APPLICATION NOTE Fundamental GNSS

Transcription

1 APPLICATION NOTE Fundamental GNSS Receiver Characterisation

2 Spirent Communications PLC Paignton, Devon, TQ4 7QR, England Web: Tel: Fax: Copyright 2011 Spirent. All Rights Reserved. All of the company names and/or brand names and/or product names referred to in this document, in particular, the name Spirent and its logo device, are either registered trademarks or trademarks of Spirent plc and its subsidiaries, pending registration in accordance with relevant national laws. All other registered trademarks or trademarks are the property of their respective owners. The information contained in this document is subject to change without notice and does not represent a commitment on the part of Spirent. The information in this document is believed to be accurate and reliable; however, Spirent assumes no responsibility or liability for any errors or inaccuracies that may appear in the document. Page 2

3 Contents Audience 4 Introduction 4 RF Simulation 5 Typical GPS Simulators 5 Fundamental Receiver Performance Parameters 5 Cold Start Time To First Fix 6 Simulator test 6 Warm Start TTFF 7 Hot Start TTFF 7 Acquisition sensitivity 8 Tracking sensitivity 8 Reacquisition Time 9 Static Navigation Accuracy 10 Dynamic Navigation Accuracy 12 Radio Frequency Interference 13 Conclusions 15 Referenced Documents 15 Glossary of Terms 16 Page 3

4 Audience This Application Note is for designers, developers, integrators and testers of GNSS receivers or systems, who need to ensure their products will perform in the intended environment. Spirent recommends you have a basic understanding of satellite navigation principles and awareness of RF simulation as a test method is desirable. Introduction There is a steady growth in the use of GNSS in new and existing markets. Consequently, there is an increasing reliance on GNSS technology. With this in mind, it is important for designers, manufacturers and consumers of these products to understand what to expect from such systems. This includes formulating an understanding of the limitations and problems of GNSS technologies. This Application Note discusses some of the fundamental receiver performance parameters applicable to GNSS systems. Complementary to this, it demonstrates how Spirent s range of GNSS Test Solutions enable you to create and run controlled and repeatable simulations and benchmark your receiver s performance when subjected to these errors. It demonstrates that a GNSS RF Simulator is able to generate the conditions required for performing suitable tests. The application determines the test criteria, and the importance of each criteria may vary significantly from one application to another. For example, short TTFF performance may be vital in automotive applications, but not so important for static position surveying. Re-acquisition is probably not a primary consideration for marine applications, where little physical external signal obscuration exists, but is important in automotive applications where tunnels and bridges frequently block signals. Page 4

5 RF Simulation An RF Constellation Simulator reproduces the environment of a GNSS receiver on a dynamic platform by modelling vehicle and satellite motion, signal characteristics, atmospheric and other effects, causing the receiver to actually navigate according to the parameters of the test scenario. By its very nature, simulation is a representation of the real world. Simulation cannot reproduce the full richness of real world conditions. A common misconception is the need to exactly replicate real world conditions for a GNSS test to be valid. However, application of representative effects via simulation is proven (over some 25 years of testing) to exercise receivers and adequately identify their limitations allowing for design centring and optimisation. More importantly, it gives complete repeatability, control and exact knowledge down to bit level of the signal stimulating the receiver. Typical GPS Simulators All the tests discussed in this Application Note can be performed using any of Spirent s multi-channel simulators. SimGEN is the control and scenario definition software for GSS6700 and GSS8000 series simulators. SimPLEX is the scenario replay and control software for the STR4500 simulator. This is not possible when using real GNSS signals for test purposes. We should look upon simulator testing as representing the real world, rather than replicating it. Spirent simulators include statistical models enabling simulation of richer multipath environments, but consideration of these is outside the scope of this document. Figure 1 shows the concept of simulation (using a GSS6700 simulator). Figure 1: RF Simulation Flow For further information on Spirent s range of Simulators, please contact your local Spirent representative, or visit com/positioning, or and click the Satellite Navigation link. Fundamental Receiver Performance Parameters This Application Note focuses on testing the key parameters to determine a GNSS receiver s general performance. Unless otherwise stated, GPS L1 C/A code signals are implied. Page 5

