VII Probe Message Processes

Roy Sumner

Probe Messages

• Intent of probes

• What a probe message contains

• How they are generated

• How they are transmitted

• How they can be managed

Intent of Probe Messages• Use vehicles as probes to generate data for

applications and users – an SAE development• Vehicles collect data, its upload to an RSE then

forwarded to the VII network • Users subscribe to this data• Certain users can actively manage the way

probe data is collected• Key constraints are:

– ensure privacy– accommodate technological changes

Content of Probe Messages

Header Snapshot1 Snapshot2 Snapshot3 Snapshot4

Lat Long Time ACID ACM Length

Lat Long Time Wipers Brakes Temp …

Header

Probe message

Snapshot

Probe Message Structure# snapshots

Probe Message Header

• Application class identifier (ACID)

• Application Context Mark

• Length of Message

• Length of Message including headers

• Recently added position/time of transmission

Position and Time• Seconds since last Probe Message sent• Universal Time Coordinated (UTC)• Time Precision • Positioning Precision• Longitude of center of vehicle • Latitude of center of vehicle• Heading• Vehicle Speed• Heading Precision• Speed Precision• Altitude Precision• Altitude• Vehicle Type

Vehicle Status Elements

• Vehicle Exterior Lights?• Wipers• Brake Applied• Stability Control (Event)• Traction Control (Event)• Anti-Lock Brakes (Event)• Sun Sensor• Rain Sensor• Air Temperature• Front Wheel Angle• Rate of Change in

Steering Wheel (Event)

• Wheel that Exceeded Vertical Acceleration Threshold (Event)

• Vertical Acceleration Value (Event)

• Longitudinal Acceleration• Lateral Acceleration• Obstacle Distance• Obstacle Direction• Yaw Rate• 100% Brake Boost

Applied (Event)

Weather Status Elements – J2735

• Wipers – off intermittent low high and sweeps per minute

• Stability Control - on• Traction Control - on• Anti-Lock Brakes - on• Sun Sensor 0-7, 0= Dark, 1=Minimal Sun

7=Maximum• Rain Sensor - on• Air Temperature -127 to +127°C

How Snapshots are generated

Snapshot are Generated:

• Definition: a snapshot is a collection of all vehicle status elements at one place and time

• Three methods of snapshot collection:– Periodically – at intervals between RSEs – subject of current

discussion– When events occur– When the vehicle starts & stops

• Vehicles have a limited snapshot buffer (currently 30)• Snapshots includes all available vehicle status elements

plus time, location, speed, & heading• Currently only one snapshot format

Periodic Snapshot Generation

• Default state is intended to evenly distribute snapshots

• At low speed snapshots (<20mph) every 6 seconds – settings can be changed

• At high speeds (>60mph) every 30 seconds

• Between 20 and 60 mph linearly spread

Speed RSE Spacing

Time between

snapshots

(<20mph 1 miles 6 secs

20-60 Linearly spread

(>60mph 10 miles 30secs

Snapshot can be Generated:

• By distance if set at the upstream RSU:

Speed Distance between snapshots

≤ S1 D1

>S1 & < S2 linear extrapolation

>S2 D2

Event Snapshot Generation

• Event snapshots are produced when a vehicle status element threshold is exceeded or when a device is activated or the value varies

• Examples:– Traction control turns on (icy conditions)– Wheel vertical acceleration exceeds threshold

(potholes)

Starts & Stops Snapshot Generation

• A start is when the vehicle speed exceeds a threshold (default = 10 mph)

• A stop is when there is no movement for some threshold stop time (default = 5s) and no other stops have occurred within a threshold time (default = 15s)

• Snapshots are not generated between stops and starts

Snapshot Temporary ID

• Generated by the OBE

• Independent of MAC address

• Allows short term vehicle trajectories to be known

• Recalculated after:– 120 seconds or– 1km

Whichever comes last

How Probe Messages are Transmitted

Probe Message Queues

Enhanced GPS

Vehicle systems

Location and time

Vehicle status elements

Snapshot # 2

Snapshot # 1

Snapshot # 4

Snapshot # 3

Snapshot # 6

Snapshot # 5

…..…..…..…..…..…..

Snapshot # 30

Probe Message QueuesHeader – this is a probe message

Snapshot # 2

Snapshot # 17

Snapshot # 4

Snapshot # 5

Header – this is a probe message

Snapshot # 1

Snapshot # 3

Snapshot # 6

Snapshot # 5

To the RSE

Snapshots reordered as follows:1. Events2. Stops & starts3. Periodic – oldest data removed as new data arrives – Note – likely to be modified

How Probe Messages can be Managed

Probe Management Concept

• Instruct OBE to generate more snapshots• Message structure:

– Temporary Snapshot Interval - time between snapshots– Probe Management Conclusion - management duration one

of:• a time – seconds• a distance - meters• out of range of current RSU after a threshold time (default = 5s) • a new RSU that supports probe management is detected

– Number of Managed Fields these are the vehicle status elements and their thresholds that can be changed to generate more snapshots.

– Time between Probe Message Transmissions

Probe Management Examples

• Collect snapshots every second – mapping

• Reduce vertical acceleration threshold – roughness measurement

• Snapshot on heading changes >45° - turning movements – comparison of snapshot IDs gives turning movements

• Report 1 sec snapshot every 4 secs – adaptive control

Current Status

• Probe message has been balloted at SAE as part of J2735

• Thresholds need definition

• Management message development is beginning