Correct Answer : Position and acceleration of satellite
Explanation : All of the satellite navigation equations have either satellite position or velocity as a variable. The linear independence of the equations, which dictate the observability of the navigation solution, is a function of the relative position of the satellites in orbit. Thus the position and the velocity of satellites is of primary importance.
Correct Answer : Line of nodes
Explanation : The line of nodes is the intersection of the orbital plane with the Earth’s equatorial plane. The ascending node and the descending node are the points where the satellite crosses the equatorial plane.
Correct Answer : 6
Explanation : Six independent constants are needed to specify the nominal orbit. These can be the three components of position and velocity at any instant of time and are used in the orbital mechanics equations.
Correct Answer : Line of apsides
Explanation : The points where the satellite is closest and farthest from the Earth is called the perigee and apogee respectively. The line connecting the apogee and the perigee is called the line of apsides.
Correct Answer : Motion of a passing comet
Explanation : Perturbations cause the orbital plane to oscillate and rotate and vary the satellite from its elliptical orbital path. These forces include spherical asymmetrical components of Earth’s gravitational field, Lunisolar perturbations, air drag, magnetic and static electric forces.
Correct Answer : Reduces velocity
Explanation : The major sources of errors are when the signal pass through the atmosphere. The troposphere causes the propagation velocity of the signal to be slowed, compressing the signal wavelength.
Correct Answer : Ultraviolet radiation from sun
Explanation : The ionosphere owes its existence to the ultraviolet radiation from the sun. The photons possess a certain amount of energy which is enough to break down electrons from their parent atom. The result is a large number of free, negatively charged, electrons and positively charged atoms and molecules called ions.
Correct Answer : Frequency
Correct Answer : Occurs mostly at day
Explanation : Ionospheric scintillation causes signal fading and effects in the equatorial and the Polar Regions. Strong scintillation effects are rare localized at certain times during the night, and usually occurs during periods of high solar activity.
Correct Answer : True
Explanation : TEC or the total electron content in the ionosphere is a function of solar radiation. As more radiation falls on Earth, more number of molecules are ionized and hence TEC is more in the daytime than at the night.
Correct Answer : Phase advance
Explanation : The result of satellite signal travelling through ionosphere is that a particular phase of the signal arrives at the receiver earlier than it would have had if the signal travelled in a complete vacuum. This early arrival is termed as phase advance.
Correct Answer : Velocity increases
Explanation : The velocity of the carrier, the pure sinusoidal wave, is increased in the presence of electrons. Thus the greater the density of electrons in the ionosphere greater the velocity of the signal.
Correct Answer : Active system
Explanation : The GPS is an all weather global navigational system. It is passive, survivable, continuous, space based system that provides any suitable equipped user with highly accurate three dimensional position, velocity, and time information anywhere on or near the Earth.
Correct Answer : 12 hours
Explanation : The GPS satellites are in approximately 12 hour orbits( 11 hours, 57 minutes, 57.27 seconds) at an altitude of approximately 11,000 nmi. At any point on Earth, 4 satellites are visible at all times.
Correct Answer : Ionosphere refraction corrections
Explanation : Each GPS satellite transmits signals at two frequencies in the L-band to permit ionospheric refraction corrections by properly equipped users. These signals are in sync with the pseudorandom noise that is unique to each satellite to provide instant ranging capabilities.
Correct Answer : Binary phase shift keying
Explanation : The L1 consists of two components: a precise and a coarse PRN code. The PRN codes and data are modulated onto the carriers using binary phase shift keying.
Correct Answer : IIF
Explanation : The IIF block of the GPS is the latest launched till date. The twelfth and final IIF launch was February 5, 2016. The third generation GPS satellites are scheduled to be launched by SpaceX in 2018.
Correct Answer : Helical array antenna
Explanation : The GPS antenna array is formed by an inner quad helices encircled by a ring of eight outer helices that provide near equal power to all terrestrial users. The helical antenna is optimum since circular polarization is least affected by weather and atmosphere.
Correct Answer : Precise position service
Explanation : GPS provides two position services, The precise position service and the standard position service. The PPS can be denied to unauthorized users while SPS is free to everyone. Users need crypto keys to access the PPS. These are generally military users.
Correct Answer : Selective availability
Explanation : The intentional degradation of GPS signal is meant to deny accuracy to unfriendly forces. It is known as selective availability. SA also denies accuracy to unauthorized users who are friendly, which is the entire civil community.
Correct Answer : FDMA
Explanation : GLONASS satellites are distinguished and differentiated by radio frequency channel rather than spread spectrum code as in GPS. They use FDMA whereas GPS uses CDMA.
Correct Answer : Beam antenna
Explanation : A uniform power density is required for using GLONASS. A shaped beam antenna is used to produce uniform power spectral density on the ground.
Correct Answer : 562.5kHz
Explanation : The unique frequency for L1 signal in the GLONASS system is given by the formula f= 1602.0 +0.5625i MHz where I=0,1,2,….,24. The separation between each of these satellites is 562.5 kHz.
