The History Of The Gps System Architecture

Published Date: 02 Nov 2017

Satellite navigation is critical to a large number of people and organisations for reasons as simple as being able to locate your position on the globe to aircraft navigation. Interference, both intentional and unintentional, is becoming increasingly more prevalent. There are many methods that have been utilised to enhance equipment performance in an interference environment. In Global Positioning System (GPS) navigation, this can be of particular importance due to GPS signals arriving at receiver with very low signal strength. Military systems especially have to deal with intentional hostile countermeasures such as jamming. This report will focus on these issues in the context of a GPS system being used for navigational purposes. However, the information and techniques presented can typically be adapted to other satellite navigation system and indeed many other signal applications.

GPS System Architecture

GPS is a satellite navigation system operated by the American military. Similar systems are GLONASS, operated by the Russian government, and the proposed EU Galileo system. They all use satellite communication to allow the appropriate receivers to locate themselves in the coverage area of the system (the earth in this case).

It consists of three connected elements, referred to as the user, space and control segments:

User Segment: The segment that interfaces directly with the user, i.e. the GPS receiver. Navigation is but one of their applications. They can come as products themselves such as an automobile ‘SatNav’ or simply as one feature of a device.

Control Segment: This is the ground facilities of the GPS network. It comprises three different elements: the master control station (MCS), the monitor stations and the ground antennas. In basic terms, the MCS communicates with and utilises the other segments to track, update and monitor necessary information to maintain functionality of the entire GPS network.

Space Segment: This is the network of satellites in operation. Twenty-four satellites are generally required for full coverage of earth; the actual number can vary due to servicing or decommissioning. Satellites orbit in groups of four in one of six orbital planes at approximately 20200km above the earth. The GPS locating process typically requires four satellites in view of a receiver, so the constellation has been optimised to ensure there is at least this amount in view anywhere on the earth at anytime.

GPS Signals

Two bands (L1 – 1575.42MHz & L2 – 1227.6MHz) are used for transmission. All satellites transmit both bands. Code division multiple access (CDMA) is used to ensure satellites do not interfere with each other. Unique codes, with low cross-correlation properties, are assigned to each satellite. When searching for satellites, these codes are compared individually by the receiver until it finds a match, after which it will be able to continue tracking the same satellite by using the same code.

There are two key parts of the transmitted signal. A ‘ranging signal’ allows calculation of the distance between receiver and satellite and the ‘navigation data’ which provides information on the satellite such as more accurate information on satellite location or its health status. Two ranging codes are used in GPS navigation: Course/Acquisition-code (C/A-code) and Precision-code (P-code). These are pseudo-random (PRN) codes; however they differ in length and sequence. For use in civilian applications, the C/A code is unencrypted, transmitted only in the L1 band and provides a slightly less accurate position. The P-code operates on both bands and is also encrypted by the ‘Y-code’. It allows for great precision by using comparison of the two separate frequencies to eliminate certain errors. Tracking of the P-code is acquired by a receiver by first obtaining lock on the unencrypted C/A code as they are transmitted together from the same location. The P-code’s greater accuracy is only for use by the American military and those approved by the American military. The differing precisions are referred to as ‘Standard Positioning Service’ (SPS) for C/A-code and ‘Precise Positioning System’ (PPS) for P-code.

Satellite Navigation

Locating a position on earth requires a frame of reference. GPS is based on an Earth-Centred Earth-Fixed (ECEF) Coordinate system; this is a Cartesian system that rotates with the earth. This allows a set of coordinates to be unique to a specific part of the earth. The specific system used is the World Geodetic System (WGS); this adds additional parameters to more accurately simulate the earth. An elliptical earth model (as earth is not perfectly spherical) and data regarding inconsistencies in the gravitational field is included in this.

