Introduction

Traditional methods of surveying and navigation resort to tedious field and astronomical observation for deriving positional and directional information. Diverse field conditions, seasonal variation and many unavoidable circumstances always bias the traditional field approach. However, due to rapid advancement in electronic systems, every aspect of human life is affected to a great deal. Field of surveying and navigation is tremendously benefited through electronic devices. Many of the critical situations in surveying/navigation are now easily and precisely solved in short time.

Astronomical observation of celestial bodies was one of the standard methods of obtaining coordinates of a position. This method is prone to visibility and weather condition and demands expert handling. Attempts have been made by USA since early 1960 s to use space based artificial satellites. System TRANSIT was widely used for establishing a network of control points over large regions. Establishment of modern geocentric datum and its relation to local datum was successfully achieved through TRANSIT. Rapid improvements in higher frequently transmission and precise clock signals along

Satellite Remote Sensing and GIS Applications in Agricultural Meteorology pp. 121150

with advanced stable satellite technology have been instrumental for the development of global positioning system.

The NAVSTAR GPS (Navigation System with Time and Ranging Global Positioning System) is a satellite based radio navigation system providing precise three- dimensional position, course and time information to suitably equipped user.

GPS has been under development in the USA since 1973. The US department of Defence as a worldwide navigation and positioning resource for military as well as civilian use for 24 hours and all weather conditions primarily developed it.

In its final configuration, NAVSTAR GPS consists of 21 satellites (plus 3 active spares) at an altitude of 20200 km above the earth's surface (Fig. 1). These satellites are so arranged in orbits to have atleast four satellites visible above the horizon anywhere on the earth, at any time of the day. GPS Satellites transmit at frequencies L1 = 1575.42 MHz and L2=1227.6 MHz modulated with two types of code viz. P-code and C/A code and with navigation message. Mainly two types of observable are of interest to the user. In pseudo ranging the distance between the satellite and the GPS receiver plus a small corrective

GPS Nominal Constellation 24 Satellites in 6 Orbital Planes 4 Satellites in each Plane 20,200 km Altitudes, 55 Degree Inclination

Figure 1: The Global Positioning System (GPS), 21-satellite configuration term for receiver clock error is observed for positioning whereas in carrier phase techniques, the difference between the phase of the carrier signal transmitted by the satellite and the phase of the receiver oscillator at the epoch is observed to derive the precise information.

The GPS satellites act as reference points from which receivers on the ground detect their position. The fundamental navigation principle is based on the measurement of pseudoranges between the user and four satellites (Fig. 2). Ground stations precisely monitor the orbit of every satellite and by measuring the travel time of the signals transmitted from the satellite four distances between receiver and satellites will yield accurate position, direction and speed. Though three-range measurements are sufficient, the fourth observation is essential for solving clock synchronization error between receiver and satellite. Thus, the term "pseudoranges" is derived. The secret of GPS measurement is due to the ability of measuring carrier phases to about 1/100 of a cycle equaling to 2 to 3 mm in linear distance. Moreover the high frequency L1 and L2 carrier signal can easily penetrate the ionosphere to reduce its effect. Dual frequency observations are important for large station separation and for eliminating most of the error parameters.

SATELLITE Z

SATELLITE 1

SATELLITE 3

Effl'

SATELLITE t.

¿V GPS-ANTENNA

¿V GPS-ANTENNA

Figure 2: Basic principle of positioning with GPS

Figure 2: Basic principle of positioning with GPS

There has been significant progress in the design and miniaturization of stable clock. GPS satellite orbits are stable because of the high altitudes and no atmosphere drag. However, the impact of the sun and moon on GPS orbit though significant, can be computed completely and effect of solar radiation pressure on the orbit and tropospheric delay of the signal have been now modeled to a great extent from past experience to obtain precise information for various applications.

Comparison of main characteristics of TRANSIT and GPS reveal technological advancement in the field of space based positioning system (Tablel).