6 Cold Start Time To First Fix TTFF is a measure of how quickly a receiver performs the signal search process. The search process, or signal acquisition, has two dimensions. The C/A code dimension associated with the replica PRN code, and the Doppler dimension associated with the carrier. When performing the search process a coldstarting receiver could have a code uncertainty of up to 1023 chips (the total number of code phase states for the GPS C/A code), and approximately +/-11 khz for the Doppler uncertainty. Some receivers use a serial search process, others a parallel (multi-correlator) process. Faster designs use matched filter or FFT techniques. The newest techniques use a combined replica of several codes instead of separate ones. A Cold Start TTFF is defined (in Reference 1) as the time between application of power to a receiver and it obtaining the first valid navigational data point, when the following criteria are met:- Time unknown Current almanac and ephemeris unknown Position unknown Simulator test Using a simulator to perform a Cold Start TTFF test is very simple. First simulate a static vehicle position with satellite power levels set so the power into the receiver s antenna is -120 dbm. With the scenario running (and the receiver connected to the simulator), power is applied to the receiver. Once the receiver has obtained its first fix, stop the scenario rewind it (if applicable) and advance the scenario time by at least 8 hours to ensure the simulated constellation geometry of visible satellites has changed. Alternatively, change the simulated location by several thousand km achieves the same effect. Clear the receiver of all navigational data and time information and remove the power. Re-run the modified scenario and re-apply power to the receiver. Repeat this process the required number of times, and average the TTFF results. If you are not certain that the receiver s memory has been completely erased, you can select very different locations and/or times (years apart) for the scenario. In this way, even if the receiver retained some prior navigation information, it would not be of use. Due to the stochastic nature of the process several (Reference 1 suggests 20) TTFF measurements are taken with different satellite geometries and then averaged. Page 6

7 Warm Start TTFF A Warm Start TTFF is defined in Reference 1 as the time between application of power to a receiver and it obtaining the first valid navigational data point when the following criteria are met:- Time is known Almanac is known No ephemeris (or the data is more than 4 hours old) Position within 100km of last fix Simulator test For a simulator test, you can use the same scenario as for the Cold Start TTFF, as the criteria (such as clearing the ephemeris, but not the almanac) are all set using the receiver. If it is not possible to clear only the ephemeris, the receiver must first be allowed to collect the almanac from the full navigation message (this takes approximately 12.5 minutes). Advance the scenario time by 4 hours (to age the ephemeris), and set the receiver time to match. Hot Start TTFF A Hot Start TTFF is defined in Reference 1 as the time between application of power to a receiver and it obtaining the first valid navigational data point when the following criteria are met:- Time is known Almanac is known Ephemeris is known Position within 100km of last fix Simulator test As with Warm Start, you can use the same scenario. There is no clearing of data from the receiver memory and no need to stop the scenario in order to alter any parameters. Power cycle the receiver for each TTFF test. As with Warm start, allow the receiver to collect the full navigation message (Approximately 12.5 minutes). Page 7

