Question

1a). For a single, handheld, GPS receiver operating in point positioning mode: i. Explain how the receiver uses signals from four satellites to determine its position on Earth. ii. Approximately what East, North and Height accuracy would you expect from this receiver? iii. What are the main factors that limit the positioning accuracy of this receiver? (b) What techniques are used to overcome the spatial accuracy limitations of a single, handheld, GPS receiver operating in point positioning mode?

Answer 1

a) For single handheld GPS receiver working in point positioning mode:

1. Working of GPS receiver using signals from four satellites to determine position on the earth:

The receiver gets a signal from each of the four satellites. The satellites transmits the exact value of time the signals are sent and it is subtracted from the time at which signals were received to give the exact distance of satellite from the point and also the exact position of the satellite. So, given the travel time of GPS signals from the four satellites and their exact position in the sky, the GPS receiver can can determine the exact position of the person.

1. Accuracy of the receiver

Accuracy of the receiver depends on various factors such as position of satellite, atmospheric behaviour, satellite clock errors etc.

Generally, with the handheld GPS 95% of the horizontal GPS position would be within 10 meters of the true location and the error in the altitude will be approximately twice the horizontal error.

1. Main factors that limit the positioning accuracy of the satellite:

​​​​​​​There are many things that affects the positiong accuracy of the satellite. Some of them are listed below:

• Signal blockage due to buildings, bridges, mountains, tress etc.
• Indoor of underground usage of the receiver
• Reflecting of signals off the building walls (known as multipath)
• Radio interfence can cause minor affects
• Solar storms can also affect signals
• Improper designed devices

b) Techniques used to overcome the spatial accuracy limitations of the receivers:

• Dual-Frequency Operation: As the first-order ionospheric delay is directly proportional to the carrier wave frequency of the signals. Therefore, the first-order ionospheric delay can be eliminated by using the combinations of dual-frequency Global navigation satellite system(GNSS) measurements.
• External Error Correction Data: This includes satellite orbit and clock corrections. For example,in the case of TerraStar service, the corrections generated are broadcasted for end-users by Inmarsat satellites.
• Modeling: The tropospheric delay is corrected using a neutral atmosphere delay model such as UNB model developed by the University of New Brunswick. The wet part of tropospheric delay is highly varying and it cannot be modeled with sufficient accuracy. Therefore, the residual tropospheric delay is estimated while estimating positions and other unknowns.