GPS-RECEIVER

System Structure

With the advent of radar stations, it became possible to measure the motion parameters and the relative location of the object reflected from the surface of the radar beam. The question arose about the possibility of measuring the parameters of object motion on the emitted signal. In 1957, the Soviet Union, a group of scientists headed by VA Kotelnikov experimentally confirmed the possibility of determining the parameters of motion of artificial Earth satellite (AES) from measurements of the Doppler frequency signal emitted by this satellite. But, most importantly, was established the possibility of solving the inverse problem - finding the coordinates of the receiver from the measured Doppler shift of the signal emitted from the satellite, if the motion parameters and the coordinates of the satellite are known [1,2].

Thus, an artificial Earth satellite radionavigation becomes the reference station, the coordinates of which change over time due to movement of a satellite in orbit, but can be calculated in advance for any time due to ephemeris information stored in the navigation satellite signals.

In the Soviet Union in 1963, work began on constructing the first low-orbit satellite navigation system "Cicada." In 1967 he was put into orbit the first domestic navigation satellite Cosmos-192 "

In the United States in 1964, creates a Doppler satellite navigation system is the first generation of "Transit". For commercial use, the system becomes available in 1967, the same way as in the "cicada" in the "Transit" coordinates of the source are calculated by the Doppler signal of one of the 7 visible satellites. Satellite systems have a circular polar orbit with altitude above the Earth about 1100 km, the period of the satellite "Transit" is 107 minutes. The accuracy of calculating the coordinates of the source in the first generation systems to a large extent depends on the error in determining the speed of the source. Thus, if the velocity of an object is determined with an accuracy of 0.5 sec, then this will in turn corrupts the ~ 500 m. For a stationary object, this value decreases to 50 m.

The main disadvantages of these systems were insufficient positioning accuracy of dynamic objects and the lack of continuity in the measurements, the time between passage of the various satellites of sight of the consumer depends on the latitude at which it is located, and may make the quantity of 35 to 90 minutes.

At the same time, the U.S. Air Force conducted its program in parallel by using broadband signals, modulated by a PN code (PRN). Correlation properties of such a code you can use a single frequency signal for all satellites, code-division signals from different satellites. Later, in 1973, two programs were merged into a single entitled "Navstar-GPS" [3]. By 1996, the deployment was completed. There are currently 28 active satellites available

In the USSR, flight tests vysokoorbitalnoy satellite navigation system GLONASS began in 1982, launching the Kosmos-1413 "[4]

Segments vysokoorbitalnyh navigation systems GLONASS and GPS
Figure 1. Segments vysokoorbitalnyh navigation systems GLONASS and GPS.

The whole system includes three functional parts

  • space segment, which includes a constellation of satellites orbiting the earth (in other words, the navigation of space vehicles);
  • management segment, ground control (GCC) constellation of satellites;
  • equipment users.

Both systems are bezzaprosnymi, so the number of users of the system does not matter. Besides the basic functions - navigation definitions - the system allows highly accurate mutual synchronization frequency and time standards for ground-based remote objects and their mutual geodetic reference.

he space segment of GLONASS and GPS
Figure 2. The space segment of GLONASS and GPS.

As mentioned above, the orbital constellation consists of 28 GPS navigation spacecraft. All of them are in circular orbits with a period of revolution around the Earth is 12 hours. The height of the orbit of each satellite is ~ 20000 km.

The structure of the radio navigation system GPS

In the GPS system uses code division signals (CDMA), so all the satellites emit signals with equal frequency. Each satellite GPS system emits two phase-shifted signal. The frequency of the first signal of L1 = 1575,42 MHz, and the second - L2 = 1227,6 MHz. Signal carrier frequency L1 is modulated with two binary sequences, each of which is formed by adding modulo 2 ranging code and transmitted by the system and navigation data generated at a rate of 50 bps. At L1 passed two quadrature components, bi-phase keying binary sequences. The first sequence is the sum modulo 2 of precise ranging code of P or a secret code, Y, and navigation data. The second sequence also is the sum modulo 2 of gross C / A (open) code and the same sequence of navigation data.

Radio signal at L2 frequency biphasic keyed to only one of the two previously considered sequences. The choice of the modulating sequence is carried out on command from Earth.

Each satellite uses a peculiar only to him rangefinder codes C / A and P (Y), which allows us to separate the satellite signals. During the formation of precise ranging P (Y) code simultaneously formed timestamp satellite signal.

Composition and structure of navigation messages GPS satellites

Structural division of the navigation information of GPS satellites is carried out on the superframe, frame, subframe and words. Superframe is made up of 25 frames and has 750 (12.5 min). One frame is transmitted for 30 seconds and has a size of 1500 bits. Frame is divided into 5 subframe of 300 bits and is transmitted during an interval of 6 seconds. The beginning of each subframe indicates a time stamp that corresponds to the beginning / end of a 6-interval with the system time GPS. Subframe consists of 10 30-bit words. In every word six LSBs are the parity-check bits.

