Appendix d: frequently asked questions (faq) – Navman 11 User Manual

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APPENDIX D: Frequently Asked

Questions (FAQ)

This appendix provides answers to frequently

asked questions about GPS in general and about

the Jupiter series of GPS receivers, it is intended

to supplement the operational description provided

in section 4.0 of this document.

1. How far and under what conditions can a passive

antenna track before it is necessary to change it to

an active antenna?

There is no simple answer to this question.

Navman generally recommends limiting cable loss

to 3 dB between the antenna and the receiver

board. If attenuation exceeds this value there may

be degraded signal acquisition and navigation

accuracy performance. GPS satellites transmit

more power than their speciﬁcation requires, but

that margin is allocated to the 3 dB cable loss. The

safest approach is to use an active antenna unless

the antenna and receiver engine are co-located.

2. Can the Jupiter receiver operate efﬁciently in an

urban location with tall structures and buildings?

Yes. By using 12 parallel channels, Jupiter

receivers maintain continuous tracking of all

visible satellites and produce an over-determined

solution, minimising the effects of signal blockage

and giving optimal performance in dense urban

environments.

3. Is there any danger to the receiver when

switching is done between active and passive

antennas?

Yes. If pre-amp power is supplied to an active

antenna and then connected to a passive antenna

there is a high probability of damage since the

passive antenna often presents a short circuit to

ground at DC. This then shorts out the pre-amp

power line and destroys the bias-tee network on

the receiver.

4. What is the criteria for choosing satellites for

navigation if more than four are visible?

The Jupiter receiver continuously tracks all visible

satellites. The measurements from these satellites

are used in an over-determined solution to provide

the most robust performance that is possible.

5. What is the accuracy of GPS with selective

availability turned on? How is the accuracy affected

by DGPS?

The U.S. Government guarantees that horizontal

accuracy will be less than 100 m (95% of the

time) and less than 300 m (99.99% of the time).

Accuracy with DGPS is primarily a function of the

quality and latency of the corrections used.

6. What is the difference between the two models

for position determination used in GPS: WGS 84

and Earth-Centred-Earth-Fixed (ECEF)?

ECEF refers to a Cartesian (rectangular)

coordinate system (x,y,z,) whose centre is at the

middle of the Earth; one axis goes through the

North Pole, one through the Greenwich meridian

at the equator, and the third passes through the

equator 90 degrees offset from the second. This

system rotates with the Earth. GPS satellites

broadcast their location in this coordinate system.

WGS-84 contains a mathematical model of the

Earth’s surface (spheroid) which is accepted

worldwide. However, the model does have some

limitations. For example, 0 m altitude may differ

from mean sea level in this model by up to ~100 m.

Position in WGS-84 is speciﬁed in latitude and

longitude and by the altitude above the WGS-84

spheroid (Earth surface model).

7. What are the addresses for U.S. FM DGPS service

providers?

• ACCQPOINT Communications Corp. 2737,

Campus Drive, Irvine, CA 92715, (800) 995-3477
• Differential Corrections Inc. (DCI) 10121 Miller

Avenue, Cupertino, CA 95014, (408) 446-8350

8. Does the Jupiter receiver provide an over-

determined solution?

Jupiter receivers provide all-in-view parallel

tracking of all visible satellites. In SPS mode

all valid measurements are used to produce an

over-determined navigation solution to minimise

position excursions arising from SA and loss of

signals. In DGPS all valid measurements with valid

DGPS corrections are used in an over-determined

solution. For example, if 8 satellites are in track,

all producing valid measurements, and DGPS

corrections are available for 7 of the 8, then 7

DGPS corrected measurements will be used in the

over-determined DGPS solution.

9. How is heading data at low speeds derived?

Shouldn’t the heading be derived from Doppler data

rather than position differences?

Navman receivers compute velocity from the

carrier loop Doppler information. Heading angle

is then computed from east and north velocity

as the arc-tangent (Ve/Vn). When on, SA

induces an error on the GPS clock and thus the

carrier Doppler information and pseudo-range is

corrupted, but the carrier data is a better source of

velocity information than position differences. The

worst heading error at 5 m/s is 1.1 degrees when

SA is off or DGPS is on. All heading determination

techniques using GPS velocities have large

uncertainties at small velocities when the velocity

approaches the magnitude of the inherent noise.

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