2003 North Texas APRS Workshop

GPS:Basics

ReceiversNew and different uses

Gerry CreagerN5JXS

n5jxs@tamu.edu

GPS Basics

● How does it work?● How accurate is it?● Who owns it?● Who pays for it?● What about other options?● Is it gonna last?

1N5JXS – NTxAPRS

GPS Basics

● How does it work?– 6 orbital planes

● 60o apart

– 24 satellites● 4 per plane● 56o inclination● May be on-orbit spares

– In latitudes lower than 60o

● 6 satellites 98% of the time; 8 satellites 95% of the time

GPS Basics: The RF Signal

● Direct Sequence Spread Spectrum● L1: 1575.42 MHz

– 19 cm ● L2: 1227.6 MHz

– 24 cm ● L5: 1176.45 MHz

– 39 cm

GPS Basics: DSSS● Pseudorandom “Gold” codes unique for each

satellite● C/A: 1023 bits, repeats each msec

– 1.023 MHz rate– 300 M integer pseudorange

● P(Y): somewhat longer, repeats about every 267 days– 10.23 MHz rate– 30 M integer pseudorange

GPS Basics: Range vs Pseudorange

● If we could measure it precisely, it'd be 'range'● Since we're estimating it, it's 'pseudorange'

– Clock errors– Relativity– Ionospheric delay and scintillation– Tropospheric (water) delay

● OK, so maybe the speed of light isn't so constant as we thought...

GPS Basics: Triangulation vs Trilateration

● Triangulation– Multiple bearings on an unknown target– Readings taken from known locations– Tends to be planar

● Trilateration– Multiple distance determinations to an unknown

target– Readings taken from known locations– 3D positioning

● GPS uses trilateration

GPS Basics: Trilateration

● How does one determine a distance (range)?– Speed of Light (not just a good idea, it's the law!)– Transit time– Pseudorange– Code phase

● Integer ambiguity● Phase measurement

● All this assumes an autonomous, code phase determination...

GPS Basics: Autonomous Positioning● How does it work?

GPS Basics: Autonomous Positioning

● SO: How accurate is it?– Does anyone remember Selective Availability?– With SA, we usually claimed we had 95 M accuracy,

95% of the time● 5% of the time, all bets were off.

– A “conservative” estimate of the C/A error budget suggests 29 M accuracy is reasonable

● Best 1970's measurement and ranging technology money can buy!

– BUT in reality, 6 M is pretty common

GPS Basics: Autonomous Positioning

● Comparison of Positions With and Without Selective Availability

– 6 1/2 Hour Data Sets

Fine Print: Data taken at the Hartsville National CORS station, National Geodetic Survey, NOAA. Data with SA were taken from 0730 to 1400 UTC on May 1, 2000. Data without SA were taken from 0730 to 1400 UTC on May 2, 2000. Both data sets were taken at 30 second intervals. Instrumentation was an Ashtech Z-12 receiver. GPS data were dual-frequency pseudorange (both L1 and L2) incorporating ionospheric correction. Data were processed in accordance with the GPS Interface Control Document ICD-GPS-200C, using the broadcast orbit parameters in the World Geodetic System WGS 84 (G873) reference system.

GPS Basics: Military's Autonomous Positioning

● Error budget suggests 14 M conservative estimates

● Reality suggests 2-3 M CEP at 2 sigma● Often observed at < 1 M accuracies

GPS Basics: Vertical Accuracy

● 1.414x the accuracy of a horizontal measurement. AT BEST.– Highly dependent on satellite geometry– You can't push on a rope– There's dirt beneath your feet

● Accuracy determinations have been the subject of a number of Masters' and PhDs' projects, and the answer is still 1.414x...

GPS Accuracy: Surveying● So how do they claim to get “survey” accuracy?

– Carrier (not Code) phase measurements

– Double Differencing● 2 satellites, 2 receivers (observers), one on a known

point● Long periods of observation

GPS Accuracy: Surveying● How good can it get?

– 1 cm 2 2Drms horizontal● 4-12 hours of data

– 3 cm 2 rms vertical● 4-12 hours of data

– Both results use long baselines (> 100km)– Both results compare favorably with long-accepted

practice (US DoC NGS)– Other practices (kinematic, “rapid static”) can achieve

2-5 cm horizontal, 3.5-8 cm vertical accuracies● Most of these are 1 statistics...

GPS: Myths● “I have an EPE reading of 2 feet”

– Garmin “pioneered” EPE to add an edge to sell receivers

– Based on Horizontal Dilution of Precision (HDOP)– Makes some assumptions

● Proprietary● Uses HDOP and URA● Almost certainly still thinks SA is on.

● More than likely, you're within 6 meters● Low numbers are good. So are low HDOPs...

GPS: Who Owns It?

● US System● Based on earlier work with TRANSIT

– tedious timing, frequency monitoring– up to 8 hours to get a fix– much more accurate than star-fixes

● Interferometry ● Doesn't have to be done with GPS satellites!

GPS: Who Owns It?