A GPS receiver ascertains its location by ‘ranging’ with every available satellite. Four is usually the minimum needed to perform position determination. Less can be needed if a dimension can be assumed such as an aircraft cruising at a constant altitude. Time of arrival (TOA) is the concept used to compute distance. The premise is that knowledge of the material the signal propagates through (the atmosphere) allows the signal velocity to be assumed. This enables use of a simple speed-distance-time calculation. Taking into account various operating errors makes more involved ranging techniques necessary in reality. This is still based on TOA, but adds complexity.

Figure : 2D diagram showing the premise behind position determinationPosition determination is easiest explained in two dimensions, it is then fairly intuitive to see how it works in three dimensions. In figure 1 the ranging signals are shown to only provide a distance, not a direction, to the receiver. This will represent a sphere around the satellite when in three dimensions. Three spheres will intersect in two spots, of which only one will make sense when location on earth is a concern. The fourth satellite is required because of unavoidable errors in the ranging process. The offset introduced by error means that the three spheres can only predict an approximate area where the receiver is located. The fourth satellite is used to synchronise the clocks of all the satellites involved in ranging which removes enough of the error to provide useful coordinates.resized coords.png

Section 2: Describe the EMC issues associated and detail the mechanisms that produce EMI

The primary concern when considering the EMC issues of a GPS navigation system is interference from radiofrequency (RF) sources. This means there is extensive variation in the possible causes of EMI to GPS receivers. This even includes the GPS satellites interfering with each other as the autocorrelation properties will not be perfect. This disadvantage is outweighed by the removal of complications associated with having each satellite in the GPS constellation operating at different frequencies. Any signal operating or containing harmonics at the L1 and L2 frequencies are potential interfering sources. Potential sources can be split into intentional and unintentional:

Intentional: This is usually what is meant by ‘jamming’. It is particularly pertinent for military applications but important in any case. A simple form is simply overpowering the GPS signal at the receiver end (spot jamming). Another, slightly more sophisticated, method is ‘spoofing’. This is the transmission of an imitation GPS signal with the purpose of arriving at the receiver with higher signal strength than the real signal [1]. This should catch the tracking process of the receiver and ultimately leads to the receiver providing incorrect positional data.

Unintentional: It can be difficult providing a comprehensive list of unintentional sources as it is possible for signals with GPS frequencies to come from anything transmitting at or around the L1 and L2 bands. Typically, the most common form will be signal harmonics. In this sense they are truly ‘unintentional’ as the source is not actually transmitting GPS frequencies on purpose. Common cases of this are AM or FM radio broadcast stations, television transmissions, citizens band (CB) transmissions and aircraft communications [2].

A GPS receiver subjected to interference can expect to suffer at least a loss of accuracy and in stronger cases complete loss of function. The receiver has to maintain a certain level of carrier-to-noise density (C/N0) ratio in order to track and then obtain measurements from a satellite – the ‘tracking threshold’. Interfering sources will reduce the receiver’s ability to do so, causing less than optimal operation. When the C/N0 at the receiver goes below the required threshold the receiver must wait until conditions have changed such that it can obtain a more favourable C/N0 again. To reacquire a tracking signal the tracking threshold has to be exceeded by a certain margin before normal navigation could resume – this is the ‘acquisition threshold’. Obtaining this will require removal of the interference by either a countermeasure of some kind or increasing the distance between receiver and interference source until acquisition threshold can be met.

The EMI Problem

To show how what effect interference can have on GPS signals a hypothetical example will be given. This will feature a GPS receiver using the C/A code from one satellite in an environment containing one jamming source operating at the same frequency. The assumptions that will be used throughout this calculation are as follows:

Both the satellite and the jammer are in line of sight of the receiver

There is no additional sources of interference other than that usually experienced in signal theory (i.e. propagation losses)

The GPS signal strength at the receiver is the minimum guaranteed for a C/A code transmission [3],

The receiver has an antenna gain of unity in all directions,

The noise figure and additional losses at the receiver will be arbitrarily assigned a generous value,

The jammer is a narrowband transmitter operating at the L1 band frequency - 1575.42MHz

The GPS receiver has a tracking threshold of 28dB-Hz

Any losses associated with the front end filter of the receiver are negligible

The jamming source is a distance of 20km from the receiver and transmits at a power of 5W

These assumptions are worst case examples or very close to. Now the C/N0 can be given by:

(1)

Where is the thermal noise reference temperature and the Boltzmann constant. To predict the tracking threshold with true accuracy requires computer simulations of a specific environment so an estimation has to be used.[ref] An approximate tracking threshold of 28dB-Hz will be used, this indicates that the receiver requires a of greater than 28dB-Hz to maintain tracking.

