Section III: Position/Navigation Satellite Systems

 

7-26 Characteristics of an Ideal Pos/Nav System

Introduction

a. Accurate, responsive position determination and navigation are essential to the conduct of all military operations. The ideal position/navigation (POS/NAV) system, as described in the Army POS/NAV Master Plan (1990) should have the following characteristics:

  • Worldwide, continuous coverage
  • User passive (No detectable electronic emissions)
  • Capable of being denied to an enemy
  • Able to support an unlimited number of users
  • Resistant to countermeasures
  • Real time responsiveness
  • Support joint and combined operations
  • No frequency allocation problems
  • Common grid or map datum reference for all users
  • Position accuracy that is neither degraded by changes in altitude nor by the time of day or year.
  • Maintained by operating personnel
  • Not dependent on externally generated signals

Pos/Nav Systems

b. In addition, an ideal POS/NAV system should not have decreasing accuracy over time or the distance traveled (no drift) and should not be dependent on the identification of precise locations to initiate or update the system.

Army Pos/Nav Systems

c. In an attempt to provide the required capabilities, the Army has fielded numerous systems, often developed to meet very specific needs. The result has been a proliferation of POS/NAV devices, many of which are incompatible with another.

GPS

d. No single system has all of these characteristics, but, Global Positioning System (GPS) comes closest. The Army POS/NAV Master Plan states GPS receivers will be issued to most units in the Army. They will be placed on all Army aircraft, many different types of tactical vehicles, weapons systems and most Army vessels. Small, lightweight portable sets will be issued to individuals.

Accuracy of Systems

e. Below shows the accuracy of various position/navigation systems in use today. GPS is 20 times more accurate than the next best navigation system, Loran-C, and Loran-C is not available worldwide. The accuracy of GPS does not degrade over time nor distance as does the accuracy of inertial or doppler navigation aids. In addition to the increased accuracy, GPS provides a common grid coordinate system based on the World Geodetic Survey 1984 (WGS 84). The coordinates used on maps, especially those prepared in foreign countries, are based on one of the more than 100 different map datums used around the world. GPS receivers contain the algorithms to convert the WGS 84 coordinates to coordinates based on local datums.

7-27 Global Positioning System (GPS)

Introduction

a. The Global Positioning System (GPS) is a space based, all weather, jam resistant, continuous operation radio navigation system. The system provides military, civil and commercial users highly accurate worldwide three-dimensional, common-grid, position/location data, as well as velocity and precision time to accuracy’s that have not been easily attainable before.

GPS Basis

b. GPS is based on the concept of triangulation from known points similar to the technique of "resection" used with a map and compass except that it is done with radio signals transmitted by satellites. The user's GPS receiver must determine precisely when a signal is sent from selected GPS satellites and the time it is received.

GPS Receivers

c. Nothing except a GPS receiver is needed to use the system so it is immediately available to soldiers as they deploy into any theater of operation. In addition, GPS receivers do not transmit any signals, therefore they are not electronically detectable. Because they only receive signals, there is no limit to the number of simultaneous GPS users.

 

GPS Accuracy

 

 

Standard Positioning Service (SPS)

Precise Positioning Service (PPS)*

 

Position

76 m SEP

40 m CEP

100 m 2 dRMS @95%

16 m SEP

9 m CEP

 

Velocity

0.5 m/sec

0.1 m/sec

 

Time

1 millisecond

100 nanosecond

 

*PPS is only available to U.S. and allied military, U.S. Government and selected civil users specifically approved by the U.S. government.

Background

d. In the 1964, the U.S. Navy's Transit space based navigation system became operational. Through the continued replacement of satellites, the system is still in operation today. Transit satellites are in low Earth orbit and the system is not available at all times. The system works by measuring the doppler shift of signals sent from the satellite. Mobile users must accurately know their velocity or significant errors will result. Except for some larger watercraft, the Army does not use Transit.

Navy and Air Force

e. Timation was a Navy space based position/navigation program that began in the 1960's. It was intended to provide 2-dimensional navigation data. At the same time, the Air Force conducted concept studies for a 3-dimensional navigation system called 621B. There was concern that these systems were duplicating capabilities. The Air Force was designated as the executive agent to consolidate the Timation and 621B concepts into a comprehensive system which could meet the requirements of all the services. The result was the Global Positioning System (GPS).

GPS begins

f. The GPS program began in 1973. The first test signals from space were transmitted from the Navigation Test Satellite 2, launched in June 1977. The basic plan was to build and launch Block I satellites in order to develop, test and refine the system, then launch Block II fully operational satellites. At the same time that Block I satellites were being developed, the control segment and user segments were being developed and constructed. GPS will be fully operational in 1994. Block IIR satellites will replace Block II satellites. Block III GPS satellites are being designed.

