A new Navigation System for Indoor Positioning (InLite)

Transcription

1 A new Navigation System for Indoor Positioning (InLite) Dr. Andreas Schmitz-Peiffer *, Dr. Andre Nuckelt **, Maik Middendorf **, and Michael Burazanis ** * EADS Astrium GmbH, Satellites, Dept. ANE 11, Navigation System Engineering. andreas.schmitz.peiffer@astrium.eads.net ** EADS Astrium GmbH, Satellites, Dept. ANB 22, Navigation Applications Abstract A new Indoor Navigation System (InLite) has been developed by Astrium Satellites GmbH which allows positioning of users inside large multi-level buildings with an accuracy of two meters without any aiding tools like inertial measurement units or other infrastructure inside the building. The Architecture is presented, and measurement results are discussed. Transmitter FPGA Memory Fast Fourier Transform X(w) Y(w)/X(w) Digital to Analog Converter Transmitter Multicarrier Replica Channel Impulse Response Estimation Transmitter Baseband 0-40MHz Receiver Extraction of Line of Sight Delay 420MHz Indoor Channel Signalpower 0dBm MHz Multipath Channel H(t,t) Keywords Indoor Navigation, TDoA, Synchronization Y(w) Fast Fourier Transform Noise I. INTRODUCTION The possibility to navigate users in case of emergency inside a building is of high interest for fire brigade, ambulance, police or military operations. However GPS signals are too low to be used for indoor navigation. Hence many indoor navigation systems proposed today use pre-installed infrastructure inside a building [RD.1]. However this does not support emergency services. There are only very few systems which do not rely on inside infrastructure [RD.2]. In comparison to the Worchester Polytechnic Institute WPI system [RD.2] in our InLite system the users in the building are passive users, they receive navigations signals. The InLite system consists of a set of 6 to 8 transmit stations positioned around the building, user terminals inside the building and a monitoring and control unit for steering the transmit stations plus for broadcasting information to the users. The transmit stations broadcast multi-carrier navigation signals with 40 MHz bandwidth from 420 to 460 MHz. The user inside the building receives the signals and calculates his position. The InLite signal design allows minimizing multipath effects so that the positioning accuracy even in massive multi-level houses made of concrete, steel and metal-shielded windows reaches 2 meters required by [RD.3]. The InLite system has been successfully tested and presented to public at places in Germany and the UK. System architecture and measurement examples are presented and an outlook for future activities is given. II. INLITE ARCHITECTURE As displayed in Fig. 1 the main functions of the Astrium indoor navigation system are explained. On the transmitter side, the navigation signal is digitally generated in a FPGA Board. In order to achieve an analogue signal, the digital samples must be converted by an ADC to the analogue baseband signal domain. Memory Samples Baseband 0-40MHz Analog to Digital Converter Lowpass 50MHz 420MHz MHz Figure 1: Functional Architecture of InLite The baseband signal is up converted to a higher frequency band, in our case 420 MHz, the carrier frequency passes the Indoor Radio Channel, where the navigation signals will be distorted by heavy multipath and external noise, and other interferers can disturb the transmitted navigations signal. On the receiver side, the signal is received by a wearable broadband antenna. Then it is necessary to band pass filter the incoming signal in order to filter out the out-of-band interferers. After filtering the signal will be down converted to the baseband, in our case 0-40 MHz. It is required to low pass the down converted baseband signal to suppress unwanted mixing products. The low passed navigation signal must be A/D converted, in order process it on a computer. For one processing step, we take samples and calculate the Fast Fourier Transform (FFT) of these samples. In the memory of the processing computer, we have stored an undistorted signal replica of the navigation signal. With this replica and the measured signal, we are able to compute the Channel Impulse Response (CIR) of the indoor channel for a specific transmitter-receiver combination at a specific time. After calculation of the CIR in the frequency domain we apply a super resolution matrix pencil algorithm to all 200 subcarrier peaks (for each transmitter) in order to extract the Line of Sight signal delay, if existent. In order to achieve the required positioning performance it is required, that all involved transmitter stations are synchronized to each other. The synchronization accuracy needs to be < 1 nanosecond /10/$ IEEE

