Global Navigation Satellite Systems (GNSS) 18

The monitoring stations check the SVs’ internally computed position and clock time at least once every 12 hours. Although the calculation of position using Keplerian laws is precise, the SV orbits are affected by the gravitational influences of the sun, moon and planets and are also affected by solar radiation, so errors between the computed position and the actual position occur. When a positional error is detected by the ground station, it is sent to the SV for the SV to update its knowledge of position. Similarly if an error is detected in the SV clock time this is notified to the SV, but since the clocks cannot be adjusted, this error is included in the SV broadcast.

The User Segment

The User Segment is all the GPS receivers using the space segment to determine position on and close to the surface of the earth. These receivers may be stand-alone or be part of integrated systems.

There are several types of receiver:

Sequential receivers which use one or two channels and scan the SVs sequentially to determine the pseudo-ranges.

Multiplex receivers may be single or twin channel and are able to move quickly between SVs to determine the pseudo-ranges and hence have a faster time to first fix than the sequential receivers.

Multi-channel receivers monitor several SVs simultaneously to give instant positional information. These include ‘all-in-view’ receivers which monitor all the SVs in view and select the best 4 to determine position. Because of the speed of operation these are the preferred type for aviation.

Figure 18.8 GPS Receiver, Control Unit

Global Navigation Satellite Systems (GNSS) 18

311

18 Global Navigation Satellite Systems (GNSS)

(GNSS) Systems Satellite Navigation Global 18

Figure 18.9 Initialization Page

Figure 18.10 Position Page

Figure 18.11 Waypoint Definitions Page

312

Global Navigation Satellite Systems (GNSS) 18

Principle Of Operation

The navigation message is contained in one frame comprising 5 sub-frames. The sub-frames each take 6 seconds to transmit, so the total frame takes 30 seconds for the receiver to receive. Frame 1 contains SV clock error, frames 2 and 3 contain the SV ephemeris data, frame 4 contains data on the ionospheric propagation model, GPS time and its correlation with UTC. The fifth frame is used to transmit current SV constellation almanac data. A series of 25 frames is required to download the whole almanac. The almanac data is usually downloaded hourly and is valid from 4 hours to several months dependent on the type of receiver.

SUBFRAME #ONE SUBFRAME = 300 BITS, 6 SECONDS 1 TLM HOW SV CLOCK CORRECTION DATA

	2	TLM	HOW	SV EPHEMERIS DATA (I)	ONE
					DATA
					FRAME
	3	TLM	HOW	SV EPHEMERIS DATA (II)	=
		25 PAGES OF SUBFRAME 4 AND 5 = 12.5 MINUTES			1500 BITS,
					30 SECONDS
	4	TLM	HOW	OTHER DATA (IONO,UTC,ETC)
	5	TLM	HOW	ALMANAC DATA FOR ALL SVS

ONE WORD = 30 BITS, 24 DATA, 6 PARITY

TLM
TELEMETRY WORD	8-BIT PREAMBLE		DATA		PARITY
HOW
	17-BIT TIME OF WEEK			DATA	PARITY
HANDOVER WORD

Figure 18.12 GPS Navigation Data Format

Because the orbits are mathematically defined, an almanac of their predicted positions can be and is maintained within the receivers. Thus, when the receiver is switched on, provided it knows its position and time to a reasonable degree of accuracy, it will know which SVs to expect and can commence position update immediately. If the almanac is corrupted, out of date or lost, or if receiver position or receiver clock time are significantly in error it will not find the expected SVs and will download the almanac from the constellation. The almanac data fills 25 frames so it takes 12.5 minutes to download. When the receiver position is significantly in error it will not detect the expected SVs. Having downloaded the almanac the receiver will now carry out a skysearch, this involves the receiver checking which SVs are above the horizon and selecting the 4 to give the most accurate fix, then commencing position fixing, this takes a least a further 2.5 minutes. Hence the time to first fix will be at least 15 minutes. If there are no problems then the first fix, on initialization, will be obtained within about 30 seconds.

The GPS receiver internally generates the PRN code and compares the relative position of the two codes to determine the time interval between transmission and reception.

Global Navigation Satellite Systems (GNSS) 18

313

18 Global Navigation Satellite Systems (GNSS)

(GNSS) Systems Satellite Navigation Global 18

RECEIVER OSCILLATOR PRODUCED
1	2	3	4	5			6	7	8	9	10
				1	2	3	4			5	6	7	8	9	10	11
												TIME
											DIFFERENCE
SATELLITE (ATOMIC CLOCKS)
PSEUDO-RANDOM
CODES

Figure 18.13 Pseudo-Random Code Time Measurement

The initial measurement of range is known as pseudo-range because it has not yet been corrected for receiver clock error.

The receiver uses four SVs and constructs a three dimensional fix using the pseudo-ranges from the 4 SVs. Each range corresponds to a position somewhere on the surface of a sphere with a radius in excess of 10 900 NM.

11 000 MILES

Figure 18.14

314

Global Navigation Satellite Systems (GNSS) 18

The intersection of two range spheres will give a circular position line.

TWO MEASUREMENTS PUTS US

SOMEWHERE ON THIS CIRCLE

Figure 18.15

The introduction of a third range sphere will produce two positions several thousand miles apart. One position will be on or close to the surface of the earth, the other position will be out in space, so it would be possible to use just three pseudo-ranges to produce a position, by rejecting the space position.

However, a fourth range position line is needed because of the way the receiver compensates for receiver time errors. The receiver has an accurate crystal oscillator to provide time. However, the accuracy does not compare with the accuracy of the SV clocks, so there will always be an error in the time measurement, and hence in the computation of range. Furthermore the receiver clock is deliberately kept in error by a small factor to ensure that the correction process can only go in one direction. This is why the initial calculated range is known as a pseudorange. As a result the position lines will not meet in a point but will form a ‘cocked hat’. For example, if the receiver clock is permanently 1 millisecond fast, then the receiver will over estimate each range by about 162 NM. So when the receiver sets about calculating the correct ranges it knows that it must reduce the pseudo-ranges.

Global Navigation Satellite Systems (GNSS) 18

315

18 Global Navigation Satellite Systems (GNSS)

(GNSS) Systems Satellite Navigation Global 18

A
	B
5
SECONDS	X
	XX
	7
	SECONDS
	9
	SECONDS
	C

Figure 18.16

4	B
SECONDS
A	6
	SECONDS

X

8 SECONDS

C

Figure 18.17

The receiver has to correct the X, Y, Z coordinates and time to produce the fix. Since it has each element provided by each SV the receiver can set up 4 linear simultaneous equations each with 4 unknown quantities (X, Y, Z, and T) which it solves by iteration to remove the receiver time error, and hence, range errors. This means that the use of 4 SVs provides a 3D fix and an accurate time reference, i.e. a 4D fix, at the receiver. The X, Y, and Z coordinates can now be transposed into latitude and longitude or any other earth reference system (e.g. the UK Ordnance Survey grid) and altitude.

Note: Some receivers can also produce a three dimensional position using three SVs with an input of altitude, the altitude simulates a fourth SV positioned at the centre of the earth. However the position produced will not be as accurate as the 4D fix.

316