7-28 GPS Segments

Introduction

a. The Global Positioning System consists of three major segments:

  • Space Segment
  • Control Segment
  • User Segment

Space Segment

b. The space segment has 24 satellites (21 functioning satellites and 3 on-orbit operational spares) in 6 circular orbital planes with an inclination of 55° at 10,900 nautical miles altitude with a period of about 12 hours. The inclined orbits and altitude result in complete global coverage. The first Block I developmental GPS satellite was launched in February 1978. By October 1985, ten Block I satellites had been launched. These did not provide continuous navigation service but did provide sufficient coverage time for the continued development and testing of the satellites, the control segment and users' receivers. The first improved Block II satellite was launched in July 1989. Four more were launched that same year and five in 1990. Only one was launched in 1991 because of problems with some of the new satellites which had to be fixed prior to launch. Launches resumed in 1992. In 1992, the GPS constellation of satellite consisted of a mix of older Block I and newer Block II satellites. Block I satellites provide accurate navigation and timing data but do not have some of the features that the Block II satellites have. The design life of GPS satellites is seven years. Some have continued to function for more than 10 years. After the full constellation of 24 operational GPS satellite is in orbit replacement satellites will be launched, as necessary, to replace ones that begin to develop problems. The GPS signals are transmitted continuously by all the GPS satellites. When fully operational users anywhere in the world will be able to receive signals from at least four satellites at all times. Usually, six GPS satellites will be in view. If one satellite should fail to provide accurate data, there will still be sufficient satellites to give total coverage. As a result, availability of the system is estimated at more than 99% of the time. By the time signals from the satellites reach the surface of the Earth they are rather weak. The signals are not able to penetrate more than a few inches of dirt, cannot penetrate through buildings or metal. Weather has little effect on the GPS signals. Clouds, rain and snow have little effect. Extremely heavy rainfall will, however, degrade the signal. Very dense, overhead vegetation can block or weaken the signals. In most forests this has not proven to be a significant problem, however, some problems may exist in dense, triple canopy jungle. A proposed procurement for a constellation of 51 2F GPS Block III series satellites has been reduced to 33 satellites. A maximum of 32 GPS satellites can be on line at one time. The new Block III satellites will have a design life of ten years.

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7-28 GPS Segments, cont’d

Control Segment

c. The control segment, operated by the 50th Space Wing of the U.S. Air Force Space Command, consists of the GPS Master Control Station (MCS) and a Monitor Station at Falcon Air Force Station, Colorado Springs, Colorado. Other Monitor Stations are located in Hawaii, Ascension Island, Diego Garcia and Kwajalein Atoll. Monitor stations track all GPS satellites in view and collect ranging data and satellite clock data passed to MCS over Defense Satellite Communications System satellites. Operators in the MCS calculate each satellite's status, ephemeris and clock data which is then sent to transmitting antennas located at the Monitor Stations (except Hawaii) where the data is uploaded to each satellite for inclusion in the navigation message transmitted by the satellites. This is done to maintain the desired system accuracy. A back-up Control Station is planned for Onizuka AFS, CA. Prior to launch, GPS satellites are checked out at a facility at Cape Canaveral AFS, FL. During Operation DESERT STORM/SHIELD, GPS MCS operators worked hard to maximize coverage over Southwest Asia.

User segment

d. The user segment of GPS consists of all military and civil users, both U.S. and foreign. There are two basic types of service available:

 

Standard Positioning Service (SPS)

SPS is available to all users around the world. Access to the SPS does not require approval by the U.S. Department of Defense. A feature called Selective Availability (SA) limits the accuracy of SPS to 76 meters Spherical Error Probable (SEP) or 100 meters 2dRMS (2-dimensional Root Mean Square). In a time of crisis this accuracy can be degraded significantly to prevent an enemy from using GPS to its advantage.

 

Precise Positioning Service (PPS)

PPS is only available to users authorized by the Department of Defense. This includes all U.S. military services, the Coast Guard and selected other government agencies, the military services of the NATO nations, Australia and other designated users. The user must load a special cryptological key (code) in order to access PPS. PPS provides the GPS receiver access to the most accurate signals from the satellites. GPS is designed to provide a PPS accuracy of approximately 9 meters Circular Error Probable (CEP) or about 16 meters Spherical Error Probable (SEP) for a three dimensional position, anywhere in the world. The velocity accuracy is 0.1 meters/second RMS. Time accuracy is 100 nanoseconds (billionths of a second).

7-29 How GPS Works

Introduction

a. For a receiver on the ground to determine its 3-dimensional position it must calculate four unknowns; latitude, longitude, altitude and time. For this reason the ground receiver must receive signals from four satellites. To calculate a 2-dimensional position (no altitude) requires three satellites to be in view of the receiver. When the full constellation is in orbit there will be six satellites in view most of the time.

GPS receivers

b. A GPS receiver has to acquire and track signals from GPS satellites, achieve carrier and code tracking, collect data from the NAV message included in the signals, and then make pseudo-range and relative velocity measurements. From these the receiver can calculate the GPS time, its position and velocity. The results are then displayed on a screen.

GPS receiver accuracy

c. The accuracy of GPS receivers is stated in statistical terms. It is important to have some understanding of these terms so that the data, particularly the accuracy of positions, is not misinterpreted. Many GPS receivers can display 10 digits in MGRS grid coordinates which equals to 1 meter resolution. This does not mean that the receiver has 1 meter accuracy. Manufacturers and the military often use different techniques to express accuracy. When comparing performance, the comparison must be made under the same operating conditions and expressed in the same terms. The U.S. Department of Defense commonly uses standards which are stated at the 50% probability. STANAG 4278 states that all navigation system performance figures in NATO documents will be stated at the 95% probability level. The DoD uses 2-dimensional Distance Root Mean Squared (2dRMS) at 95% probability to state accuracies that will be maintained when Selective Availability is on. Precise conversion between these different techniques requires a rigorous statistical solution, however, it is possible to give some approximate equivalents. A normal distribution is assumed.

Linear Error Probable (LEP)

d. Linear Error Probable (LEP) is the distance from a point on a line within which 50% of the measurements will occur. Usually LEP is used to express the vertical (altitude) error.

