This chapter covers Global Positioning System (GPS) and navigation technologies as they appear in mobile devices, a key topic under CompTIA A+ 220-1101 Objective 1.3 (Mobile Devices). You will learn how GPS works at a fundamental level, its integration into smartphones and tablets, and common troubleshooting issues. While GPS-specific questions are a small portion (approximately 2-3%) of the 220-1101 exam, understanding its principles is essential for supporting location-based services and mobile device navigation. This chapter provides the depth needed to answer exam questions accurately and to troubleshoot GPS problems in the field.
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Imagine a ship navigating coastal waters. The lighthouse is a satellite: it broadcasts a unique light pattern (signal) that includes its identity and the exact time the light flashed. The ship's navigator uses a sextant (the GPS receiver) to measure the angle between the lighthouse and the horizon, and notes the exact time the flash was seen. By comparing the time the flash was sent (from the lighthouse's broadcast) with the time it was received (from the ship's precise clock), the navigator calculates the distance to the lighthouse. If the navigator does this for three or four different lighthouses at known positions, they can triangulate the ship's exact location. The sextant's clock must be extremely accurate—any error of one millisecond translates to a 300-kilometer error in distance. GPS satellites carry atomic clocks, and the receiver corrects its own clock using signals from multiple satellites. The ship's position is computed at the intersection of spheres centered on each lighthouse with radii equal to the measured distances. This is exactly how GPS works: satellites broadcast their position and precise time, the receiver measures time-of-flight for signals from at least four satellites, and trilateration yields latitude, longitude, and altitude.
What is GPS and Why Does It Exist?
The Global Positioning System (GPS) is a satellite-based radio navigation system owned by the United States government and operated by the United States Space Force. It provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system was originally developed for military use in the 1970s but became fully operational for civilian use in 1995. Today, GPS is the most widely used global navigation satellite system (GNSS), but it is not the only one—others include Russia's GLONASS, the European Union's Galileo, and China's BeiDou. For the 220-1101 exam, you need to know that modern mobile devices often support multiple GNSS constellations to improve accuracy and reliability, but the core principles are the same.
How GPS Works Internally
GPS operates on the principle of trilateration. A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites. Each satellite continuously transmits messages that include:
The time the message was sent (based on an atomic clock)
The satellite's precise orbital position (ephemeris data)
The general system health and rough orbits of all satellites (almanac data)
The receiver measures the time delay between when the signal was sent and when it was received. Multiplying this time delay by the speed of light (299,792,458 m/s) gives the distance to the satellite. However, because the receiver's clock is much less accurate than the atomic clocks on the satellites, there is a clock error. To solve for the receiver's position (x, y, z) and the clock error, the receiver needs signals from at least four satellites. With four satellites, the receiver can solve four equations simultaneously.
Key Components and Values
Satellites: 31 operational satellites in Medium Earth Orbit (MEO) at approximately 20,200 km altitude. They orbit in six orbital planes inclined 55° to the equator, with a period of about 11 hours 58 minutes.
Frequencies: GPS uses L-band radio frequencies: L1 at 1575.42 MHz (civilian use) and L2 at 1227.60 MHz (military use). Modern receivers may also use L5 at 1176.45 MHz for higher accuracy.
Accuracy: Civilian GPS is accurate to about 5-10 meters under open sky. With augmentation systems like WAAS (Wide Area Augmentation System), accuracy can improve to <3 meters.
Time to First Fix (TTFF): The time it takes a receiver to acquire satellite signals and calculate a position. Cold start (no almanac or ephemeris) can take 12.5-30 minutes. Warm start (almanac known, ephemeris outdated) takes about 30-45 seconds. Hot start (ephemeris current) takes <10 seconds.
Assisted GPS (A-GPS): Uses network resources (cellular or Wi-Fi) to provide almanac and ephemeris data to the receiver, dramatically reducing TTFF to a few seconds. A-GPS is standard in smartphones.