2 III. SYSTEM MODULES InLite consists of the following subsystems: Master Control Station (MCS), Transmitter (TC) and User Receiver (UR). The MCS is based on a Laptop. It controls the TC s, broadcasts necessary information to the users (TC coordinates, reference barometer measurements ) and receives user information for further analyses and monitoring, and finally displays data for the operator. The TC consists of an embedded PC, a navigation signal generator, a synchronization unit, a WLAN Data Link unit, Power Supply Units (PSU) and antennas for signal transmission and reception [RD.4]. The ranging signal generator is fully initialized after powering up the system. The synchronization unit is ready some minutes after switch on. The TC embedded PC receives the status data from the synchronization unit, collects GPS raw data for later baseline calculation between all transmitter stations and controls the WLAN data link modem, which is responsible for the communication between MCS and all transmitters. The configured TC system is capable to receive command data from the MCS via the data link. The UR consists of a powerful embedded PC, a ranging signal receiver, a WLAN data link modem and a barometer. The ranging signal receiver itself consists of a RF-Frontend, which includes a 50MHz band pass filter, a down-conversion stage and an anti-alias low pass filter. The down-converted baseband will then be sampled by an analogue/digital converter. The embedded PC collects the samples via an USB bus. The UR barometer is connected to the PC and provides height information of the user. The WLAN data link modem is used to send the XYZcoordinates to the MCS. IV. SUBSYSTEM SPECIFICATION The MCS is responsible for the remote control of the GPS receiver in each TC. By pushing a button the GPS receivers will be activated via a broadcast command to start with the recording of GPS raw data over a time period of 15 minutes. After the expiration, the collected GPS raw files will be automatically transferred to the MCS. Once the raw data are available on the MCS hard disk, the baselines between the transmit stations can be calculated. As a result we achieve very accurate relative TC coordinates. The accurate absolute positions of each TC, related to a WGS84 coordinate system is obtained by using SAPOS services, otherwise a stand alone solution can have an uncertainty of about 10 meters. A further task of the MCS is to provide a communication link with the UR. As soon as the TC coordinates are calculated, they are transferred to the UR software. This process starts automatically, as soon as the UR is connected to the server (MCS). The height calculation is enhanced via differential calculations with respect to the outdoor reference barometer mounted in one TC #4. The position calculation in the UR can be started and stopped via remote desktop connection from the MCS. Once the UR has calculated the first positioning data, the data stream will be sent via WLAN to the MCS and will be visualized in a 3D model of the investigated building. The functional block diagram of a TC unit is shown in Fig.2. The task of the Synchronization Unit (SU), [RD.5] is to ensure that all engaged transmit stations are synchronized below 1 nanosecond over the whole operation time (Fig. 3, left box). TC #4 needs a reference oscillator which provides a high accurate 150MHz reference signal for the master synchronization. The synchronization triggered by this oscillator leads to an overall synchronization accuracy of 1 nsec for eight signal generators. 1 nsec time deviation corresponds to 30 cm positioning error in the transmit station coordinate. The distribution of the reference synchronization signal to both branches of the synchronization chain starts at the master synchronization unit of TC #4. Each TC receives the signal from the processor, adjusts his own oscillator and transmits the signal to the next neighbor. The main synchronization interface between the navigation signal generator and the synchronization unit is a Low Voltage Differential Signaling (LVDS) two wire Figure 2: InLite TC Architecture interface: one for the 10MHz reference clock and the other one for the 0.5Hz signal which is required for a permanent resynchronization The Navigation Signal Generator (NSG) is not a commercial-of-the-shelf product. It is specified by EADS Astrium GmbH and built by Andimedes (Fig.3, right box). The NSG has the capability to transmit signals in the 420MHz frequency band with various bandwidths up to 80MHz. The transmitted signal is a right hand SSB signal. We currently use 40MHz bandwidth in a frequency range from 420 to 460 MHz

3 Figure 3: Broadband InLite Navigation Signal Generator (right) and Synchronization Unit (left) Signal Characteristics V. SIGNAL DESCRIPTION The ranging signal is an appropriate multi carrier signal optimized for FFT. The frequency shift between the transmitters is obtained by: Sampling Rate 200 MHz = = Hz No. of Samples It equates to the frequency resolution. The frequency spacing is an integer multiple of the frequency resolution: Hz = Hz The signals characteristics are displayed in Table 1. The signal specifications of the TC are shown in Table 2. Table 1: Signal characteristics Table 2: Signal specification per TC Each signal consists of 200 subcarriers. This signal design allows eliminating multi-carrier signals. An appropriate algorithm which has been applied is the matrix pencil method. Fig. 4 shows the structure of the transmitted ranging signal over the whole bandwidth of 40 MHz consisting of 8*200 subcarriers. 1 PK CLRWR Ref 0 dbm Att 30 db * RBW 3 khz AQT 2.7 s Marker 1 [T1 ] dbm MHz * A Signal Bandwidth 40 MHz Single Side Band Signal Power in 40 MHz 10dBm bandwidth Carrier Frequency 420 MHz Number of subcarrier Center 420 MHz 10 MHz/ Date: 24.NOV :02:47 Figure 4: Transmitted InLite ranging signal Span 100 MHz Phase variation on each subcarrier Pulse width without impressed phase variation Pulse Repetition rate Unambiguousness area Normal distribution 25 ns 5.12 us 1536 Meter Ref 0 dbm PK CLRWR -20 Att 30 db * RBW 30 Hz AQT 11 s Marker 1 [T1 ] dbm MHz * A Signal Characteristics Number of carriers Impressed phase distribution on each subcarrier Transmitter #1 Normal distribution Hz Center MHz 200 khz/ Span 2 MHz Transmitter #2 Transmitter #3 Transmitter #7 Transmitter # Hz Hz Hz Hz Hz Hz Hz Hz Hz Date: 24.NOV :17:28 Figure 5: InLite ranging signals transmitted from four TCs VI. SIGNAL TRANSMISSION The TC equipment (SG, GNSS Rx, Synchronization Unit, communication unit, and an embedded PC) are mounted in a TC box. An up to 4 meters expendable telescope mast is fixed to each box as well as the WLAN antenna, the synchronization antenna, the navigation signal RF antenna and GNSS antenna. The set-up of the boxes can be performed quite easy, however the boxes are heavy and need to be carried by two persons. The advantage of the massive box leads to a good wind robustness of the TC. Fig.6 shows the set-up of the TC segment (without - 3 -

