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*Chapter 13

SATELLITE SIGNALS NAVIGATION SET AN/PSN-11



The satellite signals navigation set AN/PSN-11 (NSN 5825-01-374-6643) will provide worldwide position, velocity, and time to the field artillery surveyor. When survey control is not available and time or the tactical situation preclude the use of existing survey control, the surveyor can use the AN/PSN-11 to establish positioning data.
The AN/PSN-11 (Figure 13-1) is a receiver. It is used in the global positioning system (GPS), standard positioning system (SPS), and precise positioning system (PPS).

13-1. GLOBAL POSITIONING SYSTEM

The NAVSTAR GPS is a space-based navigation and positioning system that provides accurate, three-dimensional position and velocity information, and time. The GPS is comprised of three major segments as depicted in Figure 13-2.

a. The space segment is made up of a 24-satellite constellation that orbits the earth once every 12 hours. These satellites are deployed in six orbital planes and are configured so that four or more satellites will be in view at all times. This arrangement allows for 24-hour, three-dimensional, worldwide coverage. As with the stars, the satellites rise above the horizon about 4 minutes earlier than the previous day.

b. The control segment consists of five passive-tracking monitoring stations, active-tracking ground antennas, and the master control station located at Falcon Air Force Base, Colorado. These tracking stations, located around the world, are capable of monitoring the satellite navigation messages and time signals better than 90 percent of the time. This information is relayed to the master control station, which has the capability to effect any needed corrections to the satellite timing and navigation messages.

c. The user segment consists of navigation receivers designed for marine, aircraft, and manpack or vehicle use. The receivers must have electrical line-of-site with the satellites to receive and decode the satellite signals. The internal computer uses these satellite data to generate a precise time, velocity, and three-dimensional (3D) position data. The receiver must track four satellites to obtain a 3D position, and three satellites will yield a two-dimensional (2D) position. Current position coordinates and height are obtained from a 3D position and only current coordinates are obtained from a 2D position. The receiver needs only one satellite for precise time.

13-2. STANDARD POSITIONING SYSTEM AND PRECISE POSITIONING SYSTEM

a. The SPS is available to all GPS receivers worldwide, both military and civilian. When a receiver is in the SPS mode, almanac, navigation, and timing information are received on the nonencrypted course acquisition (CA) code satellite signal. To deny unauthorized users the full accuracy of GPS, the Department of Defense (DoD) intentionally places errors in the navigation and timing signal. This process is called selective availability (SA). The SA errors are unpredictable and can produce significant horizontal and elevation errors. This is one reason why SPS receivers are not authorized for combat operations.

b. The satellites also broadcast an encrypted precise (P/Y) code. This transmission is the basis for the PPS that is used by military GPS receivers. These receivers must have crypto keys loaded to detect and nullify the SA errors, which allows for more accurate position data. Also, the crypto keys provide a means of unscrambling the encrypted P/Y code, which is an antispoofing (AS) protection. Receivers such as the AN/PSN-11 have this capability and are considered to be PPS receivers. Only PPS receivers are authorized for combat operations.

13-3. FA SURVEY APPLICATION

a. The current AN/PSN-11 basis of issue plan (BOIP) calls for one receiver with each PADS team, each conventional survey team or party, and the survey headquarters or SPCE.

b. Field artillery survey personnel will primarily use the AN/PSN-11 to initiate surveys when existing survey control is not available. This receiver allows the surveyor to begin surveying immediately with more accuracy than assumed data or a map spot.

c. A description of all parts, components, and detailed operating procedures for the AN/PSN-11 can be found in TM 11-5825-291-13. All operators should be thoroughly trained on all functions of the system before using the AN/PSN-11. The general how-to procedures for operating the AN/PSN-11 will not be in this manual. Only those issues that deal with survey positioning and orientation or specific recommended procedures will be addressed.

(1) Setup. The setup selections allow the AN/PSN-11 operator to select several options and modes of operation. (See TM 11-5825 -291-13.) An example of a typical setup for artillery surveyors is shown in Table 13-1.

(2) Position. Whenever coordinates are being determined for a critical position such as an orienting station, howitzer location, and PADS initialization or update point, use extreme care. To meet required accuracy, the system must have crypto keys loaded, be set on the correct datum, and indicate a FOM 1 before the coordinates will be used. The AN/PSN-11 with a FOM 1 will meet or exceed an accuracy of 10 meters CEP for horizontal and 10 meters PE for vertical. The averaging mode will yield a more stable and accurate set of coordinates. After the set attains FOM 1, switch to the averaging mode, and allow the counter to increment to at least 300 counts. (This will take about 5 minutes.) Position data should always be checked by a second independent mean; for example, a second receiver, an accurate map spot, or the current coordinates on the PADS.

(3) Height. The AN/PSN-11 can determine height (elevation) relative to either the horizontal datum ellipsoid (spheroid) or mean sea level. Both modes of operation can be set for meters or feet as a unit of measurement. Normally, mean sea level and meters will be the preferred selections.

(4) Direction. The AN/PSN-11 should never be used to determine orientation azimuth for firing positions. As with the PADS, azimuth computed between two sets of GPS coordinates will produce erratic results.

(5) PADS operation. The AN/PSN-11 does not require any special installation for PADS operation. Position coordinates for initialization or update can be obtained by moving a short distance away from the vehicle and placing the set directly over the desired location. Using the system away from the vehicle will reduce the possibility of antenna masking. When the averaged FOM 1 coordinates have been recorded, move the PADS vehicle to the AN/PSN-11 position. If the AN/PSN-11 auxiliary antenna is mounted on the vehicle, the precise GPS vehicle lever arms information (X, Y, Z) must be entered during system initialization as outlined in TM 5-6675-308-12 with Change 3. The operating instructions in the current PADS TM (Chapter 3, in Change 3) applies to older GPS receivers. When using the AN/PSN-11 during PADS operations, only data determined from a FOM 1 will be used.

13-4. GPS LIMITATIONS AND CONSIDERATIONS

a. Masking. GPS receivers rely on electronic line of sight with the satellites. Therefore, dense foliage, buildings, mountains, and canyons may mask the signl. The AN/PSN-11 will initially select satellites that are 10° or more above the true horizon. If for usable satellites are not detected, the set will switch to 0° until satellite acquisition. After acquisition, the set will switch to 5° above the horizon for normal operation. If enough satellites cannot be acquired, the receiver must be moved to a more suitable location.

b. Jamming. The AN/PSN-11 is subject to jamming. When low signal to noise ratios are detected or reception is blicked altogether, jamming may be the cause. Move to a new location, and try to place something between the receiver and the suspected jammer. When the signal to noise ration is above 34 decibels (db), jamming has probably been eliminated.

c. Spoofing. The AN/PSN-11 may be subject to spoofing errors. These errors are caused by false satellite signals designed to generate errors in navigation and position data. Maximum protection against spoofing is attained by using the crypto keys and the All-Y setup selection. If the AN/PSN-11 is in a spoofing environment, the system may sense spoofer activity and generate a POSSIBLE SPOOFERS warning screen.

d. Temperature. The AN/PSN-11 operating range is -4 to +158°F. When operating in cold regins, protect the receiver by carrying it inside your outer clothing or operating it from a heated vehicle. This can be done by using the auxiliary antenna.

e. Power. The AN/PSN-11 will normally be operated by using either a BA5800/U lithium battery or 24-volt vehicle power. Operating in the continuous mode, the battery will provide adequate power for about 15 hours at 71°F. During extended missions, spare batteries must be readily available. Using vehicle power eliminates this problem; however, the correct polarity must be observed to prevent damage to the system.





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