Utilizing RTCM Corrections via DSRC to Improve Vehicle Localization

Greg LarsonCaltrans Division of Research, Innovation and System

Information (DRISI)April 17, 2018

Background

• Many connected vehicle applications rely on continuous, reliable, and accurate vehicle localization (i.e., positioning)

• Standard DSRC OBU contains GPS; however, the position information is not always reliable and accurate

• It is possible to broadcast RTCM corrections via DSRC to improve reliability and accuracy

How Does GPS Works?• The Global Positioning System (GPS) is

a network of 31 satellites orbiting the Earth– Check online the total number of

operational GPS satellites• Each satellite transmits signal about its

position and the current time at regular intervals

• A GPS receiver needs at least 4 satellites to determine its location using triangulation– Source: http://giscommons.org/chapter-2-

input/

• One satellite locates you somewhere on a sphere• Two satellites intersection places you on a circle • Three satellites intersection places you on two possible

points• The last satellite gives you the exact location

GPS Accuracy• The basic GPS service provides users

with approximately 10 meter accuracy, 95% of the time

• However, any given position may result in accuracy as low as 5 meters or up to 40 meters

• Many factors can affect the accuracy of GPS data– Number of visible satellites (at least 4,

preferable 7+, the more the better)– Satellite geometry (spread apart vs.

clustered)– Multipath effect– ……

Good Poor

Source: https://gisgeography.com/gps-accuracy-hdop-pdop-gdop-multipath/

GPS Position Correction

• Position correction techniques use two GPS receivers to improve accuracy– A stationary GPS receiver, known as the base station, takes accurate

measurements of error, and sends the correction data to a roving GPS receiver

– The roving GPS receiver applies the corrections– Both receivers need to detect the same

satellite signals• Common position correction techniques

– DGPS – Differential GPS (sub-meter accuracy)– RTK – Real-Time Kinematic (centimeters)

Source: http://www.esri.com/news/arcuser/0103/differential1of2.htmlWe are using

Networked Transport of RTCM via Internet Protocol (NTRIP)

• NTRIP is an application-level protocol for streaming position correction data (DGPS or RTK) over the Internet, in accordance with specifications published by RTCM

• NTRIP streaming system consists of– NtripSources, generating data streams from base stations;– NtripServers, transferring data streams from a source to

NtripCaster;– NtripCaster, the major system component; and– NtripClients, receiving data streams of desired NtripSources

available on the NtripCaster– Source: BKG

• NTRIP is an open standard protocol and there are open source NTRIP streaming software packages available

NTRIP Streaming Data Message Format (RTCM Standard 10403.x)

• RTCMv3 defines new message data structure for RTK applications, and supports GPS and GLONASS RTK operations– RTCM DGNSS Standards

• Message types in RTCMv3 have been structured in different groups• For proper operation, the provider must transmit at least one

message type from each of the following groups:– Observations,– Station Coordinates, and– Antenna Description

RTCMv3 RTK Message GroupsGroup Name Message Type Message Description

Observations 1001 L1-Only GPS RTK Observables

1002 Extended L1-Only GPS RTK Observables

1003 L1&L2 GPS RTK Observables

1004 Extended L1&L2 GPS RTK Observables

1009 L1-Only GLONASS RTK Observables

1010 Extended L1-Only GLONASS RTK Observables

1011 L1&L2 GLONASS RTK Observables

1012 Extended L1&L2 GLONASS RTK Observables

Station Coordinates 1005 Stationary RTK Reference Station ARP

1006 Stationary RTK Reference Station ARP with Antenna Height

Antenna Description 1007 Antenna Descriptor

1008 Antenna Descriptor & Serial Number

Minimum Requirement

Note: Within each Message Group, the higher the Message Type number, the larger the message size. A longer message contains the shorter message plus additional information to enhance the performance.

Broadcast RTCM over DSRC

• NTRIP streaming system utilizes RTCMv3

• Over-the-air RTCM utilizes SAE J2735 message frame

• Requires NtripClient to encapsulate the received NTRIP-RTCMv3 in SAE J2735 message frame– Option I – Utilizing RSU ’s RTCM

application (serve as NtripClient)– Option II – Hosting NtripClient in a

separate processor (if desired) and utilizing RSU’s Immediate Forward application for broadcast over-the-air

• DSRC RTCM Message Frame

RTCMv3

Steps to Enable Broadcast RTCM over DSRC1. Identify base station(s) near your implementation site (within 50 km and supporting

NTRIP-RTCMv3)2. Create an account for NTRIP-RTCMv3 data streaming service

– There are base stations that provide free streaming service– Global List of Real-Time GNSS Data Streams From NTRIP Broadcasters

3. Set up NTRIP streaming system on a Linux server computer– Download open source NtripServer and NtripCaster (Various options are available online. This is

what is being used in the California CV Test Bed)– Follow the README instructions to configure connections with the NTRIP-RTCMv3 streaming

service at your desired based station(s)– Start NtripServer and NtripCaster, and verify NtripCaster is receiving data streams from your

desired base station(s)• The whole process is straight-forward; no knowledge of NTRIP, RTCMv3, and SAE J2735 is

required

Implementation Options• Option I – Utilizing RSU ’s RTCM application as NtripClient

