Chapter 3. Environment Sensing (Perception) Layer

As defined in [33] there are eight attributes that characterize image sensor performance. This attributes will be explained in details in the following paragraphs:

Responsivity, the amount of signal the sensor delivers per unit of input optical energy. CMOS imagers are marginally superior to CCDs, in general, because gain elements are easier to place on a CMOS image sensor. Their complementary transistors allow low-power high-gain amplifiers, whereas CCD amplification usually comes at a significant power penalty. Some CCD manufacturers are challenging this concept with new readout amplifier techniques.

Dynamic range, the ratio of a pixel’s saturation level to its signal threshold. It gives CCDs an advantage by about a factor of two in comparable circumstances. CCDs still enjoy significant noise advantages over CMOS imagers because of quieter sensor substrates (less on-chip circuitry), inherent tolerance to bus capacitance variations and common output amplifiers with transistor geometries that can be easily adapted for minimal noise. Externally coddling the image sensor through cooling, better optics, more resolution or adapted off-chip electronics cannot make CMOS sensors equivalent to CCDs in this regard.

Uniformity, the consistency of response for different pixels under identical illumination conditions. Ideally, behaviour would be uniform, but spatial wafer processing variations, particulate defects and amplifier variations create non-uniformities. It is important to make a distinction between uniformity under illumination and uniformity at or near dark. CMOS imagers were traditionally much worse under both regimes. Each pixel had an open-loop output amplifier, and the offset and gain of each amplifier varied considerably because of wafer processing variations, making both dark and illuminated non-uniformities worse than those in CCDs. Some people predicted that this would defeat CMOS imagers as device geometries shrank and variances increased. However, feedback-based amplifier structures can trade off gain for greater uniformity under illumination. The amplifiers have made the illuminated uniformity of some CMOS imagers closer to that of CCDs, sustainable as geometries shrink. Still lacking, though, is offset variation of CMOS amplifiers, which manifests itself as non-uniformity in darkness. While CMOS imager manufacturers have invested considerable effort in suppressing dark non-uniformity, it is still generally worse than that of CCDs. This is a significant issue in high-speed applications, where limited signal levels mean that dark non-uniformities contribute significantly to overall image degradation.

Shuttering, the ability to start and stop exposure arbitrarily. It is a standard feature of virtually all consumer and most industrial CCDs, especially interline transfer devices, and is particularly important in machine vision applications. CCDs can deliver superior electronic shuttering, with little fill-factor compromise, even in small-pixel image sensors. Implementing uniform electronic shuttering in CMOS imagers requires a number of transistors in each pixel. In line-scan CMOS imagers, electronic shuttering does not compromise fill factor because shutter transistors can be placed adjacent to the active area of each pixel. In area scan (matrix) imagers, uniform electronic shuttering comes at the expense of fill factor because the opaque shutter transistors must be placed in what would otherwise be an optically sensitive area of each pixel. CMOS matrix sensor designers have dealt with this challenge in two ways. A non-uniform shutter, called a rolling shutter, exposes different lines of an array at different times. It reduces the number of in-pixel transistors, improving fill factor. This is sometimes acceptable for consumer imaging, but in higher-performance applications, object motion manifests as a distorted image. A uniform synchronous shutter, sometimes called a non-rolling shutter, exposes all pixels of the array at the same time. Object motion stops with no distortion, but this approach consumes pixel area because it requires extra transistors in each pixel. Developers must choose between low fill factor and small pixels on a small, less-expensive image sensor, or large pixels with much higher fill factor on a larger, more costly image sensor.

Speed, an area in which CMOS arguably has the advantage over CCDs because all camera functions can be placed on the image sensor. With one die, signal and power trace distances can be shorter, with less inductance, capacitance and propagation delays. To date, though, CMOS imagers have established only modest advantages in this regard, largely because of early focus on consumer applications that do not demand notably high speeds compared with the CCD’s industrial, scientific and medical applications.

One unique capability (called windowing) of CMOS technology is the ability to read out a portion of the image sensor. This allows elevated frame or line rates for small regions of interest. This is an enabling capability for CMOS imagers in some applications, such as high-temporal-precision object tracking in a sub-region of an image. CCDs generally have limited abilities in windowing.

