Global Positioning System: An Introduction - PowerPoint Presentation

Slide1

Global Positioning System: An Introduction

David Treharne, n8hku

Ford amateur radio league, November 10, 2016Slide2

Agenda

What is GPS

How does it work?

How it is incorporated into the Yaesu FTM-400 radio

Observations, issues,

etc

when using a Yaesu FTM-400 radioSlide3

Global Positioning System

A satellite-based navigation system that covers the entire Earth

Originally intended for the military, first launched in 1978

Civilian use

begain

in the 1980’s

Normally, need 24 satellites to assure complete coverage (31 currently in orbit)

Actual name is NAVSTAR for the US based system

European system is GalileoSlide4

Global Positioning System Satellites

Each Satellite is in orbit at 12,000 miles, moving at 7,000 MPH

Each satellite makes 2 orbits per day.

Frequency: 1.57542 GHz. Power: 50 watts maximum.

Weight: about 2,000 lbs.

Satellites last about 10 years, solar powered with rocket propulsion to achieve and maintain the correct orbit.

Satellites transmit at both the top of a minute or the bottom of a minute. They transmit both the civilian version and an encrypted military signal.

Each satellite contains an atomic clock for precise time keeping. It is the time keeping function that allows the receiver to use the data to determine position.Slide5

Transmission details

Data rate: 50 bits per second

Complete message: 750 seconds, or 12 ½ minutes

The

Time and Ephemeris

data is sent every 30 seconds, with part of the Almanac.

The data in the Ephemeris portion is updated every 2 hours, and is valid for 4 hours if the upload gets missed for some reason.

The Almanac data is generally uploaded every 24

hrs, although the satellite has enough memory to hold weeks worth of predicted orbital data. Slide6

How does the data get sent and received on the same base frequency?

A pseudorandom code: a known algorithm that the transmitter and receiver both know.

Think as different seeds to a computer random number generator. Each seed (satellite) produces a unique sequence)

Calld

Code Division Multiple Access: Also used by the Cell Phone network.

All of the data is transmitted with this code. The receiver has to correctly guess this sequence to decode the data. The data is a 1 or -1 multiplied by the 1 or -1 of the pseudorandom number many times. (GPS uses 1 MHz of pseudorandom digits for the 50 Hz data signal)

The 50W signal is now over 1 MHz wide!

The data looks like noise with the wrong code

Data can be demodulated with the right code.

Multiple codes can be distinguished on the same band.Slide7

Power Density of the GPS signal

Power density in

dBm

/Hz

50 watt input signal

Major dilution, even before the 12,500 miles of distance

GPS signal is below the noise floor in the receiver, but the correlation with the pseudorandom generator pulls it out.

Signal

Bandwidth

dBm

/Hz

Carrier

1

47.0

CW

25

33.0

HF

2,300

13.4

FM

12,500

6.0

GPS

1,000,000

-13.0Slide8

What gets sent?

Satellite Clock, GPS Time relationship, along with status information on the health of the satellite.

Ephemeris data: This contains the precise satellite orbit. Ephemeris means Diary in Latin, and refers to the positions of astronomical objects.

Almanac data: Information on where in space this satellite and the rest of the satellites are at any time throughout the day. Up to 32 satellites are in the Almanac. Only a couple of almanac entries are sent every 30 sec, taking 12.5 minutes to get everything.

Note: All satellites transmit at the same time. The wide bandwidth and the CDMA code allow them to be received at the same time.Slide9

Getting the Position

Knowing the position of the satellite in orbit, and how long the data took to arrive (time of flight), the receiver can calculate the location. Each satellite tells the time of the transmission precisely.

But, the receiver clock is not accurate!

With 4 satellites, the clock bias can be calculated

Once the clock is accurate, the receiver can determine with only 3 satellites.

If Z direction is ignored, then X, Y and Time can be determined with 3 satellites.

Up to 12 satellites can be seen at any given time. Algorithms handle this extra data and attempt to use the “best” satellites (those not in close together in the same space) Slide10

Location between Measurements

Issue: The exact time pulse is sent from each satellite every 30 seconds.

So, how do you track the location between measurements?

Our good friend, the Doppler shift:

The frequency gets higher as the satellite approaches, farther as the satellite recedes. The same goes if the receiver is moving.

Based upon the original measurement, the motion of the satellite is known from the Ephemeris data, so the system calculates that effect versus the actual shift. Any other shift is due to the receiver motion, so speed and direction may be obtained by calculating the motion of all of the satellites.

Most receivers do this 1x per second, some do it 10x per second.

Because of the satellite Doppler shift, the velocity is best when the satellite is moving slowly in relation to the receiver.

Sometimes, the satellites or obscured by buildings, tunnels,

etc

:

Dead Reckoning

1-3 axis gyro sensor plus 1-3 axis accelerometer accounts for receiver motion while signals are blocked.

Base Frequency

1575420000

Hz

Sattelite Speed

F doppler satellite

7000

MPH

16338

Hz

3111

m/s

Vehicle Speed

F dopplier vehicle speed

40

MPH

93.4

Hz

18

m/sSlide11

If only the Earth was a Sphere

Do not tell the flat earth society, but the Earth is not round, either. It is a geoid.

The GPS unit has mapping to find a location on the Geoid. The GPS uses the center of the Earth for a reference, but the continents are moving

Works good for US, as the North American plate is moving slowly

What about Australia?

