POSITION AND AZIMUTH DETERMINING SYSTEM
The PADS is a self-contained inertial surveying system. It can be used to rapidly and accurately determine position, azimuth, and elevation (PAE) in either ground or airborne survey operations. It is a precise and sensitive piece of equipment and should be handled with the same care as any other precise survey instrument. Detailed information on the PADS installation, operation, maintenance, and additional equipment is given in TM 5-6675-308-12.
a. System Uses. PADS is used to conduct FA surveys critical to the fire control function. PADS provides a common grid for weapon and TA systems. It can determine position, elevation, and grid or true azimuth for each point surveyed.
b. Configurations. The system (Figure 9-1) can be installed in and operated from an M151-series vehicle, a HMMWV, a commercial utility cargo vehicle (CUCV), and the small-unit support vehicle (SUSV). These vehicles must have alternators of 60 amperes or greater. PADS can also be operated from a UH-1 helicopter and an OH-58 light observation helicopter. Mounted in the M151-series or HMMWV, the PADS can also be transported or operated in a CH-47 cargo helicopter. PADS can be transferred to UH-60 (Blackhawk) helicopter. (The UH-60 needs a power converter to convert the helicopter's AC power to DC power.)
a. PADS primary pallet contains the basic electronic modules for the PADS. These are the control and display unit (CDU), power supply (PS), inertial measurement unit (IMU), and computer.
(1) The CDU (Figure 9-2) has a keyboard and an alphanumeric display for data entry and display of survey data and system commands. The CDU is the input-output device for the system.
(2) The PS (Figure 9-3) receives unregulated power from the transporting vehicle or PADS batteries. It provides controlled and regulated power to the IMU, computer, and CDU.
(3) The IMU (Figure 9-4) contains the gyroscopes, accelerometer sensors, and associated electronics needed to maintain the survey coordinate frame. It will measure distance traveled to each coordinate axis.
(4) The computer (Figure 9-5) Processes IMU data, computes survey data, and provides system control functions. The computer accepts data from and sends data to the CDU.
b. The secondary pallet is the battery box. The battery box (Figure 9-6) contains two 12-volt batteries. They are used to continue operations while the PADS is transferred from ground vehicle to helicopter or from helicopter to ground vehicle. It supplies additional power requirements for initialization and provides a backup power supply in case the vehicle power fails. It also houses cables, tools, and small hardware items.
PADS operation requires a two-man party, an assistant PADS operator (driver-radio operator), and a PADS operator (PADS party chief). Both must be qualified in FA survey.
a. Preparation for Operation. A PADS mission begins on receipt of a warning order with before-operation checks and services, followed by initialization. (See TM 5-6675-308-12.) After arrival at the initialization point (which must be within 100 meters horizontal and 10 meters vertical accuracy), activate the system by switching circuit breakers 1 and 2 on and pressing the ON-OFF button. Enter the spheroid, grid zone number, easting and northing coordinates, and elevation. The PADS then automatically aligns itself. Initialization takes from 30 to 45 minutes, depending on ambient temperature. After the PADS has completed initialization modes 0 through 8 and the CDU shows a flashing GO light, the system is ready to be updated. Drive to the SCP, and maneuver the PADS over the SCP by using the plumb bob suspended from the plumb bob arm for alignment. Enter the correct grid coordinates and height of the SCP. PADS is then ready to perform the survey mission. The SCP may not be accessible to the PADS by using the plumb bob. In this case, update the system by using a theodolite. The procedures for updating the PADS by using a theodolite are discussed in TM 5-6675-308-12, Chapter 3.
b. Zero-Velocity Corrections. Zero-velocity (Z-VEL) corrections must be performed en route to points to be surveyed and during the survey mission. The Z-VEL correction allows the PADS to correct itself to its present location. Ten minutes is the maximum Z-VEL time allowed. This is associated with battalion fifth-order survey requirements. If more accurate survey control is necessary, a 5-minute Z-VEL correction (associated with div arty or TAB fourth-order requirements) must be performed. PADS is automatically set for a 10-minute Z-VEL time. To change this time, consult TM 5-6675-308-12. The desired Z-VEL time is entered into the computer after initialization. To perform a Z-VEL correction, the vehicle must be at a complete stop and the STOP button on the CDU must be pressed. In aircraft operations, the aircraft must be stationary on the ground, but it can continue at flight idle. The PADS requests a Z-VEL correction be performed by alternately flashing the GO and STOP indicators and sounding the DS3 ALARM. This audible alarm will sound 30 seconds before the required Z-VEL correction.
