GPS for Projectiles

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fairfax-projectile positioning.pdf (ARL-TR-5994)

 

 

[GPS Reception on Projectiles]

• High seed: 1000m/s -> 5kHz Doppler shift @ 1.5GHz

• High spin: 200rps -> 500Hz Doppler, 100¨¬ phase shift @ 1.5GHz. Performance qualification at high spinning test fixture. Mayflower GPS receiver and anti-jam unit in high-spin performance test.

http://www.mayflowercom.com/images/antijam2.jpg

• "Time keeping is essential to GPS; however, the associated jerk encountered at launch by the projectile causes clock drift."[Fairfax]

• "Short time of flight limits filter settling time, in-flight calibration, and GPS acquisition."[Fairfax]

• "GPS may be lost in flight due to terrain or jamming." [Fairfax]

 

[GPS Receiver Operational Sequences]

1) Initialization: from the external reference gun GPS receiver and the local mission fire control station.

- Data transfer for hot start.

- GPS time for direct Y-code acquisition, power-on for initialization, initialization data, almanac and precise ephemeris for all SVs in view, SAASM and cryptographic keys, the gun position, target coordinate, calibration of the projector GPS reference oscillator, reference trajectory and velocity aiding data. The receiver clock is set to within 10ms (10us) of GPS Time (the smaller, the faster the acquisition)

- Maintain the initialization data accurately.

- Initialization time: minimized for high rate of fire. < 3s.

2) Hold

- Initialization power is removed and the GPS receiver is powered by the time maintenance power.

- Hold time (time maintenance interval): time from initialization to firing. a few seconds to 20min., the ref. osc. accuracy is crucial in maintaining GPS time accuracy. GPS receiver maintains initialization data set, the keys, and GPS time.

3) Firing and acquisition

- On firing, the main battery powers the GPS receiver.

- Power-up self-test is done.

- The receiver starts to capture GPS signals.

4) Fast Acquisition

- Limit the 2D search space (time = code, frequency = carrier) with hot start (initialization).

- Time search: code correlation matching

- Frequency search: for carrier phase reference (a replica of carrier phase and phase rate of the received signal) to successfully phase-demodulate the GPS signal. A variation of phase-locked loop called Costas loop is used. Acceptable error is about equal to the Costas loop bandwidth (less than 10Hz).

- Minimize frequency uncertainty: primary contribution

- Frequency uncertainty: accuracy of reference oscillator + Doppler shift caused by projectile movement (linear movement (5kHz) and spin (0.5kHz))

- Receiver's reference oscillator's frequency accuracy is very critical: reduces both time and frequency search space.

- Gun shock-induced frequency shift (the largest contributor) of the reference oscillator is accurately calibrated: removes the

  crystal aging and temperature-induced drift. Calibration is done during initialization using GPS Time.

- Use the accurately calculated projectile trajectory versus time for Doppler frequency fix (frequency fix): use all available

 initialization data - gun position, firing azimuth and elevation, muzzle velocity, drag, planned flight trajectory, elapsed time since firing to estimate the projectile velocity.

- Minimize time uncertainty:

 Use accurate projectile position.

 Calibrate out the receiver clock bias to GPS time: within 10ms at initialization.

 The receiver clock uncertainty at start-of-acquisition is the product of the receiver oscillator frequency uncertainty and the time maintenance interval (up to 20 minutes). The frequency stability of the receiver oscillator over this interval is a major factor in minimizing acquisition time since it affects both time and frequency uncertainties. The primary contributor to oscillator drift is changes in temperature. The receiver is equipped with a temperature sensor. Sensed changes in temperature are used to compensate the time keeping for known oscillator drift characteristics.

- Fast correlation:

     Use the best current estimate of projectile position, projectile velocity, receiver clock bias, and oscillator frequency.

     Use massive parallel correlation: use a large number of correlators. 100,000 effective correlators

     Acquire 1st SV and use the information from it to reduce time and frequency uncertainty in subsequent SV acquisition.

- Use many channels (more than 6).

- Realize the highest SNR:

     Minimize the hardware and software implementation losses: noise figure, filter loss, sampling loss, image rejection, signal aliasing, rejection of in-band spurious responses.

     Limit noise: minimize RF input bandwidth and tracking loop bandwidth

- Fastest possible acquisition in the presence of jammers close to the flight path.

5) Independent L1 and L2 reception

6) Anti-jamming

- Jamming effect: reduction of SNR

- Retain the best SNR: minimum implementation loss, noise figure, filter losses, sampling losses, image rejection, signal aliasing. Down conversion frequency plan free of in-band spurious responses.

