GLOBAL POSITIONING SYSTEM
Official name of GPS is NAVigational Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS). Global Positioning Systems (GPS) is a form of Global Navigation Satellite System (GNSS). First developed by the United States Department of Defense
Consists of two dozen GPS satellites in medium Earth orbit (The region of space between
2000km and 35,786 km)
GPS systems are made up of 3 segments:
Space Segment (SS)
Control Segment (CS)
User Segment (US)
It consists of 24 operational satellites evenly placed in 6 different orbits. The satellites have their own propulsion system to maintain their orbital path and can be controlled remotely. The angle between each of the 6 orbital planes and the equatorial plane i.e., Inclination, is 55°. GPS satellites fly in circular orbits at an altitude of 20,200 km and at a speed of 3.9 km/s. It takes 12 hours to complete one orbit. Each satellite makes two complete orbits each sidereal day. It passes over the same location on Earth once each day. The satellites continuously orient themselves to point their solar panels toward the sun and their antenna toward the earth. Orbits are designed so that at the very least six satellites are always within line of sight from any location on the planet.
The CS consists of 3 entities:
Master Control System
Master Control Segment
The master control station, located at Falcon Air Force Base in Colorado Springs, Colorado, is responsible for overall management of the remote monitoring and transmission sites.
Six US Air Force monitor stations are located at Falcon Air Force Base in Colorado Springs (MCS), Hawaii, Cape Carnival (Florida), Ascension Island, Diego Garcia, and Kwajalein Island.
There are aditional monitor stationa from NGA (National Geospetial-Intelligence Agency) are located at Alaska, Australia, Bahrain, Argentina, Washington (USA), Equador, United Kingdom, South Africa, South Korea and New Zeeland.
Each of the monitor stations checks the exact altitude, position, speed, and overall health of the orbiting satellites. A station can track up to 11 satellites at a time. This "check-up" is performed twice a day, by each station, as the satellites complete their journeys around the earth.
The monitoring stations track the satellites, obtain the data and pass the information to the MCS.
The MCS uses the data to compute and predict the future path of all the satellites. The MCS also determines the error of the atomic clocks in all the satellites.
The ephemeris and clocks parameters are usually updated every two hours while Almanac data is updated evevry six days through the upload station and ground antennas.
Ground antennas monitor and track the satellites from horizon to horizon and send/transmit navigation data uploads and processor program loads, and collect telemetry through S-Band communication links.
There are four dedicated GPS ground antenna sites co-located with the monitor stations at Kwajalein Atoll, Ascension Island, Diego Garcia, and Cape Canaveral.
It consists of a receiving antenna, receiver with built-in Computer and display unit. The receiver locks on to one satellite and from this satellite it obtains the almanac of all other satellites and thereby selects the four most suitable for position fixing. The fix obtained is displayed on the screen along with the course and speed made good. Receivers designed to receive only one frequency are known as single frequency receivers. To enhance position accuracy, there are dual frequency receivers that can receive both frequencies.
SATELLITE NAVIGATION DATA STRUCTURE
GPS satellites broadcast three different types of data.
Almanac – The almanac contains information about the status of the satellites and their approximate orbital information. The GPS receiver uses the almanac to calculate which satellites are currently visible. This tells the GPS receiver where each satellite should be at any time through the day. However, the almanac is not accurate enough to let the GPS receiver get a fix.
Ephemeris – To get a fix, GPS receiver requires additional data for each satellite, called the ephemeris. This data gives very precise information about the orbit of each satellite and current date & time. GPS receiver can use the ephemeris data to calculate the location of a satellite to within a metre or two. The ephemeris is updated every 2 hours and is usually valid for 4 hours.
Pseudo Random Code –Each satellite broadcasts what is called a pseudo random code for timing purposes (each satellite has its own distinct and unique pseudo random code). GPS satellites and receivers are synchronized so they're generating the same code at exactly the same time. The distance (or range) between the receiver and the satellite can be calculated from the time it takes for the satellite signal to reach the receiver. To determine the time delay, all we have to do is receive the codes from a satellite and then look back and see how long ago our receiver generated the same code. The time difference is how long the signal took to get down to us.
P Code and C/A Code: Each satellite transmits two codes, i.e. the Precision (P) code and the Coarse Acquisition (C/A) code. These codes are a very complex series of data bits in the form of 0’ and ‘1’ and hence referred to as Pseudo Random Codes or Pseudo Random Noise!
All civilian GPS receivers decode the C/A code and it also helps the military receivers to access the more accurate, encrypted, P code. These codes are modulated by phase modulation technique on two carrier frequencies, L1 and L2. The L1 signal consists of both the codes i.e. the P and the C/A codes while the L2 consists of only the P code. Data transmitted are as follows:
L1 (1575.42 MHz) - Mix of coarse-acquisition (C/A) code, encrypted precision (P) code and Navigation Message.
L2 (1227.60 MHz) – Precision (P) code only and Navigation Message.