8 Acquisition sensitivity Acquisition sensitivity is the minimum received power level at which a First Fix can occur. The sub-sets of this are separate measurements for each of the cold, warm and hot start-up conditions. Simulator test For a simulator test, you can use a simple static scenario. The simulator software allows you to control the power level of the simulated signal in various ways, to a high degree of resolution, and over a wide dynamic range. Power control can be in real-time while the scenario is running, or using a pre-scripted set of commands. Real-time control can be applied using the SimGEN GUI or using remote commands (if the simulator is being controlled by a remote system). It is possible to control the power independently on individual satellites, or on all satellites, and level can be displayed as absolute power, or relative to a reference. The resolution of power control (for the GSS6560 simulator) is very fine, being 0.1dB over the range -130 dbm (+15dB, -20dB). This allows accurate determination of a receiver s acquisition sensitivity. As with TTFF, you should run several tests with different satellite geometries (DOP) and average the results. Tracking sensitivity Tracking sensitivity is the minimum power level at which a receiver can continue to maintain lock. The tracking threshold is closely related to measurement errors due to error sources in the receiver s PLL tracking loops. Phase error, dynamic stress error and thermal noise are the dominant sources of error. Minimising these parameters will enable the receiver to continue to track signals at a much lower power. In all cases, the tracking threshold should be lower than the acquisition sensitivity. Simulator test For a simulator test, you simply lower the power on all satellite channels simultaneously until the receiver loses lock. This should be repeated for different satellite combinations and geometries, using the techniques described in section 6.1 Page 8

9 Reacquisition Time Reacquisition time is the time necessary for a receiver to regain its first valid navigational data point after total loss of all received signals. Fast reacquisition time is important for in-vehicle navigation systems. Consider a car emerging from a tunnel, in which it has lost all satellite signals. Immediately after the tunnel is a junction at which the driver must make an exit. The navigation system needs to be navigating again quickly in order for it to give the correct Exit Now instruction. Simulator test For a simulator re-acquisition time test, power must be reduced on all satellites by a minimum of 60 db. The best way to achieve this is to specify off for each satellite. SimPLEX and SimGEN allow you to turn off all satellites while the scenario is running. With SimGEN you can also create a pre-defined User Actions file which has a time-ordered list of commands. One such command is Power on/off. With SimPLEX, you can record real-time actions and replay them from a file. Set the duration for which all satellites are off so that the receiver loses complete lock to ensure a valid reacquisition. Page 9

10 Static Navigation Accuracy Static Navigation Accuracy is the accuracy to which a receiver can determine its position with respect to a known location. It can be split into three categories:- Predictable - The accuracy of a receiver s position solution with respect to the charted solution. Both the position solution and the chart must be based upon the same geodetic datum. Repeatable - The accuracy with which a user can return to a position whose coordinates have been measured at a previous time with the same receiver. Relative - The accuracy with which a user can measure position relative to that of another user of the same receiver at the same time. It is possible to perform an estimate of position error (EPE) for a receiver given certain conditions. The following formula applies: EPE (2drms) = 2 * HDOP * SQRT [URE2 + UEE2] (2) HDOP, GDOP, PDOP and VDOP are determined by the geometry of the current satellites visible above the receiver s mask angle with respect to user receiver s antenna. DOPs can be degraded (made larger) by signal obstruction due to terrain, foliage, building, vehicle structure, and so on. URE is an estimate of Signals in Space errors, such as ephemeris data, satellite clocks, ionospheric delay and tropospheric delay. These errors can be greatly reduced by differential and multiple frequency techniques. Differential correction sources include user provided reference stations, community base stations, governmental beacon transmissions, FM sub-carrier transmissions and geosynchronous satellite transmissions. EPE (1-sigma) = HDOP * UERE (1-sigma) (1) Multiplying the HDOP * UERE * 2 gives EPE (2drms) as given in equation (2) and is commonly taken as the 95% limit for the magnitude of the horizontal error. The probability of horizontal error is within an ellipse of radius 2drms ranges between 0.95 and 0.98 depending on the ratio of the ellipse semi-axes. Page 10

