Control Segment

The control segment has control and monitor stations on earth that continuously monitor and track the satellites. The control segment does these things:

· Monitor and correct satellite orbits and clocks

· Calculate and format a satellite navigation message. This message has up-to-date descriptions of the satellites future positions, and a collection of the latest data on all GPS satellites

· Update the satellite navigation message regularly.

The control segment has one master control station and five monitor stations. Three of the monitor stations are also upload stations.

The master control station is in Colorado Springs, USA. The master control station is the operational center of the GPS. The master control station controls all operations in the control segment. The master control station has an atomic clock, this clock is the reference for the GPS.

The monitor stations track the satellites 24 hours a day. The master control station remotely controls the monitor stations through on-line connections. The monitor stations are in these locations:

· Ascension island

· Colorado Springs

· Diego Garcia island

· Hawaii

· Kwajalein island.

The monitor stations receive the same information from the satellites that the GPS receiver unit in the airplane receives.

The monitor stations do these things:

· Record the accuracy of the satellite clocks

· Collect and relay to the control station meteorologic data such as barometric pressure, temperature, and dew point.

The master control station uses this data to calculate the

tropospheric signal delay

· Continuously measure the ranges to all visible satellites. The master control station uses this data to calculate and predict the satellites orbits.

The master control station uses the upload stations to send this data:

· Orbit correction commands to the satellites. The satellites use control rockets to correct their orbits

· The navigation message to the satellites.

The upload stations are on Ascension island, Diego Garcia island, and Kwajalein island.

GPS Accuracy

Civilian users have access to standard positioning service (SPS). SPS has an accuracy of 15 - 25 meters for 95% of the position fixes. For security reasons, the American Department of Defense intentionally degrades the accuracy for civilian users to 100 meters for 95% of the fixes.

Military users have access to precision positioning service (PPS). PPS has an accuracy of 18 meters or less for 95% of the fixes.

GPS - THEORY OF OPERATION – 2

Ranging

The GPS receivers use the principle of ranging to measure the distance between the receiver and the satellites. The receiver always has the location of the satellites in their orbits in memory.

The receiver measures the time it takes for a radio signal to go from a satellite to the airplane. Since the receiver knows the location of the satellite and that the radio signal travels at the speed of light, it can calculate the distance to the satellite. The receiver uses one way ranging. The receiver must know exactly at what time the satellite sent the radio signal. The receiver compares the satellite signal to a signal that the receiver makes at the same time as the satellite. The difference between the two signals (Δt) is the time the satellite signal took to get to the receiver.

Each satellite has an atomic clock to keep accurate time. All the satellites have precisely the same time. The receiver in the airplane has an internal clock but it is not atomic. It is not as accurate. Thus, it is not possible for the receiver to have precisely the same time as the satellite.

The receiver assumes that its internal clock is off by some clock bias (ΔtBIAS). This ΔtBIAS is an unknown that the receiver must calculate. The ΔtBIAS is the difference between the receiver time and GPS time.

To calculate the airplane position (latitude, longitude, and altitude) and the ΔtBIAS, the receiver must know the position of at least four satellites. The receiver then measures the distances to all the satellites at the same time. It then solves for these four unknowns with four range equations:

· Latitude

· Longitude

· Altitude

· ΔtBIAS.

GPS Time

All the satellites synchronize to universal time (coordinated) (UTC). The satellites transmit this time to the receiver. The accuracy of the satellite UTC is approximately 100 nanoseconds. The receiver transmits UTC on an ARINC 429 format. The receiver also transmits a very accurate time mark once per second.

Дата добавления: 2015-10-24