ION ITM 2014 San Diego CA Jan 27

ION ITM 2014 San Diego, CA Jan. 27 -29, 2014 Ionospheric Correction at the Southwestern Islands for the QZSS L 1 -SAIF T. Sakai, K. Hoshinoo, and K. Ito Electronic Navigation Research Institute, Japan

ION ITM Jan. 2014 - Slide 1 Introduction • QZSS (Quasi-Zenith Satellite System) program: – Regional navigation service broadcast from high-elevation angle by a combination of three or more satellites on the inclined geosynchronous (quasi-zenith) orbit; – Broadcast GPS-like supplemental signals on three frequencies and two augmentation signals, L 1 -SAIF and LEX. • L 1 -SAIF (Submeter-class Augmentation with Integrity Function) signal offers: – Submeter accuracy wide-area differential correction service; – Integrity function for safety of mobile users; and – Ranging function for improving position availability; all on L 1 single frequency. • ENRI has been developing L 1 -SAIF signal and experimental facility: – L 1 -SAIF signal achieves good accuracy less than 1 meter in an RMS manner at the mainland of Japan; – Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory; – In order to improve the accuracy at the southwestern islands during ionospheric storm, we have designed some new L 1 -SAIF messages and tested them.

ION ITM Jan. 2014 - Slide 2 QZSS Concept GPS/GEO • • • Footprint of QZSS orbit; Centered at 135 E; Eccentricity 0. 075, Inclination 43 deg. QZS • Broadcast signal from high elevation angle; • Applicable to navigation services for mountain area and urban canyon; • Augmentation signal from the zenith could help users to acquire other GPS satellites at any time.

ION ITM Jan. 2014 - Slide 3 L 1 -SAIF Signal QZS satellites Ranging Function GPS Constellation Error Correction Integrity Function • Three functions by a single signal: ranging, error correction (Target accuracy: 1 m), and integrity; • User receivers can receive both GPS and L 1 -SAIF signals with a single antenna and RF front-end; • Message-oriented information transmission: flexible contents; • See IS-QZSS for detail (Available at JAXA HP). SAIF： Submeter-class Augmentation with Integrity Function Ranging Signal User GPS Receivers

ION ITM Jan. 2014 - Slide 4 L 1 -SAIF Corrections • Example of user position error at Site 940058 (Takayama: near center of monitor station network); • Realtime operation with MSAS-like 6 monitor stations; • Period: 19 -23 Jan. 2008 (5 days); • L 1 -SAIF provides corrections only; No L 1 -SAIF ranging. Horizontal Error Vertical Error Standalone RMS GPS Max 1. 45 m 2. 92 m 6. 02 m 8. 45 m RMS 0. 29 m 0. 39 m Max 1. 56 m 2. 57 m System Standalone GPS Augmented by L 1 -SAIF Augmentation to GPS Only Note: Results shown here were obtained with surveygrade antenna and receiver in open sky condition.

ION ITM Jan. 2014 - Slide 5 Problem: Ionosphere Density (NASA/JPL) • The largest error source: Ionospheric propagation delay; • Varies on the local time, solar activity, earth magnetic field, and so on; • Cannot be predicted; Causes large effect in the low magnetic latitude region.

ION ITM Jan. 2014 - Slide 6 Accuracy at Southwestern Island LT 14: 00 At Southwestern Island (960735 Wadomari) At Northernmost City (950114 Kitami) • During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands; • The effect is not so large at the mainland of Japan; • It is confirmed that increase of the number of GMS shows a little improvement.

ION ITM Jan. 2014 - Slide 7 Actual Ionosphere Corrections PRN 28 PRN 20 5 m At Southwestern Island At Northernmost City • Ionospheric correction continuously differs from the true delay by 5 m or more; • Degradation of position accuracy during storm is due to inaccurate ionospheric correction.

ION ITM Jan. 2014 - Slide 8 L 1 -SAIF Ionospheric Correction 60 Latitude, N 60 • Vertical ionospheric delay information at IGPs ( ) located at 5 -degree grid points will be broadcast to users. 45 • User receiver computes vertical ionospheric delays at IPPs with bilinear interpolation of delays at the surrounding IGPs. 30 • Vertical delay is converted to slant delay by multiplying a factor socalled obliquity factor. 30 15 IGP 0 0 120 150 Longitude, E 180 IPP IGP

ION ITM Jan. 2014 - Slide 9 Thin-Shell Ionosphere Vertical Delay Iv IPP Slant Delay Ionosphere EL F(EL) • Iv Shell Height (350 km) Earth • • • The ionosphere model used by the L 1 -SAIF; Ionospheric propagation delay caused at a single point on the thin shell; The vertical delay is converted into the slant direction via the slant-vertical conversion factor so-called obliquity factor, F(EL).

ION ITM Jan. 2014 - Slide 10 Obliquity Factor, F(EL) Obliquity Factor 6 H=100 km Slant delay Vertical delay 4 H=350 km Elevation Ionosphere Angle Height 2 H=1000 km 0 15 30 Satellite Elevation, deg 45 Obliquity Factor = Slant / Vertical • Slant-vertical conversion factor as a function of the elevation angle; • Also a function of the shell height; The current L 1 -SAIF specifies the shell height of 350 km.

