Satellite Ephemeris Analysis

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Questions and Answers

What is the main purpose of GPS and trilateration?

To communicate with multiple users

To control satellite movements

To send signals to satellites

To fix position based on distance measurements (correct)

What is the nominal altitude of GPS satellites above Earth?

18,000 km

20,183 km (correct)

22,000 km

15,000 km

Why is GPS considered a passive system?

Users must actively send signals to GPS

Satellites only transmit signals that users receive (correct)

The system is not affected by user activity

Users provide feedback to the satellites

What does the term 'range' refer to in the context of GPS?

The distance measurement to fix position Signup and view all the answers

How does GPS measure distance to determine location?

Through trilateration using time of flight of signals Signup and view all the answers

What must the satellite do to assist the receiver in determining range?

Provide an accurate departure time of the signal Signup and view all the answers

What complicates the identification of control points in GPS?

The satellites are constantly moving Signup and view all the answers

What role does the receiver's clock play in GPS functionality?

It synchronizes with the satellite's clock Signup and view all the answers

What is the significance of the IODE in the navigation message?

It represents the issue of data ephemeris. Signup and view all the answers

How does the broadcast ephemeris affect the satellite's coordinates?

It deteriorates over time, affecting accuracy. Signup and view all the answers

Which parameter is NOT part of the ephemeris for satellite 2?

Satellite's altitude Signup and view all the answers

What does the term 'semi-major axis' signify in the satellite ephemeris?

The maximum width of the satellite's orbit. Signup and view all the answers

What parameter provides information on the rate of inclination angle in the ephemeris?

Rate of inclination angle Signup and view all the answers

Which component is specifically addressed in subframe 4 of the satellite data?

Atmospheric corrections Signup and view all the answers

What is the expected value for 'eccentricity' as represented in satellite 2's ephemeris data?

0.0139305 Signup and view all the answers

What characteristic of the broadcast ephemeris is highlighted as being imperfect?

It is based on least-squares analysis. Signup and view all the answers

What is indicated by the frequency of a modulated carrier in GPS oscillators?

The elapsed time between waveforms Signup and view all the answers

Which formula correctly represents the length of each complete wavelength in terms of frequency and the speed of light?

$rac{ca}{f}$ Signup and view all the answers

What phase angles indicate that two waves are in phase?

0⁰, 360⁰, 540⁰ Signup and view all the answers

What is the method used in GPS for addressing the cycle ambiguity problem?

Utilizing only two carriers of constant wavelengths Signup and view all the answers

In GPS, what is another term for the phase shift experienced when two waves reach different angles?

Out of phase Signup and view all the answers

What value of the speed of light is more commonly used for accuracy in calculations?

299,792,458 m/s Signup and view all the answers

What is the purpose of comparing the returning signal to a replica of the transmitted signal in EDM?

To determine the phase shift Signup and view all the answers

How are points on a modulated carrier defined in terms of angle?

By specific phase angles like 0⁰ and 180⁰ Signup and view all the answers

What was the primary purpose of Selective Availability (SA) in the GPS system?

To add artificial errors and increase security against hostile forces Signup and view all the answers

Which of the following errors could Differential GPS (DGPS) correct?

Satellite/Receiver clock errors Signup and view all the answers

What major change occurred on May 2, 2000, regarding GPS?

The removal of Selective Availability (SA) Signup and view all the answers

What is the result of having Selective Availability (SA) turned on?

Error introduced of up to 70 meters Signup and view all the answers

How does a Base Receiver in a Differential GPS system improve accuracy?

By comparing GPS signals to a known location Signup and view all the answers

What is a key benefit of using Differential GPS (DGPS) in moving applications?

It yields measurements accurate to a couple of meters Signup and view all the answers

What primary correction does DGPS provide related to atmospheric conditions?

Tropospheric and ionospheric refraction Signup and view all the answers

What information is primarily encrypted in the Anti-Spoofing (AS) approach?

Y code associated with P-code Signup and view all the answers

What is the primary purpose of the differential measurement mentioned?

To measure positional offset in both direction and distance Signup and view all the answers

What type of errors do GPS Community Base Stations primarily cancel out?

Natural and man-made errors Signup and view all the answers

Which receiver type is specifically designed for extreme accuracy?

Survey-grade receivers Signup and view all the answers

What is a key characteristic of recreational-grade GPS receivers?

Typically uses CA code and offers least accuracy Signup and view all the answers

What is the cost range for mapping-grade GPS receivers?

PhP33,600 - PhP560K Signup and view all the answers

What channel capacities are typically found in GPS receivers?

6, 8, or 12 channels Signup and view all the answers

Where is the UP GPS Community Base Station located?

On the rooftop of Melchor Hall Signup and view all the answers

What can be achieved by combining signals from Electronic GCP and user’s GPS?

Positional accuracy with several cm order in real-time Signup and view all the answers

Study Notes

Ephemeris and Broadcast Ephemeris

The user’s computer requires information from the satellite’s ephemeris to calculate its ECEF and WGS84 coordinates
The broadcast ephemeris is not perfect although it is expressed in Keplerian parameters
It is the result of a least-squares curve fitting analysis of the satellite’s actual orbit
Broadcast ephemeris deteriorates over time this is why the Navigation message uses the IODE (Issue of Data Ephemeris)

