The Global Seer

A project aimed at developing a reconfigurable sea-based relay network for ADS-B out signal for aircraft tracking over the oceans.

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Automatic dependent surveillance – broadcast (ADS–B out to be specific) are a series of periodic radio broadcasts made by aircrafts that are received by air traffic controls (ATCs) and other aircrafts allowing the broadcasting aircrafts to be tracked. Aircrafts receive navigation information (i.e. GPS location) from satellites and together with other information (i.e. aircraft type designators, course, and speed), rebroadcast them on the ADS-B out channel which is in turn received by ground-based receivers.

The received information are used to augment the less accurate aircraft positional information from primary and secondary radar. This ADS-B framework will form an essential part of the US Next Generation Air Transportation System (NextGen) and the Single European Sky ATM Research (SESAR) initiative, which promises more efficient and safer air travel by 2020.

Now the current problems faced -

The ADS-B network is currently unencrypted, making it vulnerable to attack. But that is another chapter I guess.

ADS-B is works on a microwave frequency of 1090 MHz which means that it is incapable of bouncing off the ionosphere making it essentially line-of-sight. Realistically, the broadcast will have a range of only about 300 miles or 483 km. On land this not so much of an issue as receivers can be readily deployed to increase coverage. In fact websites like and are providing free ADS-B receivers for general public to host (subjected to location). Information received by these receivers are forwarded to their server which are then made available on the internet.

Tracking aircrafts over the oceans using ADS-B is a whole new ball game. There are no comprehensive ocean-borne receiver network for this purpose. Aireon, a subsidiary of Iridium Communications Inc estimates only about 10% of the earth surface (mostly on land) has coverage for ADS-B. They plan to launch and operate satellites with ADS-B receivers providing truly global coverage by 2017.

It would be an interesting exercise to see how almost off-the-shelf technology can address this problem from now to 2017. Here lies the premise of the Global Seer Project - To develop an autonomous ocean platform with an ADS-B receiver as its payload that can be deployed to a strategic location to act as a relay for ADS-B information. Information received by the platform can be relayed via satellite/shortwave radio or from platform to platform in a mesh network configuration to land.

Now here goes :  At this early stage in the project there is a glaring lack of any details, but I will still endeavour  to come up with a functional schematic for the Global Seer scheme - 

As far as I envisage the platform will need to have the following sections:

1. Power System - Definitely involves solar power. But also at the same time toying with the idea of harnessing the energy of the waves.

2. Propulsion and Station Keeping System - Good old motor and propeller pair. May also need a good way to effect position hold on the platform.  

3. Communication System - Somekind of satellite modem. Probably will also include wifi, 3G/4G connectivity for high bandwidth data transfer near the coast. For long distance or inter-platform communcation maybe somekind of shortwave data modem.

4. Navigation/Collision Avoidance System - An onboard GPS for waypoint navigation. As big as the ocean is, subscribing to Murphy's law, the platform will inevitably collide into something. Maybe a pair of steroscopic camera for collision avoidance. 

5. Auxiliary Sensors -  I do not foresee there will be a space shortage on the platform. Possibly a suite of sensors can be included to monitor the weather (temperature, wind speed and direction, etc..., although I question the wisdom of putting a weather station on such a small platform) and the ocean (water temperature, salinity, etc...).

6. ADS-B Receiver -  The...

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  • 1 × 10W Solar Panel As the name suggest, a solar panel. Max Power 10W, Open Circuit Voltage 21.6V, Short Circuit Current 0.643A, Fuse Rating 5A, Voltage at Max Power 17.5V, Current at Max Power 0.572A, Max System Voltage 600V
  • 1 × 30A Solar Charge Controller A solar charge controller has three pairs of terminals. One for the solar panel input, one for the battery, and one for the load. The controller charges the battery and switch over to it when solar power generation drops.
  • 1 × High Power Wifi Dongle Preferably has option for external antenna.
  • 1 × 3G/4G Dongle Preferably has option for external antenna.
  • 1 × A Raspberry Pi Used as a single board computer here.

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  • Sharing the List of Tracked Planes

    Poh Hou Shun08/26/2014 at 05:48 0 comments

    Just some updates on my attempt to share the list of tracked planes on In the earlier post 'Ttracking Your Very First Planes' I indicated that I was not successful in doing so. The output from the ADSB receiver was processed by ADSB#. Despite pointing ADSB# and fr24feed ('s feeder software), fr24feed still reports the '"Too few aircraft, at least 2 required" error, despite seeing like 5 aircrafts being tracking in adsbscope. What was interesting then was that fr24feed indicated that the basestation was 'OK'. Normally if there is any configuration problem, fr24feed will spit out a big fat 'ERROR' message at the 'Basestation' entry.

