1, GPS positioning
GPS satellites continuously transmit radio signals with time and location information in the air, for receiving a GPS receiver. Since the distance of transmission factors, the timing signal received by the receiver than the time delay of the satellite transmission signal, commonly referred to as latency, and therefore, the distance may also be determined by the time delay. Satellite and receiver at the same time produces the same pseudo-random code, once the two symbol time synchronization, the receiver will be able to delay determination; delay multiplied by the speed of light, will be able to get the distance. As yet to be corrected "satellite clock and receiver clock synchronization error" because of the impact and the like, comprising a clock error factors included in the measured distance, so called "pseudorange."
Pseudorange: distance = velocity × propagation time
Of the C / A code is called pseudorange measured C / A code pseudorange, about about 20 meter accuracy, to the P code pseudorange measured referred to as P code pseudo range, accuracy of about 2 meters. Consider ionospheric, tropospheric and clock difference affect pseudorange is positioned substantially observation equation (wherein the information C / A code and the P code GPS satellites are continuously transmitted in the air, but the two different frequencies)
So that the satellite coordinates ,
= 1 ...
(
is the number of the satellite) receiver and the position coordinates of the user is
. You can get
Formulas, UAV position our position, i.e., a total of three unknowns and their three-dimensional spatial coordinates, plus the receiver clock error with the satellite clock corrections, so that a total of four unknowns. Therefore, it requires a minimum of four satellites to be able to locate the user receiver . Of course, the students will say error GPS receiver and each satellite corrections corrections Should we, like you, the answer is yes, but not the same as approximate, because the satellite clock accuracy of GPS positioning is completely up to 20ns can be seen all GPS satellites have the same time.
2, differential GPS (DGPS)
Before understanding the differential GPS first look at the errors in the GPS positioning
Differential GPS (DGPS) mainly through the elimination of error common term (drift and noise) to improve the positioning performance. The system consists of three parts DGPS reference station, and a user data link, which requires high quality reference GPS receiver on the known coordinates of the base station, base station distance estimation error components for each satellite, and each tablet satellite-satellite visible range of formation of the correction value, the correction value or the original observations, DGPS sent to all users through the data link.
I.e., as compared to a single GPS positioning, near the base station (which has been the exact geographical coordinates) to its exact coordinates to the received differential GPS coordinates, to obtain a calibration value, transmitted to the receiver by the calibration value, the receiver then the GPS coordinates and the calibration values are differentiated users get the precise geographic coordinates, the principle still sounds very simple.
3, RTK-GPS Positioning
On the basis of the above it is easy to understand the concept of the RTK-GPS, RTK that is Real - time kinematic, dynamic real-time, but also the difference of GPS, but the difference is that the base station built by ourselves, after construction to us in real time base station equipment and the receiver is to send unmanned aerial vehicles correction data, and we have built from the past, more accurate, accuracy up to 2cm!
RTK systems, the base and rover two parts, as I was involved in a multi-UAV system developed UAVs for the rover side. Rtk current purchase module, rover and the base station are the same, i.e., by the RTK-GPS antenna and a circuit chip module, when connected to the computer must first build base end to end or rover.
Back will be to add a full set of RTK content and principle, as things currently write articles and organize the school temporarily to reason things here.