Anti-blooming, the ability to gracefully drain localized overexposure without compromising the rest of the image in the sensor. CMOS generally has natural blooming immunity. CCDs, on the other hand, require specific engineering to achieve this capability. Many CCDs that have been developed for consumer applications do, but those developed for scientific applications generally do not.

CMOS imagers have a clear edge in regard of biasing and clocking. They generally operate with a single bias voltage and clock level. Nonstandard biases are generated on-chip with charge pump circuitry isolated from the user unless there is some noise leakage. CCDs typically require a few higher-voltage biases, but clocking has been simplified in modern devices that operate with low-voltage clocks.

Both image chip types are equally reliable in most consumer and industrial applications. In ultra-rugged environments, CMOS imagers have an advantage because all circuit functions can be placed on a single integrated circuit chip, minimizing leads and solder joints, which are leading causes of circuit failures in extremely harsh environments. CMOS image sensors also can be much more highly integrated than CCD devices. Timing generation, signal processing, analogue-to-digital conversion, interface and other functions can all be put on the imager chip. This means that a CMOS-based camera can be significantly smaller than a comparable CCD camera.

The image sensors only measure the brightness of each pixel. In colour cameras a colour filter array (CFA) is positioned on top of the sensor to capture the red, green, and blue components of light falling onto it. As a result, each pixel measures only one primary colour, while the other two colours are estimated based on the surrounding pixels via software. These approximations reduce image sharpness. However, as the number of pixels in current sensors increases, the sharpness reduction becomes less visible.

Figure 3.12. Principle of colour imaging with Bayer filter mosaic (Source: http://en.wikipedia.org/wiki/File:Bayer_pattern_on_sensor_profile.svg)

The most commonly used CFA is the Bayer filter mosaic as shown on Figure 54. The filter pattern is 50% green, 25% red and 25% blue. It should be noted that both various modifications of colours and arrangement and completely different technologies are available, such as colour co-site sampling or the Foveon X3 sensor.

The Navigation Signal Timing and Ranging Global Positioning System (NAVSTAR GPS, in short GPS) is a U.S. government owned utility that provides users with positioning, navigation, and timing (PNT) services. The following summary of the operating principles is based on [41].

This system consists of three segments: the space segment, the control segment, and the user segment. The U.S. Air Force develops, maintains, and operates the space and control segments.

The space segment consists of a constellation of satellites transmitting radio signals to users. The United States is committed to maintaining the availability of at least 24 operational GPS satellites, 95% of the time. To ensure this commitment, the Air Force has been flying 31 operational GPS satellites for the past few years. The extra satellites may increase GPS performance but are not considered part of the core constellation. GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km. Each satellite circles the Earth twice a day. The satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the Earth. Each plane contains four "slots" occupied by baseline satellites. This 24-slot arrangement ensures users can view at least four satellites from virtually any point on the planet.

The control segment consists of a global network of ground facilities that track the GPS satellites, monitor their transmissions, perform analyses, and send commands and data to the constellation. The current operational control segment includes a master control station, an alternate master control station, 12 command and control antennas, and 16 monitoring sites.

The user segment consists of the GPS receiver equipment, which receives the signals from the GPS satellites and uses the transmitted information to calculate the user’s three dimensional position and time.

The GPS satellites broadcast radio signals providing their locations, status, and precise time (t1) from on-board atomic clocks.

The principle of the operation is based on distance measurement. The GPS satellites broadcast radio signals providing their locations, status, and precise time from on-board atomic clocks. The GPS radio signals travel through space at the speed of light. A GPS device receives the radio signals, noting their exact time of arrival, and uses these to calculate its distance from each satellite in view. To calculate its distance from a satellite, a GPS device applies this formula to the satellite's signal:

distance = rate x time

where rate is the speed of light and time is how long the signal travelled through space

(This requires quite precise time measurement, since 1 us time deviation would result in 300m distance deviation.) The signal's travel time is the difference between the time broadcast by the satellite and the time the signal is received.