Circular Error Probable (CEP)

e. Circular Error Probable (CEP) is the radius of a circle containing 50% of the individual measurements. A receiver with an accuracy of 100 meters CEP means that 50% of the time the solution will be correct within a radius of 100 meters and 50% of the time the error will be greater than 100 meters. CEP usually refers to accuracy in the horizontal plane only without regard to vertical (altitude) accuracy.

Spherical Error Probable (SEP)

f. Spherical Error Probable (SEP) is the radius of a sphere within which there is a 50% probability of locating a point or being located. SEP includes both horizontal and vertical error.

2- Dimensional Root Mean Squared (2dRMS)

g. 2-Dimensional Distance Root Mean Squared (2dRMS), as defined in STANAG 4278, is the radius of a circle that contains 63% of all measurements. For example, 100 meter (2dRMS) means that 63% of all solutions will be within a circle with a radius of 100 meters. There are, however, some documents which base 2dRMS on a 95% probability level. An accuracy capability of 100 meters (2dRMS @ 95%) is better than 100 meters (2dRMS @ 63%).

Standard Deviation

h. Standard Deviation (Sigma or s ) is a measure of dispersion of random errors about the mean (average) value. It is calculated as the square root of the sum of the squares of deviations from the mean (average) divided by the number of measurements minus one. The percentage of measurements that represent one standard deviation depends on the number of dimensions included in the measurement. If a 3-dimensional accuracy is stated as 20 meters (1s ) then only 19.9% of the measurements can be expected to be within a sphere with a radius of 20 meters. This would indicate that stating 3-dimensional errors in terms of one standard deviation does not really tell very much about the accuracy of the solution since about four out of five times the error will be greater than that figure.

Conversions

i. The following conversions are helpful but are only approximate:

LEP = 0.6745 s CEP = 1.746 LEP = 1.18 s = 0.42 (2dRMS @ 63%)

SEP = 0.76 LEP + 0.87 CEP = 1.54 s

2dRMS at 63% = 1.32 CEP = 2.3 LEP = 1.56 s

2dRMS at 95% = 2.4 CEP = 4.19 LEP = 2.83 s

Continued on next page

7-29 How GPS Works, cont’d

Position Dilution of Precision (PDOD)

j. To get the best possible accuracy, a GPS receiver will select satellites that offer the best geometry. This is the same approach that soldiers use in selecting points to sight on when using the technique of resection with a map and compass to determine a location. A more accurate answer is obtained by sighting on two or more points that are far apart. This is also true with GPS. Satellites which appear farther apart in the sky provide a more accurate position solution than ones close together. Since the ephemeris of each satellite is known by the GPS receiver it is possible to calculate which combination of GPS satellites provide the best geometry at a given time. This is translated into a figure called the Position Dilution of Precision (PDOP). Since the satellites move across the sky relative to the user, the PDOP is always changing. A low PDOP is better. A PDOP of 4 to 6 is considered good. Position solutions calculated when the PDOP is from 6 to 10 should be used cautiously because they may have significant error. A PDOP that is above 10 indicates unacceptable error. Once all of the GPS satellites are in orbit, users should not experience a PDOP higher than 6.

Geometric Dilution of Precision (GDOP)

k. The Geometric Dilution of Precision (GDOP) is the same as the PDOP except that it includes a factor to account for any errors in the time.

Possible errors

l. As with any measuring system, GPS cannot provide absolute precision because errors are introduced from a number of different sources.

The source and typical extent of position errors are summarized below:

 

Source and Extent of GPS PSEUDO Range Error

 

 

SPS

Pseudo Range Error (1s )

PPS

Pseudo Range Error (1s )

 

Satellite clock and navigation

3.0 m

3.0 m

 

Satellite perturbations

1.0 m

1.0 m

 

Satellite ephemeris prediction model

4.2 m

4.2 m

 

Receiver noise

7.5 m

1.5 m

 

Ionospheric noise

5.0 - 10.0 m

2.3 m

 

Tropospheric delay

2.0 m

2.0 m

 

Multipath

1.2 m

1.2 m

 

Other

1.2 m

1.2 m

 

User Estimated Range Error (UERE)

(calculated by root-square sum)

10.8 - 13.9 m

6.6 m

Atomic clocks

m. Each GPS satellite has atomic clocks on board to maintain accurate time. Data on the status and accuracy of these atomic clocks are sent to the GPS control segment. Corrections are sent to the satellites whenever necessary to keep the system within specification. Atomic clocks are not nuclear powered. They get their name because they use the very stable oscillations of certain elements, often rubidium or cesium, to measure the passage of time. The accuracy is very high (+/- 1 second every 360,000 years) but it is not perfect.

Orbits of GPS satellites

n. The orbits of GPS satellites have been selected to optimize stability, longevity and coverage. The orbits are very stable, therefore it is possible to calculate each satellite's ephemeris with high precision. Even so, slight variations are introduced due to the uneven density of the Earth, magnetic fields in space, fluctuations in solar radiation and other factors external to the satellites. In addition the ephemeris prediction model is not absolutely precise. The monitor stations track each satellite and the GPS control segment updates the ephemeris frequently (usually every four hours) to remove as much error as possible.

GPS Receivers

o. GPS receivers use advanced electronic circuitry to receive, decode and process the data sent by the satellites. The receivers are not, however, perfect so a small amount of error is introduced.

Multipath Signals

p. A certain amount of error is caused by reception of multipath signals. Multipath results from signals being reflected off of objects in the vicinity of the receiver. Most receivers employ techniques to minimize the impact of multipath signals.

