Range Measurement Unit

Messenger Mercury Laser Altimeter

Basic Familiarization

Basic Function

RMU LaserFireDetectorReturns

Computer

Timing Reference5 MHz from S/C, redundant

Note can use an internal5 MHz source.

5

Measurement InformationGenerate

• Fire command– Rate: 8 Hz (1, 6, and 10 Hz available)

• Range Gate– Independent open and close commands– Referenced to TimeZero– Multiple of 200 ns– TX is not masked by the Range Gate

Range Gate (cont’d)

• Default values for Range Gate– Open: 00100016

– Close: 01300016

– Units are 200 ns per count

• Defaults restored by either:– Hardware Power-on reset– Software Reset

Signal Returns

Filter 1

Filter 3Signal

Filter 2

a

b

hg

fe

d

c

Note: Only one b-c per shot Up to 15 “little pulses” per shot

Events Input to the RMU• Six Events

– TX_Leading– TX_Trailing– Pulse_B– Pulse_C– Pulse_AEG– Pulse_DFH

• tPW = Not defined

• Lockout time = Not implemented

Noise Counters• Four Noise Counters

– NCBC

– NCAD

– NCEF

– NCGH

• tPW Not defined

• 16-bit Saturating Counters• Dead Time < 0.01%• Sampled and zeroed at RUPT

Event Counters• Five Event Counters

– TX– BC– “Little Pulses”

• AD

• EF

• GH

• tPW Not defined

• 8-bit Saturating Counters

• Enabled when Range Gate is opened

RMU - CPU Timing

125 ms

Cycle Start

Range Gate

Dump Data

Interrupt

CPU Done

max

Issue CMDs

TimeZero

Measurement Information Return Pulses per Shot

• Number: First 15 to arrive

Except High Threshold Channel 1 (one only)

• Pulse-pair Resolution: ~ 1.5 s

Measurement Information Measurements for Each Return Pulse

• Resolution: 2 ns, max

• Channels:

– CH1: 10 ns filter

two points leading edge

two points trailing edge

– CH2: 60 ns filter

one point leading edge

one point trailing edge

– CH3: 270 ns filter

one point leading edge

one point trailing edge

• Channel stored: 1st channel signal to arrive

Lockout and Channel ID provided by Detector Electronics

Shot Timing - Big Picture125 ms

Cycle Start

Fire

Range Gate

Start Return

Returns

TimeZero

Range Gate

Range Gate

Returns

• Returns arrive randomly• Process first 15 returns to arrive within Range Gate• Pulse-pair resolution ~ 1.5 s

~1.5 s

A Return Pulse

VL

VH

VL

VL

b

a

c

d

e f

g h

10/60

1/10

10/200

250/500

TimeZero

Event Time StampingSystem View

Detector

Spacecraft

RMU

Event

5 MHz

CPU

Event Time StampingRMU View

FPGAEvent

5 MHz

TOF 10 bits @ ~0.4 ns/bit(fine timing)

23 bits @ 200 ns/bit(coarse timing)

TOF = APL Time of Flight Microcircuit

Event Time StampingSystem View

Detector

Spacecraft

RMU

Event

5 MHz

CPU

Event Time StampingRMU View

FPGA

Event

5 MHz

TOF 10 bits @ ~0.4 ns/bit(fine timing)

23 bits @ 200 ns/bit(coarse timing)

TOF = APL Time of Flight Microcircuit

Event Time StampingTOF View

Event

5 MHz

TOF 10 bits @ ~0.4 ns/bit(fine timing)

TOF = APL Time of Flight Microcircuit

TOF outputs time from Event to the next rising edge of 5 MHz

Calibration Mode• Internally generated pulses can be sent to

the TOF which are based on the 5 MHz clock.– 0 ns: not guaranteed to work, STOP may

precede START– 200 ns: most important and guaranteed to work– 400 ns: not guaranteed to work, STOP-START

may exceed 400 ns.

• Available on– TX trail– Each edge of “little pulses”

1 Hz Synchronization

• Upon power-up or Software Reset, the RMU will free run and ignore the 1 Hz signal

• If 1 Hz synchronization is enabled, then the 1 Hz signal will force the RMU to immediately lock to the 1 Hz. This may result in rapid pairs of FIRE and RUPTs

Babble Bit

• The software is not supposed to pulse either the READ or WRITE lines after TimeZero and before RUPT.

• If this relationship is not met, then the BabbleBit is set, indicating a cycle slip.

• The BabbleBit may be cleared only be a power-on reset or a software reset.

• There is no lockout.

Reseting the TOF-As

• The TOF-A’s are reset with either a hardware power-on reset or a software reset.

• The TOF-A’s do have a lockup mode.• Optionally, the TOF-A’s may be reset after

every TimeZero by CYCLERESET by a command to the RMU.

• The effect of a reset on a TOF-A is a mystery, it may alter the state of the PLL.

Command Registers

• All command registers are Read/Write

• All command registers are also included in the telemetry packet.

Disabling the RMU

• The only way to disable the RMU is to turn off the power.

• There is no disable for LaserFire

• There is no disable for RUPT

Little Pulse Lockouts and Timers• The leading edge of a “little pulse” will lock out

the other channels.• Only a corresponding trailing edge will be

permitted to occur.• There is a 2 s timer.

– If the corresponding signal does not arrive, the state machine is recycled.

• Three bit, “one-hot” Channel ID– For a time out, Channel ID = 000

– More than 1 bit set is an error detection mechanism. These may occur as they can not be designed out.