    I looked up more on this issue and found the cause, I think. The data format output by ADSB# is simply just incompatible with what fr24feed requires. Here is a screen shot of the dump from ADSB# on port 47806 -

    It turns out the 'correct' software I should use is rtl1090 which was superseded by dump1090, so I went with that. dump1090 can be downloaded from -

    A good guide for setting this up can be found at -

    Since my ADSB receiver driver is working and I did not install rtl1090 previously, I only need to unzip the downloaded folder and run 'dump1090.bat'. You should see something like this -

    The screen shot of the dump from dump1090 on port 30002 is completely different -

    I then went through the routine of entering my email and approximate GPS coordinates and started fr24feed. It again return with a 'Basestation:OK' and 'Too few aircraft, at least 2 required ' status. First I have to say I am testing all these in my office. Soon I realized that my antenna is lying horizontally on a sofa below the window ledge level -

    Of course in doing so the signal is in the wrong polarization with respect to the antenna. The antenna probably cannot see any portion of the sky when placed below the window ledge. I took the antenna up and held it in my hand. The number of tracked planes short up to 5 and fr24feed managed to generate a sharing key and off I go (fr24feed at lower right corner) -

    The dump1090 software also has a web based output accessible by browsing the local ip address on port 8080 (shown above).

    After that to obtain the premium membership (non of those timeout nonsense) on flightradar24, I just created an account with the same email and click the 'I am feeding data' option.

    I noticed that not all the planes that was tracked by my receiver are visible on the website. Conversely when I put down the antenna (no I was not holding the antenna up all the time when I was doing this) so that no planes were tracked on my receiver, non of those tracks on are dropped. Maybe their server requires the same track from a number of feeds before it accepts it as a genuine track? This means a lone platform with a single receiver in the ocean may not work. Will check more on this. Meanwhile onwards with the sharing.

    Another interesting thing I found while scanning around on flightradar24 map is this -

    The two balloons on the map turns out to be balloons launched by Google for project loon - It is a project by Google to essentially strap wifi repeaters (or something like it) on balloons and with a large fleet of these balloons, try to bring internet to every corner of the earth (hemmm...).

    What is interesting is that the balloons forms a ring around some latitude of the earth in the southern hemisphere which is determined by the jet streams in the stratosphere. These jet stream crosses the oceans. Could this be part of the solution to the communication problem with the Global Seer platforms in the ocean? Cliffhanger here I guess.

  • Harnessing the Power of the Sun

    Poh Hou Shun08/21/2014 at 03:17 0 comments

    Finally I gotten around to wire up the 10W solar panel (shown below) I have laying around. I will probably get myself some higher wattage panel in the future. For the moment this should be sufficient for the land-based receiver. One thing I noticed when shopping for solar panel is that I was told by most, that these panels are extremely durable outdoor, unless something drop smack onto them. For marine usage, I have my doubts. Will need to look more into this. There are also flexible panels. Maybe they will prove to be useful.

    A solar panel alone is not really useful with the output voltage changing and all with sunrise and sunset. What is needed is a charge controller. A charge controller just regulate the output voltage for the purpose of charging a battery and powering a device when the sun is up. The load then draws the power from either the panel or the battery depending on the amount of sunshine. I went out and got myself a, 'one hung low' (hemm expression sounds familiar), 30A charge controller (shown below). The charge controller has a 'key' button to switch between a few modes. Basically this particular controller is meant for lighting purposes. It turns off the load (lights) when the sun is up and turns on the load for a set number of hours after the sunlight diminishes. These modes are EXTREMELY bad for the ocean platform as you may imagine. So I switched to a mode that continuously power the load. After I set it to the permanently powered mode, I disconnected the panel and battery. Fortunately the controller does retains the setting if it is completely powered down.

    I wired the panel and a 12V SLA battery to the controller and commenced testing. More on this in the upcoming project log.

  • Tracking Your Very First Plane

    Poh Hou Shun08/21/2014 at 02:38 0 comments

    My USB digital tv tuner (Shown below, DVB-T Realtek RTL2832U Elonics R820T 50MHz - 2200MHz RTL-SDR) a.k.a ADSB receiver finally arrived in the mail. Got it from ebay from a seller based in Hong Kong for about 10USD. But took more than three weeks to arrive. Should have bought like a dozen. Anyway this project log entry documents the installation and my first test of the receiver using Window based software. Will port it over to a Raspberry Pi running some distro of Linux (not sure which one yet) in the near future.

