GNSS Technologies. Introduction to GNSS technologies, Dr. Laura Ruotsalainen

Transcription

1 GNSS Technologies Introduction to GNSS technologies, Dr. Laura Ruotsalainen Finnish Geospatial Research Institute, National Land Survey / Aalto University, School of Engineering, Department of Real Estate, Planning and Geoinformatics

2 Content Introduction to GNSS (Global Navigation Satellite System) GNSS applications GNSS principle and system The Global Positioning System (GPS) Position determination Satellite-to-user geometry Error sources GNSS performance Next generation GNSS Shortcomings GNSS receiver design References and suggested additional material 2 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

3 Why do we need satellite-based positioning? With satellite-based positioning we can acquire Position, Velocity and Time (PVT) with 1) global coverage 2) very good accuracy 3) integrity major limitation is still the difficulty of indoor location (due to low signal power and severe multipath environments). Joint cellular and satellitebased positioning is also possible. Various applications, both civilian & military The current global market of applications and services of positioning systems is estimated to be more than 3 billion US dollars and it is expected to grow Inexpensive receivers 3 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

4 GNSS applications Applications can be summarized into 5 broad categories: Location = determining a basic position (e.g., emergency calls) Navigation = getting from one location to another (e.g., car navigation) Tracking = monitoring the movement of people and things (e.g., fleet management, workforce management, lost child/pet tracking) Mapping = creating maps of the world Timing = bringing precise timing to the world 4 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

5 GNSS applications (cont.) Personal navigation Aviation applications Automotive applications Marine applications Space applications Timing and frequency standard applications Agriculture, forestry, and natural resource exploration Geodesy and surveying Scientific applications 5 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

6 GNSS Market Segments Maritime; 0.30% Aviation; 1.00% Road; 46.20% LBS; 47% Rail; 0.10% Surveying; 4.10% Agriculture; 1.40% GSA: GNSS Market Report 6 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

7 How GNSS works?: travel distance Velocity x Time = Distance Radio waves travel at the speed of light m/s (i.e., around 3*10 8 m/s) If it took, for example, seconds to receive a signal transmitted by a satellite floating directly overhead, the travel distance of the received signal can be calculated using the above formula. Travel distance: m/s x s = m ~20086 km Precise position of the satellite at the time of signal transmission and travel time must be resolved! 7 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

8 How GNSS works?: 3D trilateration 1 Satellite 2 Satellites 3 Satellites 8 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

9 Structure of a GPS signal Source: European Space Agency, Navipedia 9 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

10 GNSS system architecture All GNSS systems are based on the same architecture (3-segment architecture): Space segment: satellites Control segment: monitoring, controlling and uploading stations => a heavy ground infrastructure required in order to deliver the right signals with the right parameters to the users. User segment: user community/gnss receivers The number of satellites and monitor stations differ according to the GNSS system (GPS, Glonass, Galileo, BeiDou,...) 10 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

11 The three segments of GNSS Ground antennas 11 GNSS Technologies

12 Tasks of space segment The space segment is formed by the satellites, also abbreviated by SV (Satellite Vehicle). The functions of a satellite are: It receives and stores data from the ground control segment. It maintains a very precise time. In order to achieve such a goal, each satellite usually carries several atomic clocks of two different technologies (e.g., cesium and rubidium), depending on the generation of the satellite. It transmits data to users through the use of several frequencies It controls both its altitude and position It may enable a wireless link between satellites 12 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

13 Tasks of control segment The main functions of the control segment are to: Monitor the satellites; activate spare satellites (if available) to maintain system availability; check the SV health Estimate the on-board clock state and define the corresponding parameters to be broadcast (with reference to the constellation s master time) Define the orbits of each satellite in order to predict the ephemeris data, together with the almanac; Ephemeris = accurate orbit and clock corrections for the satellites. Each satellite broadcast only its ephemeris data. In GPS, ephemeris is broadcast every 30 s. Almanac= coarse orbital parameters/information of the satellites (valid for up to several months) 13 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

