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GNSS Principles Tutorial: Determining the Accuracy of a GPS Device

written by: •edited by: Heather Marie Kosur•updated: 6/8/2011

While GPS systems are being used widely for a variety of services, people are seldom aware of GNSS that helps GPS determine their location. GNSS is not one or two satellites. A proper GNSS employs a minimum of four satellites to send enough information to ensure maximum accuracy of the GPS device.

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    Introduction to GNSS and GPS Accuracy

    GNSS, or Global Navigation Satellite Systems, form the base of any GPS (Global Positioning System) navigation. In short, the GNSS offer information on the positioning of any object through a network of satellites. It offers an earth-based electronic receiver (referred to as a GPS Device hereafter) with signals (information) that helps the latter determine its location in terms of longitude, latitude, and height (the three basic angles offered by GNSS satellites) within a certain radius. The fourth angle (factor) is related to error correction in GPS devices using the precise time of signal reception and/or signal origination.

    For example, if you are driving while using a GPS device, you need the fourth angle to know the exact time when your device received the signals, as you would have moved further by the time your device calculates the other three (longitude, latitude, and height) angles. If the fourth angle, the time of reception, is not taken into account, the information on your GPS device is a few meters back depending upon your speed (mph). Thus, the time of reception of GNSS signals or GNSS information plays an important role in enhancing the accuracy of GPS devices.

    Although there are a few GNSS systems that are used by several GPS devices, the NAVSTAR Global Positioning System is the only one that is functioning fully at the moment. This is why most GPS devices rely on NAVSTAR for enhanced accuracy of GPS devices. Among other GNSS are GLONASS (Russia), Galileo (Europe), COMPASS (China), and IRNSS (India). However, these are still to be converted into GNSS with full functionality. (The last three are still in the deployment stage and are coined next-generation GNSS.)

    GNSS in Short - GNSS offers information at regular intervals in form of analog signals about the longitude, latitude, and height of the GPS device so that the latter can calculate its position. The GPS device should have the feature whereby it can calculate the exact time when it received the GNSS signals. This helps in calculations that provide you with information about your location with minimum error coefficient. To ease this, GNSS also sends out time-stamps to the receptor.

    FOOTNOTE: The military GPS systems take care that they do not miss out even on fractions of millimeters for enhanced accuracy of GPS devices. For stationary GPS, the reception time can be ignored. If your GPS device is connected to a central server, the time taken to transmit the information from the earth-based server to your machine should also be considered for further accuracy of GPS devices.

    With this information in mind, we will be checking time-stamps in the coming section.

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    Importance of Time for GPS Accuracy

    Any GNSS will include a minimum of four satellites to offer proper information to the earth-based receiver. This means receiving four different measurements. This helps the GPS receiver determine its position using four factors: longitude, latitude, height, and, finally, its clock error.

    One may wonder about clock error and its necessity in measuring the accuracy of a GPS device. As explained in the first section, the GNSS does not send exact distances but rather information about the location where the GPS device is. If the GPS device is moving, the measurements may prove wrong as it changes the location by the time the GPS device or GPS server resolves the position by computing only the first three measurements without caring for the time factor.

    To resolve this problem, the time when the signal was sent is incorporated in both the GNSS satellites as well as the GPS devices or GPS servers. The GNSS receiver in the GPS device should have some feature that helps it calculate the time taken for the signal to reach it from the satellite. This leads to the necessity of the GNSS receiver to know the time when the signal left the satellite and hence the time-stamps. Along with other information sent by the GNSS satellites, the time-stamps of the signals are also sent. As the satellite positions are known, it becomes easy for the GNSS receiver to compute the time taken by the signal to reach it.

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    GNSS Clock Errors and Its Impact on GPS Accuracy

    The clock system in the GPS device’s GNSS receiver has to be precise and in accordance with the satellite. If not, the calculations will display wrong information, which may be dangerous in some fields where GPS is used.

    Normally, most of the cheap GPS devices use quartz crystal as in the digital clocks. Quartz clocks tend to get slower with each tick thereby increasing the clock error. Over time, this becomes significant, which will offer you wrong GPS information. As the speed of the signals vary in the vacuum (in space, where they travel at almost 300km/s) and decrease as they enter Earth’s atmosphere, you need to have a GNSS clock that can compute the position using the difference in signal speed across different layers of atmosphere. This can be achieved only by GNSS receivers that use atomic clocks or by having the GPS device correct its quartz clock error automatically.

    NOTE: Most GNSS satellites carry several atomic clocks to check for redundancy. They also send time-stamps that include information on the offset rate between the signal speed in the vacuum of space and Earth’s atmosphere. This helps the GNSS receivers to achieve the maximum accuracy of GPS devices to tem milliseconds, which is most accurate if not absolutely correct.

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    Military and Normal GPS

    While sending information about latitudes, longitudes, and height, the GNSS satellites append the time-stamps for each signal, clock redundancies at their end, and finally, their positions. All this information is collectively called ephemerides.

    Also, as mentioned, the speed of signals decreases when they enter the Earth’s atmosphere and further slow down as they reach the ground level to the GNSS receptor in the GPS device or GPS server. As a workaround to this, the GNSS satellites send out signals at two frequencies so that the GNSS receptor can calculate the exact location with less error coefficient in the accuracy of the GPS device.

    However, as far as NAVSTAR is concerned, the second frequency is reserved for military GPS devices only. For normal GPS devices, it is the GNSS receptor that plays an important role in correcting and computing any irregularities in the time-stamps and distances. In other words, the GNSS receptors in normal GPS devices are capable of computing the signal speed difference at different layers of the atmosphere offering the accuracy differed at most by one meter, which means the least error coefficient and maximum accuracy of the GPS device.