IELTS Academic Reading Practice Test 202

For most of human history, finding one's position on the surface of the planet depended on careful observation. Sailors measured the height of the sun and stars to establish latitude. They estimated longitude by dead reckoning, a calculation from the direction and speed of their ship that accumulated error with every day at sea. Radio beacons introduced during the Second World War allowed positions to be fixed from transmitters on land, but coverage was limited and accuracy fell sharply at great distances. A truly global system would have to be based in the sky.

The first hint of how that might be achieved came from an unexpected source. When the Soviet Union launched Sputnik 1 in October 1957, two young physicists at the Johns Hopkins University Applied Physics Laboratory in Maryland began recording its radio signal. William Guier and George Weiffenbach noticed that its frequency shifted as the satellite approached and receded, a consequence of the Doppler effect. The pattern of the shift allowed them to calculate the orbit precisely. Their supervisor then suggested the opposite problem: if the orbit were known in advance, a receiver on the ground could work out its own position from the same frequency changes.

From this insight grew Transit, the first operational navigation satellite system. Transit 1B was launched in April 1960. The system was made available to the United States Navy from 1964, chiefly so that submarines carrying ballistic missiles could update their positions before firing. A small number of satellites in low polar orbits broadcast continuously, and a receiver measured the Doppler shift as each one passed overhead, fixing its position to within a few hundred metres. The process was accurate but slow, since a user might wait an hour for the next satellite, and it was useless for aircraft moving quickly. Transit was retired in 1996.

Work on a faster alternative took place in the United States during the 1960s. The Navy's Timation programme launched satellites carrying highly accurate quartz and later atomic clocks, showing that precise timekeeping in orbit was practical. A separate Air Force project explored a signal structure that could serve many users at once. In 1973 the two were combined into a single joint programme, and an Air Force officer, Bradford Parkinson, was given the task of turning competing proposals into a working design. The system that emerged was called Navstar, later known simply as GPS.

GPS depends on timing rather than on Doppler shifts. Each satellite carries atomic clocks accurate to a few billionths of a second and broadcasts a signal that records the moment of transmission and its own orbital position. A receiver notes the time of arrival, compares it with the transmitted time and multiplies the difference by the speed of light, which gives its distance from that satellite. Three such measurements would, in principle, locate a receiver in space. The receiver's own quartz clock is far less accurate than those in orbit, so any error in timing produces an enormous error in distance. A fourth satellite supplies the correction, and the receiver solves for four unknowns at once: latitude, longitude, altitude and time.

The first GPS satellite went into orbit in 1978. In 1983 a Korean airliner strayed into Soviet airspace and was shot down, killing everyone on board. The incident prompted President Ronald Reagan to announce that the system, once complete, would be made available for civilian use. Twenty-four satellites, arranged in six orbital planes about 20,000 kilometres above the Earth, were declared fully operational in 1995. Any point on the planet was now within sight of several of them at all times.

For the first few years the civilian signal was deliberately degraded. A feature called Selective Availability added a random error to the publicly available timing data. Ordinary receivers were therefore accurate only to about a hundred metres, while the military had access to a much better one. On 1 May 2000 the United States switched this feature off permanently, and within a few years cheap receivers, accurate to a few metres, were being built into cars, phones and surveying equipment.

Other countries, unwilling to depend on a system controlled by the American military, have built their own constellations. The Soviet Union began launching GLONASS satellites in 1982, though the network fell into disrepair after the collapse of the Soviet economy and was only restored around 2011. The European Union's Galileo programme, whose first experimental satellite was launched in 2005, began offering an initial service in December 2016. China's BeiDou, which started as a regional system, was declared fully global in 2020.

All of these systems share a common weakness. Their signals arrive at the ground extremely faint, roughly comparable in power to a car headlight seen from hundreds of kilometres away, and they can be drowned out by a small jammer. They can also be deceived by a transmitter that broadcasts false information, a technique known as spoofing. Satellite positioning has quietly become essential to the timing of financial transactions, the synchronisation of power grids and the routing of traffic, so the vulnerability of the signal is now a matter of national concern. Several governments are building ground-based radio systems to provide back-up.