ION ITM Jan. 2014 - Slide 11 Limitation due to Iono-Model Observe here if H=350 km Shell Height H=350 km, EL=25 deg Observe different points if H=600 km Shell Height H=600 km, EL=25 deg • MCS assumes 2 GMS are observing same location of ionosphere; • However, if true height is not 350 km, they are looking at different locations.

ION ITM Jan. 2014 - Slide 12 Limitation due to Iono-Model • Too Simple Vertical Structure: – Assuming the thin-shell ionosphere at the fixed height of 350 km; – IPP location may differ from the actual point with the peak density; Essentially, the ionospheric delay is caused over a certain distance within ionosphere; The model may not represent the horizontal structure as well as vertical. – Obliquity factor may not reflect the true vertical structure of the ionosphere. • Linear Interpolation of Vertical Delays at IGP: – Assumption that the spatial scale of the ionosphere variation is roughly 500 km or more; – Small structure cannot, even if observed, be reflected to the delay information. • Need Alternative Ionospheric Correction Methods: – Change assumptions on the ionosphere or avoid error by some way; – Allow definition of new L 1 -SAIF messages; – Minimize modifications from the current message and correction procedure.

ION ITM Jan. 2014 - Slide 13 Candidate Methods • Maintain Single-Layer Thin-Shell Ionosphere Model: – Employ widely-used simple model to minimize modifications and to avoid complexity of user receivers; – MT 26 -like message structure: Share IGP information given by MT 18; Note: MT 26 has 7 spare (unused) bits. – Define new message as MT 55 (Message Type 55) for this purpose. • Method 1: Variable Ionosphere Height: – Broadcast the peak height of ionosphere in addition to grid delay information. • Method 2: Ionospheric Correction per Satellite: – Generate vertical delay information at the grid points per each GPS satellite. • Method 3: Ionospheric Correction per Direction: – Generate vertical delay information at the grid points per each line-of-sight direction from receiver to satellite.

ION ITM Jan. 2014 - Slide 14 Existing Message Type 26 • MT 26: Broadcast Ionospheric Vertical Delay – Contains vertical delay information at IGP; – A MT 26 message contains information at 15 IGPs; Message Type 26: Ionospheric Delay Information Repeat Content Bits Range Resolution 1 IGP Band ID 4 0 to 10 1 1 IGP Block ID 4 0 to 13 1 15 IGP Vertical Delay 9 0 to 63. 875 m 0. 125 m GIVEI 4 (Table) — 1 IODI 2 0 to 3 1 1 Spare 7 — —

ION ITM Jan. 2014 - Slide 15 New Message Design (1) • Method 1: Variable Ionosphere Height: – Broadcast the peak height of ionosphere in addition to grid delay information; – Both MCS and user receivers need to compute the ionospheric pierce point and the obliquity factor appropriately for given peak height; – MT 55 contains the information of the peak height of the ionosphere. Repeat Content Bits Range Resolution 1 IGP Band ID 4 0 to 10 1 1 IGP Block ID 4 0 to 13 1 15 IGP Vertical Delay 9 0 to 63. 875 m 0. 125 m GIVEI 4 (Table) — 1 IODI 2 0 to 3 1 1 Peak Height 2 (Table) — 1 Spare 5 — — Message Type 55 (1): Advanced Ionospheric Correction Identical to MT 26 Peak Height of Ionosphere 00: 350 km 01: 250 km 10: 600 km 11: 1, 000 km

ION ITM Jan. 2014 - Slide 16 New Message Design (2) • Method 2: Ionospheric Correction per Satellite: – Generate every grid delay information for each ranging source satellite in view; – MT 55 contains an identification of satellite; Satellite ID requires at least 8 bits, however, we have only 7 spare bits in MT 26; Here we use only GPS satellites for the experimental purpose. – May need more measurements (ground stations) for this correction. Repeat Content Bits Range Resolution 1 IGP Band ID 4 0 to 10 1 1 IGP Block ID 4 0 to 13 1 15 IGP Vertical Delay 9 0 to 63. 875 m 0. 125 m GIVEI 4 (Table) — 1 IODI 2 0 to 3 1 1 SV ID 5 1 to 32 1 1 Spare 2 — — Message Type 55 (2): Advanced Ionospheric Correction Identical to MT 26 SV ID (PRN-1)

ION ITM Jan. 2014 - Slide 17 New Message Design (3) • Method 3: Ionospheric Correction per Direction: – Generate every grid delay information for each line-of-sight direction from receiver to satellite (azimuth and elevation angle); – Divide the sky into, for example, 5 directions; Example Definition of LOS Direction 010 101 011 100 MT 55 contains the information of the direction. – Also may need more measurements (ground stations) for this correction. Repeat Content Bits Range Resolution 1 IGP Band ID 4 0 to 10 1 1 IGP Block ID 4 0 to 13 1 15 IGP Vertical Delay 9 0 to 63. 875 m 0. 125 m GIVEI 4 (Table) — 1 IODI 2 0 to 3 1 1 Direction 3 (Table) — 1 Spare 4 — — Message Type 55 (3): Advanced Ionospheric Correction Identical to MT 26 LOS Direction 000: All 001: Zenith 010: North 011: East 100: South 101: West