GPS in Mobile Devices
Smartphones and tablets integrate a GPS receiver chipset (often combined with GLONASS or Galileo support). The operating system provides location services that applications can access via APIs. Key aspects for the A+ exam: - Location Modes: On Android: High Accuracy (GPS + Wi-Fi + mobile networks), Battery Saving (Wi-Fi + mobile only), Device Only (GPS only). On iOS: similar options under Settings > Privacy > Location Services. - A-GPS: In most mobile devices, A-GPS uses the cellular network to download satellite ephemeris data. This is why a device with a cellular connection gets a GPS fix faster than a Wi-Fi-only tablet in a new location. - GLONASS/Galileo/BeiDou: Many modern devices support multiple GNSS constellations. This improves accuracy and availability, especially in urban canyons where GPS satellites may be blocked. - Wi-Fi Positioning: Uses known Wi-Fi access point MAC addresses and their signal strengths to estimate location. This is not GPS but supplements it indoors where GPS signals are weak.
Troubleshooting GPS Issues
Common problems and their causes: - No GPS fix: Often due to being indoors, in a tunnel, or surrounded by tall buildings. Check that location services are enabled and that the device has a clear view of the sky. On some devices, a hardware failure (defective GPS antenna) can cause this. - Slow GPS fix: Could be due to outdated almanac data (if the device hasn't been used for weeks) or interference. Using A-GPS (cellular or Wi-Fi) speeds up acquisition. - Inaccurate location: Caused by multipath (signals reflecting off buildings), poor satellite geometry, or atmospheric conditions. Some devices allow toggling between GPS-only and assisted modes. - Battery drain: GPS is power-intensive. The exam may ask about battery-saving location modes.
Commands and Verification
On a mobile device, you can check GPS status using:
- Android: Dial *#*#4636#*#* to access testing menu, then choose "GPS test" or use apps like "GPS Status & Toolbox".
- iOS: No direct diagnostic code; use apps like "GPS Diagnostic" from the App Store.
- Windows 10/11: Settings > Privacy > Location. In Device Manager, check for GPS driver under "Sensors".
For network engineers, GPS is not typically configured on network devices, but some enterprise access points use GPS for time synchronization (e.g., PTP). The exam does not require deep GPS configuration knowledge; focus on mobile device settings and troubleshooting.
Interaction with Other Technologies
Cellular: A-GPS uses cellular data to download satellite data. Also, cellular towers can provide coarse location (cell tower triangulation).
Wi-Fi: Wi-Fi positioning uses a database of BSSIDs and their known locations. This is faster than GPS but less accurate.
Bluetooth: Bluetooth beacons (e.g., iBeacon) can provide indoor location, but this is not part of the 220-1101 objectives.
NFC: No direct interaction with GPS.
Exam-Relevant Details
The 220-1101 exam expects you to know that GPS uses trilateration (not triangulation). Triangulation uses angles; trilateration uses distances.
Know the difference between A-GPS and standalone GPS. A-GPS uses network assistance to reduce TTFF.
Be aware that GPS requires a clear view of the sky. The exam may present a scenario where a user cannot get a GPS fix indoors, and you should identify the cause.
Understand that location services can be set to different modes (High Accuracy, Battery Saving, Device Only) and that GPS is the most accurate but drains battery fastest.
Recognize that GLONASS is the Russian equivalent of GPS, and that many devices support both.
Know that GPS satellites broadcast on L1 frequency (1575.42 MHz).
The term "almanac" refers to coarse orbital data for all satellites; "ephemeris" is precise orbital data for a specific satellite.
Summary of Key Numbers
Number of satellites needed for a 3D fix: 4 (minimum)
Number of satellites needed for a 2D fix: 3 (if altitude is known)
GPS satellite altitude: ~20,200 km
GPS satellite orbital period: ~11 hours 58 minutes
Civilian GPS accuracy: ~5-10 meters (with WAAS: <3 meters)
Cold start TTFF: up to 30 minutes
Hot start TTFF: <10 seconds
L1 frequency: 1575.42 MHz
This information is directly testable on the 220-1101 exam.