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7-29 How GPS Works, cont’d

Charged Particles

q. The ionosphere is made up of charged particles. Size and density of the ionosphere over a particular area is always changing due to sunlight, fluctuations in Earth's magnetic field, solar radiation and other factors. Radio signals transmitted through the ionosphere are slowed according to frequency of the signal and state of the ionosphere at that instant. The lower the frequency of signal, the more it is slowed by the ionosphere. Uncorrected ionospheric delay results in significant error in the position solution. PPS receivers receive the P-code on both the L1 and L2 frequencies. Since the same code is transmitted simultaneously on two different frequencies it is possible to measure the relative difference in time of arrival and calculate effect of the ionosphere. SPS receivers cannot process P-codes, therefore an equation is used to estimate ionospheric delay. This is, however, less accurate than the PPS technique.

Water Vapor

r. Water vapor in the troposphere also results in some error in the position solution. Unfortunately, this cannot be compensated for in the same way as the ionospheric delay.

Selective Availability

s. SPS can provide very good accuracy for most applications. There is concern that a potential enemy could use GPS to its advantage, therefore a feature called Selective Availability (SA) has been implemented beginning with Block II satellites to limit accuracy of GPS for users not specifically authorized by DoD. SA introduces pseudorange and doppler shift errors that appear to be random. During peacetime, DoD has agreed to keep the error introduced by SA to less than 100 meters (2dRMS) or 76 meters Spherical Error Probable (SEP). During a time of crisis, the error caused by SA can be increased to about 2,000 meters, thus making GPS much less worthwhile. SA can be applied to both C/A-code and P-code. Users authorized by DoD to use GPS are provided with a cryptological key which can be loaded into GPS receivers which are built to store it. The key removes effects of Selective Availability and allows a receiver to calculate the best solution possible.

L-Band Frequencies

t. All GPS satellites transmit pseudo random noise spread spectrum signals on the same two L-Band frequencies (L1 at 1575.42 Mhz and L2 1227.6 Mhz). L1 carries a Coarse/Acquisition code (C/A-code) and a Precision code (P-code). L2 usually carries only P-code, but can be programmed to transmit C/A-code instead. Both are a series of binary digits assembled into codes called pseudo random noise because signals seem random and similar to background noise that is always present.

C/A Code

u. The C/A-code is used to provide Standard Positioning Service. The C/A-code is relatively short and repeats itself every millisecond. A different C/A-code is assigned to each GPS satellite so that receivers can distinguish among them. Structure of C/A-code is available to all users and receiver manufacturers without restriction.

Precision Code and Anti-Spoofing

v. The Precision code (P-code) is a digital code that is 267 days long. Each GPS satellite transmits a unique seven day portion of the code. The P-code is normally transmitted simultaneously on both L1 and L2. The P-code is only available to users authorized by the Department of Defense. Anti-Spoofing (AS) encrypts the P-code into the Y-code to prevent an enemy from transmitting signals which could mimic a GPS satellite. The same cryptological key used to remove the effects of S/A is used to decrypt the Y-code into the P-code. Selective Availability can also be implemented on the P-code. S/A and AS can be implemented independently.

User Position Error

w. The position error for a user is a combination of UERE, PDOP and effects of Selective Availability and Anti-Spoofing. Assuming a PDOP of 4 to 6, typical position errors are:

 

GPS USER POSITION ERROR

 

SA

AS

 

SPS

PPS

 

OFF

OFF

C/A-code

30 m SEP

16 m SEP

 

 

 

P-code

 

16 m SEP

 

OFF

ON

C/A-code

30 m SEP

16 m SEP

 

ON

OFF

C/A-code

76 m SEP

16 m SEP

 

 

 

P-code

 

16 m SEP

 

ON

ON

C/A-code

76 m SEP

16 m SEP

NAV Message

x. A NAV message is superimposed onto both C/A-code and P-code. It contains data on GPS time, a hand-over word for transition from C/A-code to P-code tracking, ephemeris data for a particular satellite being tracked, almanac data for all satellites in a constellation, satellite health status and coefficients of ionospheric delay for SPS users. A NAV message contains 25 data pages each with 1500 bits of data. It is transmitted at 50 bits/second and takes 30 seconds to receive one data page and 121/2 minutes to receive all 25 pages. To allow receivers to reduce time needed to collect critical data, certain information is repeated in all 25 data pages.

7-30 GPS User Equipment

Introduction

a. The development and acquisition of GPS receivers for the military users is managed by the GPS Joint Program Office (GPS JPO) located at Los Angeles Air Force Base, California. The GPS JPO is staffed by personnel from the U.S. Air Force, Army, Navy, Marine Corps and Coast Guard along with representatives from the U.S. Defense Mapping Agency, Australia, and many NATO countries. The GPS JPO also provides information to manufacturers of civilian GPS receivers and processors.

Different Receivers for Different Users

b. There are many different types of GPS receivers, each intended to best meet the needs of a specific group of users. The responsiveness and accuracy of GPS receivers is determined by the electronics (hardware) and programs (software) stored in the set. Innovative approaches by manufacturers are common.

For military users, the most important feature of a GPS receiver is the ability to use the cryptological code to remove the errors introduced by Selective Availability since this would otherwise be the largest source of position error.