– Suitable for implementations that do not require a standalone processor other than RSU and traffic signal controller

– Configure RSU’s RTCM application to connect with NtripCaster for receiving NTRIP-RTCMv3 data streams. The application handles SAE J2735 encapsulation and over-the-air broadcasting

• Option II – Hosting NtripClient in a separate processor– Suitable for implementations that require a standalone processor to host CV-based

signal control applications (e.g., MMITSS)– Download and run open source NtripClient– Encapsulate NTRIP data streams in SAE J2735 message frame– Configure RSU’s Immediate Forward application for over-the-air broadcasting– California CV Test Bed uses this option to gain more control on data sources

California Connected Vehicle Test Bed

• First-in-the-nation (2005) facility for testing CV applications using DSRC on public roads

• 2.1 miles long with 11 consecutive intersections; 6 more planned

• AADT: about 50K vehicles each day• Managed by UC Berkeley PATH• Running MMITSS-CA applications bundle

• I-SIG: BSM-based vehicular phase call and extension• TSP: Transit signal priority• FSP: Freight signal priority• PED-SIG: partner with Savari SmartCross Application

California Connected Vehicle Test Bed (Cont’d)

• Test bed website: http://caconnectedvehicletestbed.org/– Real-time test bed health status– User guide and FAQ– Data samples

Current Plan for Spring 2018

# of Intersections 11 17

Roadside Unit (RSU) Version 3.1 Version 4.1

Roadside Processor Ubuntu 16.04.4 (latest)Linux kernel 4.13.0 (latest)

Broadcast Messages MAP, SPaT, SSM, and RTCM

SAE J2735 Standard Version 2016-03 (latest)

Backhaul 4G/LTE

DSRC Messaging at California CV Test Bed

Message Abbreviation From Frequency To Channel PSID (Hex)

DSRC Message

ID

Basic Safety Message BSMOBU

10 HzRSU

172

0p20 (0x20) 20

Signal Request Message SRM Asynchronous

0p80-02 (0x82)

29

MAP/GID MAP

RSU

1 Hz

OBU

18

Signal Phase and Timing SPaT 10 Hz 19

Signal Status Message SSM 1 Hz 30

RTCM Corrections Message RTCM

Type 1004 – 5 Hz

182 0p80-01 (0x81) 28Type 1006 – 2 Hz

Type 1008 – 2 Hz

RTCM-NTRIP Implementation at California CV Test Bed

• Base Station (NTRIP Server): JRSC– Cooperatively operated by UC

Berkeley and Stanford University – Located at Jasper Ridge Biological

Preserve, Stanford, CA– About 5 miles west of the CV Test Bed– RTCMv3 type 1004, 1006, and 1008– Broadcasting at 1 Hz

• NTRIP Caster located at UC Berkeley PATH Headquarters

• NTRIP Client hosted by intersection MMITSS roadside processor

NTRIP Caster

Architecture

• Networked Transport of RTCM via Internet Protocol (NTRIP)– Streaming of

differential GPS correction data over the Internet

– RTK Base Station sends corrections to NTRIP Caster, enabling the NTRIP Caster to broadcast to all connected NTRIP clients

Architecture (Cont’d)

• Roadside Unit (RSU) is equipped with NTRIP Client to receive corrections from NTRIP Caster– RTCMv3

• RTCM is encoded into SAE J2735 message and broadcast over DSRC

• Onboard Unit (OBU) receives SAE J2735 message via DSRC, and decodes the message to correct the GPS position

NMEA

Apply RTCM Corrections to OBU GPS Receiver• This is where things get a bit complicated• It requires RTK-enabled GPS receiver to utilize over-the-air RTCM

corrections• The RTK-enabled GPS receiver applies RTCM corrections to the GPS data

and outputs position with corrections• Although OBU’s GPS chip may be capable of supporting RTCM corrections,

the application has not yet been implemented and enabled• Ideally, we would need OBU’s RTCM-correction application to support

position correction• Optionally, we can connect a RTK-enabled GPS receiver to the OBU for

achieving position correction with over-the-air RTCM

Understand the Effectiveness of Position Corrections with RTCM over DSRC

• The effectiveness of position corrections with over-the-air RTCM depends on various factors

• California is conducting experimental study to quantify the effectiveness of position corrections with RTCM over DSRC, which could provide reference for other SPaT Challenge implementations

• For research purposes, an RTK-enabled GPS and a laptop are used for the experimental system

• We are expecting RTCM-correction applications to be supported by OBUs in the near future

Positioning Experiments

DGPS (WAAS) w/ RTCMv3

Static Dynamic Static Dynamic

Open Sky UCR, completed UCR, completed UCR, completed UCR, planned

Rural Canyons UCR, completed UCR, completed UCR, planned UCR, planned

• Nearly all DSRC OBU devices utilize WAAS-corrected DGPS, typically providing 2-5 meter accuracy in Open Sky

• RTCMv3 provides correction information, communicated as a DSRC message

• Positioning experiments underway: