7
Tutorial: GPS in power systems GPS applications in power systems Part 1 Introduction to GPS This series of tutorials will consider the Global Positioning System and its application in the field of electric power systems. Thefirst tutorial article is intended as an introduction to the Global Positioning System - it is not specific to power systems applications - and will give the reader an insight into the operation of GPS and an understanding of the many acronyms used in this area. Subsequent tutorials will cover existing applications of GPS in power systems and the future of GPS. by Philip Moore and Peter Crossley PS (Global Positioning System) is a system for determining a user's position in space, as such it is widely used for navigation - it is also free of charge. Since GPS communicates via radio waves which travel at a known speed (the speed of light), GPS is also a means of determining a user's position in time. GPS is now a fully operational service. It comprises three functional areas referred to as segments: 0 satellites (space segment) 0 GPS receivers (user segment) 0 ground stations (control segment) There are presently 24 satellites in the GPS constellation. Unlike satellites used for television broadcasting which always remain in the same position above the earth (geosyn- chronous), GPS satellites are constantly moving. GPS satellites are arranged in six orbital planes, take approximately 12 hours to orbit the earth and are located at a height of 10 898 nautical miles. From any position on the surface of the earth, there are always sufficient satellites in view to provide a timing and location service. Despite this, satellite availability is not always 100%; the system has been biased towards operation over the populated globe with the result that coverage suffers towards the poles (above 60"N and below 60"s). GPS provides two services: 0 Precise positioning service (PPS) is available to restricted users only (mainly USA military). The accuracy of this service is 22 m horizontally and 27.7 m vertically for positioning in space, and 200 ns for positioning in time. 0 Standard positioning service (SPS) is available for civilian use and has an accuracy of 100 m horizontally, 156 m vertically and 340 ns. Satellite versus terrestrial positioning It is useful to consider the reasons why a satellite positioning (navigation) system is preferable to a ground-based system, after all existing ground-based navigation systems are available and cost considerably less than anything involving satellites. The basis of a simple navigation system used for shipping is shown in Fig. 1. The ship receives a continuous carrier wave transmitted at the same frequency from two transmitters. If the ship is closer to, say, transmitter 1 than transmitter 2, then POWER ENGINEERING JOURNAL FEBRUARY 1999 33

GPS applications in power systems. I. Introduction to GPS

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Tutorial: GPS in power systems

GPS applications in power systems Part 1 Introduction to GPS

This series of tutorials will consider the Global Positioning System and its application in the field of electric power systems. Thefirst tutorial article is intended as an introduction to the Global Positioning System - it is not specific to power systems applications - and will give the reader an insight into the operation of GPS and an understanding of the many acronyms used in this area. Subsequent tutorials will cover existing applications of GPS in power systems and the future of GPS.

by Philip Moore and Peter Crossley

PS (Global Positioning System) is a system for determining a user's position in space, as such it is widely used for navigation - it is also free of

charge. Since GPS communicates via radio waves which travel at a known speed (the speed of light), GPS is also a means of determining a user's position in time.

GPS is now a fully operational service. It comprises three functional areas referred to as segments:

0 satellites (space segment) 0 GPS receivers (user segment) 0 ground stations (control segment)

There are presently 24 satellites in the GPS constellation. Unlike satellites used for television broadcasting which always remain in the same position above the earth (geosyn- chronous), GPS satellites are constantly moving. GPS satellites are arranged in six orbital planes, take approximately 12 hours to orbit the earth and are located at a height of 10 898 nautical miles. From any position on the surface of the earth, there are always sufficient satellites in view to provide a timing and location service. Despite this, satellite availability is not always 100%; the system has

been biased towards operation over the populated globe with the result that coverage suffers towards the poles (above 60"N and below 60"s).

GPS provides two services:

0 Precise positioning service (PPS) is available to restricted users only (mainly USA military). The accuracy of this service is 22 m horizontally and 27.7 m vertically for positioning in space, and 200 ns for positioning in time. 0 Standard positioning service (SPS) is available for civilian use and has an accuracy of 100 m horizontally, 156 m vertically and 340 ns.

Satellite versus terrestrial positioning It is useful to consider the reasons why a satellite positioning (navigation) system is preferable to a ground-based system, after all existing ground-based navigation systems are available and cost considerably less than anything involving satellites. The basis of a simple navigation system used for shipping is shown in Fig. 1.

The ship receives a continuous carrier wave transmitted at the same frequency from two transmitters. If the ship is closer to, say, transmitter 1 than transmitter 2, then

POWER ENGINEERING JOURNAL FEBRUARY 1999 33

Tutorial: GPS in power systems

1 Simple navigation receiving equipment on the ship can measure system the time difference between the two signals, Z.

Assuming that the signals travel at the speed of light, c, the ship is closer to transmitter 1 by a distance of zc. The locus of all such positions is a hyperbola; with a knowledge of the transmitter positions, the ship is able to fix its position on this locus. If three transmitters are available, the ship can determine its positon as one of two points of intersection of the hyperbolae. With four transmitters, the ship can uniquely determine its position. This is the basis of of the Omega navigating system. Of course there are many additional problems that must also be resolved such as how the ship can identify each transmitter uniquely and how the time difference T can be calculated if it corresponds to more than one cycle of the carrier. The

reader is referred to Reference 1 for further details on these points.

What is more important to understanding the advantages of satellite navigation sys..ems is to consider the frequency of the carrier signal from the transmitters. There is a conllict of interests in the choice of frequency. Thzse are summarised in Table 1.

It can be seen why navigational systents such as Omega use VLF carriers since they allow operation over large expanses of water, with a modest number of transmitting sites. 5maller coverage is available with medium frequency systems, such as Decca, with greater pofdtional accuracy, typically 200 m, but many transmitters are required and local topology, i.e. non-direct line of sight between tran jmitter and receiver, can significantly degratle the location accuracy.

With these thoughts in mind, considsr now the problem of providing a navigational system that has to cover the entire globe bL.t with positional errors of less than 50 m. You will quickly come to the conclusion that such a system cannot be realised using a ground-based system. This was precisely the view that the US government came to when considering a military positoning system and led to the evolution of a satellite-based system. In addition the US government also ,wanted features such as the ability to operate the receiving equipment on fast moving vchicles, such as aircraft, rather than just ships.

All these requirements can be rnet by Jlacing

Table 1 Summary of advantages and disadvantages of frequency choice

medium

curacy with multipath

high

34 POWER ENGINEERING JOURNAL FEBRUARY 1999

Tutorial: GPS in power systems

the navigation transmitting stations in space. All the transmitters have an unobstructed view of the earths surface. Each satellite is at an altitude of nearly 11 000 nautical miles which gives direct line of sight coverage to 42% of the earths surface. Furthermore, the higher frequency carrier waves employed (between 1 and 2 GHz in GPS) have a wavelength of 17-25 cm and so the positional error can be made small. Unlike Omega, GPS uses the time of arrival (TOA) difference technique to fix the receiver (the user) position. As we shall see, the main difference is that the satellites transmit their position to the user as well as a carrier signal.

Basis of position determination To fix a position, the user must receive a signal from the satellite, as depicted in Fig. 2 . Unlike the Omega signals, the GPS satellite signals contain digitally modulated information in the carrier wave. The signal, which will be covered in greater detail later, effectively contains a clock pulse. The user compares the clock pulse from the satellite with a local clock pulse to produce a time difference A. If the satellite and user clocks are synchronised, the distance between the two - the range - is Ac, where c is the velocity of light, since it will take A seconds for the satellite signal to reach the user. In addition to the clock pulses, the satellite signal also contains information of its current position. Hence the locus of the user’s position is the surface of a sphere which has the satellite position as its centre and Ac as the radius. Despite advances in timing technology, it is unlikely that the frequency and phase of the user’s clock will exactly match that of the satellite. This will introduce an error in the range calculation called the clock bias.

In order to locate the user’s position in three dimensions, the user’s range must be obtained from three separate satellites. Since each satellite contains a highly accurate clock that is kept at the same frequency and phase as the clocks in all the other satellites, the clock bias error observed by the user will be the same irrespective of which satellite is chosen. Therefore, by including information from a fourth satellite, the three range positions and the clock bias error can be calculated thus allowing the user’s position to be determined.

With information from four satellites, the user can now form four equations with four unknowns as shown below. The unknowns in the equations are the user’s three dimensional

position (ux, U,, U,) and the clock bias error (Cb). The satellite positions ({XI, yl, 211, { X L , y2, 2 2 1

etc.) are known from the encoded signals and the ranges ( R I , R2, Rs and &) are calculated from the clock differences. The equations are a three-dimensional appliction of Pythagoras’ Theorem; since they are nonlinear, they can only be solved by iterative means:

From the viewpoint of power engineers, whose main interest in GPS is timing, finding the user positon may not appear to be very important. However, without finding the user position, and in turn, the clock bias error, it is not possible for the user to gain access to the highly accurate satellite clock since the signal propagation delay is not known. GPS effectively allows the user access to a timing system with atomic clock accuracy - anywhere on the surface of the earth.

