Photos of Tarifa set-up here | Slashdot posting with feedback here
These technical notes, recently published on Slashdot were not intended to be published and were published without our knowledge. We were preparing a proper article to explain the context of the work done and to give full details of experiences whilst doing it. This project forms part of Transacciones / Fadaiat taking place the 22nd and 23rd of June 2004 simultaneously in Tarifa, Spain and Tangiers, Morroco.
Neither Psand.net nor Flakey.info were solely responsible for conceiving or achieving the connexion, it was achieved through the collaboration and efforts of many people, including the following: Psand.net | Fadaiat.net | Global Telesat | Arte y Pensamiento | Hackitectura | Indymedia Madiaq | OffTV | PiDiP | Archivo Transacciones | Riereta.net | Gollum | UNIA | Zemos98 | ENSA | Ayuntamiento de Tarifa
This document outlines the physical and technical issues involved in establishing a WLAN connection capable of real file transfer speeds up to 5.5mb/s from Tarifa in Spain to Tangiers in Morocco. Connecting two continents, using the free 802.11 standard and free software.
It must be noted that the calculations in this document are derived from an amalgamation of equations found on the internet. There appear to be discrepancies between the various sources found, so should be used as guidance only. I shall visit the library in order to iron out the discrepancies.
The distance from Tarifa to Tangiers: 32,000 metres, mostly over water. There is a shorter link available to Ksar-es-Seghir, Morocco which is 20,000 metres.
Tarifa antenna position: On top of the castle (Castillo de Guzmán el Bueno) 20? metres above sea level.
Tangiers antenna position: Presently unknown.
At a range of 32,000 metres.
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10 dB is on the low side of acceptable, 16 to 20 dB would be preferable. This link budget assumes that the antennas are connected to the amplifier with the minimum of cabling. If more cabling is used, the losses in this must be accounted for.
The free space loss at 32,000 metres was calculated using javascript online calculator
| Make | 11Mbps | 5.5Mbps | 2Mbps | 1Mbps |
| Orinocco cards PCMCIA Silver/Gold | -82 | -87 | -91 | -94 |
| CISCO cards Aironet 350 | -85 | -89 | -91 | -94 |
With water being the main surface for the connection, the maximum point of the first Fresnel zone will be equidistant from each antenna i.e. the obstacle (water) is in the middle of the signal (d1 = d2).
R1 approximates to SqrRoot((lambda x d1 x d2)/d1+d2)
valid for d1,d2>>R1 where:
Note: Only 60% of the first Fresnel zone (0.6 x R1) is needed for a line of sight connection.
Therefore at a range of 32,000 metres:
lambda = 0.122 m
d1 = 16,000 m
d2 = 16,000 m
R1 = SqrRoot(967)
R1 = 31.1 m
R1 x 60% = 18.7 metres
Thus, to allow for 60% of the first Fresnel Zone, the mid point of the connection will need to be 19 metres above the sea to maintain a radio connection. Therefore both antennas will need to be at least 19 metres above the top of the bulge in the sea (see below for the height of the bulge). This assumes that there are no other obstacles on land to deal with.
It seems this is not the only way to calculate the Fresnel zones, but each technique yields roughly the same answer.
h = r(1-cos((PI x d) / (2 x PI x r)))
Angles in radians
It should be noted that due to refraction the Earth curvature is reduced therefore the radius is considered to be 4/3r. This yields a result of 24 metres.
This calculation assumes the planet is a sphere, which is not quite true (very untrue in some places - but the sea is a pretty good approximation) and it appears from research that there is some discrepancy in calculating the earths curvature. It does not appear to create a great difference in the answers.
With 60% of the first Fresnel zone being 18.7 metres and the Earth bulge at the centre of the link being 24 metres, it would seem that to get a clear radio line of sight at 32 kilometres around 43 metres of clearance above the sea will be needed.
This is definately not a "brickwall" limit, a link may still be possible without this clearance, although more power / receiver sensitivity may be needed.
I have checked this result against various online information and give or take 5 metres this appears to be a good approximation.
Reflections can be a problem over water. There are various ways to remedy this.
Atmospheric absorption (haze,mist,fog,rain,or snow) is minimal at up to 5 GHz, but will still constitute a loss. Details to follow.
Unknown.Methods of avoiding noise include:
Initial aiming may be helped by telescope, compass, GPS, possibly using strong spotlights at night.
Communication during aiming, before a connection is established, may be difficult. Mobile phone? Code (Morse?) using light may be slow but reliable. Assuming times to communicate are set up.
Wind in Tarifa is fairly constant; there is a steady onshore breeze. The physical strength of the antenna and mount must be capable of maintaining aim at well above this windspeed.