Chapter 3 – Receiver Description 3.
Chapter 3 – Receiver Description DETAILED TABLE OF CONTENTS Overview ...................................................................................................................................3.4 Antenna Sense Circuit............................................................................................................3.6 Antenna Feed Current............................................................................................................3.6 Active Antenna Configuration .............
Chapter 3 – Receiver Description System Integration ................................................................................................................3.32 Interface Protocol .................................................................................................................3.32 TTL Output...............................................................................................................................3.33 Motorola Binary Format .........................................
Chapter 3 – Receiver Description About the Oncore OVERVIEW The Oncore receiver provides position, velocity, time, and satellite tracking status information via a serial port. A simplified functional block diagram of the Oncore receiver is shown in the following illustration. Figure 3.1: Oncore Receiver Functional Block Diagram Motorola GPS Products - Oncore User’s Guide Revision 5.0 08/30/02 3.
Chapter 3 – Receiver Description Simplified Block Diagram Description OVERVIEW (CONTINUED) The Oncore Receiver has an eight channel design (12-channel design for the M12+) capable of tracking eight (twelve for M12+) satellites simultaneously. The module receives the Ll GPS signal (1575.42 MHz) from the antenna and operates off the coarse/acquisition (C/A) code tracking. The code tracking is carrier aided. The Oncore receiver must be powered with regulated +5 V (Nominal 3V for M12+) power.
Chapter 3 – Receiver Description ANTENNA SENSE CIRCUIT The Oncore receiver is capable of detecting the presence of an antenna. The receiver utilizes an antenna sense circuit that can detect under current (open condition), over current (shorted or exceeding maximum receiver limits), or a valid antenna connection.
Chapter 3 – Receiver Description Antenna Feed Circuit (Continued) For the over current circuit, when the voltage drop across the current sense resistor is equal to the over current threshold (set at about 90 mA for room temperature) the output of the amplifier starts shutting down the pass transistor. At this point, the voltage to the antenna starts to decrease and a logic level is provided for the digital circuit to trigger an alarm bit that indicates an overcurrent condition. Figure: 3.
Chapter 3 – Receiver Description External Gain Range ACTIVE ANTENNA CONFIGURATION The recommended external gain (antenna gain minus cable and connector losses) for the GT Oncore R3 model is 10 to 26 dB. The recommended external gain for the UT Oncore R5 model is 10 to 33 dB. A typical antenna system might have an active antenna with 24 dB of gain and six meters of cable with 6 dB of loss. The external gain would then be 18 dB, which is within the acceptable range.
Chapter 3 – Receiver Description M12+ AND M12+ TIMER ONCORE RECEIVER ELECTRICAL CONNECTIONS The M12+ and M12+ Timer receives electrical power and receives/transmits I/O signals through a 10pin power/data connector mounted on the Oncore. Refer to Figure 3.3 for pin numbering. The following table lists the assigned signal connections of the Oncore receiver's power/data connector. For more information, refer to the Active Antenna Applications Note. Table 3.
Chapter 3 – Receiver Description M12+ ONCORE RECEIVER PRINTED CIRCUIT BOARD Figure 3.4: M12+ and M12+ Timer Oncore Printed Circuit Board Layout Motorola GPS Products - Oncore User’s Guide Revision 5.0 08/30/02 3.
Chapter 3 – Receiver Description M12+ ONCORE RECEIVER TECHNICAL CHARACTERISTICS Table 3.
Chapter 3 – Receiver Description M12+ TIMER ONCORE RECEIVER TECHNICAL CHARACTERISTICS Table 3.2: Oncore Technical Characteristics – M12+ Timer Model GENERAL CHARACTERISTICS PERFORMANCE CHARACTERISTICS SERIAL COMMUNICATION ELECTRICAL CHARACTERISTICS PHYSICAL CHARACTERISTICS ENVIRONMENTAL CHARACTERISTICS MISCELLANEOUS NOTE Motorola GPS Products - Oncore User’s Guide Revision 5.0 08/30/02 Receiver Architecture 12 channel L1 1575.42 MHz C/A code (1.
Chapter 3 – Receiver Description GT+, UT+, SL ONCORE RECEIVER ELECTRICAL CONNECTIONS The Oncore receives electrical power and receives/transmits I/O signals through a 10pin power/data connector mounted on the Oncore Refer to Figure 3.3 for pin numbering. The following table lists the assigned signal connections of the Oncore receiver's power/data connector. For more information, refer to the Active Antenna Applications Note. Table 3.