14 Tasks of user segment The main functions of GNSS receivers are: Receive the data from the satellites belonging to one or several constellations (e.g. GPS; Galileo) on one or several frequencies. If several constellation => multi-system receivers. If several frequencies => multi-frequency receivers Acquire the signal from each satellite in the sky (acquisition = identification of satellite code and coarse estimation of time delays and Doppler shifts) Track the signal received from the satellites in the sky (tracking = fine estimation of time delay and Doppler shifts) Estimate the PVT solution (position, velocity, time estimation) 14 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

15 Navstar GPS - Basics Navstar GPS was devised by the US Department of Defense for various applications (initially for military use only, later on also for civilian applications), such as fleet management, navigation, etc. It has 3 segments: Satellite constellation (Space segment): Currently 32 satellites (originally 24), positioned in 6 Earth-centered orbital planes; they provide the ranging signals and navigation data messages to the user equipment. Ground control network (Ground segment): 1 Master Control Station (MCS), 3 uploading stations, and 11 monitor (surveillance) stations; this segment tracks and maintains the satellite constellation by monitoring satellite health and signal integrity, and maintaining satellite orbit configuration. User equipment (User equipment): It receives signals from the satellite constellations and computes user Position, Velocity and Time (PVT). 15 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

16 Navstar GPS Basics (cont.) GPS has made precise navigation affordable for the masses. As a result, over the past decade the number of military, commercial, and scientific applications that use accurate positioning and timing information has increased dramatically. Many of these applications aim at using GPS in environments in which it was not originally intended to be used. That is, GPS was primarily designed for outdoor applications with clear views of the sky. Nowadays, various application utilizing GPS operate in urban areas and even some indoors. 16 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

17 Navstar GPS Basics (cont.) Satellite navigation is based on radio signals transmitted by Earth-orbiting satellites and distance measurements between satellites and a user receiver A GPS receiver 1) measures the signal travel time from the satellite to the Earth, or 2) computes the number of full carrier cycles between a satellite and a receiver range measurements A receiver receives simultaneously information from multiple satellites through multiple channels When satellite locations are known, the user receiver location can be estimated based on the range measurements GPS: SATELLITES Carriers L1 ( MHZ), L2 ( MHz) & L5 ( MHz) Modulated on the carrier: pseudorandom signals satellite orbit information 17 USERS Ranges to satellites Position, velocity and time computation CONTROL NETWORK Control and ground stations

18 GPS constellation 18 GNSS Technologies

19 GPS control segment (1) 19 GNSS Technologies

20 GPS control segment (2) The monitor stations measure signals from the Satellite Vehicles (SVs) which are incorporated into orbital models for each satellite: precise orbital data (ephemeris) and SV clock corrections 20 GNSS Technologies

21 Introduction to GNSS positioning (1) X, Y, Z, D t 21 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

22 Introduction to GNSS positioning (2) Principle of satellite navigation 22 GNSS Technologies

23 Introduction to GNSS positioning (3) Code generated by satellite 1 ms 1023 chips Code generated by receiver ( replica ) Received code at time t r t r Δt shift The received code from the satellite is delayed by Δt with respect to the code generated in the receiver that replicates the satellite transmitted code. This delay is the signal time of flight from the satellite to the receiver. When Δt is known, the distance between the satellite and receiver is obtained, r = cδt. This is called the code range. 23 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

24 Introduction to GNSS positioning (4) Pseudorange R = c (Δt+ dt r dt s ) Satellite and receiver clocks are not synchronized or in GPS time Clock corrections are necessary to both clocks: Satellite clock correction dt s is obtained from the navigation data (sent with the GPS-signal) Receiver clock correction dt r is obtained/solved for in the receiver together with the user coordinates (x r, y r, z r ) -> (x r, y r, z r, dt r ) 24 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

25 Introduction to GNSS positioning (5) Code range measurement r = c Δt = c (t r t s ) Satellite location known from the navigation data (x s, y s, z s ) (x s, y s, z s ) t s (x r, y r, z r ) t r User location (x r, y r, z r ) User time difference from GPS time dt r c = speed of light 25 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

26 Introduction to GNSS positioning (6) Example on the difference between code and carrier phase measurements on GPS L1 Source: P. Misra, P. Enge, Global Positioning System; Signals, Measurements, and Performance, 2006, 569 s. 26 GNSS Technologies

