AS 16 - Intro GPS 2 Flashcards
Review Questions
GPS consists of how many segments?
What are they?
Three
Space
Control
User
A two and three satellite GPS system will give ambiguous results, a fourth satellite will provide a definite solution and is needed to?
Absolutely resolve the ambiguity due to the two-position possibility and
resolve any errors inherent in the receiver clock
How does the MCS control GPS time?
Using Cesium clocks
Regarding the MCS, how is clock drift between updates dealt with?
Clock drift is predicted and passed to users to allow more accurate positioning
Each satellite has how many atomic clocks?
4, two cesium and 2 rubidium
What terms refers to calculating the difference in timing accuracy between satellite receiver?
Pseudo-ranging (AKA clock bias)
What does Pseudo Ranging allow for in a receiver?
The use of an inexpensive crystal clock and not an atomic clock
The C/A code is available to which users of GPS?
All users
How many C/A codes are assigned to satellites?
A total of 37 codes with each satellite having a unique code.
A C/A code is how many bits long and is transmitted at which rate?
1 023 bits long, tx @ 1.023 megabits/ sec (repeated every millisecond)
A C/A code is tx on what frequency?
L1 only
How long is the code sequence for a P code?
267 days long
Each satellite has which portion of a P code?
A unique 7 day portion
What is the tx rate of a P code?
10.23 megabits/ sec
The P Code is broadcast on which frequencies?
L1 and L2
What is the accuracy of a P code?
Less than or equal to 5.9 m 95% of the time
When the US military encrypts a P code, it becomes known as what?
Y Code
Regarding receivers, a P Code requires what compared to a C/A code?
A much more sophisticated receiver
What does a satellite clock frequency operate at?
10.23 MHz
What do the satellites transmit on?
L1 = 154 x 10.23 MHz = 1575.42 MHz L2 = 120 x 10.23 MHz = 1227.6 MHz
What are the different types of GPS capabilities?
Civilian L2 (L2C)
Military (M-Mode)
Safety of Life (L5)
New Civilian L1 (L1X)
Describe the Civilian L2 (L2C)
2nd civilian use frequency
Improved accuracy of navigation
GPS Block llR-M capability
Regarding the Military (M-Code), what freqs is it transmitted on and what does it not require, unlike P Code?
Tx on L1 and L2
Does not require C/A code to achieve lock, meaning it can calculate the users position with only the M Code
Regarding the Military (M-Code), what is the relationship between signals sent and antennas?
Two distinct signals are sent from two distinct antenna.
Regarding the Military (M-Code), the directional antenna’s signal (spot beam) is aimed at a specific region and increases what?
Local signal strength by 20 dB
Regarding Safety of Life (L5), who uses this?
Civilians involved in safety-of-life transportation and other high-performance applications.
What is special about the L5 band? What does it feature?
It is reserved exclusively for aviation safety services
It features higher power, greater bandwidth and advanced signal design.
When used in combination with L1 C/A and L2C, L5 will enable what accuracy?
Sub-meter accuracy without augmentation
Regarding Civilian L1 (L1X), which code has been modernized with newer capabilities?
C/A
Regarding Civilian L1 (L1X), backwards capability refers to what?
How Civilian L1 (L1X) can work with older C/A GPS signals
Regarding Civilian L1 (L1X), what component contains a pilot carrier to improve tracking?
The non-data signal component.
How was the Civilian L1 (L1X) designed regarding other systems?
To ensure interoperability.
What are some points on pseudo-random codes?
Satellites and receivers are synchronized and
Pseudo-random sequences are repeated every milli-second
Regarding pseudo-random codes, how does the receiver determine range?
The receiver compares the received signal to its self-generated signal.
The time difference between the two signals can be converted to range.
Ask for more detail about slide 19
I think its saying the GPS signal is so slight that after observing the background noise signal, it can be positioned within the background noise of earth and still received accurately, eliminating the need for powerful tx and large antennas
True or false
Regarding Pseudo Ranging, adding a third satellite (S3) into a situation where S1 and S2 are time-incorrect will result in a correct position.
True
What does the Navigation Message contain?
The info required by the receiver to perform the operations and computations required to navigate using GPS.
The Nav message and C/A and P codes relate how?
The Nav message is superimposed over the two codes
How long are the bits is the Nav message and how long does it take to tx an entire almanac message?