● USDoD and US Citizens– March 2000

● Clinton decrees civil availability is paramount

– 1 May 2000● SA turned off permanently

– System is considered a “Safety of Life” system– Managed and operated by USAF 2nd Space Sqdn– Directed by Interagency GPS Board

GPS: Who Pays For It?

● Congress– DoD appropriations– DoT appropriations– IGB support (line items) in Agency budgets– L5 upgrades paid by DoD

● “The right thing to do” after Congress cancelled civil funds for the upgrade (Staff didn't understand issues)

● Supported DoD mission of adding M-code● Cheap at that point in the process: $100M

GPS: Who Uses It?● DoD

– Force location– Force direction

● Civil– Aircraft– Surveyors– Cellphone companies

● Timing● E-911

– HAMS!

GPS: Who Uses It?

● GPS has become a world-wide utility● 1 May 2000 removal of SA almost certainly

followed development of selective denial capabilities

● Europe and the rest of the world don't trust us● US position is that GPS won't be turned off

– But it's still a DoD program...

GPS: Competetion● Russia/Soviet Union

– GLONASS● GPS without SA capability● Few satellites and little funding

● EU– GALILEO

● Lots of internal controversy● Expensive● Long time to build

– Still no hardware● Interesting “business model”: Subscriptions/fee for

services

GPS: Is It Gonna Last?

● Probably– BlockIII satellites now being built– WAAS fully implemented and in service– LAAS still ongoing– Ubiquitous service

GPS: Enhancing User Accuracy

● DGPS– Fixed receiver, surveyed site– Transmits corrections to receivers for incorporation in

autonomous position determination– ~10x improvement in horizontal accuracy (within

limits: range, satellite visibilities, etc)– Internet, cellphone. satellite and radio distribution

methods available – Usually subscription-based

GPS: Enhancing User Accuracy

● WAAS – Wide Area Augmentation Service– US DoT originated– Satellite-based– Full CONUS coverage– Provides data for all SV's– Lots of receiver support – Satisfactory enhancement for FAA enroute and

terminal area guidance, and non-precision approachs

GPS: Enhancing User Accuracy

● LAAS – Local Area Augmentation Service– US DoT originated– Designed to provide sufficient accuracy for Auto-

Land– Based at or near airports– May incorporate pseudolites– Still in development– Ask me about salad bowls and yagis and collinear

verticals

GPS Receivers

● 12 channel receivers are good– Are they necessary?

● 6 channel receivers are bad– Or are they?

● Code-phase autonomous positioning– How many satellites must be in view?– What's the effect of geometry on the strength of

solution?

GPS Receivers● Overdetermination

– How many satellites are required for a fix?● 2D● 3D● 1D (time)

– How many unique combinations (u) of n satellites can be found that will be useful in a priori determination?

– Using a least-squares approximation, the 4-satellite combination with the smallest residuals is chosen as “the” fix

GPS Receivers: Unique combinations of satellites

Number of Satellites Unique Combinations of Visible 4 Satellites

4 15 56 157 358 709 12610 21011 33012 49513 71514 100115 136516 1820

GPS Receivers

● What's speeded up TTFF?– Better processors– More memory– Better algorithms– Longer retention of almanac data– Longer retention of ephemeris data (!?!)– NOT more channels, or more marketeering

GPS Receivers

● How many channels are enough?– 4 channels is probably NOT enough

● No overdetermination

– 14 channels is probably too many● Rarely observe 14 satellites in the visible sky

– 12 channels is OK● Roughly 50% of the time

– 8 channels● 90%+ visible

Whose receivers are good?

● I tend to use a different receiver than you will● I do research

– L1/L2– Be the first kid on my block to have L1/L2/L5– Low-noise, plenty of available observables

● NMEA just ain't enough

● But there are PLENTY of good sources

Whose receivers are good?● Motorola

– OnCore, M-12

● Trimble – SK-8 (Lassen), SVee-6

● Rockwell– Jupiter

● SiRF– Ubiquitous, various vendors, not available directly

from SiRF

Whose receivers are good?

● Garmin– Best bang for the consumer $$$– Good receivers– Good marketing– Good customer service– Good R&D

Whose receivers are good?

● Who's missing– Magellan?– Delou?– Anyone else?

● In general, if you can't get them to tell you who made the receiver for them, it's SiRF– And that's good!

GPS: Conclusions

● Almost any receiver you find out there will be good enough for APRS

● Cost and interface are important● NMEA sentence selection helps● Differential correction isn't as important now

– WAAS is a plus, but a minor factor

● OEM board or unit with a display?● Power consumption

GPS: Conclusions● Accuracy: 6m horizontal, 9m vertical● New civil frequencies on the horizon● All sorts of non-navigation uses

– Water vapor– Timing– Ionosphere Total Electron Counts

● Aviation taking significant advantage of the technology (WAAS, LAAS, RAIM)– Autolanding– Freeflight

GPS

● Questions?● n5jxs@tamu.edu● A copy of this will be available later via

http://page4.tamu.edu under the Presentations or Publications header. I'll attempt to update it to respond to questions from today, as well.