Number of Channels and Number of Satellites

c. The number of channels in a receiver determines how many satellites it can receive signals from simultaneously. When not moving, the number of channels in a receiver is not a major factor in determining its position accuracy. When stationary, a 1-channel GPS receiver can be just as accurate as a 5-channel receiver. More channels do allow a receiver to respond and update position solutions faster. Multiple channels do allow a receiver to remain locked on the satellites in view. A multiple channel set does provide better performance when moving, especially when moving very slowly or very fast. A single channel set does not update it's data quickly enough to detect small changes in position. The result can lead to inaccurate direction of movement while moving dismounted in a field environment. A multiple channel receiver detects small changes in position with greater distinction, thus it is able to report a more accurate and consistent direction of movement. A multiple channel set will also reacquire satellites faster following a brief interruption in the signal. The number of channels in a GPS receiver used to be a significant factor in the cost of a set, however, advances in electronics have made sets with five or more channels much more affordable.

Sequential GPS Receivers

d. Sequential receivers typically have one, two or three channels. A one channel sequential receiver has to measure pseudoranges on the L1 frequency (and L2 to provide PPS). The NAV message from each must be read to obtain ephemeris information. Movement of the receiver during this time reduces the accuracy of the solution. As a result, 1-channel sequential receivers are best suited for stationary use. When stationary, a PPS 1-channel GPS receiver can have the same accuracy as other GPS receivers. There are a number of different approaches to two channel sequential receivers but typically one channel is dedicated to making pseudorange measurements and carrier tracking while the second channel reads the NAV message from each satellite in view. This reduces the Time to First Fix and allows the receiver to maintain tracking at moderate speeds.

Multiplex Receivers

e. A multiplex GPS receiver has one channel which switches at a fast rate (about 5 milliseconds) between the satellites being tracked, continuously collecting sampled data from satellites in view. In many respects, this type of receiver has performance characteristics similar to a continuous multi-channel receiver. Because a multiplex receiver only tracks each satellite for a fraction of the time they do not use all the power in the spread spectrum signal which makes them somewhat more susceptible to jamming than a continuous receiver.

Continuous Receivers

f. Continuous receivers have at least four channels so that four satellites can be tracked simultaneously. The most common are 5-channel receivers. Four channels are used to track four different satellites for 3-dimensional position solutions. The fifth channel is used to read the NAV message of the next satellite to be used in the selected constellation and, if a PPS set, to perform dual frequency measurements to compensate for the ionospheric delay. Having a channel dedicated to each satellite being used to derive position solutions enables continuous receivers to maintain lock on the satellites even during rapid turns or acceleration. These types of receivers also provide better resolution of azimuth for slow moving users. Continuous receivers typically have the fastest Time to First Fix, and better anti-jamming capabilities.

Continued on next page

7-30 GPS User Equipment, cont’d

All-in-View Receivers

g. When the full constellation of GPS satellites is in orbit most users will have six satellites in view at all times. Most receivers are programmed to select the four satellites which have the best geometric configuration (lowest PDOP/GDOP) to provide the best 3-dimensional position solution. All-in-view GPS receivers have sufficient hardware channels to simultaneously monitor all the GPS satellites in view and to quickly acquire satellites which move into view while the set is in use. Typically position solutions are derived using data from all of the satellites in view and the results are filtered by software in the set to display the most accurate solution to the user. An advantage of all-in-view receivers is that operators would not notice a change in performance even if signals from one of two satellites were temporarily blocked due to dense trees, nearby steep hills, buildings or other obstacles. In the past, all-in-view receivers were expensive, however, continued development and integration of digital signal processing components may make them more affordable.

Differential Receivers

h. The concept of Differential GPS (DGPS) is to use data gathered by a GPS receiver placed at a precisely known location to determine errors from all sources to include those induced by Selective Availability. Corrections are then broadcast to other users in the same geographical area. These corrections compensate for virtually all errors from the satellites and the control segment. Corrections of ionospheric and tropospheric delay errors are effective for GPS receivers within 150 miles (250 km) from the known location. There are two basic methods to transmit the correction data. One is to transmit the corrections on another frequency to GPS receivers equipped to receive and process GPS signals from the satellites and also the differential corrections. A pseudo-satellite is a ground transmitter that transmits the differential corrections on a GPS frequency. Since the pseudo-satellite is not listed in the almanac or the ephemeris, modifications are needed to the GPS receiver so that it will not ignore the data. Differential GPS is receiving considerable attention among civil users who are concerned with the affects of Selective Availability. The U.S. Coast Guard is installing a network of differential GPS beacons around the coast of the United States to provide ships navigating in coastal waterways, rivers and ports with enhanced position accuracy even with S/A activated. The Federal Aviation Administration has shown interest in using differential GPS for aircraft navigation, especially during takeoffs, landings and while taxiing on the ground. Differential GPS has not yet been used by the U.S. military but has potential for applications requiring high accuracy. Some examples are:

  • Field artillery and mapping survey.
  • Delivery of precision munitions.
  • Marking obstacles and cleared paths to follow.
  • All weather helicopter operations.
  • Non-precision approach for aircraft, particularly into non-instrumented airfields.
  • Autonomous operation when the control segment cannot update the satellite ephemeris.
  • Instrumented training ranges.

Military Grid Reference System

i. All receivers that have been acquired for the Army have the ability to display position in Military Grid Reference System (MGRS) coordinates, Universal Transverse Mercator (UTM) coordinates and Latitude/Longitude. They all display distances in feet, meters, miles or kilometers. Most can store position and elevation data for at least 50 waypoints. Many have the ability to have routes programmed in to them to assist the user in following a designated course. In most cases, two of the same kind of receiver can be connected by a special cable and the waypoint and route data that is stored in one can be electronically transferred to the other. This can save a considerable amount of time and virtually eliminates the chance of differences in the data in the sets.

