The 20,000km view: How GPS works

Jamey Sharp

wow. many word. so dense

Portland State Aerospace Society

Launching amateur rockets... For Science!

Commercial GPS receivers:
not built for rockets!

I'm a rocket scientist and you can too


Time-of-flight positioning

take a deep breath

GPS: What is it?

GPS: What isn't it?

GPS: What isn't it?

How does "trilateration" work?

Oops: Time-of-flight, not distance

but that's not all!

Doppler measurements provide an independent velocity estimate

Theory of relativity

Want a clock that seems to run at 10.23MHz?

Run it at 10.2299999954326MHz —IS-GPS-200G

Signal vs Noise

take a deep breath

Nearest GPS satellite:
20,000km straight up

How far is 20,000km?

How far is 20,000km?

How much power does a GPS satellite transmit?


20,000km later...


and they're all transmitting at once

how could this ever work?

Direct Sequence Spread Spectrum!

A pure sine wave has all its power at one frequency

Carefully choose a pseudo-random sequence of +1 and -1

Multiply the signal by the pseudo-random sequence

How do you get your signal back?

De-spreading piles all the energy back into one power spike

What if there's interference?

Accidental interference:


Intentional jamming:


US military "anti-spoofing"

uses a secret spreading code

History and Politics

take a deep breath

Navigation has been the product of military history

...and global time synchronization is along for the ride

1707: Scilly naval disaster

1714: Longitude prize

prizes collected by John Harrison from 1736 to 1773

1773: Harrison's Chronometer H5

October 1940

LORAN-A development begins

June 1942

LORAN-A's successor

Made In Oregon

First field test locations:

1961: Clock synchronization

1957: Navy → Coast Guard



Transit use cases

Transit limitations

Transit influence on GPS

1973: Pentagon thinks up GPS

prototype "Block I" sats launched 1978-1985

1983: KAL flight 007 shot down over USSR

1989-1994: first 24 "Block II" satellites

1995: GPS constellation declared "fully operational"

Selective Availability

GPS Modernization

given consequences of GPS failure,

can we trust GPS?

Other GNSS systems

GPS Modernization

take a deep breath

why is US military GPS more accurate than ours?

ionosphere (85km-600km) delay

bad news: GPS receiver can't measure this

good news: it's frequency dependent

for science!

side note: Measuring the Total Electron Content in the ionosphere is one piece of citizen science you can do with GPS if you already know your location

why is US military GPS more accurate than ours?

better news: GPS modernization

that's nice
but I'm impatient


codeless tracking

Data layer

take a deep breath

navigation message

50 bits per second

complete message: 30 seconds

fishing for satellites


better plan: try all satellites at once!

found the visible satellites; now what?

need each satellite's precise orbit

e·phem·er·is /iˈfem(ə)ris/

noun: ephemeris; plural noun: ephemerides

a table giving the calculated positions of a celestial object at regular intervals throughout a period.


hot start


Lock and position fix should take less than a second!

but I'm impatient!

assisted GPS

assisted GPS shortcuts

but I'm picky!

satellite ephemerides are a best-guess forecast

once per week, control segment reports what last week's orbits really were


other navigation message stuff

GPS modernization

Open hardware and software

take a deep breath

where's the openness?

good news

GPS operation is well-documented by US government

bad news

GPS hardware is the worst kind of proprietary

documentation only available with lots of money and an NDA

do we need their hardware?

1991-1992: Matjaž Vidmar's homebrew GPS/GLONASS receivers

1991-1992: Matjaž Vidmar's homebrew GPS/GLONASS receivers

Motorola 68k CPU + DSP CPU + discrete electronics

2011-2013: Andrew Holme's homebrew GPS receivers

2011-2013: Andrew Holme's homebrew GPS receivers

discrete electronics + FPGA + Raspberry Pi

other open projects using COTS hardware

current: PSAS' GPS RF frontend

GPS RF frontend possibilities

how hard is this, really?

aka, GPS in 1 minute

take a deep breath

what is GPS?

find your position by trilateration from satellites

missing features: maps and surveillance

amazing science results

amazing engineering results

positioning is still evolving

military created GPS
but we make it

come play at PSAS!

Next launch: July 20th, 2014