- Jammer sensor suite: jammer signal level and bandwidth. On board the GPS receiver.

- Optimally adjust receiver parameters: coherent/non-coherent detection integration intervals, tacking loop bandwidth, signal sampling weights and spacing. 

- Use antenna beam switching/nulling: use multiple antenna configurations.

- Separate L1 and L2 tracking channels: do not use the same down converter and multiplex it between L1 and L2. SNR loss proportional to the dwell ratios with commensurate loss in tracking threshold. increased likelihood of carrier cycle slip or drop clock.

- Omni-directional antenna: limited anti-jam performance, poor sky/ground signal separation (jamming from ground)

7) Using IMU measurements to aid GPS

- IMU derived projectile dynamics ¡æ used in GPS to limit search space. Used to reduce the bandwidth of the tracking loop. Improved S/N and reduced acquisition time ¡æ stronger anti-jamming performance.

- IMU data available quite a moment after firing: IMU depends on GPS to align itself in flight.

- IMU data latency: conversion time of raw inertial IMU data into satellite line-of-sight range, range rate and range acceleration. held less than 20ms.

8) SAASM (Selective Availability Anti-Spoofing Module)

- Mandatory in battle field operation.

- Use a dedicated ASIC chip: 40mmx40mm MCM package.

- GPS receiver output is routed to SAASM chip input. Output of SAASM chip is SV navigation message.

- Construction: GPS ASIC with 12 tracking channels each capable of tracking L1 or L2, C/A or P/Y-code, acquisition correlator ASIC, key data processor chip set, flash memory for DSP program storage.

10) Antenna switching: antenna with a null in the forward direction -> good for anti-jam capability antenna with an omni-directional pattern -> good for capturing many GPS signals.

11) High spinning projectile environment

- General receiver could find the fix only at low spin (< 6Hz). Spinning causes amplitude and phase modulation of received GPS signal. High spinning causes the satellites unlocked. Once unlocked the receiver should reacquire the satellite in a very short time which decreases with the spin rate.

 

 

- The following are remedies.

     Projectile de-spinning

     Antenna switching

    Amplitude and/or phase tracking of modulation caused by spinning [Xiao, Int. Conf. Mechtron. Automat., IEEE, Agu. 2007], [Doty, GPS World, Sept. 2004].

     Use rotation demodulator: one antenna + one receiver, multiple antennas + corresponding receivers for each antenna

http://www.gpsworld.com/files/gpsworld/nodes/2004/916/i2.jpg

 

 

http://www.gpsworld.com/files/gpsworld/nodes/2004/916/i3.jpg

 Actual amplitude & phase of rotating antenna (blue), amplitude and phase of rotation demodulator output

 

 Spin fixture: for live testing of GPS reception on high-spin

 GPS receivers with high-dynamic/spin capabilities are specially developed:

     - Rockwell-Collin ASVN(Advanced Spinning-Vehicle Navigation) development team

   - Mayflower Communications, L3 IEC, BAE Systems 'NavComp'

12) GPS-derived projectile spin measurement:

 - Rotational orientation (attitude) of a fast spinning body is required for correcting the effect of spinning body in the sensor inputs and guidance actuator controls.

 - Because the inertial sensors are spinning collectively with the projectile, they must be demodulated to remove rotational artifacts to that Coriolis and lateral acceleration components can be utilized in the trajectory solution.

13) Telemetry: during test firings only and not used in operational rounds since emitting RF signal is dangerous in battle field.

14) Packaging:

- Form factor that fits into projectile nosecone.

- Initialization coupler, GPS antennas, IMU, guidance computer, batteries, power regulator, GPS receiver, telemetry and its antenna (test model)

 

[GPS Antenna for Projectile Applications]

• Coverage requirements: 70-90% coverage of upper hemisphere irrespective of the projectile orientation

• Multiple antenna method

 - Place more than one antenna along the projectile circumference.

 - 81mm: 2 antennas, 155mm: 3 antennas

 - Non-uniform weighing is required because of overlapping patterns of individual antennas [Svendsen, JNC]

 

[GPS Anti-Jam]

• Basics

- Wideband jamming, narrow band jamming

- Jamming rejection: 20-30dB

- Antenna switching and nulling: adaptive spatial filtering, digital adaptive array

- Adaptive temporal filtering (ATF) by Mayflower Comm.

