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The physical sensor, depicted as the yellow block in Figure 35.1, is a critical part of any navigation system, and selection of the right sensor or combination of sensors is one of the most important decisions a navigation system designer can make. What comprises a navigation sensor?

At a basic level, any physical sensor that measures something which changes when the sensor is moved is a potential navigation sensor. Additionally, since clocks are an integral part of many navigation systems, we also consider clocks in this section as well. In contrast to a navigation sensor, which measures something that changes when the sensor is moved in some way, a clock is a sensor that measures how time “moves.” A summary of the major sensors covered in Volume 2 is given in Table 35.1.

Table 35.1 Sensors covered in Volume 2

Sensor	Sensed phenomenon	World model required	Other considerations
Cellular RF receiver	Cellular phone RF signals	Positions of cell towers, signal timing	Example of signal of opportunity (SoOP), reference receiver sometimes required
Terrestrial beacon receiver	Navigation signals from terrestrial beacons	Beacon locations, signal structure, signal timing	Requires dedicated infrastructure, more design flexibility than SoOP
Digital TV receiver	Digital TV signals	Transmitter locations, signal timing	Example of SoOP, reference receiver sometimes required
Low‐frequency receiver	Low‐frequency RF signals	Transmitter location or direction of arrival, local distortion effects	Susceptible to local distortions, generally less accurate than higher frequency/wider bandwidth signals
Radar	RF signals	Locations of identifiable RF reflectors for absolute positioning	Generally larger/higher power than receiver‐based systems
Low‐Earth orbit (LEO) satellite receiver	Signals from LEO satellites	LEO satellite position/velocity, signal timing (in some cases), atmospheric models	Greater geometric/signal diversity and higher received power than GNSS
Inertial	Rotation and specific force	Gravitational field	Dead‐reckoning only – drift normally requires update
GNSS	RF signals from satellites	Satellite ephemeris and clock errors, atmospheric models	Ideal for updating inertial
Magnetometer	Magnetic field (including variations)	Magnetic field map	Local (vehicle) effects calibration may be required
LiDAR	Range and intensity of laser returns	Shape/location of objects being sensed	Can be used in dead‐reckoning or absolute modes
Cameras	Intensity of light as a function of direction	Map of image features or three‐dimensional image model for absolute positioning	Can be used in dead‐reckoning or absolute modes
X‐ray detector	X‐ray signals coming from pulsars	Knowledge of pulsar directions and signal characteristics (including timing)	Positioning is based on signal time of arrival
Clock	Varies by clock type	Perhaps calibration parameters	Measures rate of time passage (frequency), and if initialized and integrated, absolute time