Consider the following statements on Global Positioning System (GPS) : 1. GPS allows accurate time-stamping on ATM transactions. 2. GPS relies on a set of satellites for transferring signals worldwide. Which of the above statements is/are correct ?
- (a)1 only
- (b)2 only
- (c)Both 1 and 2
- (d)Neither 1 nor 2
Answer
Why
Correct — C, (c) Both 1 and 2.
STATEMENT 2 FIRST, BECAUSE IT IS THE PLAINER OF THE TWO. The Global Positioning System is a CONSTELLATION of satellites in medium Earth orbit — nominally twenty-four operational satellites arranged in six orbital planes, about 20,200 kilometres up, each circling the Earth roughly twice a day. They are placed so that from any point on the Earth's surface, at any hour, several are above the horizon at once. Each satellite continuously BROADCASTS a signal carrying its own identity, its orbital position and the exact time the signal left it, and the coverage is worldwide by design. Statement 2 is correct.
STATEMENT 1 IS THE ONE CANDIDATES DOUBT, AND IT IS ALSO CORRECT. GPS is not only a positioning system; it is the world's most widely used source of PRECISE TIME. Every satellite carries atomic clocks, and the whole constellation is kept to a single system time by the ground control segment. A receiver anywhere on Earth can recover that time to within tens of nanoseconds from a small antenna, at no cost — something no other technology delivers so cheaply or so universally.
That is exactly what a banking network needs. Automated teller machines, the switches that route their messages, and the settlement systems behind them are spread across the country and must agree on WHEN each transaction happened, so that a sequence of events can be reconstructed, so that duplicate or disputed transactions can be resolved, and so that logs from different sites can be laid side by side. Networks of this kind are therefore synchronised to a common reference, and a GPS-disciplined clock is the standard way of providing it. The same reasoning puts GPS timing behind telecommunication networks, stock exchange trade records and the protection systems of electricity grids.
THE TWO STATEMENTS ARE ACTUALLY THE SAME FACT SEEN TWICE, which is the insight worth carrying. A receiver finds its position by measuring how long each satellite's signal took to arrive and multiplying by the speed of light. Three such measurements would fix a position in three dimensions IF the receiver's own clock were perfect — and it is not, being a cheap quartz oscillator. So the receiver takes a FOURTH satellite and solves for four unknowns instead of three: the three coordinates AND its own clock error. Correcting that error is not a by-product of finding position; it is part of the same solution. That is why an ordinary GPS receiver ends up holding time of atomic-clock quality, and why statement 1 follows from statement 2 rather than sitting oddly beside it.
This item prints its two statements and then the question sentence, with no separate line of codes — the layout several statement items on this booklet use.
Why the others are wrong
- (a)1 only — This accepts the timing claim and rejects the satellite claim, which is the harder of the two to doubt. GPS is a satellite system by definition: the space segment is the constellation of satellites in medium Earth orbit, the control segment is the network of ground stations that tracks them and keeps their clocks and orbital data correct, and the user segment is the receivers. Nothing in the chain works without the satellites, and it is the satellites that make the coverage global — a ground-based system of transmitters could not reach the oceans, the poles or the middle of a continent uniformly. A candidate may hesitate over the wording 'transferring signals', since the satellites BROADCAST one way and the receiver only listens, never transmitting back. That is a fair observation about the phrasing and it does not make the statement false: the signals that carry the service to a user worldwide do come from the satellites.
- (b)2 only — This is the option a candidate reaches who thinks of GPS purely as a navigation aid — a map on a phone — and cannot see what it has to do with a cash machine. The link is TIME. Precise timing is not an accessory to GPS; it is the mechanism by which positioning works, because position is computed from the travel time of signals from atomic clocks in orbit. A receiver that solves for position necessarily also solves for its own clock error against system time, and the result is a source of accurate time available everywhere for the price of an antenna. Financial networks, telecommunication networks and power grids all use it to keep geographically separated equipment on a common clock, which is what allows transactions to be time-stamped consistently and their order to be established afterwards. Rejecting statement 1 mistakes the most widespread industrial use of GPS for a fanciful one.
- (d)Neither 1 nor 2 — Rejects both, and so rejects both halves of what GPS is: a constellation of satellites broadcasting worldwide, and a source of precise time used far beyond navigation. It is the option chosen by elimination when a candidate is unsure of statement 1 and has talked themselves out of statement 2 as well. There is a general discipline worth applying to two-statement items that offer 'Both' and 'Neither': judge each statement on its own and write the verdict down before looking at the four choices, because the choices are arranged so that hesitation on one statement tends to contaminate the other. Here each statement is separately and straightforwardly true, and the answer is the one that says so.
