A description of sources of errors in GPS

1. Sources of GPS Error in a Glance
pg. 1
Sources of GPS Error in a Glance
Complied By: Atiqa Ijaz Khan
Dated: 26-08-2016

2. Sources of GPS Error in a Glance
pg. 2
SOURCES OF GPS ERROR
IN A GLANCE
This article delves into GPS error sources that make it paramount for a professional surveyor
to either use a base station or land correction networks as the case may be.
1. Satellite Clock/ Receiver Clock
Each satellite carries with it an atomic clock that ticks with an accuracy of 1 nanosecond (1
billionth of a second). The GPS receiver also has its own clock. A GPS position calculation
depends on measuring signal transmission time from satellite to receiver which in turn depends
on knowing time on both ends. Even though the atomic clock is accurate it can drift up to a
millisecond which is enough to make an accuracy difference. To overcome this, survey-grade
receivers use some clever mathematical trickery to account for the time-keeping error based on
how the signals from three or more satellite signals are detected by the receiver, which
essentially allows the receiver to reset its clock. It is important to mention that the Satellite
clock error is the largest contributor of the GPS error budget.
2. Ionosphere
The ionosphere is the part of the atmosphere that occupies an altitude of 50-500km above the
earth. It mainly consists of ionized particles. As a GPS signal passes through this layer it is
delayed and deflected. The density of this layer varies in more places than others. The delay
also depends on how close the satellite is to being overhead which implies the distance it will
pass through the layer is least. A significant part of this error is removed through mathematical
modeling by the base or reference station to be transmitted to the roving receiver. The
significant part of the bias that is removed accounts for three quarters of the total ionospheric
error making it the second largest contributor to the GPS error budget.
3. Satellite Orbit
GPS receivers calculate coordinates relative to the known locations of satellites in space, a
complex task that involves knowing the shapes of satellite orbits as well as their velocities,
neither of which is constant. The GPS Control Segment monitors satellite locations at all times,
calculates orbit eccentricities, and compiles these deviations in documents called ephemerides.
An ephemeris is compiled for each satellite and broadcast with the satellite signal. GPS
receivers that are able to process ephemerides can compensate for some orbital errors. Check
whether your receiver can process ephemerides. If it does not then it does not compensate for
satellite orbit errors.

3. Sources of GPS Error in a Glance
pg. 3
4. Troposphere
Troposphere is the atmosphere layer placed between the earth’s surface and an altitude of about
60 km. The effect of the troposphere on the GNSS signals appears as an extra delay in the
measurement of the signal traveling from the satellite to receiver. This delay depends on the
temperature, pressure, humidity as well as the transmitter and receiver antennas location.
Signals from satellites close to the horizon are delayed the most, since they pass through the
most atmosphere.
5. Multi-path
Ideally, GPS signals travel from satellites through the atmosphere directly to GPS receivers. In
reality, GPS receivers must discriminate between signals received directly from satellites and
other signals that have been reflected from surrounding objects, such as buildings, trees, and
even the ground. Antennas are designed to minimize interference from signals reflected from
below, but signals reflected from above are more difficult to eliminate. One technique for
minimizing multipath errors is to track only those satellites that are at least 15° above the
horizon, a threshold called the ‘elevation mask angle.’ Also, most GPS manufactures boast that
their receivers possess advanced multipath mitigation technology. Multipath errors are
particularly common in urban or woody environments, especially those with large valleys or
mountainous terrain, and are one of the primary reasons why GPS receivers work poorly or not
at all in large buildings, underground, or on narrow city streets that have tall buildings on both
sides.
NOTE:
So the next time you set up your RTK, think about how the base receiver is modeling all these
errors mathematically in a matter of a few nanoseconds to give you a ‘fix’ in your roving
receiver. How wonderful technology is!
COURTESY TO: LinkedIn Post at 26-08-2016