Global Positioning System (GPS) is actually a …edge.rit.edu/edge/P09233/public/GPS Report...

Heidi Morgan GPS Receiver P09233 Airframe Measurements

1) GPS Receiver:

Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting

satellites where 24 of them are in operation and the other three are extras in case one

fails. The GPS receiver's job is to locate four or more of these satellites, calculate the

distance to each. Using this information, the receiver derives its own location. This

process is based on a simple mathematical principle called trilateration.

The GPS receiver must find the distance to three satellites of known positions to

locate it. If the receiver finds that it is X miles from one satellite, it knows that it must be

somewhere on an imaginary sphere, with the satellite as the center and a radius of X. if

the receiver can generate these spheres for two satellites, it knows it can only be located

where the surfaces of the two spheres intersect. The two spheres overlap in a ring of

possible receiver positions. By generating a sphere for a third satellite, the receiver

narrows its possible positions down to two points. The receiver ejects the point located in

space, leaving only one possible position.

The GPS receiver has to know two things to make this calculation, the location of

at least three satellites above you , and the distance between you and each of those

satellites.

The GPS receiver analyzes high-frequency, low-power radio signals from the

GPS satellites to calculate both of these things required. The receiver can figure out how

far the signal has traveled by timing how long it took the signal to arrive.

The receiver begins running a digital pattern which has been transmitted from the

satellite along digital pattern. When the satellite's signal reaches the receiver, its

transmission of the pattern will be delayed a bit behind the receiver's playing of the

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pattern. The delay length is equal to the signal's travel time. The receiver multiplies this

time by the speed of light to calculate how far the signal traveled.

To make this measurement, the receiver and satellite both need clocks that can be

synchronized down to the nanosecond. To make a satellite positioning system using only

synchronized clocks, all the satellites would need to have atomic clock and the receiver

itself uses an ordinary quartz clock, which it constantly resets. Quartz clock is been

choosen for the receiver instead of the atomic clock because the atomic clock is

expensive in the range of $70,000 which wouldn’t be in the reach of eac individual. The

receiver looks at incoming signals from four or more satellites and estimates its own

inaccuracy so there is only one value for the current time that the receiver can use. The

correct time value will cause all of the signals that the receiver is receiving to align at a

single point in space. That time value is the time value held by the atomic clocks in all of

the satellites. So the receiver sets its clock to that time value, and it then has the same

time value that all the atomic clocks in all of the satellites have.

When the distance to four located satellites are measured, four spheres can be

drawn that all intersect at one point. Three spheres will intersect even if the numbers are

way off, but four spheres will not intersect at one point if the measured values were

incorrect. Since the receiver makes all its distance measurements using its own built-in

clock, the distances will all be proportionally incorrect. The receiver can easily calculate

the necessary adjustment that will cause the four spheres to intersect at one point. Based

on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does

this constantly whenever it's on, which means it is nearly as accurate as the expensive

atomic clocks in the satellites.

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Once the receiver makes this calculation, it can tell you the latitude, longitude and

altitude of its current position. To make the navigation more useful and easier to follow,

most receivers plug this raw data into map files stored in memory.You can use maps

stored in the receiver's memory, connect the receiver to a computer that can hold more

detailed maps in its memory.

2) Design Specification:

A standard GPS receiver will not only place you on a map at any particular

location, but will also trace your path across a map as you move. If you leave your

receiver on, it can stay in constant communication with GPS satellites to see how your

location is changing. With this information and its built-in clock, the receiver can give

you several pieces of valuable information such as, how far you've traveled (odometer) ,

how long you've been traveling , your current speed (speedometer) , your average speed,

a "bread crumb" trail showing you exactly where you have traveled on the map, and the

estimated time of arrival at your destination if you maintain your current speed .

3) Pugh Analysis:

A big search was done to find a GPS Receiver that meets our requirements. After

narrowing down our choices, we ended up with ten GPS receivers. A comparison was

done for the ten GPS receivers according to the following features: accuracy, voltage

supply, power consumption, battery backup pin, built in antenna, baud rate, dimensions,

weight, price, and number of channel tracking. The best four GPS receivers were picked

out of the ten after the comparison was done. See attached spreadsheet in the back.

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Concept screening matrix was the next step, where the four GPS receivers were

rated with a + or – or 0 on each selection criteria in each GPS receiver. The 0 means it

doesn’t have any effect on our ranking decision, that is because it is either all the GPS

receivers have the same feature or there is no big difference between them. The + means

it is ranked the best in this criteria over the rest of the other GPS receivers. The – means

this GPS receiver has a weakness in this criteria or it is not preferred according to the

other GPS receivers which have a better rank in this criteria. The ranking is based on the

following selection criteria: Cost, size, weight, resolution, range, power consumption,

interface, and baud rate. When each GPS receiver has been ranked for each criteria, we

sum the ranking rates +’s, -‘s, and 0’s for each GPS receiver. Then the sum from the –‘s

get subtracted from the sum of the +’s, and that will be the net score. According to the net

score we rank each GPS receiver, highest net score means the top ranked and lowest net

score means ranked the last favored. The highest ranked GPS receivers are picked to

continue to the next concept-screening matrix. See attached spreadsheet in the back.