Finding the jammer-to-signal power () ratio is a measure of the relative intensities of the jammer and desired signals. The performance of the receiver will give some indication of its ability to operate under interfering conditions. First taking the equation for the equivalent density ratio:

(2)

Where is a gain factor depending on spectrum of jammer (1 for narrowband) and is the chipping rate of the GPS signal (chips/sec for C/A code). Also, subscript ‘S’ denotes the desires signal whilst subscript ‘J’ refers to the jamming signal. Equation (2) can yield the ratio by first expressing in dB-Hz, then rearranging for and using the tracking threshold as :

(3)

This is used to compute maximum allowable jamming signal:

(4)

Using the transmitted power of the jamming signal and the range to the receiver the actual jamming signal strength can be found:

(5)

Where is the wavelength of the jamming signal (in this case equal to C/A code). Equation (5) shows that this somewhat diminutive jamming signal still produces more than enough interference to stop the receiver functioning, as it is 11.3dB above the strength required to stop the tracking function. Clearly, measures must be taken to ensure this does not occur.

For the simple situation presented the values are in the realm of worst case scenario. This leads to over estimations of the severity of jamming elements and under estimations on the magnitudes of desired signal terms. Some assumptions lead to slight simplifications, usually in terms of losses, that affect the result marginally either way. The aim with every assumption was to exaggerate jamming signals and depreciate the desired signal. Despite this the values calculated are within the realms of possibility, so it is useful in determining the performance of the GPS receiver.

Section 3: Methods to improve systems EMC performance

There are a number of possible solutions to the interference problem. Barring physical elimination of the jamming source there are two approaches: simply increasing the distance between receiver and jammer or the more broad approach of utilising one or many of a selection of signal processing techniques. The problem of distance is simple and is dealt with later in the section. The signal processing methods deserve more of an introduction.

Many of the methods are based on the premise that GPS signals arrive at the receiver well below the noise floor of the environment. If the receiver assumes such a condition it can automatically reject any signal with strength above the noise. On its own, this process is still very susceptible to more subtle interference sources, particularly spoofing. This method is often an element of a more complex countermeasure system. Other methods, ranging in complexity, include:

Front-End Filtering: The purpose of this is usually to reject signals based on the frequency band. Ideally it would a very narrowband filter only allowing the desired signal through. While effective for this exact purpose it has two main drawbacks. Firstly, very high power signals present the possibility of harmonics overwhelming the filter and rendering it incapable of usual function. Secondly, it will not be able to differentiate between the desired signal and an interfering signal at a similar frequency. This means this solution is also quite susceptible to well executed spoofing.

Navigation Augmentation: There are a number of ways of enhancing accuracy and robustness using additional sensors in tandem with GPS positioning. The purpose is to make use of more information regarding the receiver’s location in terms of attributes like trajectory and velocity. This is commonly used by GPS receivers. It usually allows the system to help predict its location in a future instance of measurement. Sophisticated examples of this might even allow some level of navigation or position determination when there is temporarily no connection to the GPS constellation.

Adaptive Arrays: This is an advanced and very broad category of signal enhancement and interference suppression making use of spatial filtering. It revolves around using multiple antennas as a receiver and adapting the gain pattern of the array in some fashion to improve system performance. The approaches to this can vary. Some of the more simple methods involve affecting nulls towards detected interference sources. The next stage is improving the gain characteristic in the direction of desired signals and then, the most advanced form, adaptively adjusting the beam pattern of the antenna to both direct nulls towards interference and increase gain for desired signals. The various methods of adapting antenna directivity are often referred to as beamforming [4].