Chapter 3 – Receiver Description GT+, UT+, SL ONCORE RECEIVER PRINTED CIRCUIT BOARD Figure 3.5: Oncore Printed Circuit Board Layout UT, GT and SL Motorola GPS Products - Oncore User’s Guide Revision 5.0 08/30/02 3.
Chapter 3 – Receiver Description GT+ ONCORE RECEIVER TECHNICAL CHARACTERISTICS Table 3.
Chapter 3 – Receiver Description UT+ ONCORE RECEIVER TECHNICAL CHARACTERISTICS Table 3.
Chapter 3 – Receiver Description SL ONCORE RECEIVER TECHNICAL CHARACTERISTICS Table 3.
Chapter 3 – Receiver Description 1PPS SIGNAL DEFINITION 0 to 5 V live pulse (0 to 3 V for M12+) 1 PPS time mark is synchronous with the mid point of the rising edge of the pulse rising from 0 V to 5 V Rise time is approximately 20 to 30 ns 5 V pulse width is approximately 200 ms ± 1 ms The falling edge will occur approximately 200 ms after the rising edge Accurate to < 500 ns (1 sigma) in stand alone mode (with SA on) UT Oncore accurate to < 130 ns (1 sigma) in stand alone mode (with SA on) UT Oncore accur
Chapter 3 – Receiver Description RF Jamming Immunity (continued) Figure below compares the selectivity of the R5 model of the UT Oncore with the B8 model of the VP Oncore. An additional 30 dB of rejection (an improvement of 1000:1 in power) has been achieved at the first image O/S 110 dB). The improvement is 15 dB at the second image (J/S 87 dB). The jamming immunity of the GPS receiver and antenna system will be further improved with the additional margin provided by the filtering in the active antenna.
Chapter 3 – Receiver Description RF Jamming Immunity (continued Test results have demonstrated that this approach is effective at providing an additional 10 dB of jamming immunity both in the GPS band and out-of-band. The combined results of the additional filtering and the adaptive tracking loops in the UT Oncore make it very effective at improving RF jamming immunity, thus making installation in timing applications more flexible and robust.
Chapter 3 – Receiver Description 100PPS OUTPUT (UT MODEL ONLY) With the UT Oncore 2.x firmware, the timing output can be selected between 1PPS and 100PPS. This is done using the Pulse Mode command (@@AP). See chapter 6 for information on the format of this command. When selected, the 100PPS signal is output on the same pin as the 1PPS. The 100PPS signal has the same accuracy and stability characteristics as the 1PPS signal.
Chapter 3 – Receiver Description TIME RAIM ALGORITHM DESCRIPTION (M12 TIMING AND UT MODELS ONLY) Time Receiver Autonomous Integrity Monitoring (RAIM) is an algorithm in the Oncore timing GPS receivers (M12 Timing and UT) that uses redundant satellite measurements to confirm the integrity of the timing solution. The RAIM approach is borrowed from the aviation community where integrity monitoring is safety critical.
Chapter 3 – Receiver Description Table 3.7: Blunder Detection Example Trial Measurement (m) Residual (mm) Status New Residual (mm) 1 9.998 14.5 OK 2 2 10.001 11.5 OK -1 3 9.999 13.5 OK 1 4 10.000 12.5 OK 0 5 10.002 10.5 OK -2 6 10.100 -87.5 removed 7 9.999 13.5 OK 1 8 10.001 11.5 OK -1 Average 10.0125 Motorola GPS Products - Oncore User’s Guide Revision 5.0 08/30/02 10.000 3.
Chapter 3 – Receiver Description RECEIVER MODULE INSTALLATION Your Oncore receiver has been carefully inspected and packaged to ensure optimum performance. As with any piece of electronic equipment, proper installation is essential before you can use the equipment. When mounting the Oncore receiver board into your housing system, special precautions need to be considered.
Chapter 3 – Receiver Description Installer Caution (Continued) RF Shielding The RF circuitry sections on the Oncore GPS receiver board are protected with a tin plate shield to guard against potential interference from external sources. When a design calls for the Oncore to be near or around RF sources such as radios, it is recommended that the Oncore be tested and tried in the target environment to identify potential interference issues prior to final design.
Chapter 3 – Receiver Description PCB Mounting Hardware ONCORE RECEIVER MOUNTING INSTRUCTIONS (GT+/UT+ ONLY) Mounting Hardware Design Guidelines For all the design validation and process validation tests that were conducted and completed successfully by Motorola, the Oncore PCBs were mounted on round or hex female threaded metal standoffs and screwed/tightened down with metal english or metric screws. The mounting standoffs are available with english or metric threads.