- Digital spatial temporal adaptive processing (STAP) by Mayflower Comm., miniature GPS array, excellent nulling    performances

• Mayflower Comm.: research from late 1980's, RF ASIC chip BeaconTM. For C/A and P(Y) codes, based on ATF, ASF and STAP algorithms, gun-hardened to 20kg, high dynamic operation optimized for high-speed spinning applications, RF/digital interface to GPS receiver, up to 4 antenna elements, 80mm, protection against multiple jammers of different kinds

 

[Military GPS Receivers]

Common technical features:

 - PPS(precise pos. serv.) units(P-code): exportable only to GPS Memorandum of Understanding Countries

 - PPS security module (Y-code): obtained through Foreign Military Sales(FMS) procurement

 - Freq.: L1 C/A and P or Y, L2 P or Y

 - Greater accuracy with 24 channels.

 - Dynamics > 10g

Direct P(Y)-code acquisition:

 - GPS P-code: more accurate positioning, higher tolerance to broadband and CW jamming

 - GPS Y-code: used in place of the P-code when the anti-spoofing mode of operation is enabled.

 - Accurate time transfer to the receiver (initialization), time maintenance using a stable oscillator, massively parallel  correlator

 - Small time error: leads to large code search space

 - 32,000 time-frequency bins in parallel

 - Operational conditions: acquisition probability 0.9, initial time error ¡¾1ms, initial frequency error ¡¾0.2ppm, J/S 50dB, 20.46MHz signal bandwidth

 - Merits of direction acquisition of Y-code: higher tolerance to broadband and CW jamming

 - Code search resolution cell (code phase search): 1/2 PN chip. In the worst case, all the resolution cells in the entire uncertainty region must be tested before the signal is detected. Statistically half of the total number of resolutions cells must be tested. 511 resolution cells in time are tested in parallel. Code phase search is same as shifting the phase of the replica PRN code generated by the receiver until it correlates with the received satellite PRN code. 

 - Frequency search resolution cell (carrier phase search): reciprocal of the coherent integration period, use FFT to extend 511-time resolution cells to 64-frequency resolution cells. 511x64 = 32704 time-frequency resolution cells. Carrier phase search is same as changing the receiver frequency until it correlates with the received satellite carrier frequency plus Doppler.

Military GPS receiver products:

• STS Y-Express: direct P-code reception using 511 time resolution (by correlator) and 64 frequency resolution cells (by FFT) resulting in parallel processing of 32704 time-frequency resolution cells. P-code is decoded in less than 250s with ¡¾1ms timing uncertainty and ¡¾0.2ppm frequency uncertainty. [wolfert-sts-yexpress.pdf, li-p-code acquisition.pdf]

• Rockwell-Collins NavStorm Artillery G-Hardened GPS Anti-Jam System: 12-ch., artillery, enhanced direct Y-code acquisition/cold start, SAASM, high-rate aiding, 80-90dB J/S performance scalable anti-jam, up to 4 RF signal inputs, single(L1 or L2) or dual (L1 & L2) freq. tracking, high-g vib. & shock, field clock recalibration for extended storage, 6000 correlators with Panther ASIC, high anti-jamming immunity, antenna masking selection, precision time transfer, simultaneous ionospheric correction, carrier loop aiding(in future upgrade), carrier phase measurement output, fast initial acquisition, extended range correlation(in future upgrade); TTFF = 77s cold start w/o initialization data, < 25s with time uncertainty less than 25ms, < 8s with time uncertainty less than 10us; designed for high-g shock; used in ERGM RR, CMATD, Team STAR, LCCCM

• Rockwell-Collins NavStrike 3.3 Munitions Receiver: 12-ch., JDAM, enhanced direct Y-code acquisition/cold start, 5th generation SAASM, single(L1 or L2) or dual (L1 & L2) freq. tracking, high-g vib. & shock, field clock recalibration for extended storage, 6000 correlators with Panther ASIC, high anti-jamming immunity, antenna masking selection, precision time transfer, simultaneous ionospheric correction, carrier loop aiding(in future upgrade), carrier phase measurement output, fast initial acquisition, extended range correlation(in future upgrade); TTFF = 77s cold start w/o initialization data, < 25s with time uncertainty less than 25ms, < 8s with time uncertainty less than 10us; Shock 386g

Mayflower Gun-Hard C/A Code GPS Receiver: 12-ch., embedded data link, embedded anti-jam with Mayflower patented ATF technology, digital/RF interface to Mayflower anti-jam solution, gun-hardened at 20,000g, interface to passive antenna arrays (up to 4 elements), 10mCEP, size 40mm dia., TTFF = 15s(hot), 38s(warm), 50s(cold), hot start TTFF tested in high-speed spinning applications, used in GIF and BTERM II

• Mayflower NavAssureTM 100 SAASM GPS Receiver: L1 and L2, 12-ch., up to 4 antennas, 20kg gun hardened, 40mm dia.