Table 1. TRANSIT vs GPS

Details

TRANSIT

GPS

Orbit Altitude

1000 Km

20,200 Km

Orbital Period

105 Min

12 Hours

Frequencies

150 MHz 400 MHz

1575 MHz 1228 MHz

Navigation data

2D : X, Y

4D : X,Y,Z, t velocity

Availability

15-20 minute per pass

Continuously

Accuracy

n 30-40 meters (Depending on velocity error)

n15m (Pcode/No. SA 0.1 Knots

Repeatability

n1.3 meters relative

Satellite Constellation

4-6

21-24

Geometry

Variable

Repeating

Satellite Clock

Quartz

Rubidium, Cesium

GPS has been designed to provide navigational accuracy of ±10 m to ±15 m. However, sub meter accuracy in differential mode has been achieved and it has been proved that broad varieties of problems in geodesy and geodynamics can be tackled through GPS.

Versatile use of GPS for a civilian need in following fields have been successfully practiced viz. navigation on land, sea, air, space, high precision kinematics survey on the ground, cadastral surveying, geodetic control network densification, high precision aircraft positioning, photogrammetry without ground control, monitoring deformations, hydrographic surveys, active control survey and many other similar jobs related to navigation and positioning,. The outcome of a typical GPS survey includes geocentric position accurate to 10 m and relative positions between receiver locations to centimeter level or better.

SEGMENTS OF GPS

For better understanding of GPS, we normally consider three major segments viz. space segment, Control segment and User segment. Space segment deals with GPS satellites systems, Control segment describes ground based time and orbit control prediction and in User segment various types of existing GPS receiver and its application is dealt (Fig. 3).

Table 2 gives a brief account of the function and of various segments along with input and output information.

Table 2. Functions of various segments of GPS

Segment

Input

Function

Output

Space

Navigation message

Generate and Transmit code and carrier phases and navigation message

P-Code C/A Code L1,L2 carrier Navigation message

Control

P-Code Observations Time

Produce GPS time predict ephemeris manage space vehicles

Navigation message

User

Code observation Carrier phase observation Navigation message

Navigation solution Surveying solution

Position velocity time

GLONASS (Global Navigation & Surveying System) a similar system to GPS is being developed by former Soviet Union and it is considered to be a valuable complementary system to GPS for future application.

Space Segment

Space segment will consist 21 GPS satellites with an addition of 3 active spares. These satellites are placed in almost six circular orbits with an inclination of 55 degree. Orbital height of these satellites is about 20,200 km corresponding to about 26,600 km from the semi major axis. Orbital period is exactly 12 hours of sidereal time and this provides repeated satellite configuration every day advanced by four minutes with respect to universal time.

Final arrangement of 21 satellites constellation known as "Primary satellite constellation" is given in Fig. 4. There are six orbital planes A to F with a separation of 60 degrees at right ascension (crossing at equator). The position of a satellite within a particular orbit plane can be identified by argument of latitude or mean anomaly M for a given epoch.

GPS satellites are broadly divided into three blocks : Block-I satellite pertains to development stage, Block II represents production satellite and Block IIR are replenishment/spare satellite.

Figure 4: Arrangement of satellites in full constellation

Under Block-I, NAVSTAR 1 to 11 satellites were launched before 1978 to 1985 in two orbital planes of 63-degree inclination. Design life of these prototype test satellites was only five years but the operational period has been exceeded in most of the cases.

The first Block-II production satellite was launched in February 1989 using channel Douglas Delta 2 booster rocket. A total of 28 Block-II satellites are planned to support 21+3 satellite configuration. Block-II satellites have a designed lifetime of 5-7 years.

To sustain the GPS facility, the development of follow-up satellites under Block-II R has started. Twenty replenishment satellites will replace the current block-II satellite as and when necessary. These GPS satellites under Block-IR have additional ability to measure distances between satellites and will also compute ephemeris on board for real time information.

Fig.5 gives a schematic view of Block-II satellite. Electrical power is generated through two solar panels covering a surface area of 7.2 square meter each. However, additional battery backup is provided to provide energy when the satellite moves into earth's shadow region. Each satellite weighs 845kg and has a propulsion system for positional stabilization and orbit maneuvers.

