Mitigation of the Effects Early Skywaves Ben Peterson, Peterson Integrated Geopositioning & Per Enge, Stanford University Funded by Federal Aviation Administration, Mitch Narins, Program Manager International Loran Association, November 5, 2003

Outline Review the environment & 1986 MOPS (TSO C60b) –29 & 29 Oct 2003 sample data Shift tracking points for phase & ECD earlier –Analysis of noise and bias in phase & ECD measurements vs tracking point & pulse rise time Receivers w/ causal & non-Causal (block processing) filtering –Change in time differences with shifted tracking points Transmitting pulses with carrier frequencies of 96, 100, and 104 kHz Augmentation (monitors & warnings via LDC)

From Peter Morris (curves for 1 mmho/m) ECD M * 5 usec TOA (N+1/2) * 5 usec 1 mmho/m)

690 NM

441 NM

591 NM

Simultaneous loss of WAAS vertical guidance & Loran horizontal is not operationally significant. Ionosphere never gets bad enough that WAAS HPL > 556 m Loran exists to address other vulnerabilities (jamming, spoofing, interference, etc)

Geomagnetic Latitude Auroral Zone 60deg

Basic Problem Statement Pulse design and 1986 MOPS address everyday skywaves Issue is abnormally early skywaves caused by solar event We can easily detect existence of skywaves, tough part is to distinguish skywave with 25 us delay from one with 35 us, 23 us from 33 us, etc. in a user receiver –With < 1e-7 integrity & –With < 1e-4 to 1-e3 false alarms This can be done in a monitor receiver

Current requirement

No RF filtering. Curves will still apply when we filter, we just need to correct for the group delay & the NEBW re the 30 kHz used here. Entire pulse phase noise will not change with NEBW.

8 th order Butterworth, 28 kHz -3 dB bandwidth (NEBW = 28.9 kHz)

8 th order Butterworth, 28 kHz -3 dB bandwidth, group delay = 30 usec

8 th order Butterworth, 28 kHz -3 dB bandwidth, group delay = 30 usec (Vertical axis ECD bias in usec, x axis skywave delay in usec)

Cascade of 2 nd order analog filter, 64 kHz BW w/non-causal digital filter

Cascade of 2 nd order analog filter, 64 kHz BW w/non-causal digital filter ECD Bias

Cascade of 2 nd order analog filter, 64 kHz BW w/non-causal digital filter, 50 usec rise time

Cascade of 2 nd order analog filter, 64 kHz BW w/non-causal digital filter, 50 usec rise time, ECD Bias (Green Locus LRS IIID data)

Field strength predictions of BALOR code for Seneca-Little Rock path Undoes the effect of differentiation

In near far field, zeros crossings are < 5 usec apart

After propagation has cancelled frequency response of differentiation

At long differences, zeros crossings are > 5 usec apart

Frequency modulation Transmit mixture of 96, 100, & 110 kHz pulses in known pattern SSX controller switches L or C but not during pulse as in IFM Measure ECD & TOA for each frequency & compare Early skywave will cause different biases at the different frequencies Issues: Spectrum Effect on legacy receivers Ability to detect early skywaves

Rise Time = 72 usec

Lengthening pulse permits balanced distribution among frequencies & still meeting spectrum requirement 2 96 & 104 kHz 5 96 & 104 kHz

Lengthening pulse makes leading edge of average pulse the same to legacy receivers

 = approx. 10% of bias

Horizontal: Candidate detection statistic, Vertical: Bias to detect

Assumptions Time constant (after Doppler has been measured and removed) = 20 sec GRI = 9990, 1800 pulses (1600 unmodulated) in 20 sec 96 kHz, 104 kHz, kHz, & kHz SNR = -10dB –SNR = +17 dB after average of 500 –SNR = +22 dB after average of 1600 Phase  = usec /(N x SNR)  = usec for average of -10dB SNR  = usec for difference between 2 frequencies  = usec for average of -10dB SNR ECD  = 30.4 usec/(N x SNR)  = 4.29 usec for average of -10dB SNR  = 6.07 usec for difference between 2 frequencies  = 2.41 usec for average of -10dB SNR

Candidate test statistics for TOA bias Delta TOA to detect TOA bias –Can’t use 104 re 96 kHz because delta goes to 0 at bias max (22.5 & 27 usec) –For max of 104 re 100 kHz and 96 re 100 kHz Delta TOA = 0.1 x TOA bias For probability of false alarm = 1e-3, 3.3 x delta TOA  = 3.3 x usec = 0.74 usec, corresponding bias is 7.4 usec or 2,200 meters Delta ECD to detect TOA bias –Delta ECD = 40 x TOA bias –For probability of false alarm = 1e-3, 3.3 x delta ECD  = 3.3 x 6.07 usec = 20 usec, corresponding bias is 0.5 usec or 150 meters –At 0 dB SNR, this bias becomes 0.17 usec or 50 meters

Candidate test statistics for ECD bias Delta ECD to detect ECD bias –Can’t use because all deltas go to 0 at bias max (24.5 usec) Delta TOA (104 re 96) to detect ECD bias –Delta TOA = x ECD bias –For probability of false alarm = 1e-3, 3.3 x delta TOA  = 3.3 x usec = 0.74 usec, corresponding ECD bias is 26 usec

79 Paths of < 900 nm using only LORSTA’s With addition of Dunbar Forest, dense enough to see PCD. Dunbar Forest??

Conclusions Problem is much worse at high geomagnetic latitudes (Alaska) but occasionally exists in large portion of northern CONUS –Problem for NELS, Russia, but probably not FERNS Problem can be detected with existing monitor infrastructure –Monitoring at LORSTA’s (plus Dunbar Forest) only is OK Receiver issues –Faster rise time is possible and helps –Non causal digital vice causal analog or digital filtering helps –Moving tracking point earlier helps, phase and ECD measurement noise get worse but are reasonable –Moving tracking point affects TDs, issue for maritime, less for aviation –Frequency modulation is elegant but preliminary analysis indicates poor performance at low SNR Simultaneous loss of WAAS vertical guidance & Loran horizontal is not operationally significant.

Options (in order of cost) A. New receiver MOPS Probably not enough B. Real time warnings via data channel (& new MOPS) –Hopefully enough but need to study impact on availability C. Faster rise time (& new MOPS) D. Frequency modulation (& new MOPS) A, B & C or A, B & D

Options (in order of cost) A. New receiver MOPS Probably not enough B. Real time warnings via data channel (& new MOPS) –Hopefully enough but need to study impact on availability C. Faster rise time (& new MOPS) D. Frequency modulation (& new MOPS) A, B & C or A, B & D

Acknowledgements, etc. Funded by Federal Aviation Administration –Mitch Narins – Program Manager For additional info: -Note- The views expressed herein are those of the authors and are not to be construed as official or reflecting the views of the U. S. Federal Aviation Administration, or the U.S. Departments of Transportation and Homeland Security.