Concept
THE GLOBAL POSITIONING SYSTEM is a satellite-based navigation and timing service operated by the United States, and it is one member of a family now called GNSS — Global Navigation Satellite Systems.
THE THREE SEGMENTS, which is the standard way the system is described:
SPACE SEGMENT — the constellation, nominally twenty-four operational satellites in six orbital planes at about 20,200 km, each with an orbital period of roughly twelve hours, arranged so that at least four are visible from anywhere on Earth at any time. More than the nominal number are usually in service. CONTROL SEGMENT — a master control station and a worldwide network of monitoring and uplink stations that track the satellites, correct their orbital data and keep their atomic clocks aligned to system time. USER SEGMENT — the receivers, which only listen. A GPS receiver transmits nothing, so the system serves an unlimited number of users at once and reveals nothing about who is using it.
HOW A FIX IS COMPUTED. Each satellite broadcasts its identity, its position and the precise time of transmission. The receiver measures the delay, multiplies by the speed of light to get a range, and intersects the ranges from several satellites. Because the receiver's own clock is imprecise, a fourth satellite is needed: the solution finds three coordinates and the clock error together. Hence position and time come out of the same calculation.
WHY THE TIMING MATTERS SO WIDELY. Atomic clocks in orbit, disciplined to a single system time, make GPS the cheapest universally available source of precise time. Telecommunication networks use it to synchronise base stations; electricity grids use it to time-stamp measurements across a wide area; financial systems use it to order and record transactions; scientific instruments use it to combine observations made far apart.
ACCURACY AND ITS LIMITS. Ordinary civilian accuracy is a few metres, degraded by the ionosphere and troposphere, by signals reflected off buildings, and by poor satellite geometry. Augmentation systems improve it by broadcasting corrections. The deliberate degradation of the civil signal known as Selective Availability was switched off in May 2000, and civilian accuracy improved sharply at that moment.
THE OTHER SYSTEMS. GLONASS is Russia's, Galileo the European Union's and BeiDou China's — all global. India's is NavIC, the operational name of the Indian Regional Navigation Satellite System, a constellation of seven satellites covering India and a region around it rather than the whole world. India also operates GAGAN, a satellite-based augmentation system developed by ISRO with the Airports Authority of India, which broadcasts corrections to improve accuracy for civil aviation over the Indian region.
Technology questions on this APFC paper are asked at the level of an informed newspaper reader, and this one is typical: two short statements about a familiar system, one of them obvious and one designed to be doubted. Thirty items on this booklet print a numbered statement list, which makes statement analysis the paper's dominant habit rather than an occasional format.
The way such an item is built is worth understanding. One statement is included because almost everyone will accept it; the other is included because it describes a real but less familiar use, and the examiner is testing whether the candidate knows the system or only knows what it is popularly for. The wrong choices are then simply the four combinations. This means the work is entirely in judging the two statements separately — and that a candidate who is confident about one and unsure about the other should resist letting the doubt spread.
The general lesson from statement 1 is that infrastructure technologies usually have a second life that is invisible to the public. GPS is known for showing a blue dot on a map; the industry that depends on it most heavily depends on its clocks. The same pattern shows up elsewhere on these papers — a communications satellite used for broadcasting and for education, a payments network used for subsidies, a biometric identity system used for authentication far beyond its original purpose.
One feature of this paper's layout is worth noting so it is not misread as a printing error: several statement items, this one included, print the question sentence immediately after the numbered list without a separate 'select using the codes given below' line. Two-statement items here answer with 'Both' and 'Neither' rather than with numeric codes.
Key facts
- GPS is a satellite-based navigation AND timing system operated by the United States, with a nominal constellation of twenty-four operational satellites in six orbital planes at about 20,200 km.
- Each satellite carries atomic clocks and broadcasts its identity, its orbital position and the exact time of transmission; receivers only listen and never transmit.
- A receiver needs signals from four satellites: three would fix position if its own clock were perfect, and the fourth solves for the receiver's clock error.
- Because position is computed from travel times, precise time falls out of the same calculation — which is why GPS is the world's most widely used source of accurate time.
- Banking and ATM networks, telecommunication networks, stock exchanges and electricity grids all use GPS-disciplined clocks to time-stamp events and synchronise equipment at separate sites.