The map is off by 1.5 meters, or about 5 feet

This is due to the fact the continent is moving about 2.2 inches per year, which has built up since the map was fixed in 1994 (GDA94 datum)

They plan to fix this in 2020. Slide12

Relativity

Einstein was right about Space-time and gravity

A clock in a high gravitational field will run slower, so that the Earth based clocks seem slow to the GPS satellites. (General Relativity)

A clock moving in relation to a second clock will also run slower, so that the Earth based clocks seem fast to the GPS satellites. (Special relativity)

The satellite speed and orbit were not set up to precisely counteract the two effects, so the GPS satellite clocks are updated on a regular basis. Slide13

Throwing out Other Errors

That pseudorandom code can help toss out errors due to multipath and bouncing off of buildings: How?

At 1 MHz, the code will be 50% out of phase after a distance change of 300/2 or 150 meters. If the receiver can see the phase change of just 10%, then that is 30m of bounce, or the extra path from a building a block away.

Atmospheric errors: How much “air” does the satellite signal go through? Any storms creating more charged particles in the ionosphere, slowing the signals? Solution: use more satellites

Military solution: decode the civilian and military data: Ionospheric delay is a function of the frequency and total electron content of the path, so the time delta between the signal received between the 2 frequencies allows the error to be calculated and compensated for!

Geographic distribution: If all the received satellites are in one area of the sky, the accuracy will be reduced. This happens with mountains or “urban canyons” of buildings. This also occurs with

recievers

in cars or in buildings.

Speed: A moving vehicle introduces errors in measuring speed. Aircraft have to use compensation methods, such as measuring the Doppler shift of the received signals. Slide14

How about Earth Time versus Atomic time?

GPS-Time (GPST) was started with a match to UTC in 1980. Since then, it has not been changed. GPS time is 17 seconds ahead of UTC.

GPS date is expressed as a week number, and seconds into the week time. The date is sent with a 13 bit signal now, so that the date will not repeat for 157 years. The prior GPS data listed 19.6 years, resetting to 0 on August 21, 1999, from a start from January 6, 1980.Slide15

Why does it take so long for the Yaesu GPS unit to synchronize sometimes

The system can pick up immediately, or with seconds, or it can take 12-24 minutes to lock onto satellites.

It can seem to get things wrong for a bit, then finally correct. When the satellites appear correctly, they appear in groups. (The GPS has 12 “channels” that are trying to guess and decode satellite data. As soon as one is achieved, the stored Almanac data can be used to figure out where the rest of them are to be overhead, allowing the channels to synchronize.

The Yaesu radio powers off the GPS receiver when off, but an on-board Lithium battery keeps the clock running for a few days at least.

To acquire a signal, many chips need a signal greater than -130dBm. Once acquired, most can keep track down to -160dBm. Slide16

Yaesu FT-400 XDR Times to Sync

Another user has been trying to reverse engineer the GPS unit, and has run some tests:

Leaving the radio switched off for 4 hours requires about 45 seconds for the GPS to acquire a lock.

Leaving it switched off all night requires 5 to 10 minutes

(I have not had this consistency with my unit: I can sometimes lock on within seconds, even after 12 hours of vehicle off time)

I have had it take 20 minutes before locking, where I am almost at work.

Some have

jumpered

the GPS receiver to be on whenever power is on the radio, which draws 29mA. It eliminates the time delay under most uses. Our cars run about 20mA of key off current, so this approach would double that

I think the GPS unit is a bit weak, although the newer one in the XDR is better than the original 400 DR. There does not seem to be a firmware fix, it is just the unit itself, which is a stand alone chip running 4800 baud data to the main CPU. Slide17

Other Uses for GPS

The precise timing of GPS clocks allow for use with:

Power grids, financial transactions, Doppler weather station timing, earthquake networks.

The signals can be blocked or jammed: Devices are sometimes used by commercial vehicle drivers to avoid getting tracked during lunch or breaks.

There is a great concern that the reliance on GPS may leave the cell phone network and utilities vulnerable to disruption if GPS is lost due to intentional jamming or a solar storm that disables the satellites.

Backups have not been tested. Some systems have neglected to even install backup methods with their latest designs.

We hams know not to rely on one communications technology exclusively. Slide18

Future work with Multi-GNSS: (Multiple Frequency Global Navigation Satellite System)

GLONASS of Russia, Galileo of Europe,

BeiDou

(Compass) of China, SBAS (Satellite Based Augmentation System with Geosynchronous satellites and US WAAS (Wide Area Augmentation System run by the FAA)) is starting. Japan has started QZSS (Quasi-Zenith Satellite System)

New receiver chips pick up more satellites, improving accuracy and availability.Slide19

Conclusions

The Global Positioning System is here to stay.

Worldwide systems designed to provide an alternative to GPS plus provide more accuracy are making positioning measurements better

The GPS inside the Yaesu FT-400 XDR transceiver is fairly simple and has some issues, an external unit attached to the device would provide better accuracy and coverage when in tunnels, etc.

Designers of equipment using GPS need to incorporate back up plans for when the system is not operational. Slide20

Bibliography

https://en.wikipedia.org/wiki/Global_Positioning_System

http://www.physics.org/article-questions.asp?id=55

https://en.wikipedia.org/wiki/Ephemeris

https://en.wikipedia.org/wiki/Chip_(CDMA)

http://www.atlasobscura.com/articles/australias-entire-gps-navigation-is-off-by-5-feet

http://forums.qrz.com/index.php?threads/yaesu-ftm-400-gps-issues-and-possible-solutions.483742/

http://www.furuno.com/en/gnss/technical/tec_what_gps#BasicstructureofGPS

https://www.e-education.psu.edu/geog862/node/1786

(GPS and GNSS for Geospatial Professionals)