C/A-Code is modulated over L1 carrier frequency by phase modulation.
It is transmitted at the rate of 1.023 Mbps in the form of ‘0’ and ‘1’ called a ‘Chip’.
Length of a Chip is 1 micro-sec, and terms of length measures 293 metres.
1023 chips make a complete C/A code.
Thus a complete C/A code is of 1 millisecond, and terms of length measures about (1023 x 293 ~) 300 km.
Each Satellite has its own unique C/A code.
The GPS receiver locks on to this C/A code, synchronizes itself with code and then uses it for timing and ranging purposes.
P-Code is modulated over L1 and also L2 carrier frequency by phase modulation.
It is transmitted at a faster rate of 10.23 Mbps in the form of ‘0’ and ‘1’ also called a ‘Chip’.
Length of a Chip is 0.1 micro-sec, and terms of length measures 29.3 metres.
A complete P-code is of 267 days, and is of 2.35 × 1014 bits in length (approx. 26.716 Terra Bytes).
The code length being so large, each satellite is assigned only a weekly segment of the master P-Code (of 267 days length).
In order to facilitate GPS receivers to synchronize faster with this long code, the start time of the code is transmitted every 6 seconds.
This helps the GPS Rx to identify which part of the code is being transmitted currently (and synchronize) .
However, P-Code is same for all the satellites as against the C/A codes which are unique for each satellite.
Navigation Message: Essential purpose of the navigation message transmission by satellites is to determine its position by the GPS receiver. Each satellite transmits a navigational message of 30 seconds in the form of 50 bps data frame. This data, which is different for each satellite, is previously supplied to the satellites by master control station and is divided into 5 sub-frames. Each sub-frame commences with a telemetry word (TLM) containing satellite status followed by a hand over word (HOW) data for acquiring P code from C/A code. The sub-frames are:
The 1st sub-frame contains data relating to satellite clock correction.
The 2nd and 3rd sub-frames contain the satellite ephemeris defining the position of the satellite.
The 4th sub-frame passes the alpha-numeric data to the user and will only be used when the upload station has a need to pass specific messages. Otherwise, it contains the almanac and health data of the standby satellites (SV25 to 32) to be used by monitoring stations and master control stations.
The 5th sub-frame gives the almanac of all the other satellites which includes the identity codes thus allowing the user the best choice of satellites for position fixing.
A GPS receiver can tell its own position by determining the ranges to three satellites and using Trilateration. To get the distance, each satellite transmits a signal.
These signals travel at a known speed. The system measures the time delay between the transmission of the signal by the satellite and its reception by the receiver.
The signals are carrying information about the satellite’s location. The GPS receiver determines the position of and distance to, at least three satellites (to reduce error) and computes the position using Trilateration.
The coordinates are calculated according to the World Geodetic System WGS84 coordinate system.
The satellites are equipped with atomic clocks while GPS receiver uses an internal crystal oscillator-based clock that is continually updated using the error correction signals from the satellites. To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time. Receiver identifies each satellite's signal by its distinct C/A code pattern, then measures the time delay for each satellite.
The distance is obtained by multiplying the travel time by the speed of light. Orbital position data from the Navigation Message is used to calculate the satellite's precise position. Knowing the position and the distance of a satellite indicates that the receiver is located somewhere on the surface of an imaginary sphere centered on that satellite and whose radius is the distance to it.
When distances from four such satellites are measured at the same time, the point where the four imaginary spheres meet is recorded as the location of the receiver. This method is called Trilateration.
Pseudo Range: In order to obtain range of satellites with accuracy, the exact position of the satellite has to be known to the GPS Rx and also the clocks of the satellite and the Rx has to be perfectly synchronized. Both of these are obtained from the Navigation Message of the satellite transmission. However, some error remains in the synchronization of the two clocks. As a result, the range of satellite calculated by the GPS Rx has an element of error due to the clocks difference. This range (with time error) is called Pseudo Range.
The following range equation eleiminates the error from the pseudo range:
R1 = C x t – C x Δt = √ ((x1 –x)2 + (y1-y)2 + (z1-z)2)
R1 = True range of satelliet no.1
C= Speed of radio waves
t = total time taken by the signals to reach (includes clock error)
Δt = clock error (difference in satellite and user clocks)
x1, y1, z1 = 3D co-ordinates of the satellite-1
x, y, z = 3D co-ordinates of the GPS receriver
Similar process is repeated for three more visible satellites and with four such equations the values of x, y and z are computed by the receiver. As can be seen, true position can be calculated despite error in range due to clock error.
The carrier frequency is also used to determine the speed of the user by the measurement of Doppler shift, i.e. change in the frequency of radio waves received when the distance between the satellite and user is changing due to the relative motion between the two. The position and velocity of the satellite as well as the position of the user are known to the user’s receiver.
The velocity vector of the satellite can be resolved in two ways:
i) In the direction towards the user
ii) In the direction perpendicular to (i).