The second concept-screening matrix is a refinement for the first Pugh’s matrix

where only the highest ranked ones from the first Pugh’s matrix will be in this and the

weighting will be added for each criteria. Weights are selected according to how

important each feature to out design. The scale will be refined in this stage from 1 to 5,

where 5 is the best and 1 is the worst. An average criteria is selected for a reference, and

then a comparison is made for each criteria for each GPS receiver to reference criteria.

Summing weighted scores ranks concepts. Highest ranked will be the best concept that

will be purchased. All three GPS receivers, which were selected to continue to the second

concept-screening matrix, have the same size, weight, range, power consumption, and

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interface; the only difference was in the channel tracking, cost, and baud rate. Choice B

and c have the same default baud rate, and same number of channels, however B is

cheaper than C. There fore choice B is better than C. Choice A has a higher baud rate and

more channels than B. A has 32 channels compared with B which has 20 channels. Even

though choice A cost $99.95 and B cost $61, choice A worse the $29 extra since it has

almost double the channels which means more reliable. Therefore choice A was ranked

the highest. See attached spreadsheet in the back.

4) Components Specification:

The best concept, which was purchased, is the 32 Chan San Jose Nav., part number GPS-

08266 from Spark Fun Electronics. It has a 3.3m accuracy, 3.3-5V-voltage supply, 33-

59mA-power consumption; it has a battery backup pin, built in antenna, baud rate of

38400 bps, with dimensions of 1.2 x 1.2 x 0.3'', and weights 15 grams, it has 32 channel

tracking and cost $99.95. This receiver has a 5Hz update rate.

Please note at 5Hz the serial stream is large enough the default baud rate is

38400bps - but the unit is completely configurable! The unit configuration is stored in

volatile memory and is reset when power cycled. Use a battery backup to maintain user

settings.

The MiniGPS software that was provided by San Jose Navigation allows for an easy to

use GUI to reconfigure the FV-M8 as well as data log incoming NMEA sentences! It is

able to track over 9 satellites indoors.

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Features

*5Hz max update rate

*Default 38400bps

*Selectable update rate from 1 to 5Hz

*Selectable baud rate from 4800 to 115200bps

*Support for DGPS, WAAS, EGNOS, MSAS

*32 parallel channel receiver architecture

*-158dBm sensitivity

*3.3 m accuracy

*2.6 m accuracy with DGPS

*3.3 to 5V supply

*59/42/33mA power consumption

*2 NMEA serial ports

*Battery backup pin

Dimensions

30x30x8.5mm (1.2x1.2x0.3")

15grams

100mm cable included

Documents

FV-M8 Datasheet

FV-M8 User Manual

MTK Packet User Manual

EB-230 Packet Guide

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Mini GPS Software

GPSTrace Software

GPS Tutorial

Block Diagram

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Pin Definition

Pin# Signal Name Type Description 1 RF Input I Antenna port, L1, 1575.42MHz, 50 ohm 2 GND P Ground 3 V30D P Digital power supply, 3.0V±10% 4 GPS status/GPIO0 O GPS status, blink when GPS has position fix General input/ output 5 1PPS O 1Hz pulse 10% duty cycle when GPS has position fix

6 TXD O 3V CMOS level, data output from EB-2307 RXD I 3V CMOS level, data into EB-2308 GND P Ground9 GND P Ground10 GPIO6/SO I/O* General input/ output, SPI data output11 GPIO4/SCS1 I/O* General input/ output, SPI chip select 112 GPIO5/SCS0 I/O* General input/ output, SPI chip select 0

13 GPIO7/SIN I/O* General input/ output, SPI data input 14 GPIO8/SCK I/O* General input/ output, SPI clock 15 V12RTC P RTC power 1.0~1.5Vdc, 500uA max 16 V30A P Analog power supply, 3.0V±10% 17 GND P Ground 18 GND P Ground

P: Power I: Input O: Output I/O*: Input or Output, Open if not used

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Application circuit

5) Test Plan:

Simulate experimental conditions on the ground by raising the plane to various levels and moving it forward and turning right and left. By programming ‘waypoints’ to be followed and moving the plane through the points manually we can see if the data is being read and responded to.

6) Test Results:

7) Final Design:

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Global Positioning System (GPS) is actually a …edge.rit.edu/edge/P09233/public/GPS Report...

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