IEC (L-3 Comm. subsidiary) TruTrak Evolution (TTE) GPS receiver

 - Anti-jamming: 4s acquisition at 55dB J/S, track at 65dB J/S and at 90dB J/S with optional FaSTAP(with Deep Integration with IMU)

 - Accuracy: 3m(1 sigma) pseudorange, 0.025m (1s) delta range

 - TTFF: <6s (direct Y, hot start, no jamming), <5s (reacquisition)

 - Navigation: unfiltered GPS-only least squares, deeply integrated Kalman filter, IMU-less attitude determination

 - Hardware: 24-ch., L1 C/A, L1 & L2 P(Y), 4 embedded processors, synchronization on pulses, 0-100,000ft altitude, 20kg 16ms gun setback, 5kg 1ms lateral balloting

 - Dynamics: <1200m/s velocity, < 10g acceleration

 - Used in PGK and Excalibur.

 

Projectie acceleration:

 

GPS acquisition window:

 

 

 

 

ECF(European Correcting Fuze), CCF(Course Correction Fuze):

• BAE Systems

• 2 drag brakes = range correction, spin brakes = reduce spin to reduce the yaw of repose. ¡æ range and deflection control

 1D course correction

M549A1

• GPS C/A receiver

• Antenna: revolution symmetric radiation pattern

 

TopGun:

IAI, Eurosatory Symposium & Exhibition in Paris June 14th-18th 2010

GPS/INS 2D course correction, 4 fins

Compatible with 2" thread well 155mm artillery projectile

20m CEP for all ranges

 

PGK(Precision Guidance Kit), M1156

burke(2010)-pgk overview.pdf

 

• US Army, ATK

• Screw-on fuze replacement, $3000 low-rate initial production

50m CEP (initial), 30m CEP (final)

155mm howitzer fired, later applied to 105mm system also(2008), IOC at FY12

• 150-275Hz spin, 330-830m/s speed, 20kg setback, 6-27km range

Deep fuze well:

No battery: generates its own power

2-D GPS SAASM guidance

Fixed canard guidance: ATK

Inductive mission setting: EPIAFS

Rockwell Collins GPS receiver

Draper Labs IMU

 

 

 

 

Antenna cylinder: dia 62mm, h 45mm, patch: 31*31mm, single antenna

Unit cost: 3000ºÒ ÀÌÇÏ

¼öÃâ: È£ÁÖ 2013.8 ¾à 4000¹ß ¹× ºÎ´ëÀåÁö, 5800¸¸ºÒ(660¾ï¿ø, ¹ß´ç 1650¸¸¿ø)

http://upload.wikimedia.org/wikipedia/commons/9/98/XM1156-PGK.jpg

 

IEC (L-3 Comm. subsidiary) TruTrak Evolution (TTE) GPS receiver

- Size: 1.75"x2.45", 3.07"x0.93", 2.45"x2.45" for various projectile applications

- Low power consumption: 3W @ highest performance mode, 0.7W @ low power mode, 20mW @ standby mode 

- Anti-jamming: 4s acquisition at 55dB J/S, track at 65dB J/S and at 90dB J/S with optional FaSTAP(with Deep Integration with IMU)

- Accuracy: 3m(1 sigma) pseudorange, 0.025m (1s) delta range

- TTFF: <6s (direct Y, hot start, no jamming), <5s (reacquisition)

- Navigation: unfiltered GPS-only least squares, deeply integrated Kalman filter, IMU-less attitude determination

- Hardware: 24-ch., L1 C/A, L1 & L2 P(Y), 4 embedded processors, synchronization on pulses, 0-100,000ft altitude, 20kg 16ms gun setback, 5kg 1ms lateral balloting, SAASM

- Dynamics: <1200m/s velocity, < 10g acceleration

- Used in Excalibur also.

TruTrak Evolution.jpg

 

MGK(Mortar Guidance Kit):

• US Army, ATK

120mm mortar:

 

VAPP(Very Affordable Precision Projectile:

• US Army

• Lower the cost of guided munitions by 10 times. Use low-cost guidance solution. Use COTS(commercial-off-the-shelf) components.

• Retrofitting existing munitions: reduced cost but limited performance due to the narrowed design space.

• Conventional maneuver mechanisms (servomotor-driven fins = carnards) do not have the frequency response to actuate at high spin rates (hundreds of rps).