Ephemeris Parameters for Satellite 2

PRN number: 2
Issue of ephemeris data: 224
Semi-Major Axis (meters): 2.65603E+07
C(ic) (rad): 1.88127E-07
C(is) (rad): -1.00583E-07
C(rc) (meters): 321.656
C(rs) (meters): 87.6875
C(uc) (rad): 4.36418E-06
C(us) (rad): 2.70829E-06
Mean motion difference (rad/sec): 5.04521E-09
Eccentricity (dimensionless): 0.0139305
Rate of inclination angle (rad/sec): 4.11089E-10
Inclination angle @ ref.time (rad): 0.950462
Mean Anomaly at reference time (rad): -2.62555
Corrected Mean Motion (rad/sec): 0.000145859
Computed Mean Motion (rad/sec): 0.000145854
Argument of perigee (rad): -2.56865
Rate of right ascension (rad/sec): -8.43857E-09
Right ascension @ ref time (rad): 1.75048
Sqrt (1 - e^2): 0.999903
Sqr root semi-major axis, (m^1/2): 5153.67
Reference time ephemeris (sec): 240704

Atmospheric Corrections

Subframe 4 addresses atmospheric correction

GPS Satellites

GPS Satellites are operational 95% of the time
Block II/IIA/IIR/IIR-M: 31 operational GPS satellites

GPS and Trilateration

GPS and trilateration measure distances to fix positions
Range is the GPS term for distance
GPS ranging measures distance from satellites orbiting at a nominal altitude of 20,183 km above earth

A Passive System

GPS signals are broadcast in the microwave part of the electromagnetic spectrum
It is a passive system because only the satellites transmit
There is no limit to the number of users

One-way Ranging

Distance is a function of speed of light, signal frequency, and elapsed time
The GPS signals do not return to the satellite
A clock in the satellite marks its departure time
A receiver’s clock marks the arrival time
The range depends on the time the GPS signal takes to travel from satellite to receiver

Control

The satellites are the control points in GPS
The satellite’s movement complicates the process

EDM Distance Determination

GPS oscillators are sometimes called clocks
The length of a complete wavelength is: λ=ca/f
- λ = the length of each complete wavelength (m)
- ca = the speed of light corrected for atmospheric effects
- f = frequency in hertz

Phase Shift

The fractional part of a wavelength is determined by comparing the returning signal to the transmitted signal to determine the phase shift
Two waves that reach the same phase angle at the same time are in phase
Two waves that reach the same phase angle at different times are out of phase
In GPS, the process is called carrier phase ranging and the measurement is done on the carrier itself

The Cycle Ambiguity Problem

The determination of the fractional part of a wavelength can be solved, but the number of full wavelengths is still a problem
GPS ranging uses a different technique to solve the cycle ambiguity problem because the satellites broadcast only 2 carriers of constant wavelengths in one direction.

Denial of Accuracy (DOA)

The US military uses two approaches to prevent the use of the full resolution of the GPS system:
- Selective Availability (SA): noise is added to the clock signal and the navigation message contains inaccuracies
- Anti-Spoofing (AS): the P-code is encrypted with Y code

Selective Availability (S/A)

The government introduced artificial errors to reduce GPS position accuracy
This discouraged hostile forces from using the system
It was the largest source of error
Eventually removed in May 2000

How S/A Works

The Space Command control center offsets satellite clocks and introduces ephemeris errors
Only the military has the correction information

S/A Error

The error introduced by S/A is up to 70 meters
Users with S/A active can expect accuracy of around 100 meters
Users with S/A inactive can expect accuracy of around 20 to 40 meters

Selective Availability Removed

Selective Availability was removed on May 2, 2000

Differential GPS

Differential GPS is an effective way to correct various inaccuracies in the GPS system.
Differential GPS or “DGPS” yields measurements accurate to a couple of meters in moving applications and even better in stationary situations
This improved accuracy makes GPS a powerful measurement system.

DGPS: What Errors can be Corrected?

Satellite/receiver clock error
Satellite ephemeris error
Atmospheric Refraction (Ionospheric and Tropospheric)
Receiver Noise
Multipath
Selective Availability

DGPS: How does it work?

Two GPS receivers are used:
- A base receiver or base station is placed at a known controlled point of reference such as a National Geodetic Survey marker.
- A rover receiver collects information autonomously.
Software/hardware at the base station calculates the difference between the known position and the GPS position. This differential is a measurement of positional offset in both direction and distance
The differential data is used to correct the positional errors in the rover GPS receivers in real-time or after the fact.

GPS Community Base Stations

They make GPS even more accurate by canceling out most natural and man-made (SA) errors

Public GPS Stations

Public GPS stations: provide continuous observation and dissemination of the received signals
By combining signals from Electronic GCP and user GPS receivers, positional accuracy on the order of centimeters can be achieved real-time

GPS Community Base Station in UP

Located on the rooftop of Melchor Hall (College of Engineering)
Beneath the rooftop is a Coast and Geodetic Survey control point
The antenna is connected to a receiver and a computer

GPS Equipment - Antenna

Mounted on a roof, a pole, a truck or a person

GPS Equipment - Receivers

Receivers are designed for casual users (recreationists), navigation, mapping, and surveying
Each satellite tracked requires a “channel” in the receiver
Receivers generally have 6, 8, or 12 channels
Recreation-grade receivers: typically use CA code and have the least accurate positioning
Navigating receivers: similar to recreation receivers but designed for portability and long battery life
Mapping-grade receivers: offer more features, higher accuracy, and store more data (including attribute data). They typically use L1 and L2 code.
Survey-grade receivers: designed for extreme accuracy. They are larger and heavier than other types, and are very expensive. They also typically use L1 and L2 code.

GPS Receivers - Cost

Recreation Grade: PhP 4,200 - PhP 33,600
Mapping Grade: PhP 33,600 - PhP 560,000
Survey Grade: PhP 500,000 - PhP 3,000,000

GPS Receivers - Performance

Accuracy varies depending on the type of receiver

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Description

This quiz focuses on understanding satellite ephemeris and its parameters, specifically the broadcast ephemeris used for computing ECEF and WGS84 coordinates. It covers key concepts such as Keplerian parameters, IODE, and the impact of time on the accuracy of satellite navigation data.