11 UEE includes receiver noise, multipath, antenna orientation, EMI/RFI. Receiver and antenna design can greatly reduce UEE error sources. Position error can range from tens of metres (recreational applications) to a few millimetres (survey applications) depending on equipment, signals and usage. Professional mapping and survey equipment often includes user-settable minimum thresholds for parameters such as SNR, mask angle, DOP, number of SVs used, etc. UERE is User Equivalent Range Error, and is computed (for L1 C/A) as shown in Table 1 Error source Bias Random Total DGPS Ephemeris Data Satellite Clock Ionosphere Troposphere Multipath Receiver measurement UERE (1-sigma RMS) Filtered UERE, RMS Vertical 1-sigma errors (VDOP = 2.5) Horizontal m1-sigma errors (HDOP=2.0) Table 1 L1 C/A User Equivalent Range Error (UERE) Simulator test For a simulator test, you can eliminate UERE and certain UEE errors by disabling the effects of the atmosphere, and deliberately not including any ephemeris, clock errors, multipath or EMI/ RFI in the simulation. This is not possible using real satellite signals. For this reason, it is not possible to physically measure a receiver s absolute navigation accuracy with any method other than using a GNSS simulator. To measure Static Navigation Accuracy you define a scenario, with a static position ( 0 degrees Latitude, 0 degrees Longitude and 0 metres height is always a good location as it is easy to see any divergence that will be highlighted in non-zero digits). The scenario should run for at least 24 hours (as stated in Reference 1) and the power level must not exceed -160 dbw (the Received Minimum RF Signal Strength as defined in Reference 3). Some receivers have a mode that fixes the position if the detected velocity falls below a certain threshold. This so-called static mode is useful for in-car receivers, to prevent display jitter when the vehicle is stationary. However, when performing a static position accuracy test, this mode must be disabled to prevent a falsely good result. Reference 4 gives a detailed explanation of how to measure accuracy performance. Page 11

12 Dynamic Navigation Accuracy Dynamic Navigation Accuracy is the same as Static Navigation Accuracy, except the receiver is undergoing motion in either or all of the three axes of movement x, y, z Simulator test For a simulator test, define a scenario with a simple trajectory defined using SimGEN s internal vehicle model commands. To ensure data de-correlation, run the test over a minimum of three time periods of duration no less than one hour each. Each period must contain at least 1000 valid navigation data points. The three tests should be equally spaced during a 24-hour period. For application specific dynamic accuracy performance, a simulator can perform almost any dynamic trajectory profile you desire. The high dynamic performance of Spirent s simulators enables testing of receivers for any application where dynamic motion is required, from emergency beacons drifting in the sea, to military ordnance shells spinning at several hundred revolutions per second. Page 12

13 Radio Frequency Interference RFI is defined as any unwanted signal that causes degradation in performance, partial loss, or full loss of navigation. Such signals are often referred to as Jamming signals. Jamming, being a widely used term describing a signal that prevents the wanted radio communication from being received. Jamming can either be intentional or un-intentional. Intentional Jamming is a result of a deliberate attempt to deny a GNSS receiver use of the GNSS system. An example may be found within the theatre of war, where an enemy is trying to attack the other s capability. An advanced form of intentional jamming, called Spoofing involves re-transmission of GNSS-like signals that make a receiver think it is navigating in a way which, in reality it is not. To emphasise the vulnerability of a receiver to jamming, it is estimated that an airborne 1-watt CW jammer signal on the L1 frequency ( GHz) can deny GPS tracking to an already locked receiver at up to 10km away, and prevent an un-locked receiver acquiring lock at up to 85km away. Reference 5 discusses this in detail. Simulator test For a simulator test, there are several options. Simple non-coherent jamming is easily achieved with Spirent s GSS6700 and GSS8000 series of simulators, as they have an RF Jammer input port that allows you to inject an external RF signal into the main GNSS signal path in a controlled way (using a directional coupler within the simulator). Figure 2 shows this concept. Un-intentional jamming results from unknown interference. It is less specific, and can come from a wide variety of sources. Examples include: harmonics from commercial TV broadcast stations or pulsed interference from Aircraft VOR/DME navigation beacons. The main vulnerability of GNSS signals with respect to both types of jamming is that they are extremely low power when arriving at the receiver s antenna (typically -120 dbm). The signals are, in fact some 20 db below the level of background noise, necessitating the use of signal processing gain (correlators) in the receiver to extract the signal from the noise. Figure 2 Typical Interference Simulation Set-up Page 13