In the 1 -, 2 - and 3-m subframe transmits data about the parameters of the correction of clock and data satellite ephemerides, which is connected to. The content and structure of these subframe remain unchanged on all pages superframe. The 4 - and 5-m subframe contains information about the configuration and status of all spacecraft systems, spacecraft almanacs, special reports, parameters describing the relationship GPS time with UTC, and so on.

Definition of coordinate consumer

To determine the coordinates of the consumer needs to know the coordinates of the satellites (at least 4) and the distance from the user to each visible satellite. In order that the consumer can determine the coordinates of the satellites, navigation signals emitted by them are modeled by messages about the parameters of their movement. In consumer equipment is a selection of these messages and positioning satellites at the right time.

Coordinates and velocity components are changing very rapidly, so that messages about the parameters of satellite motion does not contain information about their coordinates and velocity vector components, and information about the parameters of a model that approximates the trajectory of the spacecraft at a sufficiently long time interval (about 30 minutes). Parameters of the approximating models are changing slowly enough, and they can be considered constant over the interval of approximation.

Parameters of the approximating models are part of the satellite navigation message. In the GPS system used by the Keplerian motion model with osculating elements. In this case, the trajectory of the spacecraft is divided into regions approximating the duration of one hour. In the center of each plot is given a nodal point in time, the value of which is reported to the consumer navigation information. In addition, Consumer Reports model parameters osculating elements at the nodal point in time, as well as parameters of functions that approximate the changes of model parameters osculating elements over time as the previous nodes, and following him.

In consumer equipment allocated time interval between the moment of time that is necessary to determine a satellite's position and the key moments. Then, using the approximating functions and their parameters, extracted from the navigation message, calculate the values of model parameters osculating elements at the right time. In the last step by conventional formulas of Keplerian models determine the coordinates and velocity components satellite.

Differential mode

Satellite navigation systems allow the consumer to get the coordinates to an accuracy of 10-15 m. However, for many tasks, especially for navigation in urban areas, require greater accuracy. One of the main methods of increasing the accuracy of locating an object based on the application known in the navigation principle of differential navigation measurements.

Differential DGPS (Differential GPS) allows you to set the coordinates of up to 3 m in the dynamic navigation environment and up to 1 m - in stationary conditions. Differential mode is realized by controlling the GPS-receiver, called a reference station. It is located in the point with known coordinates, in the same area as the main GPS-receiver. Comparing the known coordinates (resulting from high-precision geodetic survey) with the measured, base station computes corrections that are passed to consumers via radio in the pre-specified format.

Consumer equipment receives from the base station differential corrections and takes them into account when determining the location of the consumer.

Hardware implementation

Figure 4. Generalized structure of the receiver
Figure 4. Generalized structure of the receiver

Generalized structure of the receiver
Figure 5. Generalized structure of the receiver

As a rule, the typical signal receiver GLONASS / GPS consists of five functional parts:

  • antenna system;
  • RF unit;
  • digital correlation processing unit;
  • navigation processor.
  • User Interface

Block diagram Acutime 2000
Figure 6. Block diagram Acutime 2000

Figure 6 presents the internal block diagram used in the GPS receiver firm Trimble Acutime 2000, specifically designed to work with systems of exact time. To do this, this receiver is an output (1 pulse per second, PPS), which served timestamps 1 time per second.

This unit is an 8-channel single-frequency GPS receiver. The internal oscillator is used crystal oscillator with a frequency of 12.504 MHz.

Error seconds pulse
Figure 8. Error seconds pulse

When using such a generator errors "quantization", see Fig. 8, but they are smaller than the error location. Error in determining the origin is large enough and this is due to the fact that this unit uses an open, loose ranging code. The accuracy of determining the time of the order of 50 ns, or the origin of the order of 15 meters.

Used GPS receiver uses for communication protocol RS-422, so the connection to a PC by using the converter levels RS - 422 - RS - 232. Communication protocol Trimble serial input protocol (Tsip) represents opportunities configuration and configure the receiver.

For the initial inspection and configuration of the GPS receiver can use a standard program supplied by Trimble. Exterior window is shown in Figure 9.

software from Trimble
Figure 9. software from Trimble

Literature

  1. Povalyaev E., S. Khutornoi Satellite navigation systems GLONASS and GPS, Chip News № 10,2001.
  2. Radio systems. Ed Kazarinov Yu.M. Moscow: Higher School, 1990.
  3. Solovyev JA Satellite navigation system. M.: Eco-Trendz, 2000.
  4. Global satellite navigation system GLONASS. Ed. Kharisova VN, Perov AI, Boldin, VA MM: IPRZHR, 1998.