Figure 5: Schematic view of a Block II GPS satellite

GPS satellites have a very high performance frequency standard with an accuracy of between 1X10-12 to 1X10-13 and are thus capable of creating precise time base. Block-I satellites were partly equipped with only quartz oscillators but Block-II satellites have two cesium frequency standards and two rubidium frequency standards. Using fundamental frequency of 10.23 MHz, two carrier frequencies are generated to transmit signal codes.

Observation Principle and Signal Structure

NAVSTAR GPS is a one-way ranging system i.e. signals are only transmitted by the satellite. Signal travel time between the satellite and the receiver is observed and the range distance is calculated through the knowledge of signal propagation velocity. One way ranging means that a clock reading at the transmitted antenna is compared with a clock reading at the receiver antenna. But since the two clocks are not strictly synchronized, the observed signal travel time is biased with systematic synchronization error. Biased ranges are known as pseudoranges. Simultaneous observations of four pseudoranges are necessary to determine X, Y, Z coordinates of user antenna and clock bias.

Real time positioning through GPS signals is possible by modulating carrier frequency with Pseudorandom Noise (PRN) codes. These are sequence of binary values (zeros and ones or +1 and -1) having random character but identifiable distinctly. Thus pseudoranges are derived from travel time of an identified PRN signal code. Two different codes viz. P-code and C/A code are in use. P means precision or protected and C/A means clear/acquisition or coarse acquisition.

P- code has a frequency of 10.23 MHz. This refers to a sequence of 10.23 million binary digits or chips per second. This frequency is also referred to as the chipping rate of P-code. Wavelength corresponding to one chip is 29.30m. The P-code sequence is extremely long and repeats only after 266 days. Portions of seven days each are assigned to the various satellites. As a consequence, all satellite can transmit on the same frequency and can be identified by their unique one-week segment. This technique is also called as Code Division Multiple Access (CDMA). P -code is the primary code for navigation and is available on carrier frequencies L1 and L2.

The C/A code has a length of only one millisecond; its chipping rate is 1.023 MHz with corresponding wavelength of 300 meters. C/A code is only transmitted on L1 carrier.

GPS receiver normally has a copy of the code sequence for determining the signal propagation time. This code sequence is phase-shifted in time step-by-step and correlated with the received code signal until maximum correlation is achieved. The necessary phase-shift in the two sequences of codes is a measure of the signal travel time between the satellite and the receiver antennas. This technique can be explained as code phase observation.

For precise geodetic applications, the pseudoranges should be derived from phase measurements on the carrier signals because of much higher resolution. Problems of ambiguity determination are vital for such observations.

The third type of signal transmitted from a GPS satellite is the broadcast message sent at a rather slow rate of 50 bits per second (50 bps) and repeated every 30 seconds. Chip sequence of P-code and C/A code are separately combined with the stream of message bit by binary addition ie the same value for code and message chip gives 0 and different values result in 1.

The main features of all three signal types used in GPS observation viz carrier, code and data signals are given in Table 3.

Table 3. GPS Satellite Signals

Atomic Clock (G, Rb) fundamental frequency

10.23. MHz

L1 Carrier Signal

154 X 10.23 MHz

L1 Frequency

1575.42 MHz

L1 Wave length

19.05 Cm

L2 Carrier Signal

120 X 10.23 MHz

L2 Frequency

1227.60 MHz

L2 Wave Length

24.45 Cm

P-Code Frequency (Chipping Rate)

10.23 MHz (Mbps)

P-Code Wavelength

29.31 M

P-Code Period

267 days : 7 Days/Satellite

C/A-Code Frequency (Chipping Rate)

1.023 MHz (Mbps)

C/A-Code Wavelength

293.1 M

C/A-Code Cycle Length

1 Milisecond

Data Signal Frequency

50 bps

Data Signal Cycle Length

30 Seconds

The signal structure permits both the phase and the phase shift (Doppler effect) to be measured along with the direct signal propagation. The necessary bandwidth is achieved by phase modulation of the PRN code as illustrated in Fig. 6.

Figure 6: Generation of GPS Signals

Structure of the GPS Navigation Data

Structure of GPS navigation data (message) is shown in Fig. 7. The user has to decode the data signal to get access to the navigation data. For on line navigation purposes, the internal processor within the receiver does the decoding. Most of the manufacturers of GPS receiver provide decoding software for post processing purposes.

