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.