Satellite Broadcasts Signal
Each GPS satellite continuously transmits a navigation message on L1 frequency (1575.42 MHz). The message includes the satellite's precise ephemeris (orbital parameters), the almanac (coarse orbits of all satellites), and the exact time the message was sent, derived from an onboard atomic clock. The signal is encoded using Code Division Multiple Access (CDMA) so that all satellites share the same frequency but use unique pseudorandom noise codes. The receiver must first synchronize with the satellite's code to extract the navigation data.
Receiver Measures Time of Flight
The GPS receiver generates a replica of the satellite's pseudorandom code and shifts it in time until it correlates with the received signal. The amount of shift required is the propagation delay—the time it took for the signal to travel from the satellite to the receiver. Multiplying this time by the speed of light gives the pseudorange. However, because the receiver's clock is not synchronized with the satellite's atomic clock, this pseudorange includes a clock error term. The receiver must solve for this error using multiple satellites.
Collects Ephemeris from Four Satellites
To compute a 3D position (latitude, longitude, altitude) plus the receiver's clock error, the receiver needs pseudoranges from at least four satellites. It decodes the ephemeris data from each satellite to know the satellite's exact position in space at the time of transmission. The receiver then sets up a system of four equations: for each satellite, the measured pseudorange equals the geometric distance between the satellite and the receiver plus the clock error (converted to distance). Solving these equations yields the receiver's position and clock offset.
Computes Position via Trilateration
The receiver uses an iterative algorithm (often a least-squares solution) to solve the nonlinear equations. Geometrically, each satellite defines a sphere centered at its position with radius equal to the corrected pseudorange. The intersection of three spheres gives a point (and an ambiguous second point), and the fourth satellite resolves the ambiguity and provides the clock error. The algorithm converges to a solution typically within a few milliseconds on modern processors. The computed position is then converted to a geodetic coordinate system (WGS84) for output.
Updates Position Continuously
Once the receiver has a fix, it continues to track the satellites and update its position at a typical rate of 1 Hz (once per second). The receiver also updates the ephemeris data as new broadcasts are received (ephemeris is updated every 2 hours). If the receiver loses lock on satellites (e.g., entering a tunnel), it may reacquire using stored almanac and ephemeris data (hot start) or require a new download (cold start). The receiver outputs NMEA 0183 sentences (standard format) that applications use for navigation.
Enterprise Scenario 1: Fleet Management
A logistics company equips its delivery trucks with GPS-enabled telematics devices. Each device contains a GPS receiver that reports vehicle position, speed, and heading every 30 seconds over a cellular connection. The central server aggregates this data to optimize routes, monitor driver behavior, and provide estimated arrival times to customers. The devices use A-GPS to ensure fast fixes even when trucks leave garages. Challenges include GPS signal loss in urban canyons (solved by integrating GLONASS) and battery drain (devices are hardwired to vehicle power). Misconfiguration, such as setting the GPS update interval too high (e.g., every 5 minutes), can result in inaccurate route tracking and customer complaints.
Enterprise Scenario 2: Precision Agriculture
Modern tractors use GPS with Real-Time Kinematic (RTK) correction to achieve centimeter-level accuracy for planting and harvesting. RTK uses a fixed base station that broadcasts differential corrections to the rover (tractor) via a radio link or cellular network. The base station calculates the error in the GPS signal and sends corrections, allowing the rover to correct its position in real time. This requires a clear view of the sky and a reliable communication link. If the correction signal is lost, the system degrades to standard GPS accuracy, which may cause overlapping rows or gaps. Proper configuration includes setting the base station coordinates accurately and ensuring the radio link has sufficient range.
Enterprise Scenario 3: Time Synchronization
Data centers and financial trading platforms rely on GPS for precise time synchronization. GPS receivers provide a Pulse Per Second (PPS) signal and time-of-day information via NMEA sentences. Network Time Protocol (NTP) servers use this GPS source to synchronize all servers within microseconds. For compliance (e.g., MiFID II), trading platforms require timestamping accuracy within 100 microseconds. A common issue is GPS antenna placement: if the antenna is installed near metal structures or under a roof, it may lose satellite lock, causing the NTP server to drift. Proper installation requires a clear view of the sky and a lightning surge protector for outdoor antennas.