Between Sputnik's simple beeps and the constellations now orbiting the Earth lies a remarkably short span of history. An idea devised to track a satellite became the means by which its descendants tell us where we are.

A red blood cell is about seven micrometres across, a micrometre being one thousandth of a millimetre. The devices that engineers call microrobots are built on a similar scale, ranging from a few micrometres to about a millimetre. They are designed to move through the body under their own power rather than being carried passively by the bloodstream. The ambition behind them is straightforward. A machine that can swim to a diseased tissue could deliver a drug exactly where it is needed, take a sample, or perform a small surgical task while leaving the rest of the body untouched.

At this scale, physics behaves strangely. For a bacterium or a microrobot, water feels as thick as honey, a point made by the American physicist Edward Purcell in a celebrated lecture in 1977. The Reynolds number, which compares inertial with viscous forces, is tiny for a swimmer of this size, so that when propulsion stops the device halts almost immediately. In such conditions viscosity dominates inertia, and a swimmer that simply moves back and forth repeats the same motion in reverse and goes nowhere. This is Purcell's scallop theorem, and it means that a microrobot must move asymmetrically, with a corkscrew tail or a flexible flagellum that beats in a wave.

The most widely used solution is to attach the robot to an external magnetic field. A helical device a few micrometres long, coated with a thin magnetic layer, rotates and drills forward when placed in a rotating field. The field is generated by coils outside the body, so the device needs neither battery nor fuel, and magnetic fields pass through tissue almost unchanged. By adjusting the direction and strength of the field an operator can steer the robot along a chosen path, and switching the field off leaves it stationary.

Other groups have turned to sound, light or chemistry. Ultrasound can set a bubble or a small flagellum vibrating and drive it forward, and it has the advantage of already being used in hospitals. Light can propel particles through a transparent medium, though it penetrates tissue poorly. Chemical propulsion, in which a catalyst decomposes a fuel to produce thrust, works impressively in a dish but has struggled in the body, where the available fuels are dilute and the products may be harmful. Each method has its advocates, and none has yet proved decisive.

A more biological approach borrows an existing motor. Certain bacteria, first described in the 1970s, contain chains of magnetic crystals that align them with the Earth's magnetic field, and they swim by rotating a helical flagellum. Coated with drugs or attached to a small load, they can be guided by magnets and, unlike synthetic devices, they generate their own thrust and can sense chemical gradients. The difficulty is control: living organisms are hard to standardise, and the immune system treats them as an infection.

Seeing where a microrobot has gone is as difficult as moving it. A device smaller than a grain of rice is invisible on most hospital scanners, and X-rays show only dense materials. Magnetic particles can be detected with sensitive sensors, and some designs are visible in an MRI scanner, but the magnetic fields that drive the robot can interfere with the images. Much current work therefore combines propulsion and imaging in a single system, so that the position of the device is known while it moves.

The devices closest to routine use are not swimmers at all but pill-sized capsules. A camera in a capsule that a patient swallows can photograph the lining of the small intestine, a stretch of gut that older instruments could not reach. Magnetically controlled capsules can now be steered around the stomach with a joystick. Magnetic guidance is also being applied to catheters. A thin tube whose tip can be bent by an external field can be threaded into the heart to treat an irregular rhythm, reducing the need for the doctor to manipulate it by hand under X-rays.

Beyond the technical obstacles lie biological ones. Anything introduced into the blood is inspected by the immune system and filtered by the liver and spleen. A device must therefore be small enough to pass, or made of a material that dissolves harmlessly once its work is done. Magnesium and zinc alloys, and polymers that break down into harmless acids, are being tested for this purpose. Delivering a precise dose is a further problem: a swarm of a million particles may carry less drug than a single conventional injection, and the amount released is hard to control.

Researchers are consequently more interested in swarms than in single machines, and in forms of control that do not require an operator watching a screen. In one approach the robot responds to the chemistry of its surroundings and releases its payload only where the tissue is acidic, as tumours often are. Others are working on biodegradable helical swimmers driven by ultrasound, which might one day be used in the narrow vessels of the brain. Regulatory agencies have yet to decide how such devices should be tested.