14 A signal injected from a third-party signal generator would not be coherent with the simulators GNSS signal. However, the Spirent GSS7765 Interference Simulation System option is available for GSS6700 and GSS8000 series simulators. This allows specific signal generators to be controlled using SimGEN in either a coherent or non-coherent way with a variety of signal modulation types and with modelled power, which simulates the relative distance effects of the interference source with respect to the simulated GNSS position (for example, a jamming source flying over a receiver s location). For more information regarding the GSS7765 interference simulation option, see Reference 6. Page 14

15 Conclusions This Application Note describes the fundamental performance parameters that apply to all GNSS receivers. These parameters must be optimised at an early stage in a receiver design. Optimisation requires suitable testing. This Application Note shows that a GNSS simulator allows you to develop tests that optimise receiver design. SimGEN offers very high resolution control of signals and bit-level manipulation of data, reproducing the most complex error effects while its easy-to-use interface allows straightforward tests to be carried out with the same powerful modelling taking place in the background. It shows that there are no practical alternatives to simulator testing in situations where the receiver must be tested while undergoing high-dynamic motion. Referenced Documents 1. ION STD 101 Recommended Test Procedures For GPS Receivers, [I.O.N] 2. IS-GPS-200D Navstar GPS Space Segment/Navigation User Interface Specification, Revision D, 7th Dec ICD-GPS-200 Navstar GPS Space Segment/Navigation User Interface Control Document, Revision IRN- 200C-004, 12th April GPS SPS Signal Specification, Annex C, Means of measuring GPS performance 5. Vulnerability assessment of the transportation infrastructure replying on the Global Positioning System [John A. Volpe, NTSC, Aug 29th, 2001] 6. MS3055 GSS7765 Interference Simulation System Product Specification. 7. DGP00902AAA Latest Issue SimPLEX Standard Scenario Descriptions Page 15

16 Glossary of Terms C/A Code Chip DOP EMI EPE FFT GNSS PPS PRN RF RFI Scenario SNR SPS SV TTFF UEE UERE Valid Navigational Data Point The GPS SPS Coarse Acquisition ranging code The time between transitions in the C/A code (not referred to as a bit because the code does not carry information) Dilution of Precision (GDOP = Geometric DOP, HDOP = Horizontal DOP, VDOP = Vertical DOP, PDOP = Position DOP, TDOP = Time DOP Electro-Magnetic Interference Estimated Position Error Fast Fourier Transform Global Navigation Satellite System For example: GPS, Glonass & Galileo GPS Precise Positioning Service, employing Pseudo-Random Number code. A code which appears completely random when a portion of it is viewed, but which in reality repeats (for the GPS C/A code the repetition is 1mS, the GPS P-code takes 7 days to repeat) Radio Frequency Radio Frequency Interference In this context, a GNSS simulation running on either SimGEN or SimPLEX simulator control software. Signal-to-Noise Ratio GPS Standard Positioning Service, employing the C/A-code GPS Satellite Vehicle Time To First Fix User Estimated Error User Estimated Range Error A single, time-tagged estimate fix of Latitude, Longitude and Altitude referenced to the WGS-84 (for GPS) coordinate system, computed while in 3D navigation mode VOR/DME VHF Omni-directional Range/Distance Measuring Equipment Page 16

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18 CONTACT US DAN003 ISSUE 1-03 Got a smartphone? If you have a smartphone download a QR Code reader and then point your phone camera at the QR Code to read the graphic. We are adding new content to our website on a regular basis. Bookmark this link: Visit the Spirent GNSS blog, there are currently over 90 posts with 2 to 3 new posts added each week. Catch up on what s new. Need more information? gnss-solutions@spirent.com Why not share this document? Facebook LinkedIn Twitter Technorati Google Buzz Digg Delicious Reddit Stumbleupon Spirent Communications globalsales@spirent.com Spirent Federal Systems info@spirentfederal.com Rev. 1.0 Jul 2011