Figure 3.24. Position calculation method based on 3 satellite data. (Source: http://www.e-education.psu.edu)

If distances from three satellites are known, the receiver's position must be one of two points at the intersection of three spherical ranges (see Figure 54). GPS receivers are usually smart enough to choose the location nearest to the Earth's surface. At a minimum, three satellites are required for a two-dimensional (horizontal) fix if our receiver’s clock would be perfect. In this theoretical case all our satellite ranges would intersect at a single point (which is our position). But with imperfect clocks, a fourth measurement, done as a cross-check, will not intersect with the first three. Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver's clock back into sync with universal time. Once it has that correction it applies to all the rest of its measurements and now we've got precise positioning. One consequence of this principle is that any decent GPS receiver will need to have at least four channels so that it can make the four measurements simultaneously.

The modernization program of the GPS was started in May 2000 with turning off the GPS Selective Availability (SA) feature. SA was an intentional degradation of civilian GPS accuracy, implemented on a global basis through the GPS satellites. During the 1990s, civil GPS readings could be incorrect by as much as a football field (100 meters). On the day SA was deactivated, civil GPS accuracy improved tenfold, benefiting civil and commercial users worldwide. In 2007, the government announced plans to permanently eliminate SA by building the GPS III satellites without it. The GPS modernization program involves a series of consecutive satellite acquisitions. It also involves improvements to the GPS control segment, including the Architecture Evolution Plan (AEP) and the Next Generation Operational Control System (OCX). The main goal is to improve GPS performance using new civilian and military signals. In addition GPS modernization is introducing modern technologies throughout the space and control segments that will enhance overall performance. For example, legacy computers and communications systems are being replaced with a network-centric architecture, allowing more frequent and precise satellite commands that will improve accuracy for everyone.

From the point of view of the transportation the Second and the Third Civil Signals (L2C and L5) will be the most significant changes in the modernized GPS system. L2C is designed specifically to meet commercial needs. When combined with L1 C/A in a dual-frequency receiver, L2C enables ionospheric correction, a technique that boosts accuracy. Civilians with dual-frequency GPS receivers enjoy the same accuracy as the military (or better). For professional users with existing dual-frequency operations, L2C delivers faster signal acquisition, enhanced reliability, and greater operating range. L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it easier to receive under trees and even indoors.

L5 is designed to meet demanding requirements for safety-of-life transportation and other high-performance applications. L5 is broadcast in a radio band reserved exclusively for aviation safety services. It features higher power, greater bandwidth, and an advanced signal design. Future aircraft will use L5 in combination with L1 C/A to improve accuracy (via ionospheric correction) and robustness (via signal redundancy). In addition to enhancing safety, L5 use will increase capacity and fuel efficiency within U.S. airspace, railroads, waterways, and highways.

Both signals are in launching phase. The L2C will be available on 24 GPS satellites around 2018 and the L5 around 2021.

GLONASS (Globalnaya Navigatsionnaya Sputnikovaya Sistema or Global Navigation Satellite System) is a GNSS operated by the Russian Aerospace Defence Forces. Development of GLONASS began in 1976, with a goal of global coverage by 1991 [42]. Beginning on 12 October 1982, numerous rocket launches added satellites to the system until the constellation was completed in 1995. GLONASS was developed to provide real-time position and velocity determination, initially for use by the Soviet military in navigating and ballistic missile targeting. With the collapse of the Russian economy GLONASS rapidly degraded, mainly due to the relatively short design life-time of the GLONASS satellites. Beginning in 2001, Russia committed to restoring the system, and in recent years has diversified, introducing the Indian government as a partner. This plan calls for 18 operational satellites in orbit by 2008 and 24 satellites (21 operational and 3 on-orbit spares deployed in three orbital planes) in place by 2009; and a level of performance matching that of the U.S Global Positioning System by 2011. Finally by 2010, GLONASS has achieved 100% coverage of Russia's territory and in October 2011, the full orbital constellation of 24 satellites was restored, enabling full global coverage. It both complements and provides an alternative to the United States' Global Positioning System (GPS) and is the only alternative navigational system in operation with global coverage and of comparable precision.

