERP , RX NF, etc.  The difference will in a typical case be the time duration of a reflection, as the higher frequency usually has a much higher gain antenna, narrowing the volume of airspace available for the airplane (assuming that the antennas are static, not tracking the airplane).

Doppler shift.

When an airplane is travelling exactly along the bore sight between two stations, and the airplane is in the central part of the path, there will be no Doppler. But as soon as the airplane deviates from the bore sight, there will be some Doppler. At longer distances, e.g. 100 km or more, the Doppler will be small, assumed that the airplane travel in the general direction from one station to the other. If however an airplane is crossing the LOS at an angle that is perpendicular to bore sight the Doppler from that airplane is very audible, going from a higher frequency as the airplane approaches the bore sight. When crossing the bore sight the transmitted frequency will be equal to the Doppler. When moving away from the bore sight, the Doppler will be lower in frequency, a behaviour much as we would expect. When passing perpendicular, the Doppler will shift rapidly, when passing at an obtuse angle, the shift will be of longer duration. The Doppler frequency is typically of +/- 200-300 HZ on 3cm and each station using 60cm dishes, equivalent of a theoretical antenna gain of approximately 33dB. At times, several Doppler signals may be heard due to multiple airplane reflections.

Equipment setup.

The basic SBS-1 receiver is placed in the shack. Power feeding and signalling is via the USB cable to you PC. As I noted that the SBS-1 bare foot RX has some performance to improve,the input ccthas too low selectivity, e.g. transmitting on 144 causes interference to the RX (ok, I have 1kW on 144, but anyway…). So a high dynamic preamp preceded by a 3-pole coaxial filter mounted close to the antenna fixed all interference problems. The original antenna was quickly put back again in the original package, my recommendation, make your own GP and tune it to 1,090GHz, it works much better. After installing my own external filter and high dynamic preamp, I can TX with 100W on 1296 with no QRM to the SBS1.

As my junk box had a piece of 1/2 inch hard line with an angle 7/16 connector on, it had to do when making my own antenna. The white stuff on the antenna is just some water sealing compound.

Some real life examples of how to use SBS-1. 

When using the SBS-1 together with  Spectran, a SW to detect weak signals, and tuning to a weak signal source, in my case SK6YH/B on 3cm, at a distance of 393km a correlation between signal strength increase and airplane locations could easily be found.A typical signal enhancement is in the region 10-30 dB up on a weak tropo signal.

The Screen has been centred at the midpoint between my QTH and the SK6YH beacon. The airplane is marked with a red line from the centre of the screen. This event show the position of the airplane when the reflected signal had zero Doppler, i.e. the airplane is on the bore sight line. The reflection lasted about 40 second with maximum Doppler +200Hz and minimum -200Hz. The signal was enhanced about 10 dB with some QSB. The path travelled by the airplane during the reflection does coincide roughly with the beam width of my antenna.

Here is another interesting situation. We have three prospective airplanes that may act as a reflector point. The strong signal from SK6YH is propagated via RS, hence the wide spectrum, while the airplane signal has a pure tone, offset by about 30 Hz on the low side and 50 Hz on the high side. The reflected signal kept QSBing up and down for a good 10 minutes, most likely because the airplanes travel very close to the bore sight. As can be seen on the Spectran trace, there are two airplane reflections and one Rain scatter reflection.

Note, as this reflection occurs very near the bore sight and is placed about 1/3 of the total distance between SK6YH and me, it possible because both ends have LOS to the airplane. I have tried to track the airplane as it drift away from the bore sight. The attempt to follow the airplane has never been successful to the extent that I can say it is worth the effort.