The 2nd component is not considered as speed in this direction will not cause Doppler shift.
The receiver calculates the velocity vector of the satellite in the direction towards the user.
If the relative approach speed between the satellite and the user’s speed (based on the Doppler shift measurement) is not equal to the satellite speed vector towards the user; the difference can only arise due to user’s speed towards or away from the satellite.
Similarly with the help of the other two satellites, the receiver can calculate two additional speed vectors and these speed vectors will be towards or away from their respective satellites. These velocity vectors are resolved into three other vectors, i.e. x, y and z co-ordinates and with these three vectors the course and speed of the user is calculated.
ERRORS OF GPS
1. Atmospheric Error: Changing atmospheric conditions change the speed of the GPS signals as they pass through the Earth's atmosphere and this affects the time difference measurement and the fix will not be accurate.
Each satellite transmits its message on two frequencies and hence a dual frequency receiver receives both the frequencies and correction is calculated and compensated within the receiver thus increasing the accuracy of the fix.
Effect is minimized when the satellite is directly overhead.
Becomes greater for satellites nearer the horizon. The receiver is designed to reject satellites with elevation less than 9.5 degrees.
2. User Clock Error: If the user clock is not perfectly synchronised with the satellite clock, the range measurement will not be accurate. The range measurement along with the clock error is called pseudo range. This error can be eliminated within the receiver by obtaining pseudo range from three satellites and is done automatically within the receiver.
3. Satellite Clock Error: This error is caused due to the error in the satellite’s clock w.r.t. GPS time. This is monitored by the ground based segments and any error in the satellites clock forms part of the 30 seconds navigational message.
4. GDOP Error: GDOP (geometric dilution of precision)
describes error caused by the relative position and thus
the ‘angle of cut’ of the GPS satellites at the receiver.
Basically, the more signals a GPS receiver can “see” (spread apart versus close together), the more precise it can be. Wider the angular separation between the satellites, better the accuracy of the fix. Or, conversely said, the lower the GDOP value, the greater the accuracy of the fix. The GDOP value is indicated on the display unit.
5. Multipath Error: This error is caused by the satellite signals arriving at the ship’s antenna both directly from the satellite and those that get reflected by some objects. Thus two signals are received simultaneously which will cause the distortion of signal from which range measurement is obtained. Siting the antenna at a suitable place can minimize this error.
6. Orbital Error: The satellites are monitored and
their paths are predicted by the ground based
segment. However, between two consecutive
monitoring of the same satellite, there may be
minor drifts from their predicted paths resulting in
small position inaccuracy.
Receiver Channel Noise
All receivers have internal equipment noise. If the receiver noise is high in comparison to the satellite signal received, the usability of those signals is reduced in the same ratio. This comparison is expressed as “Signal-to-Noise” Ratio, denoted by “SNR”. The current S/N should be displayed by the GPS Rx.
The situation of poor SNR develops if either the receiver noise is high (a low-quality Rx) or if the satellite signal level goes down (Satellites nearing the horizon). A value of above 20 is good SNR.
A GPS Rx can be configured to a pre-set value of SNR below which it will not use the satellites’ signals, as low SNR degrades the accuracy of the position fix.
ACCURACY OF GPS
The accuracy of the position your GPS reports is influenced by a number of factors, such as the positions of the satellites in the sky, atmospheric effects, satellite clock errors and ephemeris errors etc., as explained above. Under ideal conditions, this may be 5, or even 3 metres. GPS units often show on the screen an accuracy figure.
The accuracy of a GPS can be improved by using secondary data from external reference stations. One such method is Augmented GPS called Differential GPS.
DIFFERENTIAL GPS (DGPS): DGPS uses a network of ground stations located at precisely known positions. The DGPS reference station is situated at a fixed location and from this position the GPS receiver tracks all the satellites within its sight, obtains the data from them and calculates the corrections for each satellite.
These corrections are then broadcast by the DGPS stations on MF frequencies for use by the DGPS receivers in the vicinity.
There are two methods employed for enhancing DGPS position accuracy.
1. Position Correction
2. Range correction.
In the first method, the reference station knows its exact position and compares it to the position obtained by the GPS and determines the corrections based on the actual and GPS calculated positions. These corrections are then broadcast to the users in terms of co-ordinates (i.e. x, y and z). The accuracy decreases as the distance of the user from the reference station increases.
Besides, the reference station and the user must select the same satellites, which is practically not possible as the user does not have the option of manually selecting the satellites.
In the second method, the reference station receives signals from all the visible satellites and measures the pseudo range to each of them. The reference station also calculates the true range of all the selected satellites. By comparing the calculated true range and the measured pseudo range, corrections to each satellite can be determined. These corrections are then broadcast to the users and applied to their pseudo range measurements before calculation of the position.
As the above explanations warrants, the DGPS will be working only in coastal areas where such services are provided.