None of this will replace a surgeon or a pill in the near future. But medicine has always advanced by finding ways to reach places that were previously out of reach, and machines that swim through the body are beginning to do exactly that.

In the past fifteen years the small camera has become a normal piece of equipment. Police officers in many countries wear one on the chest, and cyclists and delivery riders wear one on a helmet. A growing number of people record large parts of their own lives for no reason beyond wanting a record. Advocates promise accountability, safety and a better memory for details. Critics see the spread of surveillance into places where it was never expected. My own view is that the case for cameras is strongest where recording is deliberate, limited in time and known to everyone involved — and that this is precisely what many schemes get wrong.

The evidence that first persuaded police forces came from a small city in California. In 2012 the police department of Rialto equipped half its patrol officers with cameras that they were required to switch on at every encounter with the public. Over the following year, reports of officers using force fell by about half in the department as a whole, and complaints against officers fell even more sharply. The results were widely reported, and forces in Britain and the United States began buying cameras in large numbers.

Larger and more rigorous trials have been less encouraging. In Washington DC, a large randomised experiment reported in 2017 found no significant reduction in the use of force. A series of trials in several British forces reached similar conclusions. Researchers offered an explanation: the earlier results may have depended on officers knowing that their footage would be reviewed. Where nobody checks the recordings, the camera changes nothing. Its power lies not in the lens but in the certainty of being watched afterwards.

None of this means the cameras are worthless. Recordings settle disputes about what happened during an arrest, and they allow trainers to show new officers how a situation escalated. They can also support prosecutions in cases of domestic violence, where an early account given at the scene is often the best evidence available. In each case the camera produces a record that is more complete than a notebook and harder to dispute. Studies of complaints suggest that both officers and members of the public behave differently when they know a camera is running. The effect on the use of force, however, is smaller than was first thought.

The difficulty is that a camera records only what someone decides to record. Officers forget, batteries fail, and the rules on when to switch on are complicated. A domestic argument in a kitchen may be intensely private, yet the same incident may later be the subject of a trial. Decisions about when to record therefore fall to the officer in the street, and the moments at which the camera is switched off are precisely those where a record would matter most.

Less discussed is the position of the people who are filmed without choosing to be. A woman whose home is entered at four in the morning, a teenager stopped in the street, a person in a mental health crisis: all become, briefly, the subjects of a permanent record. That record may be copied, stored and, in some countries, released under freedom of information laws. Because storage is cheap, the easiest policy is to keep everything, and the easiest policy is rarely the right one. Consent is impossible in such encounters, which is why the rules about retention matter more than the rules about recording.

The personal versions of the same technology raise different questions. A camera worn on a strap can capture a photograph every few seconds. For a small number of patients with severe memory loss, such records have proved genuinely valuable, allowing them to revisit an afternoon that would otherwise have vanished. Yet the same device records everyone who comes near the wearer, in shops, in classrooms and at family meals, and none of them agreed to appear in a database. The wearer's memory is improved at the cost of other people's privacy.

The greatest risk, however, lies in what happens to the footage afterwards. Once recordings exist, there is a standing temptation to search them: to run facial recognition over them, to look for known faces at a protest, or to mine them for patterns of behaviour. Each of these steps is technically easy, and each transforms a tool for accountability into a system of surveillance. The safeguards that matter are therefore institutional rather than technical: limits on how long footage is kept, independent audit, clear rules on who may search it, and a refusal to attach facial recognition at all.

Cameras should be kept, then, but on terms that are written down and enforced. The test for any scheme should be whether it makes the powerful answerable without turning ordinary life into evidence. On the present evidence, the camera passes that test only when it is switched on predictably, its footage is reviewed, and the people it films have reasons to trust the rules. Rules that exist only as guidance, and are never enforced, will be quietly abandoned as soon as they become inconvenient. That is why legislation, rather than a voluntary code, is needed.