Subframes

Subframes

Figure 7: Structure of Navigation data

With a bit rate of 50 bps and a cycle time of 30 seconds, the total information content of a navigation data set is 1500 bits. The complete data frame is subdivided into five subframes of six-second duration comprising 300 bits of information. Each subframe contains the data words of 30 bits each. Six of these are control bits. The first two words of each subframe are the Telemetry Work (TLM) and the C/A-P-Code Hand over Work (HOW). The TLM work contains a synchronization pattern, which facilitates the access to the navigation data.

The navigation data record is divided into three data blocks:

Data Block I appears in the first subframe and contains the clock coefficient/bias.

Data Block II appears in the second and third subframe and contains all necessary parameters for the computation of the satellite coordinates.

Data Block III appears in the fourth and fifth subframes and contains the almanac data with clock and ephemeris parameter for all available satellite of the GPS system. This data block includes also ionospheric correction parameters and particular alphanumeric information for authorized users.

Unlike the first two blocks, the subframe four and five are not repeated every 30 seconds.

International Limitation of the System Accuracy

Since GPS is a military navigation system of US, a limited access to the total system accuracy is made available to the civilian users. The service available to the civilians is called Standard Positioning System (SPS) while the service available to the authorized users is called the Precise Positioning Service (PPS). Under current policy the accuracy available to SPS users is 100m, 2D-RMS and for PPS users it is 10 to 20 meters in 3D. Additional limitation viz. Anti-Spoofing (AS), and Selective Availability (SA) was further imposed for civilian users. Under AS, only authorized users will have the means to get access to the P-code. By imposing SA condition, positional accuracy from Block-II satellite was randomly offset for SPS users. Since May 1, 2000 according to declaration of US President, SA is switched off for all users.

The GPS system time is defined by the cesium oscillator at a selected monitor station. However, no clock parameter are derived for this station. GPS time is indicated by a week number and the number of seconds since the beginning of the current week. GPS time thus varies between 0 at the beginning of a week to 6,04,800 at the end of the week. The initial GPS epoch is January 5, 1980 at 0 hours Universal Time. Hence, GPS week starts at Midnight (UT) between Saturday and Sunday. The GPS time is a continuous time scale and is defined by the main clock at the Master Control Station (MCS). The leap seconds is UTC time scale and the drift in the MCS clock indicate that GPS time and UTC are not identical. The difference is continuously monitored by the control segment and is broadcast to the users in the navigation message. Difference of about 7 seconds was observed in July,

1992.

Figure 8: Data Flow in the determination of the broadcast ephemeris

GPS satellite is identified by two different numbering schemes. Based on launch sequence, SVN (Space Vehicle Number) or NAVSTAR number is allocated. PRN (Pseudo Random Noise) or SVID (Space Vehicle Identification) number is related to orbit arrangement and the particular PRN segment allocated to the individual satellite. Usually the GPS receiver displays PRN number.

Control Segment

Control segment is the vital link in GPS technology. Main functions of the control segment are:

- Monitoring and controlling the satellite system continuously

- Determine GPS system time

- Predict the satellite ephemeris and the behavior of each satellite clock.

- Update periodically the navigation message for each particular satellite.

For continuos monitoring and controlling GPS satellites a master control stations (MCS), several monitor stations (MS) and ground antennas (GA) are located around the world (Fig. 9). The operational control segment (OCS) consists of MCS near Colorado springs (USA), three MS and GA in Kwajaleian Ascension and Diego Garcia and two more MS at Colorado Spring and Hawai.

The monitor station receives all visible satellite signals and determines their pseudorages and then transmits the range data along with the local meteorological data via data link to the master control stations. MCS then precomputes satellite ephemeris and the behaviour of the satellite clocks and formulates the navigation data. The navigation message data are transmitted to the ground antennas and via S-band it links to the satellites in view. Fig. 9 shows this process schematically. Due to systematic global distribution of upload antennas, it is possible to have atleast three contacts per day between the control segment and each satellite.

0 0

Post a comment