4 GNSS antenna, which would be placed on the very top). Fig.7 shows the TC rear panel with cable connections. Fig.8. displays the modules and components in the 19" rack. The TC power supply is performed by using four LiPo accumulators with a voltage of 14.8V and 10 Ampere hours (Ah). This leads to availability of the system of about 8 hours before recharging is necessary. Figure 8: TC modules and components in the 19" rack. Figure 6: View of the InLite transmit station VII. USER RECEIVER The InLite test User segment receives the transmitted signal of all eight transmit stations. The core module is an embedded PC, connected to an AD Converter. A data link connection establishes an interface to the Master Control Station, in order to send the calculated three dimensional receiver coordinates. The AD Converter sends the data samples via an USB-Interface to the embedded PC. The Block diagram of the RF-Frontend is shown in Fig.9. First we have a band pass filter with 60MHz bandwidth, to suppress out of band interferers. After filtering, the signal must be amplified, with a low noise amplifier. The gain must be in an adequate range, to avoid an overdriving of the following mixer. The down converted signal must be band limited, to avoid aliasing products in the connected AD-Converter. The low pass filter has also the function of noise power reduction. The 10MHz reference input must be tied to the 10MHz local oscillator output, to avoid beat frequencies. The output samples (each batch has a size of samples) of the AD converter arrive via an USB bus in the embedded PC memory, for estimating the channel impulse response, and the line of sight delays of each involved transmitter. Fig. 10 shows the Astrium receiver. Figure 7: TC back view cable connections Fig. 8 shows the TC modules and components placed in the 19" Rack. Figure 9: Block Diagram of the User Receiver Architecture - 4 -

5 Figure 9: Overall InLite Architecture Figure 10: Front view of the Astrium user receiver The light weight wearable receiver antenna is carried on the human body in front of the user and on the back side of the backpack Receiver. The overall antenna diagram is quasi-omnidirectional and allows the user to walk around in the building. The following figure shows the user receiver and the master control station. IX. TEST CAMPAIGNS Several test campaigns were run in 2008 and 2009 in Germany and in the UK in order to test the indoor performance if the InLite system. We present the results from a test campaign in and around a multi-level massive not used building in Ottobrunn near Munich made of reinforced concrete with metal shielded windows. The other campaign was performed in Newport, Wales in and around a hotel in order to show the performance in a place where people live. The positioning performance is comparable for both sides. Fig. 11 shows the Ottobrunn test building. This building was selected to represent a realistic challenging mission environment. Figure 9: InLite Test Receiver (left) and MCS (right) VIII. OVERALL ARCHITECTURE Fig.10 displays the overall InLite Architecture. The three signal links are clearly visible. The transmission of broadband navigation signals ( MHz) starts from the eight TCs. The synchronization chain is initiated by TC 4 triggered by a master clock. WLAN communication between MCS, TCs and user receiver is started by the MCS. Fig.11: Test building with reinforced concert (60 cm thickness) and metal covered windows to protect the heat in summer. The following figures show the results. Fig.12: Track of the user receiver on the ground floor in the Ottobrunn building - 5 -

6 The performance analysis shows that the track of a person within the building has an accuracy of roughly 2 meters. The location of the person can be followed even when the person is moving upstairs. Successful measurements were made up to the 5th floor of the building. X. SUMMARY AND OUTLOOK The next steps in a follow-on programme will be Research and tests to analyse the impact of the near-far effect on the positioning performance of the user receiver Research and tests to investigate and separate the different error sources impaction the positioning performance. Research and tests to analyse the operating distance and performance of the Astrium indoor system for example with several buildings in between the TC setup. Investigations to miniaturize the user receiver. ACKNOWLEDGMENT This work was co-funded by the German Aerospace Center DLR, Germany. The team thanks Dr. Michael Heyl from DLR for supporting the work during the 3 year period. The Newport and Ottobrunn test campaigns were supported by EADS. Fig.13: Three-dimensional track of the user inside the Ottobrunn building. The blue pin points in the upper part of the figure show the locations of the transmit stations. REFERENCES [1] The Potential of UWB for Local Augmentations to GNSS Positioning, Barry Darby, Pierre Diederich, Ewan Frazer, Dave Harmer, Gethin Morgan-Owen, 20th AIAA International Communications Satellite Systems Conference May 12-15, 2002 [2] J. Duckworth at al, 2007: WPI Precision Personnel Locator System - Evaluation by First Responders, ION-GNSS Session D3, Indoor Positioning, Forth Worth, TX, p [3] InLite Mission Requirements Document IN-AST-DDD-0001, 17.April 2007 [4] ANDIMEDES System Specification for an InLite Transmit Station, 2008 [5] SYMEO Specification of the Synchronization Unit, Fig.14: Track of the user in the Newport Hotel during the UK test campaign in