- The system has three segments: space (the satellites), control (the tracking and uplink stations) and user (the receivers).
- Selective Availability, the deliberate degradation of the civilian signal, was switched off in May 2000.
- The other global systems are GLONASS (Russia), Galileo (European Union) and BeiDou (China).
- India's NavIC is the operational name of the Indian Regional Navigation Satellite System, a seven-satellite constellation covering India and a surrounding region, not the whole world.
- GAGAN, developed by ISRO with the Airports Authority of India, is a satellite-based augmentation system that broadcasts corrections to improve accuracy over the Indian region.
Study next
Common traps
- Thinking of GPS only as navigation. Precise timing is its other major service and the one industry depends on most.
- Assuming a GPS receiver transmits its position back to the satellites. It only receives, which is why the system can serve unlimited users.
- Believing three satellites suffice for a fix. Four are needed, because the receiver's own clock is an unknown.
- Confusing NavIC, which is regional, with the four global systems — GPS, GLONASS, Galileo and BeiDou.
- Confusing NavIC with GAGAN: one is a navigation constellation, the other an augmentation system that broadcasts corrections.
- Letting doubt about one statement decide the other. In a two-statement item each must be judged on its own before the choices are read.
Science and technology on EPFO papers arrives either as a one-line definition or, as here, as two or three short statements to be judged true or false and answered through 'Both', 'Neither' or a numeric code. The statements are usually a mixture of the widely known and the slightly specialised, and the specialised one is normally a genuine application rather than an invention — which means the safer instinct is to ask whether a claimed use is real, not to reject it for being unfamiliar. Satellite technology, telecommunications and digital payments recur because they connect to governance and to the work of a public office. Preparation that pays is knowing, for each major system, who runs it, what it is made of, what it is used for beyond the obvious, and how India's own equivalent differs.
Related PYQs
EPFO_APFC_2016_Q80Consider the following statements : Cellular technology evolves in stages called Generation (G), where 1. A Generation represents the number of subscribers; higher Generation has more subscribers. 2. 2G technology has two standards CDMA and GSM. 3. 2G technology has CDMA standard and 3G has GSM standard. Which of the above statements is/are correct ?
- (a) 1 and 3 only
- (b) 1 only
- (c) 3 only
- (d) 2 only
Answer(d) 2 only
Cellular technology and what a Generation means, immediately after this item on the same paper — the neighbouring statement-analysis question in the same technology strand.
EPFO_APFC_2016_Q25Which of the following are incorporated in the Government approved National Telecom Policy, 2012 ? 1. Broadband for all with a minimum download speed of two megabits per second 2. India's rural tele-density to be improved from 39% to 70% in the next five years 3. Roaming charges shall be scrapped Select the correct answer using the codes given below :
- (a) 1 and 2 only
- (b) 1 and 3 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer(d) 1, 2 and 3
What the National Telecom Policy, 2012 incorporates — technology policy asked as a three-statement item, the same format applied to a government document.
EPFO_EOAO_2017_Q53ISRO in August 2016 successfully test launched Supersonic Combustion Ramjet (Scramjet) Engine. Which of the following statements with regard to Scramjet Engine is/are correct? 1. It can efficiently operate both in subsonic and supersonic combustor modes. 2. India is the first country to demonstrate the flight testing of a Scramjet Engine. Select the correct answer using the code given below.
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(a) 1 only
Two statements about ISRO's Scramjet engine test — space technology judged statement by statement and answered through the same 'Both / Neither' option pair.
Practice
- practice — not a real PYQ
What is the minimum number of satellites from which a GPS receiver must acquire signals in order to determine its position in three dimensions and correct its own clock ?
- (a)Two
- (b)Three
- (c)Four
- (d)Six
Answer(c) Four — there are four unknowns to solve for, the three coordinates of position and the receiver's own clock error. Three satellites would be enough only if the receiver carried a perfect clock, which it does not.
- practice — not a real PYQ
NavIC is the operational name of which one of the following ?
- (a)India's regional navigation satellite system
- (b)The European Union's global navigation satellite system
- (c)China's global navigation satellite system
- (d)India's satellite-based augmentation system for civil aviation
Answer(a) India's regional navigation satellite system — NavIC is the operational name of the Indian Regional Navigation Satellite System, covering India and a surrounding region. The European system is Galileo, the Chinese one is BeiDou, and India's augmentation system for civil aviation is GAGAN.