User velocity determination Of less interest to power engineers, but for completeness, Fig. 3 shows how GPS can determine the instantaneous velocity of the user. As a satellite moves through its orbit it will spend most of its time either moving away from, or towards, the user. Due to the Doppler effect, the frequency of the satellite carrier signal will appear slightly higher as the satellite

2 Reception of single satellite signal

POWER ENGINEERING JOURNAL FEBRUARY 1999 35

Tutorial: GPS in power systems

moves towards the user and slightly lower as the satellites recedes. Only when the satellite is exactly overhead will the carrier frequency be correct. The difference between the true and observed carrier frequencies is equal to the relative velocity vector divided by the speed of light. The relative velocity vector describes how fast, and in which direction, the satellite is moving relative to the user. Since the satellite signal contains the current satellite position and orbit, the user’s instantaneous velocity can be calculated.

Satellites The Navstar satellites have been built over many years. Initial work began in the 1970s. The initial satellites were referred to as Block I satellites - the last remaining Block I satellite ceased service in 1995. These satellites proved the GPS system.

3 Doppler effect on Block I1 satellites were initial production the satellite carrier satellites and a total of nine units were built, signal launched and are currently in service. An

upgraded version of the Block I1 satellites have been produced - these are Block IIA satellites. There are 15 Block IIA units in service and another four have been built and await launch when replacements are required. When the Block IIA satellites are used up, the replenishment satellites - Block IIR - will be used. Twenty Block IIR units have been purchased. Beyond Block IIR, another series of satellites have been planned - sustainment satellites Block IIF:

In general, newer satellites have extra features which allow the satellites to operate for longer without ground updates. The typical lifespan for a satellite is 7% years (equivalent to 580 million miles), although many have lasted longer than this.

Satellite signals Introduction We will now concentrate on the structure of the satellite signals. Each satellite continuously transmits on two L band frequenciw L1: 1575.42 MHz and L2: 1227.6 MHz. Al-hough in other broadcast networks, such as a cellular mobile phone network, it is importait that transmitters do not use the same frequencies, in GPS the use of a technique called code division multiple access allows the same Irequericies to be used on each satellite. Inherent to the operation of code division multipLe access is the use of random binary sequences or code 5. To be accurate these are not truly random codiis, they are pseudorandom, that is they are ge ierated deterministically by hardware and are hence repeatable. GPS uses two codes: the coarse acquisition (C/A) code and the precision (P) code. The C/A code can be utilised by anyone; the P code is for restricted users only - mainly the military.

Code division multiple access ma1:es use of a technique called spread spectrum, which allows the user to distinguish incividual satellite signals. Although the carrier frequencies are common to the GPS constellation, the code, or to give it its technical term pseudorandom binary sequence (PRBS), is different for each satellite.

Spread spectrum techniques In a spread spectrum system, the useful data to be broadcast is modulated (using an e-cclusive or gate) with the PRBS (Fig. 4). For the rlata we speak of bits whereas the PRES consists of chips. This might sound confiising because both data and chips are made up of 1s and 0s. However, since the PRBS will repeat w ien the sequence ends, the code itself does not convey any useful information other than to allow a user to receive the data. To reinforce this distinction, we refer to chips for the PRBS and bits for the data. The rate of broadcast of the PRBS - the chipping rate - 1s usually lo3 times faster than the data rate. This means that more carrier bandwidth is uscd for transmitting the data than is strictly necessary - hence the term spread spectrum. Although apparently wasteful of bandwidth, spread spectrum techniques allow users to discern individual signals when many transmitters are using the same carrier frequency - this is a little like listening to someone talking in a crowded and noisy restaurant Additionally spread spectrum allows great immunity to

