Learning Objectives:

1
Lesson 20

• Learning Objective(s)
• Know the operating principles of the Navigation
  Satellite System (NAVSAT),Global Positioning
  System GPS, Inertial Navigation system (INS).
  Navigation Sensor System Interface (NAVSSI), and
  bottom contour navigation.
• Applicable reading Hobbs pp. 530-546, 553-558,
  565-591.

2

• NAVSAT The NAVSAT system consists of eight
  operational satellites orbiting the earth plus
  several spares, a network of ground tracking
  stations, a computing center, an injection
  station, Naval Observatory time signals, and the
  shipboard receiver-computer combination. The
  system is based on the shipboard receivers
  ability to measure the Doppler shift of a
  transmitted signal from the satellites and derive
  a position from these signals.
• A satellite transmits a signal that provides the
  time of transmission and orbital information to a
  receiving unit.
• The transmitted signal is compressed as it
  approaches the receiving unit and elongated as it
  recedes. The time of no Doppler shift is the
  point of closest approach of the satellite.
• The receiving unit can determine Its position
  using the time and positional information
  corresponding to the signal with no Doppler
  shift.
• The advantages of the NAVSAT system are 1) It
  provides accurate coverage, 2) It is worldwide,
  3) It is passive, and 4) It can provide
  information to surface, subsurface and air
  platforms. Its disadvantages are 1) Satellite
  jamming or destruction is possible during times
  of conflict, and 2) NAVSAT satellites do not
  provide continuous fixes - shipboard computers
  need 10-15 minutes to process signals (the
  AN/SAN-25 is installed on Navy vessels. The
  actual receiver is the WRN-5). Additionally, the
  opportunity for a satellite transmission may
  exceed 95 minutes, and a highly accurate speed
  input to the system is required. The NAVSAT
  system is accurate to 200 yds.

3

• The basic function of the components of the
  NAVSAT system are\
• Ground tracking stations - compares known
  position data to position data received from the
  satellite. This information is forwarded to the
  computer center.
• Computer center - takes the information from the
  tracking station and computes future orbital
  parameters for the satellites.
• Injection Station - transmits the new orbital
  parameters to the satellite.
• Naval observatory time signals - provide accurate
  time to the ground tracking stations, computer
  center and injection stations.
• Shipboard Receiver - receives the signal and
  computes ships position.
• A diagram of the NAVSAT system appears below.

4
(Overhead 20-1)
5

• The NAVSTAR Global positioning System This is
  the top of the line navigation system. It
  provides worldwide coverage and constant position
  updates. It consists of 18 satellites grouped
  into six different orbits providing coverage
  anywhere on earth. The fixes provided by GPS are
  accurate to within 8 meters horizontally and 10
  meters vertically. GPS can also provide velocity
  information that is accurate to within .1 kts.
• GPS enables ships to fix their positions using
  range LOPS from the orbiting satellites.
• GPS satellites are approximately 11,000 miles up
  in space because of this significant separation
  from the earth, they assume a reasonably uniform
  orbit that is easy to track. DOD stations track
  the satellites constantly and transmit positional
  data to the satellite, so it can compensate for
  minor positional errors. The satellites in turn
  transmit positional information to the ships
  receiver.
• In order to obtain a range LOP, the receiving
  ship must know the time the satellite transmitted
  its signal and the actual position of the
  satellite. GPS satellites have highly accurate
  atomic clocks onboard which allow them to
  transmit a certain code at a precise moment. The
  satellite and receivers are designed to be
  synchronized, so they will generate the same
  codes at the same time. In theory, this would
  allow the shipboard receiver to easily determine
  the time it took for a satellite transmission to
  reach the ship (the receiver checks the received
  code then determines the time difference between
  the received time and the time the same code was
  internally generated).