Each message is 37 500 bits long
Takes 12.5 minutes to tx.
A Nav message is broken down into how many frames and sub frames?
25 frames and 5 sub-frames per message
What is the ephemeris portion of the Nav message entail?
The predictions/ updates of current position of an individual satellite’s orbital position, that is tx to the user in the Nav message.
What is the almanac portion of the Nav message entail?
A catalogue of ephemeris data, for all the satellites, continually updated in the user receiver as new ephemeris data is received.
What is the almanac portion of the Nav message tell the user?
The approximate position of each satellite
The ephemeris portion of the Nav message is updated how often?
Usually hourly by the MCS
The initial start-up for a satellite uses what data?
Almanac data and/or entered position, velocity and time (including date) (PVT)
Upon start-up, a satellite will choose?
the best four satellites within its horizon
If the receiver does not have PVT or almanac data what mode will it enter?
“Searching the Sky” mode
Using a preprogrammed sequence, the receiver picks what?
A particular satellite and begins searching for its unique C/A code.
True or false
The inability to find a satellite precludes other satellites due to constellation geometry.
True
Once a particular satellite is found, the memory will?
Tell the receiver which other satellites are available.
In multi-channel receivers, the GPS can do what?
Search for many satellites at the same time and choose the best based on geometry.
After the receiver chooses four satellites it knows the approximate distance to each and their respective C/A codes, it will estimate what?
Phase shift req’d for the time difference
Doppler shift req’d for the relative speeds
After receiving the estimated shifts (phase and doppler) the code and carrier tracking loops can perform what?
Signal acquisition by scanning the incoming code from the satellite and matching it with its own receiver-generated code.
What is a Tracking Loop?
A system within a receiver that generates its own signal then tries to match it with the incoming signal.
To successfully lock onto the satellites signal, the tracking loops must compensate for what two factors?
The Phase Shift (code-tracking loop) and the Doppler Shift (carrier tracking loop)
Regarding the Code Tracking Loop, the C/A code search commences at what point? Why?
At the phase shifted point. This allows for the time lag from satellite to receiver.
The receiver will adjust as necessary to obtain a code lock which also concedes with what? Why?
Highest signal to noise ratio
The amplification (Highest signal to noise ratio) allows the reception of weak signals and helps offset gain limits from the small antenna
What is Code Lock?
When the two signals match and is also the point of highest signal to noise ratio.
After Code Tracking is complete, the Carrier Tracking Loop begins to what?
Narrow in on the correct frequency from the satellite.
The Carrier Tracking Loop uses a crystal oscillator to what?
To develop a frequency, which it adds to the incoming frequency, to come up with the original satellite frequency.
The receiver extracts what from the carrier?
The navigation message.
After the receiver has code and carrier lock on all viable satellites, any changes will be a result of what?
Receiver movement
After the receiver has code and carrier lock on all viable satellites, the carrier and code do what?
The carrier tracking loop assists the refining of position by providing velocity measurements to the code tracking loop to help it maintain lock-on.
After the receiver has code and carrier lock on all viable satellites, the receiver refines what?
The pseudo ranges it originally calculated.
After the receiver has code and carrier lock on all viable satellites, a fourth pseudo code is used to what?
Offset the clock error from its clock to the more accurate GPS system time.
Once a receiver has the required satellites, clock bias and the nav message, what can it do?
Navigate
Review Questions
What technique does the GPS use to calculate the difference in timing accuracy between satellite and receiver?
Pseudo-ranging
Review Questions
How long does it take to receive the entire Navigation message?
12.5 minutes
Review Questions
How many Gold codes (C/A) are there?
37
Review Questions
What are the two carrier frequencies?
L1 = 154 x 10.23 MHz = 1575.42 MHz
L2 = 120 x 10.23 MHz = 1227.6 MHz
Review Questions
How does the GPS initially select satellites for navigation?
Uses almanac data and/or operator entered PVT data
Chooses 4 best satellites within its horizon
If it doesn’t have almanac or PVT data, it enter “Searching the Sky” mode
Review Questions
How does GPS acquire satellites?
The receiver now knows what C/A codes it will search for
Estimates the phase shift required for the time difference
Estimated the doppler shift needed for the relative speeds
The code and carrier tracking loops now start to lock onto the signals