Altering the Distance Between Jammer and Receiver

This is a very simple look at how the distance affects the level of interference. This first point that must be considered is that after losing the tracking signal of the satellite, the receiver must then gain the acquirement threshold before beginning to function again. This threshold is typically 6-8dB above the tracking threshold [2]; 8dB will be used to continue the worst case scenario. Therefore, using equation (3) and replacing the tracking threshold with the new threshold of gives the ratio required to regain function:

(6)

Then, using equation (4) trivially allows calculation of the reduced jamming signal . The next stage is to rearrange equation (5) to obtain the distance this signal occurs:

(7)

This would require moving 220km directly away from the jammer. Clearly this presents many impracticalities; if travelling past the region the user would still be without a functioning receiver for many hundreds of kilometres after this exact point and a similar amount before as they were approaching the interference. There is also a high chance that if a user is located close to this jammer it is because they have need to be there and perhaps require the use of a GPS receiver. This is particularly relevant in a military application as an apparently intentional jammer, as used in this example, is far more likely to occur. In the case of a military operation they cannot simply get up and move several hundred kilometres until their equipment begins to work again; there will be an important objective located within the jammer’s influence. Therefore, more advanced techniques are required.

Adaptive Array Solution

Adaptive array techniques are too complex to cover their exact processes within the scope of this report. Therefore this section will attempt to show how they can be beneficial to the situation by dealing solely with their actual effect: that being the gain pattern of the GPS receiver. In this process the receiver gain can no longer simply be assumed as unity in every direction, it must now be split into two elements. These are the gain in the direction of the satellite and the gain in the direction of the jammer. From equation (6) it is know that the new required . Expanding equation (4) to contain parameters for the antenna gain in the relevant direction and rearranging for gives:

(8)

This is significant as it implies the required can be achieved without altering distance by manipulation of the gain parameters. Now by using new with the original and and rearranging equation (8) the required difference in gain can be found:

(9)

This seems reasonable until it is considered in linear terms where . This is quite a high margin of difference. In an ideal adaptive array system a null gain could be pointed towards the jammer, effectively rendering its effect on the receiver system to zero. Then the gain applied to the desired signal is not so important. Ultimately the array system will attempt to introduce the difference in effective gain to at least the value given in equation (9) by altering the directional gain pattern of the receiver as appropriate. To provide an example, should the receiver adapt its beam pattern such that there is a reasonably low linear gain of 0.05 towards the jammer, it will require a gain in the satellite direction of:

(10)

This is not an unreasonable value, but, it highlights the benefit directing as close as possible to a null gain towards the jamming source. Although the method presented is a very simplified view of a beamforming process it shows a far higher level of practicality in allowing the receiver to function around a moderate interference source. Barring the presence of any counter-countermeasures it should enable the operation of a typical GPS receiver in this overly simplified environment whether the user was a hiker utilising GPS for recreation or as part of a navigation system within sophisticated military equipment.

Section 4: Testing the System for verification of meeting the required EMC standards

Any product must meet the relevant standards in electromagnetic emissions and immunity. For the situation presented in sections 2 and 3 the most important aspect of a GPS receiver's EMC compliance is the immunity aspect. For this reason discussion will focus on immunity standards. This section will aim to detail the more important types of testing that the GPS system would be exhibited to and will then discuss the relevant EMC standards that must be adhered to.

Types of Tests Required

There are four three elements of RF immunity to test for conventional telecommunication equipment: radiated RF immunity, conducted RF immunity and transient immunity [5]. In the context of the example from the previous sections, it is the radiated RF immunity that is of prime importance in the presence of a jammer.