Chapter 3 – Receiver Description Mounting Hardware (Continued) The recommended screws for the standoffs that will secure the Oncore to the standoffs are metal screws with 4-40 threads or M3 threads. The nominal torque to assemble the Oncore PCB with screws to the standoffs is 6 in-lb each with a maximum of 7 and minimum of 5 in-lb. Washers are not required nor recommended for use with the Oncore PCB.
Chapter 3 – Receiver Description Table 3.8: List of Threaded Standoff Suppliers Mounting Hardware (Continued) No. Company Name Part description of metal standoffs Outside diameter 1 Keystone Electronic Corp. Plain female standoffs 4-40 threads available in lengths from 0.250 to 1.0 in. 0.187 in. round or hex Plain female standoffs M3x0.5 mm threads available in lengths from 5 to 25 mm 5 mm hex Plain female standoffs 4-40 threads available in lengths from 0.250 to 1.0 in. 0.187 in.
Chapter 3 – Receiver Description Mounting Hardware (Continued) Sturdiness and Reliability of Metal Standoffs The Oncore PCB mounted on standoffs 0.375 or 0.500 in. long passed the vibration test successfully. The mechanical test is conducted in three axes, one hour each, at 7.7 Gs random vibration. In the final analysis this is a severe military specification as per MILSTD 810E. After the vibration test leg of the design validation, the screws lose about 60% to 80% torque, which is expected as per design.
Chapter 3 – Receiver Description Mounting Hardware (Continued) Design Worksheets (Continued) Figure 3.10: Sample layout of GPS Oncore PCB and the application PCB independently mounted on a baseplate Motorola GPS Products - Oncore User’s Guide Revision 5.0 08/30/02 3.
Chapter 3 – Receiver Description MEAN TIME BETWEEN FAILURE (MTBF) The MTBFs for the Oncore family of GPS receivers have been computed using the methods, formulas, and database of MIL-HDBK-217. Table 3.
Chapter 3 – Receiver Description Operational Overview SYSTEM INTEGRATION Interface Protocol Description INTERFACE PROTOCOL The Motorola Oncore receiver is an intelligent GPS sensor intended to be used as a component in a precision positioning, navigation or timing system. The Oncore receiver is capable of providing autonomous position, velocity, and time information over a serial TTL port. The minimum usable system combines the Oncore receiver, antenna, and an intelligent system controller device.
Chapter 3 – Receiver Description TTL OUTPUT The serial interface signals, RXD and TXD, are available for user connection. A ground signal is also required to complete the serial interface. There is no additional protection or signal conditioning besides the internal protection of the microprocessor since they are connected to the microprocessor directly. TXD and RXD are regular TTL signals with voltage ranges from (0v-3v or 0v-5v depending on the receiver).
Chapter 3 – Receiver Description Motorola Binary Format The binary data messages used by the Oncore Receiver consist of a variable number of binary characters. These binary messages begin with the ASCII @@ characters and are terminated with the ASCII carriage return and line feed . The first two bytes after the @@ characters are two ASCII characters that identify the particular structure and format of the remaining binary data.
Chapter 3 – Receiver Description Description of Motorola Binary Format (Continued) Motorola Binary Format (Continued) You must take care in correctly formatting the input command. Pay particular attention to the number of parameters and their valid ranges. An invalid message could be interpreted as a valid unintended message.
Chapter 3 – Receiver Description Motorola Binary Format (Continued) Status messages are output at the selected update rate (typically, once per second) for those messages that contain position, velocity, or time, or can be commanded to output the data one time upon request. The rate at which the data is output in the continuous output mode is dependent on the type of data in the message.
Chapter 3 – Receiver Description Motorola Binary Format Input/Output Processing Time NMEA Support The GT Oncore 2.x firmware supports the NMEA 0183 format for GPS data output. Output of data in the NMEA-0183 standard format allows a direct interface via the serial port to an electronic navigation instrument that supports the specific output messages. The following NMEA output messages are supported as per the NMEA0183 Specification Revision 2.0.1.
Chapter 3 – Receiver Description NMEA 0183 Format Overview (Continued) For the case where more than one output message is scheduled during the same one second interval, the GPS receiver will output all scheduled messages but will attempt to limit the number of bytes transmitted each second to 400 bytes. For the case of multiple output messages, if the next message to be sent fits into the 400 byte length goal, then the message will be output.