    A very comprehensive guide for the process is avaliable at -

    Basically install the driver from the tiny cdrom included with the receiver. Plug in the receiver so that the driver takes hold. Use the software Zadig from -

    to replace the stock driver for Windows 7.

    Now here is where I departed from the instruction from I downloaded the files from -

    and unzipped them into a folder. Go into the folder 'sdrsharp' and launch the program 'ADSBSharp.exe'. ADSBSharp.exe is sort the software back end that interfaces with the receiver and then pipe the result via a TCP port. You should see something like this - 

    I left the 'Port' setting at its default value of 47806. Next time if there are multiple ADSB server running, this port setting can be changed. Click start to start crunching plane information. Next I downloaded the software 'adsbscope' from  -

    Once again unzip all the files into a folder. Launch 'adsbscope27_256.exe' in the folder 'adsb_all\pc_software\adsbscope\27\' This is what you should see next.

    As this is not the first time I am running this, I have reset the software's default receiver location. You can do the same by centering your receiver location on the map and choosing 'set Receiver Location' from the 'Navigation' menu. Once that is done select 'save default' from the 'File' menu. Now go  menu 'other' -> 'Network' -> 'Network setup', click on the 'ADSB#' and 'local' button. You should see this - 

    Now go menu 'other' -> 'Network' -> 'Raw-data Client Active' and start tracking airplanes. After I connected up my homemade antenna and played around with some of display settings, this is what I saw - 

    Surprising I can see any planes at all sat in my office waving around a 60cm PVC pipe. 

    Some other tests I did yield unexpected result. I brought the receiver to the roof top a few days later but actually detected less planes. Maybe I just happen to hit timing with minimal air traffic?  I also tried to share the receiver output on flightradar24 using their fr24feed software, but gotten a 'too few aircraft' error despite plenty are displayed on adsbscope. Will cover more on these tests in future log once I figure out where is the problem.

  • The Ear of the Global Seer?

    Poh Hou Shun08/20/2014 at 19:36 0 comments

    I think many would have guessed correctly that is a project log entry on the construction of an ADSB antenna. The digital tv tuner (shown below) a.k.a the ADSB receiver in the component list comes with a dinky little wire antenna with a MCX jack (more details on the tuner coming up in future blog).  For antenna, bigger (up a certain length) is better, at least for this case.

    An excellent guide for making your own ADSB antenna can be found here -

    What I am actually building is called a Coaxial Collinear Antenna. The parts needed is a length (2m for my case) of 75Ohm, RG-6U coaxial cable (shown below, left)

    Firstly I need to calculate what is the actual length each of the segments that I need to cut from the coaxial cable. The segments need to be half the wavelength of the received signal. Let's start from the fact that the ADSB signal is broadcasted on a frequency of 1090Mhz. So the wavelength is given by 

    wavelength = c/frequency where c  = 299792458 m/s (this is exact) is the speed of light in vacuum.

    Substituting a frequency of 1090Mhz in the above formula, we get the wavelength to be 0.275m. Thus half the wavelength will be 0.138m. Now this value of 0.138m is only applicable for the case for a radio signal traveling in vacuum. In an dielectric medium like in a coaxial cable, radio signal travels slower. This is a similar effect as in refraction as light travels slower in glass. Since the frequency remains unchanged, the wavelength must also decrease. From the datasheet of the cable, the 'Nominal Velocity of Propagation' is listed as 83%. Thus  the magical length to cut the coax cable to 0.138 X 83% giving me 0.114m.

    Now the next question is how many segment do I need? Ideally, the greater the number of segments, the more sensitive the antenna pickup is. Above a certain number of segments, the losses in the cable dominates and one no longer sees an increase in sensitivity. I believe in Hak5 they went for a 16 segment antenna here -

    For me the choice of the number of segments to use is based on a more mundane consideration. The antenna is supposed to be mounted on an ocean platform. So I would say antenna length of about 60cm is acceptable. Thus I went with a four segment antenna as a test.

    The antenna is assembled by inserting the core of one cable into the sleeve of the other.  According to the balarad website, it is recommended to leave 5cm of the core uncut. With that consideration the length of each of the segment I need is 0.114 + 0.05 + 0.05 = 0.214m. With this number and my trusty vernier caliper, I commenced cutting. I have use marking tape on the cable to mark the places I need to cut. Result of this exercise is shown below.

    I then stripped off 5cm of the outer layer leaving only the central core (shown below).

    The core of each segment is then inserted into the sleeve of the other with electrical tape for insulation (shown below). The cables are then taped together with electrical tape.