36 POWER ENGINEERING JOURNAL FEBRUARY 1999

Tutorial: GPS in power systems

multipath effects. In spread spectrum broadcasting the user

knows the PRBS being broadcast and can hence attempt to correlate the broadcast signal with a locally generated version of the known PRBS. Fig. 5 shows a very simple (though impractical due to the small number of chips) example of a PRBS and the correlation attempts by the user. The code sent by the satellite consists of ten chips - in practice a minimum of 1000 would be used. When receiving the code, the user will not know initially at which point in the sequence the current chips represent. Hence, the user correlates all combinations of the code with the current signal. The correlator gives an output of 1 if the received and local code chips are the same (i.e. both Os or both 1s) and gives a zero output otherwise. In Fig. 5 we see three attempts to find the correct sequence. Only in the third attempt does the correlator output give 10, showing that all chips are correct. In a PRBS consisting of a 1000 or more chip sequence, the difference between the correct correlator output and the 999 incorrect ones would be far greater than that shown in Fig. 5. The PRBSs chosen for each satellite in the GPS constellation have little correlation with each other and hence the user can never mistake one satellite for another. Once the correlator output indicates that it has found the correct point in the sequence of the PRBS, the receiver is said to be locked on to the satellite signal. This means that the receiver can receive the modulated data from the desired satellite, even in the presence of other satellite signals.

The U A code from the satellites is relatively short, consisting of 1023 chips, which are transmitted at the rate of 1023 000 chips per second. The C/A code, therefore, is repeated almost 1000 times every second and has purposely been designed to be easily recognisable. The P code, Eiowever, is sent at a rate of 1 2 . 2 3 ~ 1 0 ~ chips per second, consists of a sequence of 6 .1871~10’~ chips and is repeated every seven days. It is interesting to note that, if it was desired to record the P code using a PC with a hard disc capacity of 8GB, a total of 90 PCs would be required! For this reason the P code used in military receivers is generated using hardware rather than recorded on a storage device.

Generation of satellite signals Fig. 6 shows how the L1 and L2 satellite signals are generated. Each satellite contains a highly accurate atomic clock (actually to be pedantic

output

4 Modulation of data in a spread spectrum system

each satellite has four of them!) which provides a base frequency of 10.23 MHz. This is multiplied by 120 and 154 to achieve the two L band transmission frequencies. The L1 signal contains both the P and C/A codes which are modulated by the satellite data. The L2 signal contains only the P code and data, hence the L2 signal is not available for civilian use.

Data structure Despite the high carrier frequencies and high chipping rates of the PRBSs, the data stream from each satellite occurs at the very sedate rate of 50 bits per second. The data is split into five subframes, each of which is 6 seconds in length. The subframes are:

1 Clock correction factors - current error and rate of change of the satellite’s clock frequency. 2 Ephemeris - data describing the satellite’s current position and orbit. 3 As 2. 4 Navigation messages and health status - further information regarding the accuracy of 5 Correlation anempa the satellite data. by receiver

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Tutorial: GPS in power systems

5 Almanac - data of the position and orbits of all satellites in the GPS constellation.

Note, subframes 4 and 5 consist of 25 consecutive frames which take 12.5 minutes to receive in full.

GPS errors Despite the foregoing, other errors can degrade the user’s positional fix if left uncompensated. These are:

0 Ionospheric delay: The ionosphere is an upper region of the earths atmosphere and consists of charged particles which can disperse satellite signals and cause a delay The delay is proportional to the transmitted frequency and accurate compensation can be achieved by the use of two satellite signals at different frequencies; this is one reason for the use of the L1 and L2 signals in GPS. Unfortunately, the L2 signal cannot be decoded by civilian users and so calculation of the delay can only be accomplished by restricted users. Civilian users have to compensate for the ionospheric delay by modelling its effect mathematically, a technique which is not as accurate as using two carrier frequencies. The coefficients needed to perform the mathematical modelling are transmitted in the satellite’s data stream. 0 Tropospheric delay: The lower troposphere affects both L1 and L2 signals equally It can only be compensated by modelling. 0 Relativistic effects: Since the user and satellites are located in different strength gravity fields, according to the Theory of Relativity, the satellite clocks will run faster than identical clocks located with the user. To overcome this effect the satellite clocks are actually set to 10.22999999545 MHz prior to launch - this

6 Generation of satellite signal

L2

L1

+ CIA code data generator generator xo, 1

will appear to the user as 10.23 MHz when in orbit. Unfortunately the orbits experienced by the satellites are not circular but ellipti:al, and hence a correction needs to be made when the satellites move closer to or away fron their nominal height.

We must not forget that GPS originated from a military impetus and so a brief consideration of the military operational aspects of GPS is instructional. Since GPS is of immense use in a military situation, account must be taken of the fact that interference of the GF‘S signals may occur. Two main modes are anticipated:

0 Spoofing - in this mode a signa which appears to be a geniune GPS sigr.al, but containing spoof information, is broadcast. To combat this, the P code can be encrypted (barring its reception by unauthorised users), which affords good anti-spoofing protection. 0 Jamming - in this mode a strong signal at the GPS carrier frequencies is broadcas. in an attempt to ‘drown’ the geniune satellite signals. This mode of interference is easier to organise than spoofing and is considered I he mo-e likely. The use of spread spectrum techniques helps in this situation although anti-jamming measures are more a feature of receiver design. Military receivers will, inevitably, be better equipped to cope with jamming signals than civilian receivers.

