Satellites and Services - Satellite Applications
Navigation Using Satellites
c) GPS system used by Android phones for location information
Android phones worldwide use the GPS (Global Positioning System) for location information, which is one of the global navigation satellite systems providing precise location and time information. In addition to GPS, some Android phones also support other satellite systems such as GLONASS, Galileo, and BeiDou, which can provide additional location information and improve accuracy in areas with poor GPS reception.
d) GPS working on your mobile device
When you enable GPS (Global Positioning System) on your mobile phone, your device uses a combination of signals from satellites, cellular networks, and Wi-Fi hotspots to determine your precise location. Here's a general overview of what happens:
1. Your mobile device starts searching for signals from GPS satellites orbiting the Earth. These satellites broadcast a signal that includes information about the satellite's position and the exact time the signal was transmitted.
2. Your mobile device receives these signals and uses the information to calculate its distance from each satellite.
3. By comparing the distances from at least four satellites, your mobile device can calculate its precise location in three dimensions (latitude, longitude, and altitude).
4. In addition to GPS, your mobile device may also use signals from nearby cellular towers and Wi-Fi hotspots to help determine your location. This is known as assisted GPS (A-GPS).
5. Your mobile device then uses this location information to provide various services, such as navigation, geolocation, and location-based services for apps and websites.
It's worth noting that the accuracy of GPS location can vary depending on various factors such as the number of satellites in view, the strength of the signals, and obstructions such as tall buildings or mountains.
Location identification
When a mobile phone receives a broadcast signal from a satellite, it uses a process called trilateration to calculate the distance between the satellite and the phone. Trilateration is a mathematical technique that involves measuring the time it takes for a signal to travel from the satellite to the phone, and then using that time to calculate the distance between the two.
Here's how it works:
1. The satellite broadcasts a signal that includes information about the satellite's position and the exact time the signal was transmitted.
2. The mobile phone receives this signal and notes the exact time it was received.
3. The mobile phone uses the difference between the time the signal was transmitted and the time it was received to calculate the signal's travel time.
4. Since the signal travels at the speed of light, the mobile phone can use the travel time to calculate the distance between the satellite and the phone.
5. By repeating this process for signals received from at least four satellites, the mobile phone can determine its precise location in three dimensions (latitude, longitude, and altitude) using trilateration.
It's worth noting that the accuracy of the location calculation can be affected by various factors such as atmospheric conditions, the quality of the signal, and the geometry of the satellites in view. To improve accuracy, mobile phones often use additional sources of location information, such as Wi-Fi or cellular networks, in addition to satellite signals.
To give a glimpse of time intervals involved in location calculation, consider that you have travelled a distance of 10 KM, and we compute how it translates in to time difference in signal received by the mobile phone from it's intial location?
The speed of light is approximately 299,792,458 meters per second, which means that it takes approximately 3.33564095 × 10^-9 seconds for light to travel one meter. Therefore, the time difference in a signal received by a mobile phone due to a distance of 10 kilometers (10,000 meters) would be:
(10,000 meters) / (299,792,458 meters per second) = 0.0000333564 seconds
In other words, the signal would arrive at the mobile phone 0.0000333564 seconds later than it would if the phone were located at the same position as the transmitter. This time difference is incredibly small and would not be noticeable to the user of the mobile phone. However, it is still a factor that must be taken into account when calculating the distance to a satellite for the purpose of location services.
Mobile phones use highly accurate clocks and specialized hardware and software to measure the small time intervals required for location computation using GPS and other satellite systems.
Typically, mobile phones use a quartz crystal oscillator as the reference clock for timing measurements. These oscillators can be very accurate, with a frequency stability of up to a few parts per billion (ppb). The phone's clock is continuously calibrated against the GPS time provided by the satellites to ensure accuracy.
In addition, the phone's GPS receiver uses sophisticated digital signal processing algorithms to accurately measure the time delay of the received satellite signals. The signals are digitized and correlated with a local replica of the satellite signal to determine the time difference between the satellite's transmission time and the time of reception at the phone.
To achieve higher accuracy, the GPS receiver can track multiple satellite signals simultaneously and use advanced techniques such as signal smoothing and carrier phase measurements.
Overall, the mobile phone's ability to compute such small time intervals is a result of the combination of high-precision timing components, advanced digital signal processing, and the ability to simultaneously track multiple satellite signals.
Many modern smartphones use carrier phase measurements from satellite signals to calculate relative location and speed. This technique is known as carrier-phase tracking and is used to improve the accuracy of GPS and other satellite-based location systems.
When a GPS receiver is tracking a satellite signal, it compares the phase of the signal at the receiver with the phase of the signal transmitted by the satellite. This comparison is used to measure the time delay of the signal and, hence, the distance to the satellite. By tracking the phase of the carrier signal, the receiver can also measure the relative speed between the satellite and the receiver.
To use carrier-phase tracking for precise location determination, the receiver must track the carrier phase over a long period of time to resolve the phase ambiguities that arise from the periodicity of the carrier signal. This requires a stable clock and specialized signal processing techniques.
Many smartphones and other devices now incorporate carrier-phase tracking to improve the accuracy of their location services, especially in environments with poor signal quality or obstructed views of the sky.
Carrier phase tracking and time-difference measurements are two complementary techniques used by GPS and other satellite navigation systems to determine position and speed.
Most GPS receivers, including those in mobile phones, use a combination of time-difference and carrier-phase measurements to determine location and speed. When the receiver first starts tracking a satellite signal, it uses time-difference measurements to quickly calculate an initial estimate of its position. As it continues to track the signal, it switches to using carrier-phase measurements, which provide higher accuracy but require longer processing times to resolve the signal ambiguities.
In carrier-phase tracking, the receiver measures the difference in phase between the carrier signal transmitted by the satellite and the carrier signal received by the receiver. The change in phase is proportional to the distance the signal has traveled. By tracking the carrier phase over time, the receiver can obtain highly accurate measurements of the distance to the satellite and, hence, its position.
Carrier-phase measurements can also be used to determine the relative speed between the receiver and the satellite, as the frequency of the carrier signal is shifted by the Doppler effect when the receiver and satellite are in motion relative to each other.
So, to answer your question, a mobile phone may use carrier-phase measurements to compute the relative location and speed, but it typically uses a combination of carrier-phase and time-difference measurements to obtain accurate and reliable position information.