6

• Once the time difference is determined, it can be
  converted into a range. The position of the
  satellite and the rate of travel of the signal
  are known, so the equation D S x T will yield a
  range. Plotting two ranges (three if altitude is
  desired) would then yield an accurate fix.
• Unfortunately, atomic clocks are not installed on
  ships, subs and aircraft due to their huge cost.
  Consequently, errors may occur when the shipboard
  receiver attempts to identify the travel time of
  the satellite transmission.
• This problem can be solved by triangulation -
  adjusting the range arc from each of three
  satellites by the same increment until they meet
  at one point

(Overhead 20-2)
7

• Because the ranges derived from the satellite
  signals must be corrected in order to form a fix
  they are often referred to as pseudoranges.
• In practice, the receiver solves this problem by
  using inputs from three (or four if height /
  altitude is desired) satellites and applying
  algebra to solve for the time error.
• Differential GPS Increased reliability can be
  obtained from GPS by using a differential
  technique. This technique utilizes a receiving
  station located at a fixed position. This
  station compares information obtained from GPS
  satellites to its known position. Any
  difference in the GPS position and known position
  is due to GPS error. The receiving station
  calculates this positional error and transmits it
  to a mobile unit. The mobile unit then corrects
  for this error and obtains a more accurate
  position. Moving vessels can obtain position
  accuracy of 2-5 meters using the differential
  technique. The figure below shows an example of
  the differential technique.
• The military can use GPS for purely navigational
  purposes or as an input to various weapons
  systems targeting and fire control systems.

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(Overhead 20-3)
9

• The Ships Inertial Navigation System (SINS)
  Inertial navigation is defined as the process of
  directing the movements of a vessel based on
  sensed accelerations in known spatial directions.
  Specially designed instruments convert these
  accelerations into velocity and position.
• SINS utilizes gyrocompasses which react to any
  change (force) applied to them. Devices known as
  accelerometers measure the force exerted on the
  gyro by the ships movement. Since the gyro is
  of known mass and the accelerometer can measure
  the force exerted by a ships movement, the
  acceleration of the ship in a particular
  direction can be measured (Newtons Second Law of
  Motion, FMA). This measured acceleration is
  converted into a velocity by the SINS system. The
  ships gyrocompass provides the direction of
  ships movement, so the ships position can be
  determined by the SINS highly accurate version of
  a DR.
• The main disadvantage of this system is that the
  ships position must be periodically updated by
  an external source (GPS or NAVSAT). Errors in
  SINS generated positions can be caused by such
  things as the daily rotational motion of the
  earth or friction in the gyrocompass support
  systems. These errors are cumulative and will
  get worse as time goes on unless an accurate
  update of ships position is introduced.

10

• Navigation Sensor Systems Interface (NAVSSI)
  This interface can utilize all available
  information to determine ships position, and
  pass this information to other systems such as
  JOTS, Outboard, and Tomahawk. Other features of
  NAVSSI include the future utilization of digital
  charts which may ultimately replace paper charts,
  and program software that can compute tides and
  current, set and drift, and celestial phenomenon.

11

• Bathymetric Navigation This method of navigation
  establishes position by using the geographic
  features of the ocean floor.
• An echo sounder (fathometer) is used to produce a
  trace of the ocean floor beneath the vessel.
• The echo sounder consists two components
• A transducer (usually located at or near the
  keel) which is comprised of a projector to send a
  signal to the floor below and a hydrophone to
  receive the reflected signal.
• A recorder (usually located in the chart room) to
  produce a continuous trace of the signals.
• The echo sounder transmits a sound pulse which is
  reflected from the bottom of the sea. The
  reflected signal is received and turned into an
  electromagnetic signal that is used by the
  recorder to calculate depth (the time of signal
  transmission, signal receipt and the rate at
  which sound travels through water are all known,
  so depth can be calculated).
• The echo sounder uses a mechanical arm to plot
  these depths on trace paper. The trace can then
  be compared to a bottom contour chart (shown
  below) to establish position.

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(Overhead 20-4)
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• Factors affecting the accuracy of the bathymetric
  DR plot
• Water depth. Multiple bottom return can occur in
  shallow water (e.g. reverberations between the
  bottom and surface in shallow water or a return
  from the top layer of mud then a return from the
  harder layer beneath it).
• Scattering caused by a suspension of matter (such
  as biologics) between the bottom and surface.
• Type of bottom (e.g. mud vs rock, flat vs
  varied)
• Side echo. The signal sent out by the fathometer
  is conical and propagates outward. Therefore,
  the return that is used for depth may not be
  directly under the vessel.
• Rolling and pitching of the ship. Creates the
  same problem as side echo.