Unfortunately, civilian use of GPS is further degraded by the selective aviiilabili y (SA) feature. SA can be switched on or off on demand. SA has two effects. Firstly i t can dither (i.e. introduce an error int.0) the satellite clocks thus degrading the time reference. Secondly, the ephemeris data can be altered to show the satellite in slightly the wrong position. Hence the user’s position fix is also in error. Military receivers, namrally, do not suffer these effects.

There has been much speculation in recent times that the SA feature would be disabled thus allowing all civilian users highly accurate positioning. An announcement made in March 1998 by USVice President A1 Go-e’ was expected to reveal more details on this. In the event the announcement made 110 reference to SA but promised that civilian users would be able to receive a second GPS signal. This would allow compensation for the ionospheric delay described above. How and when the second signal is to be made available was. not dixussed.

38 POWER ENGINEERING JOURNAL FEBRUA3Y 1999

GPS receivers The generic outline of a GPS receiver is shown in Fig. 7. The receiver aerial does not have to be accurately lined up with the satellite since the satellite signals are circularly polarised. The received signal is converted down to a lower intermediate frequency and passes through two loops. The code loop correlates the satellite’s PRBS to allow the receiver to lock on to the satellite, thus providing the timing and data information. The carrier loop tracks the carrier frequency and determines the Doppler shift. From these signals the processor calculates positional, timing and velocity information which is displayed to the user.

There are essentially three types of receiver.

Continuous tracking receivers have a separate channel for each satellite, e.g. a six channel receiver is common. These receivers can fix the user position faster than other types. They are also better in moving platforms, such as cars boats and planes, and offer better anti-jamming performance.

Slow sequencing receivers time share the receiver front end amongst the different satellites, typically allocating 1 second to each.

Fast sequencing receivers operate similarly but allocate 5 ms per satellite.

Control of GPS Left on their own, satellites will eventually get lost and forget what time it is. Hence there is a need to track the satellites and update new position and time information - this is the control segment described earlier. Each satellite receives an update at least once per day. To keep track of time, identical satellite clocks are present on earth and are referenced to other accurate time sources so that clock errors can be calculated. To track the satellites’ positions, a technique called inverted navigation is used. Here the L1 and L2 signals are used by ground stations to locate the satellite positions with high accuracy. There are five ground stations for this purpose which are mostly unmanned. The master control station is located in Colorado and performs the satellite updating.

GPS applications Finally, we will consider some of the main GPS applications; the power systems applications will be described in later tutorials. First and foremost GPS has a military use for all aspects of combat and reconnaissance. Navigation is a

Tutorial: GPS in

display

large user of GPS from locating small pleasure boats and light aircraft through to steering large tankers and civil airliners. Timing applications include power systems uses but also encompass uses by local and wide area network providers through to cellular phone operators.

Producers of maps and GIS information are also big users of GPS where its use for large- scale surveying saves valuable time. However, small-scale surveying applications, such as building site surveying, are also possible through the use of dgerential GPS. In differential GPS, a stationary receiver transmits GPS error offsets to other mobile GPS receivers. Millimetre accuracy is possible using this approach. It is also finding uses for aircraft landing systems.

References 1 ‘Electronic navigation signals’, Philco Training

Manual, 1955 2 This document is located on the White House web

server. Obtain this document by going to the following web address and instructing the search engine to find all documents relating to ‘global positioning system’

http://www.pub.whitehouse.gov/search/everything.html The document in question was dated 30 March 1998.

Bibliography The following books were consulted for this article:

‘The Navstar Global Positioning System’, Tom Logsdon, Van Nostrand Reinhold, 1992 ‘Understanding GPS - principles and applications’, Elliot D Kaplan (Editor), Artech House Publishers, 1996 ‘Guide to GPS positioning’, David Wells, Canadian GPS Associates, 1986 ‘Using GPS’, Conrad Dixon, Adlard Coles Nautical, 1994

OIEE: 1999

Philip Moore is with the University of Bath and Peter Crossley is with UMIST, Manchester.

power systems

7 GPSreceiver structure

POWER ENGINEERING JOURNAL FEBRUARY 1999 39