At present GLONASS provides real time position and velocity determination for military and civilian users. The fully operational GLONASS constellation consists of 24 satellites, with 21 used for transmitting signals and three for in-orbit spares, deployed in three orbital planes. The three orbital planes' ascending nodes are separated by 120° with each plane containing eight equally spaced satellites. The orbits are roughly circular, with an inclination of about 64.8°, and orbit the Earth at an altitude of 19,100 km, which yields an orbital period of approximately 11 hours, 15 minutes. The planes themselves have a latitude displacement of 15°, which results in the satellites crossing the equator one at a time, instead of three at once. The overall arrangement is such that, if the constellation is fully populated, a minimum of 5 satellites are in view from any given point at any given time. This guarantees for continuous and global navigation for users world-wide.

Galileo is Europe’s own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control, which is introduced from [43].

By offering dual frequencies as standard, Galileo will deliver real-time positioning accuracy down to the metre range. It will guarantee availability of the service under all but the most extreme circumstances and will inform users within seconds of any satellite failure, making it suitable for safety-critical applications such as guiding cars, running trains and landing aircraft. On 21 October 2011 came the first two of four operational satellites designed to validate the Galileo concept in both space and on Earth. Two more followed on 12 October 2012. This In-Orbit Validation (IOV) phase is now being followed by additional satellite launches to reach Initial Operational Capability (IOC) by mid-decade.

Galileo services will come with quality and integrity guarantees, which mark the key difference of this first complete civil positioning system from the military systems that have come before. A range of services will be extended as the system is built up from IOC to reach the Full Operational Capability (FOC) by this decade’s end. The fully deployed Galileo system consists of 30 satellites (27 operational + 3 active spares), positioned in three circular Medium Earth Orbit (MEO) planes at 23 222 km altitude above the Earth, and at an inclination of the orbital planes of 56 degrees to the equator.

The four operational satellites launched so far - the basic minimum for satellite navigation in principle - serve to validate the Galileo concept with both segments: space and related ground infrastructure. On March 12th, 2013, a first positioning test based on the 4 first operational Galileo satellites was conducted by ESA. This first position fix returned an accuracy range of +/- 10 meters, which is considered as very good, considering that only 4 satellites (out of the total constellation of 30) are already deployed. The Open Service, Search and Rescue and Public Regulated Service will be available with initial performances soon. Then as the constellation is built-up beyond that, new services will be tested and made available to reach Full Operational Capability (FOC). Once this is achieved, the Galileo navigation signals will provide good coverage even at latitudes up to 75 degrees north, which corresponds to Norway's North Cape - the most northerly tip of Europe - and beyond. The large number of satellites together with the carefully-optimised constellation design, plus the availability of the three active spare satellites, will ensure that the loss of one satellite has no discernible effect on the user.

The BeiDou Navigation Satellite System (BDS) is a Chinese satellite navigation system. It consists of two separate satellite constellations – a limited test system that has been operating since 2000, and a full-scale global navigation system that is currently under construction.

The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System and also known as BeiDou-1, consists of three satellites and offers limited coverage and applications. It has been offering navigation services, mainly for customers in China and neighbouring regions, since 2000.

Figure 3.25. Long March rocket head for launching Compass G4 satellite (Source: http://www.beidou.gov.cn)

The second generation of the system officially called the BeiDou Satellite Navigation System (BDSNS) and also known as COMPASS or BeiDou-2, will be a global satellite navigation system. According to the plans the full constellation will consist of 35 satellites. Since December 2012 there are 10 satellites providing local services to the Asia-Pacific area and it is planned to cover all over the world serving global customers before 2020. [44]

Figure 3.26. BeiDou 2nd Deployment Step(Source: http://gpsworld.com/the-system-vistas-from-the-summit/)

The COMPASS Phase II has successfully established as planned. On December 27, 2013, the first anniversary of BeiDou Navigation Satellite System providing full operational regional service was held in Beijing. At the meeting, China Satellite Navigation System Management Office Director Ran Chengqi announced the BeiDou Navigation Satellite System Public Service Performance Standard.