Radiated Immunity Testing

This test assesses the performance of the receiver when subjected to an interfering RF signal. It is typically performed in an ‘anechoic chamber’ which should nullify any external noise and signal reflection. This creates a simulation environment similar to the example, where signals representing the jammer and the GPS satellite can be used. A slight complication for testing the discussed array antenna solution is the need for two signals in the test chamber. Both the jammer and the GPS signal need to be simulated to allow for testing the receiver’s ability to maintain lock on the GPS signal in the presence of the jamming signal. A genuine GPS signal could not be used

The main equipment required, in addition to the receiver under test:

Two signal sources to represent GPS and jamming signal, must be adjustable in terms of relative location and signal strength

Equipment to monitor operation of the receiver

A power supply for the receiver set-up to minimise effect on testing

Figure : Very basic diagram showing the set-up for a radiated immunity test of the GPS receiverThe signal sources will not be transmitting at the same strengths due to their proximity in the test environment. The aim is to adjust their power to reflect all the operating conditions at the receiver that are required for complying with specifications. Typically, the equipment under testing will be rotated or moved relative to the RF signal. However, in this case, it is important that the signal sources are moved. The receiver must be tested for a wide variety of operating conditions in terms of the relative position of GPS and jammer signals to each other. Simply rotating the receiver would only test the directivity; it must be tested instead for as many different locations of signal sources up to the maximum jamming signal desired for testing. The level of jamming to be tested will depend on specifications; military users may require very high ability to resist strong jamming signals [6].anechoic chamber.png

Other Immunity Tests

Conducted interference is a measure of disturbance, usually through the power supply, from a wide variety of natural electromagnetic disturbances typically acting on power lines that supply the mains power. Mobile GPS receivers are usually not connected to mains supplies, especially not during operation so this interference usually does not have too much effect. Where it might come into effect is when a GPS receiver is part of a more permanent device that is hypothetically connected to the mains or some other supply.

Transient immunity tests take a number of forms. Electrostatic discharge (ESD) tests assess the ability of the equipment to withstand very high strengths in a single very short burst. Electrical fast transient (EFT) tests gauge the ability of the equipment to deal with repetitive pulses of reasonable strength. Finally, surge testing assesses the ability of the equipment to continue operating after being subjected to a build up of high energy over a longer time period relative to the other transient forms [6].

As is often the case, militaries tend to have different standards for equipment compared to civil applications.

Relevant EMC Standards

In the EU the two directives relevant to telecommunication equipment, such as a GPS receiver, are the 1995/5/EC R&TTE directive and the EN 55 020. The European directive mentioned regards immunity to RF signals, which has been considered to be more important in this report for a navigation system context. The corresponding European directive for unwanted RF transmission is the EN 55 022. The R&TTE directive applies to any telecommunications equipment intending to connect to telecommunications networks, barring some exceptions. Not all receive only devices are covered by the directive [7], however, as GPS receivers are not for sound or TV reception they are within the scope of the 1995/5/EC. The directive is less concerned about whether the receiver serves its function, more that it is sufficiently immune from interfering signals and does not unreasonably interfere with other devices. Some of the important objectives the receiver and any electrical component must satisfy are [8]:

Not posing a health and safety risk to users

Exhibit sufficiently good characteristics in terms of RF emission and immunity

Other general criteria that must be met for equipment under the ‘telecommunications’ category, that are of particular relevance to GPS receivers [8]:

Reasonably remains within the relevant allotted frequency spectrum

Does not cause unacceptable degradation in performance of the relevant network

The directive also contains a slew of other requirements that must be met. Many are quite general, such as the information on required documentation of proof of compliance and details on the appropriate tests for the equipment being tested. Proof of compliance also involves providing users of the equipment with all the necessary information for operating the device safely, where it can be used and knowledge of applicable network. The already mentioned directives only apply to the UK and/or the EU; however, they will be very similar to the equivalent directive of other locales. An example would be the American FCC part 15 (47 CFR 15). Additionally, as with testing, military organisations often have slightly different criteria for their EMC standards. These may also very similar to civil applications, but, will be amended with requirements applicable only to military applications. The American military has the MIL-STD-188 document detailing its standards for telecommunications equipment.