Correct Answer : i=-7 to +4
Explanation : The GLONASS use i= -7 to +4 frequency channels for normal operation. This was done so that the signal does not interfere with the radio astronomy ground station which uses the same frequency band.
Correct Answer : Avoid future MSS interference
Explanation : The Frequency of the GLONASS channel was shifted down in 2005. This shift down in frequency is to avoid interference from future Mobile Satellite Services.
Correct Answer : UTC(USNO)
Explanation : The GPS segment provides correlations so that GPS time can be related to UTC(USNO) modulo 1sec to within 90nsec, whereas the GLONASS control segment provides correlations so that GLONASS time can be related to UTC(Moscow) to within 1μsec.
Correct Answer : 437.5kHz
Explanation : The unique frequency for L2 signal in the GLONASS system is given by the formula f= 1246.0 +0.4375i MHz where I=0,1,2,….,24. The separation between each of these satellites is 437.5kHz.
Correct Answer : ECEF coordinates
Explanation : The GLONASS ephemeris data are broadcast as ECEF or Earth Center Earth Fixed coordinate system cartesian coordinates in position and velocity with lunar and solar acceleration perturbations parameters that are valid about 0.5 hour.
Correct Answer : Acceleration
Explanation : The accelerometer is a type of inertial instrument that measures the acceleration in any given axes. The measured acceleration can be integrated over time to obtain velocity and distance.
Correct Answer : 8g
Explanation : Accelerometer is used to measure acceleration of the vehicle and thus must be corrected for the Earth’s gravitational effect. If the accelerometer is being accelerated upward with an acceleration of 7g, then (a-G) = 7g – (-1g), and the instrument reads 8g.
Correct Answer : Flexure pivot accelerometer
Explanation : The flexure pivot accelerometer is the most commonly used in aircraft systems. The sensitive element consists of a pendulum with a torquer coil and a pick off supported by torsional spring or flexure.
Correct Answer : Thermal characters
Explanation : Accelerometers that include fluid damping exhibit reduced bandwidth and additional thermal sensitivity due to changes in fluid characteristics. Thus flexure pivoted accelerometers are better than floated instruments.
Correct Answer : Voltage forcing
Explanation : In potential forcing, a potential is applied to the pendulum and to one or both the electrodes. The voltage establishes electric fields that induce the charge of the nonconducting pendulum. This causes a net force to act on the proof mass.
Correct Answer : Metalised wafers
Explanation : The frame, proof mass and hinges are made with single crystal silicon. Metalised wafers are used to enclose this and also serve as electrodes for sensing proof mass and rebalancing.
Correct Answer : Heating
Explanation : Heating of torquer coil due to rebalance current can lead to rectification of vibration inputs and must often be compensated. A pulse rebalance torquer maintains constant heating.
Correct Answer : Flexure must provide maximum resistance in the direction of input axis
Explanation : The pivot or flexure supporting the pendulum must provide minimal restraint for the pendulum in the direction of the input axis while exhibiting high stiffness in the other two directions. The spring constant of the flexure or pivot generates a restoring force that reduces the gain of the electronic restoring loop.
Correct Answer : Space-stabilize the accelerometer
Explanation : The purpose of gyroscope or gyro in an inertial navigation system is to space stabilize the accelerometers. In gimballed platforms, the gyros measure the rotation of the platform, which is angularly isolated from the vehicle’s motion.
Correct Answer : High drift rate
Explanation : Gyroscopes are used as heading references. The spinning wheel, large angle, unfloated instruments with ball bearings that have drift rates on the order of 50deg/hr . and hence are useless for navigation.
Correct Answer : Three gimbals are used
Explanation : In SDF floated gyros or single degree of freedom floated gyros, a spinning wheels were mounted inside a single gimbal that was floated at neutral buoyancy. A magnetic pickoff was used to sense rotation and they were primarily used in space launch vehicles.
Correct Answer : Electrostatically suspended TDF gyro
Explanation : In electrostatically suspended TDF gyro, the spinning wheel is suspended electrostatically hence reducing friction and achieving 0.001 deg/hr accuracy but at costs orders of magnitude higher than the floated TDF gyros.
Correct Answer : Low run up time
Explanation : The major disadvantage of spinning wheel or spinning rotor gyros is that they are mechanically complex and have a high probability of failure due to the moving components. They also require high powers and have high run up time. Run up time is the time taken for the gyro to start spinning at the required RPM after engine start.
Correct Answer : Coriolis effect
Explanation : MEMS Micro Electro Mechanical System gyro exploit the effects of Coriolis force. Coriolis force is experienced when a vibrating mass is subjected to a rate of rotation about an axis in the plane of vibration.
Correct Answer : High maintenance
Explanation : Micro machined vibrating mass rate gyros exploit semiconductor manufacturing technology for mass production. They are extremely reliable and rugged devices with low power consumption and offer the major advantage of relatively low cost. They also do not require the very high maintenance that spinning rotor gyros require.