ION ITM Jan. 2014 - Slide 18 Experiment: Configuration • Experiment Using L 1 -SAIF Master Station (L 1 SMS): – Upgrade to support new messages (MT 55) for Methods (1) to (3); – For this experiment, L 1 SMS operates in off-line mode; No realtime connection to GEONET and QZSS MCS; RINEX files from GEONET; – Evaluate augmentation performance of new messages by receiver software also upgraded to support MT 55. GPS Satellites g in g n Ra GEONET Na v M es sa ge Measurements IF A -S 1 L L 1 SMS al n g Si L 1 -SAIF Message k lin Up nd ba K- Operates in Off-Line Mode al n g Si QZSS MCS GSI Server ENRI JAXA TKSC (Tokyo) (Tsukuba) Evaluation by User Receiver Software

ION ITM Jan. 2014 - Slide 19 Experiment: Configuration • Upgrade of L 1 -SAIF Master Station (L 1 SMS): – Support new messages (MT 55) for Methods (1) to (3); – Accept additional measurements from IMS (Ionosphere Monitor Station) sites to increase the number of measurements (IPPs) for Method (2) and (3); – User receiver software is also upgraded to decode and apply MT 55. GPS Satellites Ra ng in g Si gn al GEONET RINEX Files GMS Data GMS+IMS Data GMS/IMS Measurements Clock/Orbit Corrections Ionosphere Corrections L 1 SMS Upgraded for MT 55 L 1 -SAIF Message Performance Evaluation Receiver Software MT 26/55 User Algorithms User Measurements

ION ITM Jan. 2014 - Slide 20 Experiment: Monitor Stations • Observation Data from GEONET: – GPS network operated by Geospatial Information Authority of Japan; – Survey-grade receivers over 1, 200 stations within Japanese territory. • Monitor Stations for Experiment: – 6 GMS (Ground Monitor Station) near MSAS GMS locations for clock/orbit and ionospheric corrections; – 8 IMS (Ionosphere Monitor Station) for Method (2) and (3) ionospheric corrections. • User Stations: – Selected 5 stations from North to South: (1) to (5) for performance evaluation.

ION ITM Jan. 2014 - Slide 21 Baseline Performance LT 14: 00 At Southwestern Island (User #4) At Northernmost City (User #1) • During severe ionospheric storm condition (Kp~7+), position accuracy with differential correction largely degrades at the Southwestern Islands; • The effect is not so large at the mainland of Japan; • All corrections are derived by measurements from 6 GMS.

ION ITM Jan. 2014 - Slide 22 Variable Ionosphere Height LT 14: 00 At Southwestern Island (User #4) At Northernmost City (User #1) • The ionosphere shell height of 600 km improves position accuracy a little; • However, some degradation is observed at the north and during quiet conditions; The effect is limited; • All corrections are derived by measurements from 6 GMS.

ION ITM Jan. 2014 - Slide 23 Variable Ionosphere Height Storm Condition (11/10/23 to 11/10/26) Quiet Condition (12/7/22 to 12/7/24) • The ionosphere shell height of 600 km may improve the balance of the accuracy between North and South; • The effect is not so large; Need more investigation.

ION ITM Jan. 2014 - Slide 24 Iono-Correction per Satellite LT 14: 00 At Southwestern Island (User #4) At Northernmost City (User #1) • Reduces position error by roughly 40% at the Southwestern Islands, while maintains the accuracy at other regions; • Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.

ION ITM Jan. 2014 - Slide 25 Iono-Correction per Direction LT 14: 00 At Southwestern Island (User #4) At Northernmost City (User #1) • This method also has a capability to reduce position error at the Southwestern Islands; • Desirable behavior at other regions; • Clock/Orbit corrections by 6 GMS; Ionospheric corrections by 6 GMS + 8 IMS.

ION ITM Jan. 2014 - Slide 26 Iono-Correction per SV/Direction Storm Condition (11/10/23 to 11/10/26) Quiet Condition (12/7/22 to 12/7/24) • These methods have similar performance on ionospheric corrections; • In terms of the number of messages to be broadcast, Method (3) correction per direction has the advantage.

ION ITM Jan. 2014 - Slide 27 Conclusion • ENRI has been developing L 1 -SAIF signal: – Signal design: GPS/SBAS-like L 1 C/A code (PRN 183); – Planned as an augmentation to mobile users. • Ionosphere disturbance is a concern: – L 1 -SAIF signal achieves good accuracy less than 1 meter in an RMS manner at the mainland of Japan; – Ionosphere disturbance sometimes degrades the position accuracy, especially at the Southwestern Islands of Japanese territory; – In order to improve the accuracy at the southwestern islands during ionospheric storm, we have designed some new L 1 -SAIF messages and tested them. Method (3) corrections per direction has a good property. • Further Investigations will include: – Validation of performance against historical storm events at many locations; – Performance at other Asian Countries; – More investigation of other correction methods against ionospheric disturbances.