Where to use

j. All GPS sets can be connected to an external antenna. GPS signals cannot penetrate so that the receiver can be used inside vehicles and aircraft which have metal roofs, sides or supports. The GPS signal penetrates Kevlar and canvas with very little loss in power.

Security with Receivers

k. Receivers developed solely for military use are able to store the required cryptological key required to compensate for S/A and A-S. The key is loaded by KYK-13 or KOI-18 data loaders. A PPS Security Module (PPS-SM) has been developed so that the cryptological key can be stored in a GPS receiver yet the set remains unclassified.

7-31 GPS Receivers in the Army

U.S. Army GPS Receiver Models

a. The table below summarizes the features of many of the GPS receivers found in the Army.

 

U.S. Army GPS Receiver Models

 

Model

Channels

Type Service

Platform

On Hand

 

AN/PSN-8 or AN/VSN-8

1

PPS

Manpack or Vehicle

247

 

AN-PSN-9 or AN/VSN-9

5

PPS

Manpack or Vehicle

100

 

AN/ASN-149 (V1 or V2) (RCVR UH or OH

2

PPS

Helicopter

535

 

AN/ASN 149 (V3) (RCVR C4)

2

PPS

Fixed Wing Aircraft

0

 

AN/PSN-10 SLGR

2 or 3

SPS

Handheld

8,500

 

NAV PRO 1000M

1 or 2

SPS

Handheld

1,000

 

AN/WRN-6 (RECVR 3S)

5

PPS

Vessels

55

 

Miniature Airborne GPS Receiver (MAGR)

5

PPS

Aircraft

see text

 

Precision Lightweight GPS Receiver (PLGR)

5

PPS

Handheld or Vehicle

see text

AN/PSN-8 GPS Receiver

b. The AN/PSN-8 Manpack GPS receiver is a 1-channel PPS set that weighs about 17 lbs. The set is commonly called the Rockwell-Collins manpack, in reference to the manufacturer. It is designed to be carried on a rucksack frame. A cryptographic code can be loaded into the set, thus giving it the capability to provide PPS. The AN/VSN-8 is the vehicular version of the AN/PSN-8. There are 247 of these receivers in the Army.

AN/ASN-149 (V1) GPS Receiver

c. The AN/ASN-149 (V1) GPS Receiver, also referred to as Receiver UH, is a 2-channel PPS set built to MIL-STD specifications. It is intended for installation on UH-60A, AH-1S (FM), EH-1 and the CH-47D aircraft. The receiver/processor is mounted in an equipment rack. Only a control/display unit (CDU) is mounted in the cockpit. This receiver can be operated as a stand-alone system or in conjunction with a Doppler navigation system.

AN/ASN-149 (V2) GPS Receiver

d. The AN/ASN-149 (V2) GPS Receiver, also referred to as Receiver OH, is a 2-channel PPS set built to MIL-STD specifications. It is intended for installation on OH-58D, AH-64, MH-60, MH-47, OV-1 and EH-60 aircraft. The receiver connects to Control/Display Unit in the cockpit through the aircraft's 1553 data bus.

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AN/ASN-149 (V3) GPS Receiver

e. The AN/ASN-149 (V3) GPS Receiver, also referred to as Receiver C4, is a 2-channel PPS set built to MIL-STD specifications. It was intended for installation in selected Army fixed wing aircraft. The current Army GPS program does not provide for any of these receivers. It is likely that requirements will be met by the MAGR or an equivalent set.

AN/PSN-9 GPS Receiver

f. The AN/PSN-9 is a 5-channel PPS receiver that weighs about 9 pounds. It can be carried in a rucksack or over the shoulder. A small Control/Display Unit (CDU) is connected by a cable to the receiver. The antenna module can be separated from the receiver to allow remote operations. Position and velocity updates are provided once per second. The AN/VSN-9 is a vehicular version of the same receiver. The AN/VSN-9 is also installed in some small Army watercraft. There are 100 of these receivers in the Army.

AN/WRN-6 GPS Receiver

g. The AN/WRN-6 GPS Receiver, also referred to as Receiver 3S, is a 5-channel PPS set built to MIL-STD specifications. This set was developed primarily for use by the U.S. Navy but is also installed in 55 larger, ocean-going Army vessels such as the Landing Ship Vehicle (LSV), Landing Craft Utility (LCU) and large tugs.

AN/PSN-10, Small Lightweight GPS Receiver (SLGR)

h. The SLGR is a hand-held C/A-Code developed as the "Trimpack" by Trimble Navigation. It is only capable of providing SPS. The receiver weighs 4 lbs. It uses an internal lithium battery or can be connected to vehicle power. The sets were used by tactical units during field training exercises in 1989 and 1990 in order to gain operational experience and to demonstrate how GPS could support the Army. They have proven to be very rugged. In August 1990, they were shipped to units deployed in support of Operation DESERT SHIELD. They were in such great demand that the Army purchased thousands more on an emergency basis. The initial SLGRs have two channels. Newer models have three channels. There are now about 8,500 SLGRs in the Army.

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Miniature Airborne GPS Receiver (MAGR)

i. The MAGR, also referred to as Receiver 3M, is a 5-channel PPS GPS receiver. It is a smaller, lighter and faster version of the Receiver 3A developed for Air Force aircraft. It connects to the aircraft data bus through a standard 1553 multiplex bus. It is intended for use in selected Army and Marine Corps aircraft. Ten have been provided for integration and testing. Approximately 250 are programmed to be purchased.