Correct Answer : Halteres
Explanation : Every flying object needs to maintain its orientation and insects that fly use halteres. Halteres work in the same principle as MEMS gyro, the vibrate and use Coriolis force to know their orientation.
Correct Answer : Space launch vehicles
Explanation : MEMS gyros are being exploited not only in avionic applications such as flight control, standby attitude and heading reference systems, and missile mid-course guidance but also in the automobile industry in car stability enhancement systems. Space launch vehicles use ring laser gyros which are more accurate.
Correct Answer : Reduce base motion
Explanation : The vibrating mass rate gyro is a simple instrument suffers from the unacceptable characteristic that the smallest linear motions applied to its base cause unacceptably large errors. To overcome the effect of base motion, it is necessary to use balanced oscillations in which the oscillations of one mass are counter-balanced by equal and opposite motion of a second equal mass as in the case of tuning fork gyro.
Correct Answer : Ring laser gyro
Explanation : Optical gyroscopes such as the ring laser gyro and the fibre optic gyro measure angular rate of rotation by sensing the resulting difference in the transit times for laser light waves travelling around a closed path in opposite directions.
Correct Answer : Sagnac effect
Explanation : The Sagnac effect is a relativistic phenomena of light in a rotating reference system. When laser beams circulate in a closed path that is rotating in an inertial space, the optical length seen by the co-rotating beam appears longer than that seen by counter rotating beam.
Explanation : In a Ring laser gyro, the counterpropagating beams form resonant nodes within the gyro cavity and create an electromagnetic standing wave that remains fixed with inertial space. When the housing of the gyro rotates, a detector can count nodes of the standing waves, each of which represents an angle.
Correct Answer : Rate biasing
Explanation : The method of rate biasing employs a turntable that applies a constant rotation to the gyro. An angular encoder measures the relative angle between the instrument and its base.
Correct Answer : Backscatter
Explanation : Lock-in phenomenon leads makes the RLG insensitive to low angular rates. The cause of the lock-in phenomenon is backscatter within the cavity, usually resulting from imperfections in or particulates in the mirror surfaces.
Correct Answer : Spontaneous emission
Explanation : The gas medium that supplies the gain for the laser also occasionally emits photons which are unrelated to the laser signal. This is known as spontaneous emissions are leads to noise and random walk in the RLG angle output.
Correct Answer : Interference intensity
Explanation : In a fiber optic gyro, The counterpropagating beams are launched into an optical path and are recombined as they exit. The interference generated by the recombination depends on the optical phase difference between the two beams and therefore provides a measure of rotation.
Correct Answer : Strapdown system
Explanation : In a strapdown navigator, Gyroscopes and accelerometers are rigidly mounted to a sensor assembly that is usually mounted to the vehicle on a set of strap mounts. The gyroscopes track the rotation of the body and compute the orientation of the body.
Correct Answer : Angular rate sensors
Explanation : In a strapdown system, the gyros do not act as null sensors as in a gimballed system but act as inertial angular rate sensors. Angle and orientation are computed from the measured angular rates by the algorithms that calculate heading and orientation.
Correct Answer : Provide inputs to air data computer
Explanation : The purpose of strapdown computations are to calculate the vehicle’s attitude relative to the navigational coordinates using the gyro measurements, to transform the accelerometer measurements from vehicle axes into navigational coordinates, and to perform the dead-reckoning computations. Air data computer is independent of inertial navigation system.
Correct Answer : Quaternion
Explanation : Describing rotation motion in the strapdown system employ more than the minimum three parameters. A quaternion is a four element entity ting of a scalar part and a vector part. It defines the instantaneous axis of rotation.
Correct Answer : Normalization constraint
Explanation : Quaternion integration algorithms usually make use of the normalization constraint to control error growth in the computations. The sum of the squares of the quaternion elements are subtracted from unity to yield the normalization error.
Correct Answer : Linear motion of the vehicle
Explanation : The ability of stable element to remain non-rotating relative to a space or Earth coordinate frame depends on drift rate of the reference gyros, accuracy of gyro torquers, linear motion of the vehicle, orthogonality of the components on the platform.
Correct Answer : Middle gimbal approaches 90°
Explanation : The condition in which the middle gimbal angle approaches 90° is called gimbal lock. In this condition, the innermost and outermost axis coincide, thus depriving the platform one of its degrees of freedom.
Correct Answer : Euler angles are not easily readable from the gimbals
Explanation : One of the main purposes of gimballed system is to provide a convenient readout of vehicle attitude by reading each of the gimbals angles separately, thereby yielding convenient Euler angle outputs.
Correct Answer : Coning algorithm
Explanation : Pre-processing the gyro data at a higher rate than the quaternion iteration improves the approximation in the vector change in angle in order to follow the actual motion of the rotation axis closely. The pre-processing algorithm is also called as coning algorithms.
Correct Answer : Limit vibration errors
Explanation : The gyros and accelerometers are rigidly mounted on a block with temperature sensors. The block is usually shock mounted to the vehicle to control the bandwidth of the motion sensed at the instruments. Shock mounting limits vibration rectification errors within the errors themselves.
