US-Mexico border: Efforts to build a virtual wall
When darkness descends on the desert separating Mexico from the state of Arizona in the US, the hustle and bustle of illegal immigrants, drug traffickers and patrols intensifies as a game of hide-and-seek is played out among the metal fences and concrete walls.
A third-party observes the scene in silence: an army of electronic sensors acting as watch guard in a place where the arm of the law would not otherwise reach.
One false move and what seems like simple rocks and stones spring into life, sending alarm signals to a nearby tower. In a matter of seconds radar and infra-red cameras sweep the area in search of intruders.
Information travels in microwaves to a control room and human patrol men are sent co-ordinates and images to let them detect whether the subject is armed.A bittersweet dream
What's just been described is an American dream, or at least the US Border Authorities' dream. For several years it has been using Arizona as a test ground for a so-called "virtual fence".
The project was sanctioned by George W Bush's government in 2006 under the name of SBInet. It aimed to install the surveillance technology 3,185 km (1,979 miles) along the border.
It was intended to be fully operational by 2011. Instead it became a major headache for both the Bush and Obama administrations.
Technical problems plagued the project, and plans to complete it were formally ditched at the start of last year.
According to the Department of Homeland Security, taxpayers have spent $1bn (£632m) on the technology which only protects 85km (53 miles) of border.
When conditions are windy it sometimes fails to distinguish between a tree and a person, and it is often too slow at relaying the information it has gathered.
It is not the first such fiasco: two earlier attempts to roll out alternative technologies in 1998 and 2005 also failed. Only 1% of the alarm calls they created led to arrests.
But the US is not about to throw in the towel. It now wants to invest another $750m in an alternative project called "lntegrated Fixed Towers" (IFT).
Based on existing technology it aims to avoid the need to invest in research and development.
"The idea is to combine physical infrastructure, like walls and fences, with existing sensor technology to identify what's going on," Mark Borkowski, from the US Customs and Border Protection Department, told the BBC.
"So we could distinguish whether someone was loading a weapon or carrying their belongings. One kind of person is obviously more of a threat than the other.
"The third element is the (border) officers. We want to increase their effectiveness and use technology to give us an indication if something is going on, and enough information to allow patrols to react accordingly."Spying rocks
According to the US government, some 7,500 sensors were acquired between 2003 and 2007, to create a movement detection perimeter.
Known as Unattended Ground Sensors (UGS), the technology has been around since the 1970s. But advances mean the sensors are now tiny, solar powered and capable of operating for decades.
They are backed up by watchtowers fitted with infra-red radars and optical sensors purchased under SBInet before it was cancelled.
"The radars can detect activity and set off the cameras," Mr Borkowski explained.
"A lot of technologies like UGS are not able to identify what is moving out there. It could be an animal or a person. Thanks to these gadgets we can free the patrols from the task of searching the camera monitors so they can deal with other threats."Desert phantoms
The sensors are supported by drone aircraft known locally as "desert phantoms". The unmanned planes are capable of detecting people and vehicles from a height of 6,000 metres (19,685ft).
The Predator comes equipped with radar, seven video cameras, an infra-red sensor and a powerful zoom. Each one costs $20m and, according to the Department of Homeland Security, nine of them patrol the skies over Arizona, Florida, Texas and North Dakota.
Mr Borkowski said there were also plans to test self-controlled surveillance blimps equipped with cameras. The "floating eyes" have already been used in Afghanistan.
The Department of Homeland Security has also invited private companies to tender to build six new radar and camera-equipped towers, to be installed at different points along the border by 2020. It says they must be capable of detecting a person within a radius of eight kilometres.
US government sources indicate the revised virtual fence should detect between 70% to 80% of intruders along the border, reducing the need for so many border patrol officers.
But it has not yet said how it will fund the project - an issue in this period of austerity.
And bearing in mind the speed at which technology advances and the length of time the authorities typically take to approve such budgets, the virtual border fence may evolve again before it is finally completed.