NAVPRO 1000M Magellan GPS Receiver

j. The original NavPro 1000M is a single channel C/A-code GPS receiver capable of providing only SPS. It is slightly smaller than the Trimpak. The set can be powered by AA size batteries or through a vehicle power adaptor. Later models have 2-channels. About 1,000 of these sets were purchased for the Army for use during Operation DESERT STORM.

Precision Lightweight GPS Receiver (PLGR)

k. The PLGR is the next generation of handheld, portable GPS receivers for the Army and other military services. A production contract was awarded in 1993. Initial deliveries will begin in mid-1993. The set will be a 5 channel PPS receiver. The handheld version is powered by disposable batteries. A mount and external antenna will also be available to install the set in vehicles. When installed in a vehicle, power can be provided by the vehicle's power system. The initial production quantity is for 11,000 sets. The validated Army requirements are more than 70,000, even a smaller Army force structure.

7-32 Army Use of GPS Receivers

Early Use

a. From September 1989 through July 1990, the Army Space Command, assisted by the U.S. Army Space Institute, conducted demonstrations of the capabilities of GPS receivers through the Army Space Demonstration Program. Approximately 400 Small Lightweight GPS Receivers (SLGRs) were temporarily issued to a variety of tactical units for their use during normal tactical training at Fort Irwin, CA; Fort Chaffee, AR; Grafenw`hr, GE; Korea and other locations. These were not formal tests of the SLGRs but rather opportunities for tactical units to use GPS during their normal tactical operations. Soldiers of all ranks and many different Military Occupational Specialties (MOS) used the SLGRs to accomplish a wide variety of tasks and missions. GPS receivers had never before been used in such a high density in as many tactical units. Valuable operational experience was gained through this portion of the Army Space Demonstration Program that was used to develop requirements and specifications for the PLGR.

DESERT STORM Use

b. Following Iraq's invasion of Kuwait in August 1990, many of the units which had participated in the Army Space Demonstration Program were deployed to Saudi Arabia. These units, and others which had learned of GPS's capabilities, immediately requested GPS receivers be issued to them. The Army Space Command<$IArmy Space Command> collected more than 500 SLGRs and provided them to many of the deploying units. In addition, 377 AN/PSN-8's, 170 AN/PSN-9's were also deployed. Demand far exceeded the number of available sets, therefore the Army, through the GPS JPO, purchased about 7,500 more Trimpacks from Trimble Navigation and about 1,000 NavPro 1000M receivers from Magellan Systems. Since these sets had to be built, shipped to the theater of operations and then distributed, only slightly more than half were delivered to users before the end of hostilities.

Desert Navigation

c. Navigation on the desert is very difficult. There are few distinct permanent landmarks to relate to and few roads. Trails crisscross frequently making it difficult to follow specific ones. Blowing sand and dust limit observation. Weather conditions can vary significantly.

Maps of the Kuwait theater of operations were out of date. Different versions varied significantly. The open dessert terrain with very few terrain features to orient on, very limited road network and bad weather conditions made position determination and navigation very challenging. The Persian Gulf War was the first combat operation in which GPS was used to provide support to all services of the U.S. military. Although GPS was not fully operational, a sufficient number of satellites were in service to provide almost 24 hours of 2-dimensional coverage and about 18-20 hours of 3-dimensional coverage. The average position accuracy during the conflict was 8.3 meters SEP and 4.5 meters CEP which was better than the system specifications. Due to the large number of C/A-code receivers in use by the U.S. Army and the U.S. Marine Corps, and the fact that Iraq did not have GPS receivers, Selective Availability was not activated.

GPS Enhanced Coordination

d. GPS receivers provided very accurate position data to users throughout the theater of operations. It was instrumental in the execution of the high speed advance of forces across the desert. GPS receivers also provided a common grid reference system which enhanced coordination between the Army, Marine Corps, Navy, Air Force and the military forces of other allied nations. Whenever accurate position data and precise navigation was needed, GPS was used with outstanding results. In addition, GPS receiver were used to instantly convert Military Grid Reference Systems (MGRS) coordinates into latitude and longitude commonly used by the Air Force and the Navy.

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Examples of GPS Receivers

e. Some examples of the use of GPS receivers are:

  • Special Forces teams were inserted behind enemy lines using GPS to navigate and to report their location and the location of enemy units and facilities. Knowledge of their precise position aided in their resupply or extraction.
  • Helicopters equipped with GPS were able to know their position accurately at all times and to navigate with precision along predetermined routes.
  • Reconnaissance forces were able to accurately report on enemy positions and forces. Patrols were able to navigate accurately and with confidence at night without landmarks.
  • Tanks and other armored vehicles, using external antennas, knew their exact position, direction of movement and speed, even when the hatches were closed to protect the soldiers inside.
  • Large formations of combat units were able to conduct coordinated, rapid advances without unwanted dispersion.
  • Fire support teams reported their positions accurately, even while moving rapidly.
  • Field artillery units were accurately positioned with less need for surveyed firing points.
  • Accurate indirect fire was provided more quickly with fewer rounds used for adjustment.
  • GPS prevented friendly units from erroneously navigating into the sectors of adjacent units or into areas mined by the enemy.
  • Once a breach in an obstacle was made by an advancing coalition force, the location was determined using GPS and reported to the other units that planned to pass through that breach. GPS gave them the ability to navigate directly to the breach.
  • Air support, both close-in tactical support and high altitude bombing was more accurate and timely.
  • Attack helicopters were able to navigate at night to planned firing positions to engage precisely located targets.
  • Logistics units were able to accurately navigate over long distances to precise locations, thus providing support more quickly.
  • The location of casualties was determined using GPS and reported. Medical evacuation teams navigated directly to the exact location of the casualties thus medical support and evacuation was provided more quickly.