• Main development considerations: precision, range, angle-of-fall, cost

• System features: rolling and fin-stabilized airframe, single-axis maneuver mechanism, ballistic-based guidance and flight control

• System parameters:

1) Roll rate = 5-30Hz (fin-stabilized)(for simplified and cost-reduced maneuver mechanism, use slipping band obturator for disengagement from gun rifling, and tail fiins depolyed after launch and generating appropriate aerodynamic center of pressure location),

2) GNC(guidance, navigation and control) = GPS receiver and antenna (position, velocity, time, roll orientation with a discrete pulse generated when the GPS antenna is oriented toward the earth), guidance and flight control algorithm, maneuver system, axial accelerometer for initiation of GNC at launch, telemetry transmitter and antenna for obtaining data during flight.

4) Guidance and flight control: use GPS data, with reference to up pulse (=maneuver direction), canard amplitude and phase wrt up pulse, impact point calculation using a closed-form flight dynamic model of projectile airframe and GPS-supplied position and velocity, trajectory shaping in glide and endgame phases of the trajectory, rotate canards by desired positive and negative angles during one roll cycle. Active damping based on expensive IMU is not used.

5) Airframe: 105/120/155mm, interior ballistics(projectile weight, chamber volume, muzzle velocity, roll rate, propellant), sub-system packaging, aeroballistics (wind tunnel experiments, CFD predictions, free-flight experiments, aerodynamic behavior prediction, flight stability, six degree-of-freedom Monte Carlo flight simulations with such parameters as physical properties, aerodynamics, launch conditions, and atmospheric conditions), structural integrity, a classic multi-disciplinary design problem with competing requirements, compact GNC(size, weight, battery power) to provide more space for warhead or rocket motor.

6) Gun hardening: electrical and mechanical components' survival through gun launch environment, FEM modeling, extensive structural dynamics simulation, verified through state-of-the-art soft-catch and ground-truth data acquisition-based gun firings, ensured structural integrity

7) GPS: L3 IEC, SAASM-enabled, gun-hardened, direct acquisition of encrypted Y-code, RHCP patch antenna, integration of antenna with GPS receiver, have to cope with reduced signal level of Y-code, active beam-forming not applicable, use shielding for EMI effects on antenna and RF front-end, assess GPS performance for projectile trajectories, Monte Carlo simulation of the receiver with the GPS simulator gives GPS errors and the Kalman filter gains to be tuned for specific maneuverability. field test = to confirm navigation and up-finding (finding sky direction) + mounted to cantilevered roll drive to check the GPS estimate of roll angle.

8) Performance evaluation on rolling platform: check GPS performance and canards deflections throughout a roll cycle, use high-speed camera.

9) IMU: not used to reduce the cost.

10) Maneuver system: single-axis maneuver control, phasing of the canard deflections wrt roll orientation permits maneuvers in any direction, used DSP with feedback, 2-4 canards on projectile head, tail fins for maintaining roll rate, wind tunnel experiments, test firing, range can be adjusted ¡¾30% by maneuvering. 

11) Hardware-in-the-loop experiment: maneuver system, guidance and control algorithm, GPS and telemetry are fully integrated.

12) Test Firing: guide-to-hit flight experiments, 120mm mortar fired in Mar. 2010 at the Aberdeen Proving Ground. 155mm in July 2010.

 

 

-120mm mortar: 62-deg quadrant elevation with muzzle velocity 233m/s, target at 3.8km, downrange(X) and crossrange(Y) error converge to zero. max. trajectory height 1.5km, projectile landed with 10m from target.

-155mm artillery: 50-deg. quad. elevation with muzzle velocity 696ms/, target at 16.4km, projectile landed within 1m of target.

 

 

Mk 64:

 

http://www.globalsecurity.org/military/systems/munitions/images/cm-compare-proj.jpghttp://www.globalsecurity.org/military/systems/munitions/images/cm-atd3.jpg

 

ERGM(Extended Range Guided Munition, XM171:

• US Navy, 5" gun-fired, 15-60nm range, 127mm dia. 1.55m length, 50kg, 10m CEP

• Roll-stabilized, fin-stabilized glide

• GPS antenna: 2 units on the skin.

 

 

 

 

DCI(Digital Communication Interface): for initialization, two-way serial communication, dual coil design, provides power to GPS receiver

- Power: 20kHz 50% duty, 80W

- Data: 500kHz Manchester encoded data, CRC except crypto, 0.02% BER

 

 

• GPS receiver: L3 IEC TruTrack receiver.