27 s s s z y x 2 2 2,, r r s r s r s z z y y x x r s s s z y x 3 3 3,, r s r s r s z z y y x x r s s s z y x 1 1 1,, r s r s r s z z y y x x r User position(x r, y r, z r ) Finding (x r, y r, z r ) so that the three equations stand, when r and (x s, y s, z s ) known Position Position computation illustration: from ranges to position GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

28 Uncertainty in satellite-based positioning (1) e 3 User position (x r, y r, z r ) e = error Error space: Size of the error space i.e. the uncertainty of the position depends on 1) number of satellites and 2) user-to-satellite geometry 3) sizes of measurement errors e e 1 r 2 e 2 28 The more satellite measurements and the further away satellites are from each other, the more accurate the user result will be

29 Uncertainty in satellite-based positioning (2) PDOP (position dilution of precision) reveals the user-satellite geometry Typically: < 3 = optimal > 7 = poor geometry Poor geometry Good geometry 29

30 Uncertainty in satellite-based positioning (3) Good geometry: small error space Worse geometry: Larger error space with the same measurement error values 30 GNSS Technologies

31 Error sources Satellite measurements are noisy and erroneous since the signals attenuate on their way from the satellite to the receiver and bounce off e.g. the ground and buildings Satellite related errors: Satellite clock errors Ephemeris errors Atmospheric related errors: Ionospheric delays Tropospheric delays Receiver and its surrounding: Receiver noise Multipath 31 GNSS Technologies

32 GPS Error budget Standard error model - L1 C/A (sources: Samuel J. Wormley E. Kaplan and J. Hegarty: GPS Principles and Applications, 2 nd edition, 2006) Error source One-sigma error, m Ephemeris data Satellite clock Ionosphere Troposphere Multipath Receiver measurement Ionospheric effects are the main error source for line-of-sight signals 32 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

33 GNSS signal multipath Sometimes the signals are reflected from the objects near the receiver before they hit the receiver. This causes multipath reception - one of the major error sources in GNSS positioning White = true path Green = GPS positions Rarely 4 satellites acquired Red = Path with GPS 33 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

34 Error mitigation (1/2) Currently the largest and most persistently troublesome of GNSS errors are those caused by the unknown delays and other disturbances added by the Earth s atmosphere One way to reduce these errors is by using differential corrections (DGNSS) Reference station (known position) Compute Range corrections Range corrections Rover (mobile receiver) Range corrections applied 34 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

35 Error mitigation (2/2) The errors induced by the ionosphere can be significantly reduced when measurements on two different carrier frequencies are used, e.g. L1 and L2. This results in a significant improvement of the positioning accuracy. Also satellite-based augmentation systems (SBAS) through geostationary satellites to provide atmospheric corrections Wide Area Augmentation System (WAAS) in USA European Geostationary Navigation Overlay System (EGNOS) in Europe 35 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

36 Positioning performance metrics Accuracy: measure of the level of positioning error Integrity: measure of the trust that can be placed in the correctness of the information supplied by a navigation system Continuity: the probability that the specified system performance will be maintained for the duration of the operation, presuming that the system was available at the beginning of the operation Availability: fraction of time a navigation system is providing position fixes to the specified level of accuracy, integrity, and continuity 36 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

37 GNSS accuracy (1) Accuracies obtainable: 10 m 1 m Navigation; code measurement; one receiver DGPS; code measurement + base station 0.1 m RTK; phase observations + base station 0.01 m 0.001m 37 Static positioning; phase observations, network of base stations, post processing Permanent stations; time series Issues affecting GNSS accuracy: Receiver technology used Location and environment of the antenna Weather conditions

38 GNSS accuracy (2) GPS offers reliable positioning performance in open outdoor environments Positioning accuracy from even a few millimeters to tens of meters depending on the environment, weather, and technology used one or multiple frequency usage code or phase measurements one or multiple receivers GNSS signals are however not hearable in deep indoor environments Alternative means for localization required 38 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

39 Trends in receiver requirements Period Receiver type Main use case Primary feature Secondary feature Early 2000s Personal Navigation Device (PND) Urban Canyon Sensitivity Time-To-First-Fix (TTFF) Mid 2000s Mobile phone E911 TTFF Sensitivity 2010-today Smart phone LBS Power Availability (MultiGNSS) Near future Wearables / IOT Continuous Location Energy Availability (Hybrid) Based on: Greg Turetzky, GPS World 9/2015, Receiver design for the future How the Internet of Things Now Drives Location Technology 39 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