Use in Europe

f. The military forces of Great Britain, France, Italy, the Netherlands, Denmark, Canada and Belgium also had some GPS receivers. Arab coalition forces did not have GPS receivers but the U.S. Army Special Forces teams attached to them were equipped with SLGRs.

Conclusion

g. In summary, virtually every task on the battlefield that directly or indirectly required position location or navigation was more easily and more accurately accomplished through the use of GPS receivers.

In addition to improved stand-alone GPS receivers, efforts are on-going to integrate or imbed GPS into other battlefield systems such as inertial navigation devices, survey equipment, combat net radios, Army Tactical Command and Control System (ATCCS) equipment, and numerous other systems that have a requirement for position determination, velocity or precise time.

7-33 Other Uses of GPS

Civilian Use of GPS

a. The civilian market for GPS receivers is expanding rapidly. Many companies, both U.S. and foreign, are developing receivers to fill specific needs. Regardless of how many channels and other features to improve performance, all commercially developed sets are C/A-code receivers and are not able to provide PPS because they cannot store the cryptographic codes needed to compensate for SA and cannot decrypt the Y-code AS is turned on.

GLONASS

b. The Soviet Union began on-orbit testing of it Global Navigation Satellite System (GLONASS) in 1982. It is very similar to the U.S.'s GPS. Russia has continued to support the GLONASS program. When fully operational, probably in 1995, GLONASS will consist of 24 satellites in three orbital planes. GLONASS satellites are launched three at a time by the SL-12 booster rocket. The GLONASS satellites are similar to the GPS satellites except that they do not have on-board atomic clocks. Precise timing is maintained at ground stations and periodically transmitted to the satellites. In spite of being less complex than GPS satellites, GLONASS satellites have demonstrated an operational life of only a few years, therefore the satellites need more frequent replacement than GPS satellites. In 1988 and 1989, the USSR released specifications on the signal and data structure of the transmissions from the satellite. This provided the information needed for anyone to access to the system since without this information it was not possible to know how to process the data received from the satellites in order to calculate a position solution. GLONASS satellites transmit almost identical navigation data from each satellite, but each satellite transmits at a slightly different frequency. GPS satellites all transmit on the same frequencies. It is possible to build user sets that receive data from both GPS and GLONASS.

TRANSIT

c. The U.S. Navy Navigation Satellite System, more commonly referred to as TRANSIT, provides passive, all weather, worldwide position information to ships (military and commercial) and to fleet ballistic missile submarines. The position update is two-dimensional and requires accurate knowledge of the user's velocity and antenna height. The TRANSIT constellation consists of twelve satellites in low earth orbit, of which seven are operational and five are spares. Because of the TRANSIT satellite constellation configuration, the waiting time between fixes varies from eight to 20 minutes, depending on the user's latitude. Accuracy is typically 200 meters without detailed post processing. There are thousands of TRANSIT receivers/terminals in use. They are deployed on every Navy surface ship and submarines and on thousands of commercial and private vessels. The only TRANSIT receivers in use by the Army are on ocean-going vessels. The receivers are produced commercially and are not portable. Although TRANSIT has served the Navy well, it does not meet the needs of the Air Force, Army and Marine Corps. GPS will eventually replace Transit.

TSIKADA/ NADEZHDA

d. The Soviet Union developed the Tsikada navigation satellites as functional equivalents of the U.S. TRANSIT satellites. Another set of very similar constellation of satellites was launched for use by the Soviet military, primarily the Navy and merchant marine. In 1989, a COSPAS/SARSAT search and rescue receiver/transponder was added to a civilian Tsikada navigation satellite and was renamed Nadezhda, Russian for "hope." These systems are helpful for stationary users or ships. Accuracy and availability of service from GPS and GLONASS are better.

COSPAS/ SARSAT

e. COSPAS/SARSAT is an international satellite based search and rescue system. COSPAS is the Russian acronym for Space System for Search of Vehicles in Distress. SARSAT is the acronym Search And Rescue Satellite Aided Tracking. The U.S., France, Canada and the USSR established the system in 1979. Russia has continued the USSR's role in the system. COSPAS/SARSAT sensors are carried on U.S. NOAA weather satellites and on Russian Nadezhda navigation satellites. The first launch was on a Russian satellite in 1982.

A special distress radio transmitter is now carried on most ocean going vessels, commercial aircraft and many smaller civilian aircraft. The transmitter is activated manually or automatically by sensors which detect submersion in water or shock impact. When a satellite with a COSPAS/SARSAT module on board passes within range (about 2,500 km) it detects the signal from the distress radio transmitter and relays it to the nearest Local User Terminal on the ground. The Local User Terminal calculates the location of the distress signal to within 10-15 km. Search and rescue aircraft and ships can then be directed to a specific area to conduct a detailed search.

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7-33 Other Uses of GPS, cont’d

Worldwide Terminals

f. There are Local User Terminals located in the United States, Canada, Russia, France, Great Britain, Brazil, Sweden, Norway, Spain, India, Pakistan, Singapore, Hong Kong, Indonesia, Japan, Australia, New Zealand, Chile, Uraguay and Brazil. The result is nearly worldwide coverage.

More than 2,500 people have been rescued around the world, often in remote land and ocean areas where search operations would have been difficult in the past.

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