    It seems that the recommended 5cm length for the exposed core is probably too long. The core have a tendency of perforating the sleeve of the other cable. So I would say a 2.5cm length for the exposed core would probably be sufficient. It is prudent to do a continuity check after every segment is installed. For every even number of segments, check for continuity between the core and sleeve on the opposite ends. For odd number of segments, check between the cores on the opposite ends and similarly for the sleeves. At one end of the cable I spliced in a patch of coax with the connector. With an application of heat shrink tubing, here is the final result.

    However when I tried to characterize the frequency response of the antenna using a spectrum analyzer in the lab, I got an almost flat response. I strongly suspect that the connection between the...

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  • The Three Step Road Map

    Poh Hou Shun08/20/2014 at 18:54 0 comments

    After giving it literately a few minutes thought, given the scale of the build it will probably be useful to set up some incremental/intermediate build goals that will lead to the final build. Conceivably, in this way, various sections can be tested individually and much needed experience in dealing with each subsystem can be gained. With that, here is the three step road map/build goals - 

    1. Build a solar powered land based ADSB receiver station. The receiver station should be completely off the power grid and have GPS capability. ADSB information received will be shared on firstly via wifi then 3G/4G network. I will list the component needed (or rather what I have) in the component list.

    2. Build a solar powered sea based ADSB receiver station on board of a floating bouy near the coast. This will add on to the previous build and is where the satellite/shortwave communication comes in. Hope I can pick up some skill in making 'marine-grade' fiberglass hull here.

    3. The final goal, build the Global Seer Platform. By this stage all the ADSB components should be fairly well understood. The main task will be to build the autonomous ocean-faring platform.  

  • Conceptualisation

    Poh Hou Shun08/20/2014 at 04:59 0 comments

    The is the first entry in the Global Seer project log. Before I get into the nitty gritty of any actual build I will always engage in bouts of blue-sky thinking about the project in question. For this case, they resulted in 'approximate' platform design in the cover photo (shown again below but with legends this time round). Although it probably will not resemble the final iteration in any shape or form, this flight-of-fancy design does allows me to have an overall view project and points to a direction to head towards...

    The first target of my blue-sky thinking is the design of the ocean platform. I guess what is needed here is a platform that is compact/streamlined  (so that it can braves the storm with relative ease?) and have a relatively large payload capacity. Speed in this case, though preferable, is not really required in this case.  From the way I see it, a Global Seer platform will probably be undergoing a long range trek to the destination. Once at the destination,  it will only be required to make short distance positional changes to improve coverage. Therefore I went with a rather conventional single hull design (show below) for the platform. As it stands now, the hull from bow to stern has a length of about 1.3m and a width of 308mm. This is largely determined by the size of the sample solar panel that I have got which is 390mm X 254mm . From the deck to the keel is about 150mm. This is determined by the height of the sealed lead acid batteries. More studies will need to be conduced to find out the actual water displacement of the hull, address stability issues and carry out refinement of the keel design.

    The next choice issue that I will tackle is the selection of a propulsion system. For this case I have opted for use of pair of azimuth thrusters (shown below) as oppose to the traditional rudder system. These are basically motor and propeller pair attached to a swiveling mount. This arrangement is supposed to be more efficient compared to inboard mounted motors as torque lost through a long drive shaft is eliminated. There is increased maneuverability as the thrust can be directed  in any direction even in reverse. As the motors are outboard, there is also more space in the hull for various payloads. Two thrusters instead of one for redundancy consideration. Mechanically  it is also simpler as the number of rotational coupling through the hull is reduced from 4 to 2. The only case when I cannot think of a clear advantage over a normal rudder is what happens when the swivel mount fails? There may also be issue in the design of the rotational coupling through the hull as there is a need to feed power to the motors.

    Another problem that is related to the propulsion system is the issue of position holding/station keeping. Strictly speaking the area coverage of the ADSB signal does not require high accuracy position holding to effect. Even for the purpose of multilateration (MLAT), the determination of the position of an aircraft from the minute time delay (referenced with GPS clock signal) between signal received from it at different receivers, is dependent  mostly on the accuracy of the GPS (typical about 10m). However if the Global Seer platforms are placed in a mesh network for communication relay the issue of position holding becomes important. In order to bridge a certain communication distance with as little number of platform as possible, it would be helpful to place the platform just within radio communication range of each other thus requiring highly accurate position hold.

    While the azimuth thrusters excel in providing excellent maneuverability, they are very slow to react to fast positional drift. This is because in order to redirect the thrust, the whole azimuth assembly needs to be rotated. While trying to source for a solution to this problem, I stumbled upon a propeller configuration that I...

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