40 Next generation GNSS The future European Galileo, the Russian Glonass, and the Compass/Beidou are similar systems with GPS Glonass is however currently a FDMA system when GPS is and Galileo will be CDMA Glonass to be modernized to CDMA Also GPS is being modernized: new civil and military signals on L2 and L5 GPS January 2016: 30 SV operational Galileo January 2016: 4 IOV satellites and 8 FOC satellites, 5 SV operational Glonass January 2016: 24 SV operational Compass / BeiDou2: 16 SV operational 40 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

41 Satellites visible Benefits of several GNSSs: Improved availability GPS GNSS Multiple GNSSs may provide an added level of redundancy and, thus, an added degree of robustness to GNSS applications in addition to better availability and accuracy Time 41 GNSS Technologies

42 Galileo system The Galileo Program is a joint initiative of the European Commission (EC) and the European Space Agency (ESA) to provide Europe with its own independent global civilian controlled satellite navigation system. It is an autonomous system, interoperable with GPS and globally available. It is based on the same technology as GPS (i.e., DS-CDMA) and provides a similar - and possibly higher - degree of precision, thanks to the structure of the constellation of satellites and the ground-based control and management systems planned. 42 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

43 Galileo landmarks 1998: European Union decides to develop its own satellite navigation system, which it called Galileo (so named to honor the great European scientist Galileo Galilei) 2000: Definition phase of Galileo starts (was completed in 2003) Mar 2002: statement on GPS-Galileo cooperation Nov 2003: Joint statement between European Commission and US regarding GPS-Galileo: a mutually acceptable modulation for Galileo PRS service discussed June 2004: EU and US signed GPS-Galileo agreement: Common civil signal: Binary Offset Carrier BOC(1,1) modulation was selected as the baseline for both Galileo and GPS future OS signals 2005: first Galileo test satellite launched on orbit (GIOVE-A) May 2006: First Galileo standardization documents (for Open Service signal only) were made available 2008: second Galileo test satellite (GIOVE-B) launched on orbit 2011: first two IOV Galileo satellites launched (October 2011) 2012: Two IOV Galileo satellites launched (October 2012) 2013: ESA informs that Galileo will have full constellation in : Two FOC Galileo satellites launched but resulted in failure (August 2014) Several delays due to political and funding problems 43 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

44 In-Orbit Validation (IOV) Satellites The last two out of total four Galileo IOV (In-Orbit Validation) satellites were successfully launched on October 12, 2012 using a Soyuz rocket at the Guiana Space Center in French Guiana Currently only 3 operational 44 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

45 Full Operational Capability (FOC) Satellites 1/2 The first two Galileo FOC (Full Operational Capability) satellites were launched in August 2014 using a Soyuz rocket at the Guiana Space Center in French Guiana The launch experienced an anomaly and the satellites were injected into an incorrect orbit On satellite has been recovered, but its orbit still nonnominal 45 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

46 Full Operational Capability (FOC) Satellites 2/2 6 Galileo FOCs launched in status: 3 IOVs operational, 1 not available 2 FOCs operational 4 in correct orbit, start transmitting soon 2? 46 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

47 Galileo benefits Technical Better availability if combined GPS/Galileo signals are used (compared to GPS); especially in urban environments System certification, liability and guarantee of service Economic (location-based services; potential significant revenues) Social: services aiming at increasing safety and quality of life (e.g., advanced in-car navigation systems, seamless road tolling, improved transportation systems) Strategic: European civil control; opportunities of greater political, economic and social cohesion across Europe, etc. 47 GNSS Technologies

48 Galileo services Galileo satellite-only services Open Service (OS): free for everyone; mass market applications, simple positioning Safety of Life (SoL): for professional applications; integrity; authentication of signal Commercial Service (CS): for maritime, aviation and train applications; encrypted; high accuracy; guaranteed service Public Regulated Service (PRS): encrypted; governmentregulated; integrity; continuous availability Support to Search and Rescue service (SAR): humanitarian purpose; near real time; precise; return link feasible Other Galileo-related services: Galileo locally assisted services (use some local elements to improve performance, e.g., differential encoding, more carriers, additional pilot tones), Galileo combined services (combination with other navigation or communication systems). 48 GNSS Technologies