Chapter 3 – Receiver Description EXCLUSIVE-OR CHECKSUM CREATION This application note describes the procedure to calculate checksums used in the serial messages of the Oncore GPS receivers. An example message is used to illustrate the procedure. Command name: Position/Status/Data Output Message (eight channel) Command in Motorola binary format: @@EamC In this message, ‘m’ indicates the response message rate (ie. 1 = once per second, 2 = once every two seconds, etc.), and ‘C’ is the checksum.
Chapter 3 – Receiver Description MILLISECOND TO DEGREE CONVERSION The primary output message of Oncore receivers is the Position/Status/Data Message (@@Ea). In this message, the latitude and longitude are reported in milliarcseconds. This note describes how to convert milliarcseconds to degrees. One degree of latitude or longitude has 60 arcminutes, or 3600 arcseconds, or 3,600,000 milliarcseconds.
Chapter 3 – Receiver Description Motorola Binary Format Input/Output Processing time INPUT/OUTPUT PROCESSING TIME The Oncore receiver always operates in position fix mode and the input buffer data is serviced once a second. When powered on and available satellites are tracked, the current receiver position is available. If no satellite signals are received, the last known position is output.
Chapter 3 – Receiver Description DATA LATENCY The Oncore receiver can output position, velocity, and time data on the TTL port once each second. The start of the output data is timed to closely correspond with the receiver measurement epoch. The measurement epoch is the point in time at which the receiver makes satellite range measurements for the purpose of computing position.
Chapter 3 – Receiver Description DATA LATENCY (CONTINUED). Figure 3.11: Position/Status/Data Output Message Latency To compensate for the one second computational pipeline delay, a one second propagated position is computed that corresponds to Tk based on the position and velocity data computed from measurements taken at time Tk -1. In this way, the position data output on epoch Tk will most closely correspond with the receiver true position when the data is output on the TTL port.
Chapter 3 – Receiver Description Position DATA Latency The position data output in the current data packet (i.e., at time Tk) is the result of a least squares estimation (LSE) algorithm using satellite pseudorange measurements taken at time Tk-1. The resulting LSE position corresponding to time Tk-1 is then propagated one second forward by the velocity vector (the result of an LSE fit using satellite pseudorange rate measurements taken at Tk-1). The resulting propagated position is output at the Tk epoch.
Chapter 3 – Receiver Description The computed time is relative to UTC or GPS time depending on the time type as specified by the user by the Time Mode. The Oncore system timing is designed to slip time when necessary in discrete one millisecond intervals so that the receiver local time corresponds closely to the measurement epoch offset.
Chapter 3 – Receiver Description Output Data Timing Relative to Measurement Epoch (Continued) The Position/Status/Data Message and the Time RAIM Setup and Status Message are the only output messages containing time information. If enabled, these messages will be output from the receiver shortly after a measurement epoch. Generally, the first data byte in the first message will be output between 0 to 50 ms after a measurement epoch.
Chapter 3 – Receiver Description OPERATIONAL CONSIDERATIONS When powered on, the Oncore Receiver automatically acquires and tracks satellites; measures the pseudorange and phase data from each of up to eight satellites; decodes and collects satellite broadcast data; computes the Oncore receiver's position, velocity, and time; and outputs the results according to the current I/O configuration selected.
Chapter 3 – Receiver Description Initialization When powered up, the Oncore receiver executes the satellite acquisition and tracking algorithms and will compute position when it acquires at least three satellites. For each of the user-requestable outputs, the receiver (if battery backed) remembers the previously requested message state (continuous or onetime) and rate.
Chapter 3 – Receiver Description ROLLOVERS IN TIME In August of 1999, the GPS week number will rollover from 1023 to 0 due to the limited length of the GPS week field in the navigation data stream. Motorola Oncore receivers (M12+, M12 Timing, GT+, UT+, VP, XT, and Basic) have been designed and tested to properly distinguish the correct 20 year window (1024 weeks is just shy of 20 years). They will not need reprogramming or replacement come 1999.
Chapter 3 – Receiver Description RECEIVED CARRIER TO NOISE DENSITY RATIO (C/NO) The Position/Status/Data Message outputs C/No, which can be used to determine the relative signal levels of received satellite signals (refer to Figure below). C/No is the received carrier to noise density ratio. The units are dBHz, where No is the noise density ratio received in a 1 Hz bandwidth. The plot in Figure 3.13 is linear. The satellite signal strength is measured at the antenna input.