Exactly What 220-1101 Tests on GPS
CompTIA A+ 220-1101 Objective 1.3 states: "Given a scenario, install and configure mobile device hardware and software." Within this, GPS appears as a feature of mobile devices that may need to be enabled or troubleshot. The exam does not test deep GNSS theory, but expects you to:
Understand that GPS requires a clear view of the sky to get a fix.
Know that A-GPS uses cellular or Wi-Fi networks to assist in acquiring satellite signals faster.
Recognize that location services can be set to different modes (High Accuracy, Battery Saving, Device Only) and which mode uses GPS.
Identify that GPS is separate from Wi-Fi positioning (which uses access point MAC addresses).
Common Wrong Answers and Why Candidates Choose Them
"GPS uses triangulation" – Many candidates confuse trilateration with triangulation. Triangulation uses angles; GPS uses distances (trilateration). The exam may present a distractor that says "triangulation" – the correct term is trilateration.
"A-GPS requires an active cellular data connection" – While A-GPS typically uses cellular data to download satellite data, it can also use Wi-Fi. The exam might say "A-GPS only works with cellular" – that is false.
"GPS works indoors without issue" – GPS signals are weak and cannot penetrate most buildings. A common scenario: a user complains they cannot get a GPS fix indoors. The correct answer is that GPS requires a clear view of the sky.
"Battery Saving mode uses GPS" – Battery Saving mode disables GPS and uses only Wi-Fi and mobile networks. High Accuracy mode uses GPS. Candidates often mix these up.
Specific Numbers and Terms That Appear Verbatim
L1 frequency: 1575.42 MHz – This exact number may appear in a question about GPS frequencies.
Number of satellites for 3D fix: 4 – The exam may ask the minimum number of satellites needed for a 3D position.
Accuracy: 5-10 meters – Typical civilian GPS accuracy.
A-GPS – Assisted GPS, reduces time to first fix.
GLONASS – Russian GNSS; many devices support both.
Edge Cases and Exceptions
If a device has GPS disabled but location services enabled, it may still get a rough location via Wi-Fi and cellular towers. The exam may test that GPS is not required for all location services.
In airplane mode, cellular and Wi-Fi are disabled, so A-GPS cannot download satellite data. The device may still get a GPS fix if it has recent ephemeris data (hot start), but it will take longer.
Some devices have a hardware switch for GPS (e.g., on some rugged tablets). The exam may present a scenario where GPS is not working due to a physical switch being off.
How to Eliminate Wrong Answers
When answering GPS questions, first identify whether the question is about enabling a feature or troubleshooting. For troubleshooting, check if the user is indoors – that is the most common cause of GPS failure. For configuration, remember that High Accuracy uses all sources (GPS, Wi-Fi, cellular), Battery Saving uses only Wi-Fi/cellular, and Device Only uses only GPS. If the question mentions "fastest fix" or "uses network assistance," the answer is A-GPS. If it mentions "most accurate," the answer is GPS.
GPS uses trilateration, not triangulation, to determine position.
A minimum of 4 satellites is required for a 3D fix (latitude, longitude, altitude, and time).
Civilian GPS accuracy is typically 5-10 meters under open sky.
Assisted GPS (A-GPS) uses network resources to reduce Time to First Fix (TTFF) to a few seconds.
GPS signals (L1 at 1575.42 MHz) are weak and require a clear view of the sky; they do not work well indoors.
High Accuracy location mode uses GPS, Wi-Fi, and cellular; Battery Saving mode uses only Wi-Fi and cellular; Device Only mode uses only GPS.
GLONASS is the Russian equivalent of GPS; many devices support both for improved reliability.
Cold start TTFF can be up to 30 minutes; hot start is under 10 seconds.
These come up on the exam all the time. Here's how to tell them apart.