49 GLONASS Dual-use (civil/military) system provided by Russia Civil signals provided to all, free of user fees It started in 1983; now it s updated 24 active satellites (in total 29 satellites, as of Jan 10, 2015) GLONASS modernization directive, as of January 2006: Constellation of 18 SV by end of 2007 Full operational capability by end of 2009 Comparable performance with GPS and Galileo by 2010 Fully operational since December 2011 GLONASS uses frequency division multiple access FDMA (different from all the other CDMA-based satellite navigation systems) 49 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

50 BeiDou (a.k.a. COMPASS) System under development by China 30 MEO satellites + 5 GEO satellites MEO= Medium Earth Orbit GEO= Geostationary Earth Orbit Claimed accuracy: positioning, 10 meters Could institute user charges Currently, they have already launched 16 satellites (GEO/MEO and IGSO); more are planned to be launched in the coming years 50 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

51 System comparison GPS GALILEO GLONASS COMPASS First launch Full Operational Cabability (FOC) Number of satellites ? ? Orbital planes Multiple Access Current Status 30 operational CDMA CDMA FDMA/CDM A 5 operational 24 operational CDMA 16 operational satellites, full coverage on Asia pacific region 51 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

52 GNSS systems 52 GNSS Technologies

53 Regional satellite navigation systems QZSS: Quasi-Zenith Satellite System (Japan): Civil system for Asia Pacific region Signals on the same frequency bands as GPS developed in collaboration with US Initial objective of 3 satellites, extension to 4 decided in 2013 The first satellite 'Michibiki' was launched on 11 September, Full operational status is expected by IRNSS: Indian Regional Navigational Satellite System (India): Constellation of 7 GEO satellites, due for completion in next 1-2 years, 1 st launched in 2013 Ground segment: 1 master control station + 4 monitor TT&C stations 53 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

54 GNSS shortcomings Signal s susceptibility to unintentional or malicious radio frequency interference (RFI) or jamming GNSS signals are typically too weak to be observable indoors GNSS signals need to be augmented with external sensors to function accurately indoors Signal cannot provide an orientation solution easily, a feature that is indispensable in many vehicle navigation and guidance applications GNSS and integrated navigation: Inertial navigation systems (INSs) have been integrated with GNSSs with considerable success. This fusion between GNSSs and INSs is complementary: INS helps mitigate the shortcoming of the GNSS and vice versa. Other sensors are also commonly integrated with GNSS (e.g. other radio frequency (RF) signals, magnetometer, LIDAR, barometer). 54 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi

55 References and additional material Books: S. Gleason, D. Gebre-Egziabher (Eds.): GNSS Applications and Methods, Artech House Inc., Misra, Enge: Global Positioning System: Signals, Measurements and Performance, 2 nd edition, El-Rabbany: Introduction to GPS: The Global Positioning System, (2nd edition), Hofmann-Wellenhof Lichtenegger Wasle: GNSS Global Navigation Satellite Systems, Springer, Leick: GPS Satellite Surveying (3rd edition), Wiley & Sons, Poutanen: GPS-paikanmääritys, Ursa, Online, e.g.: European Space Agency The reference for Global Navigation Satellite Systems: Peter Dana's Global Positioning System Overview, a classic: A simple course on GPS positioning, by C. Rizos (Univ. New South Wales, Australia): Mike Craymer's collection of GPS resources: Contact details: Prof. Heidi Kuusniemi, Dept. of Navigation and Positioning, FGI, NLS, heidi.kuusniemi@nls.fi Dr. Laura Ruotsalainen, Dept. of Navigation and Positioning, FGI, NLS, laura.ruotsalainen@nls.fi Dr. Zahidul Bhuiyan, Dept. of Navigation and Positioning, FGI, NLS, zahidul.bhuiyan@nls.fi 55 GNSS Technologies heidi.kuusniemi@nls.fi, laura.ruotsalainen@nls.fi, zahidul.bhuiyan@nls.fi