c. Survey Operations. On arrival at the position to be surveyed, maneuver the vehicle with PADS over the point to be established. The PADS operator must know what data the user requires. He must decide which marking procedure (plumb bob or theodolite) will be used. If an orienting line is required in the mission, a two-position azimuth mark must be performed. The two-position azimuth mark using the plumb bob requires that the two points beat least 100 meters apart and intervisible. The PADS automatically computes the grid azimuth from the second marked position back to the first marked position. The coordinates that the PADS computes are unadjusted. This unadjusted data may be given to the users with the understanding that the adjusted data will be given upon completion of the mission. The procedures for performing a two-position azimuth mark and adjusting data are also discussed in TM 5-6675-308-12, Chapter 3. Because of terrain or unit SOP, two-position azimuth marks may not be the preferred method. In this case, the second method, marking position, azimuth, and elevation by using the theodolite, would be used, since there is no distance requirement.
Note. Do not use PADS coordinates of two stations to compute the azimuth between those stations. The error incorporated within the computed azimuth could cause artillery rounds to miss the target. This could result in the rounds impacting on friendly units. Only use azimuth from a two-position mark or a theodolite mark.
d. Shutdown Procedures. On completion of the survey mission, the PADS must be shut down. The PADS can be operated for an unlimited amount of time, but you must perform an update every 7 hours of operation. If the PADS is shut down and you are required to do an additional mission after shutdown, you must wait 2 minutes, then reinitialize. After initialization is complete, update over a known SCP. Before shutdown, close out the survey over a known SCP by updating the system and recalling the adjusted data. The first step in the shutdown procedure is to conduct a PADS battery test. This test checks the PADS battery operation under load. After the battery test, turn off the system. (Detailed shutdown procedures are listed in TM 5-6675-308-12, Table 3-2.)
a. Field artillery surveyors are accustomed to describing survey accuracies in terms of survey specifications for fourth-order (1:3,000) and fifth-order (1:1,000) surveys. These specifications are described in Appendix B and serve as rejection criteria for conventional survey techniques. PADS operations are similar to a conventional traverse. The computer determines the position of a point by using input from the accelerometers (compared to measuring the distance with a tape or distance-measuring equipment). Conventional traverse errors usually increase during the survey because of systematic errors in the measurement of angles and distances. PADS errors also generally increase as the PADS proceeds along the survey route. However, absolute rejection criteria cannot be applied to PADS position data. This is because the error will vary with mission length, duration, system characteristics, Z-VEL correction periods, and other factors. In units that require fourth-order conventional specifications, the PADS uses 5-minute Z-VEL corrections. In units that require fifth-order conventional specifications, it uses 10-minute Z-VEL corrections. Acceptability of a conventional survey is determined by comparing the radial error of closure with a maximum allowable error in position and similar criteria for azimuth. PADS performance is checked by using an update rejection procedure.
b. A PADS survey should be started on fourth-order or higher conventional survey control or on a PADS SCP established by using 5-minute Z-VEL corrections. It should be closed on a second SCP of an equal or higher order survey. If a second known SCP does not exist, a PADS survey must be closed on the starting point. If PADS accepts the data, then the survey is good, if not, refer to TM 5-6675-308-12, Table 3-3.
Entering accurate position and/or elevation data into the PADS is called updating. Updates should be performed over fourth-order or higher conventional survey control. If known control is not available, update over a PADS SCP established by using 5-minute Z-VEL corrections, or when no known PADS SCPs are available, update by using the precise lightweight GPS receiver (PLGR). (Refer to TM 5-6675-308-12, Change 3, page 3-36.12). The PADS must be updated after initialization before it can provide accurate survey data. The first update after initialization is called the initial update. The survey mission starts with the initial update. The PADS must also be updated at the end of a survey mission. This closing update should be on a second known SCP. lf a second known SCP does not exist, a PADS survey can be closed on the starting point, or when no known SCPs are available, by using the PLGR. As in conventional survey, if a survey is closed on the starting point (or GPS), there is no check on the starting control. Care should be taken to verify these data by using all available means (another GPS, maps, and so on). The PADS will either accept or reject the update. If it accepts the update, the PADS automatically adjusts all data recorded since the previous update. The adjusted data replace the unadjusted data stored in the system computer memory. The adjusted data must be provided to all users.