GPS (Global Positioning System)
Operated by the United States
31 satellites in Medium Earth Orbit (~20,200 km)
Uses CDMA on L1 (1575.42 MHz) and L2 frequencies
Civilian accuracy ~5-10 meters
Widely supported in consumer devices
GLONASS (Global Navigation Satellite System)
Operated by Russia
24 satellites in Medium Earth Orbit (~19,100 km)
Uses FDMA on L1 (1602 MHz) and L2 frequencies (modern satellites also use CDMA)
Civilian accuracy ~5-10 meters (similar to GPS)
Often combined with GPS in dual-constellation receivers for improved performance
Mistake
GPS uses triangulation to determine location.
Correct
GPS uses trilateration, not triangulation. Triangulation measures angles; trilateration measures distances (time of flight) from multiple satellites to compute position.
Mistake
A-GPS requires a constant internet connection to work.
Correct
A-GPS uses network assistance to download satellite ephemeris and almanac data, which speeds up the initial fix. However, once the receiver has the data, it can compute positions without an active connection. The connection is only needed for the initial data download.
Mistake
GPS works perfectly indoors.
Correct
GPS signals are very weak (about -125 dBm) and cannot penetrate most building materials. Indoors, the signal is often too weak to achieve a lock. Some devices may get a fix near windows, but generally GPS does not work indoors.
Mistake
Battery Saving location mode uses GPS but less frequently.
Correct
Battery Saving mode disables GPS entirely and uses only Wi-Fi and mobile networks for location. High Accuracy mode uses GPS, Wi-Fi, and cellular. Device Only mode uses only GPS.
Mistake
All GPS receivers are equally accurate.
Correct
Accuracy depends on the receiver's quality, number of channels, support for multiple GNSS constellations, and use of augmentation systems like WAAS. Consumer-grade receivers typically achieve 5-10 meters, while survey-grade receivers with RTK can achieve centimeter accuracy.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
GPS needs a minimum of four satellites for a 3D fix (latitude, longitude, altitude, and time). With three satellites, you can get a 2D fix (latitude and longitude) if altitude is known, but the receiver's clock error requires a fourth satellite to solve for time. On the 220-1101 exam, remember that four is the magic number for a full 3D position.
A-GPS (Assisted GPS) uses cellular or Wi-Fi networks to download satellite ephemeris and almanac data, dramatically reducing the time to first fix (TTFF) from minutes to seconds. Standalone GPS must download this data directly from the satellites, which is slow (up to 30 minutes on a cold start). A-GPS also helps in weak signal areas by providing aiding data. The exam tests that A-GPS is faster and uses network assistance.
GPS signals are extremely weak (around -125 dBm) and cannot penetrate most building materials like concrete, metal, or thick glass. Indoors, the signal is often too attenuated to be detected by the receiver. Even near a window, you may get a weak signal, but a reliable fix typically requires a clear view of the sky. The exam may present this as a troubleshooting scenario.
GPS L1 frequency is 1575.42 MHz. It is the primary civilian frequency. The exam may ask about this exact number in a question about GPS frequencies or interference. L2 (1227.60 MHz) is used for military and some civilian dual-frequency receivers.
On Android, go to Settings > Location and toggle location on. You can choose mode: High Accuracy (GPS + Wi-Fi + cellular), Battery Saving (Wi-Fi + cellular only), or Device Only (GPS only). On iOS, go to Settings > Privacy > Location Services and toggle on. You can also set per-app permissions. The exam may test that location services must be enabled for GPS to function.
GLONASS is the Russian global navigation satellite system. It operates with 24 satellites in Medium Earth Orbit at about 19,100 km altitude. Unlike GPS which uses CDMA, GLONASS traditionally uses FDMA on L1 (1602 MHz) and L2 frequencies, though newer satellites also use CDMA. Many modern receivers support both GPS and GLONASS to improve accuracy and availability, especially in challenging environments like urban canyons.
Yes, Wi-Fi-only tablets often include a GPS receiver. However, without a cellular connection, they cannot use A-GPS to download satellite data quickly. The first fix may take longer (cold start). Once the receiver has the ephemeris, it works normally. The exam may ask about this: a Wi-Fi-only tablet can still get a GPS fix, but it may be slower initially.
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