u-blox 6

navBbrMaskfield, where Hot, Warm and Cold starts can be initiated, and also other combinations thereof.
13.3 Aiding / Assisted GPS (A-GPS)
The Challenge of Stand-alone GPS
GPS users expect instant position information. With standard GPS this is not always possible because at least
four satellites must transmit their precise orbital position data, called Ephemeris, to the GPS receiver. Under
adverse signal conditions, data downloads from the satellites to the receiver can take minutes, hours or even
fail altogether.
Assisted GPS (A-GPS) boosts acquisition performance by providing data such as Ephemeris, Almanac, accurate
time and satellite status to the GPS receiver via mobile networks or the Internet. The aiding data enables the
receiver to compute a position within seconds, even under poor signal conditions.
13.4 Aiding Data
The following aiding data can be submitted to the receiver:
• Position: Position information can be submitted to the receiver using the UBX-AID-INI message. Both,
ECEF X/Y/Z and latitude/longitude/height formats are supported.
• Time: The time can either be supplied as an inexact value via the standard communication interfaces,
suffering from latency depending on the baud rate, or using hardware time synchronization where an
accurate time pulse is connected to an external interrupt. Both methods are supported in the UBX-AID-INI
message.
• Frequency: It is possible to supply hardware frequency aiding by connecting a periodic rectangular signal
with a frequency up to 500 kHz and arbitrary duty cycle (low/high phase duration must not be shorter than
50 ns) to an external interrupt, and providing the applied frequency value using the UBX-AID-INI
message.
• Orbit data: Orbit data can be submitted using UBX-AID-ALM and UBX-AID-EPH.
• Additional information: UBX-AID-HUI can be used to supply health information, UTC parameters and
ionospheric data to the receiver.
13.5 Aiding Sequence
A typical aiding sequence comprises the following steps:
• Power-up the GPS receiver
• Send UBX-AID-INI (time, clock and position) message.
• Send UBX-AID-EPH (ephemeris) message.
• Apply optional hardware time synchronization pulse within 0.5 s after (or before, depending on the
configuration in UBX-AID-INI) sending the UBX-AID-INI message if hardware time synchronization is
required. When sending the message before applying the pulse, make sure to allow the GPS receiver to
parse and process the aiding message. The time for parsing depends on the baud rate. The processing time
is 100 ms maximum.
• Send optional UBX-AID-HUI (health, UTC and ionosphere parameters) message.
• Send optional UBX-AID-ALM (almanac) message.
13.6 AssistNow Online
AssistNow Online is u-blox' end-to-end Assisted GPS (A-GPS) solution that boosts GPS acquisition performance,
bringing Time To First Fix (TTFF) down to seconds. The system works by accessing assistance data such as
Ephemeris, Almanac and accurate time from our Global Reference Network of GPS receivers placed around the
GPS.G6-SW-10018-A Public Release Page 34 of 201