The result comparison with GPS created by John Lavrakas (Advanced Research Corporation) [45] shows mixed results. In some cases, the commitments from BeiDou were stronger (URE accuracy, the table to show green for the GNSS service committing to a more stringent standard over the other vertical position), and in other cases the commitments from GPS were stronger (continuity of service, advance notice of outages).

Figure 3.27. Comparison of BeiDou with GPS(Source: http://gpsworld.com/china-releases-public-service-performance-standard-for-beidou/)

Differential correction techniques are used to enhance the quality of location data gathered using global positioning system (GPS) receivers. Differential correction can be applied either in real-time (directly in the field, during measurement) or when post-processing data in the office (later during data evaluation). Although both methods are based on the same underlying principles, each accesses different data sources and achieves different levels of accuracy. Combining both methods provides flexibility during data collection and improves data integrity.

The underlying assumption of differential GPS (DGPS) technology is that any two receivers that are relatively close together will experience similar atmospheric errors. DGPS requires that one GPS receiver must be set up on a precisely known location. This GPS receiver is the base or reference station. The base station receiver calculates its position based on satellite signals and compares this location to the already known precise location resulting in a difference. This difference then is applied to the GPS data recorded by the second GPS receiver, which is known as the roving receiver. The corrected information can be applied to data from the roving receiver in real time in the field using radio signals or through post-processing after data capture using special processing software.

Figure 3.28. Differential GPS operation (source: http://www.nuvation.com)

Real-time DGPS occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The correction is received by the roving receiver via a radio signal if the source is land based or via a satellite signal if it is satellite based and applied to the position it is calculating. As a result, the position displayed and logged to the data file of the roving GPS receiver is a differentially corrected position.

In the US, the most widely available DGPS system is the Wide Area Augmentation System (WAAS). The WAAS was created by the Federal Aviation Administration and the Department of Transportation for use in precision flight approaches. However, the system is not limited to aviation applications, and many of the commercial GPS receivers intended for maritime and automotive use support WAAS. WAAS provides GPS correction data across North America with a typical accuracy less than 5 meters. Similar systems are available in Europe.

The European Geostationary Navigation Overlay Service (EGNOS) is the first pan-European satellite navigation system. It augments the US GPS satellite navigation system and makes it suitable for safety critical applications such as flying aircraft or navigating ships through narrow channels.

Consisting of three geostationary satellites and a network of ground stations, EGNOS achieves its aim by transmitting a signal containing information on the reliability and accuracy of the positioning signals sent out by GPS. It allows users in Europe and beyond to determine their position to within 1.5 meters. (Source: [46])

Another high precision GPS solution is the real-time kinematic GPS (RTK GPS). RTK is a GBS-based survey that utilizes a fixed, nearby ground base station that is in direct communication with the rover receiver through a radio link. RTK is capable of taking survey grade measurements in real time and providing immediate accuracy to within 1-5 cm. RTK systems are the most precise of all GNSS systems. They are also the only systems that can achieve complete repeatability, allowing the returning to the exact location indefinitely. All this precision and repeatability comes at a fairly high cost. But there are other drawbacks to RTK also. RTK requires a base station within about 10-15 km of the rover. [47]

Assisted GPS describes a system where outside sources, such as an assistance server and reference network, help a GPS receiver perform the tasks required to make range measurements and position solutions. The assistance server has the ability to access information from the reference network and also has computing power far beyond that of the GPS receiver. The assistance server communicates with the GPS receiver via a wireless link (generally mobile network data link). With assistance from the network, the receiver can operate (start-up) more quickly and efficiently than it would unassisted, because a set of tasks that it would normally handle is shared with the assistance server. The resulting AGPS system, consisting of the integrated GPS receiver and network components, boosts performance beyond that of the same receiver in a stand-alone mode. It improves the start-up performance or time-to-first-fix (TTFF) of the GPS receiver. The assistance server is also able to compute position solutions, leaving the GPS receiver with the sole job of collecting range measurements. The only disadvantage of AGPS is that it depends on the infrastructure (requiring an assistant service and an on-line data connection). (Source: [48])