Note. Position and/or elevation updates can be performed independently.
a. Update Procedures With Plumb Bob or Theodolite. Procedures used to update position and/or elevation with the plumb bob or theodolite are listed in TM 5-6675-308-12, Table 3-2, steps 3, 4, 5, and 6.
b. Update Acceptance. After the operator has entered the known trig list positions and/or elevation data of the SCP into the PADS, the system automatically tests the difference between the update coordinates (trig list data) and the PADS position coordinates. If the difference (error) is within the built-in calibration tolerance parameters, the update is accepted and the CDU displays ID * PAE U-U. This display tells the PADS operator that the PADS has met the specified accuracy needed for that update and all surveyed stations established back to the previous update. When the update has been accepted by the PADS, the survey is considered a closed survey within the prescribed accuracy.
c. Update Rejection. If the difference (error) is outside the built-in calibration tolerance parameters, the PADS recues for the update data by displaying a flashing E on the CDU, followed by the data the operator entered. The flashing E indicates a possible update rejection in position. The operator then checks the probable reason for the update rejection and takes the appropriate corrective action as outlined in TM 5-6675-308-12, Table 3-3.
Because of the speed and accuracy of the system, extensive planning and a map reconnaissance must be performed. The mode of travel, route, weather, terrain, and tactical situation must be considered. Because of time and distance limitations, SCPs nearest the area of the PADS mission must be used for starting control. For detailed information on PADS planning and use of the PADS in FA units, refer to Chapters 14 and 15.
The following are special situations in which the PADS can be used. No additional personnel or equipment is required.
*a. Establish a Declination Station. When the PADS is used to establish a declination station, the criteria for the predetermined site is the same as that described in paragraph 14-5d. The preferred procedure, time and tactical situation permitting, is to travel directly from an update point and determine the mean of two azimuths for each azimuth line (as a check) by following normal PADS procedures for determining an azimuth. The azimuths should agree within 0.4 mil. To close out the declination station survey, update as soon as possible and record adjusted data. Record and include measured vertical angles with the declination station data. (The vertical angle is not used with PADS but will be used at the declination station to determine a vertical angle correction when the aiming circle is declinated.)
*b. Using the PADS With Assumed Data. The PADS can be operated in areas where known survey control or GPS data are not available. In such cases, the PADS operator must know the spheroid and UTM grid zone and must use all existing support elements (S2, SPCE) to determine the data needed to initialize the system. The initialization data used should be as accurate as possible. When conducting a PADS operation under these conditions, the PADS operator will update the system over the initialization point (assumed point) and all control will be extended from this point to ensure all elements are on a common grid. To close out this type of survey, the PADS must be updated over the initialization-initial update point (assumed SCP). This procedure should be used only in special missions where known survey control is not available.
c. PADS Operation at Night. The PADS can be operated at night under blackout conditions as long as the CDU keyboard can be read with the lamps dimmed or with a flashlight with strict light control. Only the CDU status indicators and data display can be dimmed. The keyboard is either fully illuminated or dark. If autoreflection is to be performed, the theodolite front sight can be painted white with typewriter correction fluid or can be illuminated with the hand lamp. The end of the azimuth line must be illuminated. Under blackout conditions, the two-position mark method is preferred in establishing an azimuth line. The only illumination required for this method is to aid in marking the station. For tactical reasons, the PADS team must be thoroughly familiar with all indicators and controls and the CDU keyboard.
d. PADS Decontamination. While in a survey mission, the PADS can continue to operate in a nuclear, biological, chemical (NBC) environment with partial decontamination. To partially decontaminate the system, the PADS operator should use the M13 decontamination kit. The CDU, plumb bob arm, porro prism cover, circuit breaker covers, flashlight, and battery box latches should be decontaminated. When the survey mission is complete and the unit has established a decontamination point, the entire system can be decontaminated with soapy water. When the entire system is to be decontaminated, extreme caution must be used to prevent exposing the system to high-pressure water. The PADS operator also must ensure that the circuit breaker covers are closed and that any unconnected cable connector is capped or taped.
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